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The Structure of the Proton in the Higgs Boson Era

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Juan Rojo gave a seminar at NIKHEF in Amsterdam on January 22, 2015 about the structure of the proton in the Higgs boson era. He discussed how the discovery of the Higgs boson completed the Standard Model but also opened new questions. He explained that the Large Hadron Collider will play a key role in exploring these questions over the next 20 years. Accurately determining the parton distribution functions of the proton is vital for phenomenology at the LHC.

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Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 ! The Structure of the Proton ! in the Higgs Boson Era Juan Rojo! STFC Rutherford Fellow! Rudolf Peierls Center for Theoretical Physics! University of Oxford! ! ! Theory Seminar ! NIKHEF, 22/01/2015
! ! ! ! ! ! Exploring the high-energy frontier:! The Large Hadron Collider Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! The Higgs Boson is the most important discovery in particle physics in 25 years ! The Higgs completes the extremely successful Standard Model of particle physics, but at the same time opens a number of crucial questions for the field that we need to address! The LHC will play a crucial role in exploring the energy frontier in the next 20 years Particle Physics in the headlines Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! Standard Model of Particle Physics Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1" Higgs"boson"and"EWSB" !  Is"mH"natural"or"fine8tuned"?" """if"natural:"what"new"physics/symmetry"?" !  does"it"regularize"the"divergent"WLWL"cross8secEon" """"""at"high"M(WLWL)"?"Or"is"there"a"new"dynamics"?" !  elementary"or"composite"Higgs"?" !  is"it"alone"or"are"there"other"Higgs"bosons"?" !  origin"of"couplings"to"fermions""" !  coupling"to"dark"maKer"?"" !  does"it"violate"CP"?" !  cosmological"EW"phase"transiEon"" Neutrinos:" !  ν"masses"and"and"their"origin" !  what"is"the"role"of"H(125)"?""" !  Majorana"or"Dirac"?" !  CP"violaEon"" !  addiEonal"species"""sterile"ν"?" Dark"maKer:" !  composiEon:"WIMP,"sterile"neutrinos,"" """""axions,"other"hidden"sector"parEcles,".." !  one"type"or"more"?"" !  only"gravitaEonal"or"other"interacEons"?" The"two"epochs"of"Universe’s"accelerated"expansion:" !  primordial:"is"inflaEon"correct"?"" """""which"(scalar)"fields?"role"of"quantum"gravity?""" !  today:"dark"energy"(why"is"Λ"so"small?)"or" """""modificaEon"of"gravity"theory"?" Quarks"and"leptons:" !  why"3"families"?" !  masses"and"mixing" !  CP"violaEon"in"the"lepton"sector" !  maKer"and"anEmaKer"asymmetry" !  baryon"and"charged"lepton"" """""number"violaEon"" Physics"at"the"highest"E8scales:" !  how"is"gravity"connected"with"the"other"forces"?" !  do"forces"unify"at"high"energy"?" Outstanding**Ques-ons*in*Par-cle*Physics*circa%2014* …"there"has"never"been"a"beKer"Eme"to"be"a"parEcle"physicist!" List from Ian Shipsey Many of these crucial questions can be addressed at the Large Hadron Collider
! ! ! ! ! ! Standard Model of Particle Physics Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1" Higgs"boson"and"EWSB" !  Is"mH"natural"or"fine8tuned"?" """if"natural:"what"new"physics/symmetry"?" !  does"it"regularize"the"divergent"WLWL"cross8secEon" """"""at"high"M(WLWL)"?"Or"is"there"a"new"dynamics"?" !  elementary"or"composite"Higgs"?" !  is"it"alone"or"are"there"other"Higgs"bosons"?" !  origin"of"couplings"to"fermions""" !  coupling"to"dark"maKer"?"" !  does"it"violate"CP"?" !  cosmological"EW"phase"transiEon"" Neutrinos:" !  ν"masses"and"and"their"origin" !  what"is"the"role"of"H(125)"?""" !  Majorana"or"Dirac"?" !  CP"violaEon"" !  addiEonal"species"""sterile"ν"?" Dark"maKer:" !  composiEon:"WIMP,"sterile"neutrinos,"" """""axions,"other"hidden"sector"parEcles,".." !  one"type"or"more"?"" !  only"gravitaEonal"or"other"interacEons"?" The"two"epochs"of"Universe’s"accelerated"expansion:" !  primordial:"is"inflaEon"correct"?"" """""which"(scalar)"fields?"role"of"quantum"gravity?""" !  today:"dark"energy"(why"is"Λ"so"small?)"or" """""modificaEon"of"gravity"theory"?" Quarks"and"leptons:" !  why"3"families"?" !  masses"and"mixing" !  CP"violaEon"in"the"lepton"sector" !  maKer"and"anEmaKer"asymmetry" !  baryon"and"charged"lepton"" """""number"violaEon"" Physics"at"the"highest"E8scales:" !  how"is"gravity"connected"with"the"other"forces"?" !  do"forces"unify"at"high"energy"?" Outstanding**Ques-ons*in*Par-cle*Physics*circa%2014* …"there"has"never"been"a"beKer"Eme"to"be"a"parEcle"physicist!" List from Ian Shipsey For the next 20 years, LHC will be at the forefront of the exploration of the high-energy frontier
6 Juan Rojo PDF@CMS Kick-off Workshop, CERN, 07/05/2012
The inner life of the protons! The Large Hadron Collider collides proton, but these are not fundamental particles: really what the LHC is doing is colliding quarks and gluons! The distribution of momentum that the quarks and gluons carry is quantified by the Parton Distribution Functions (PDFs), determined by non-perturbative dynamics: cannot be computed from first principles and need to be extracted from experimental data! An accurate determination of PDFs is of paramount importance to be able to do precision physics at hadronic colliders as the LHC! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
LHC collisions in a nutshell Drawing by K. Hamilton Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
Drawing by K. Hamilton Initial State:! Parton Distribution Functions (PDFs) Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 LHC collisions in a nutshell The accurate determination of the Parton Distribution Functions of the proton is a vital ingredient for LHC phenomenology
QCD: The Toolbox for Discoveries at the LHC ! ! ! ! ! ! Campbell, ICHEP12 My key message • The days of “guaranteed” discoveries or of no-lose theorems in particle physics are over, at least for the time being .... • .... but the big questions of our field remain wild open (hierarchy problem, flavour, neutrinos, DM, BAU, .... ) • This simply implies that, more than for the past 30 years, future HEP’s progress is to be driven by experimental exploration, possibly renouncing/reviewing deeply rooted theoretical bias • This has become particularly apparent in the DM-related sessions: • Direct detection experiments and astrophysics are challenging the theoretical DM folklore as much as the LHC is challenging the theoretical folklore about the hierarchy problem. • But great opportunities lie ahead, and the current challenges are simply hardening theorists’ ingenuity, creativity and skills 3 Mangano, Aspen14 Improving our quantitative understanding of the Standard Model is essential in this new era for HEP, where we need to hunt, unbiased, for answers to the big questions of our field! Now, more than ever, sharpening our QCD tools could be the key for new discoveries at the LHC Prime example: extraction of Higgs couplings from LHC data soon to be limited by QCD uncertainties! ! More accurate determination of PDFs ! ! Improved indirect sensitivity to New Physics via deviations of Higgs couplings from SM expectations Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
Parton Distributions and LHC phenomenology ! ! ! ! ! ! 2) Very large PDF uncertainties (>100%) for new heavy particle production Supersymmetric QCD 1) PDFs fundamental limit for Higgs boson characterization in terms of couplings 3) PDFs dominant systematic for precision measurements, like W boson mass, that test internal consistency of the Standard Model Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! The inner life of protons :! Parton Distribution Functions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
13 Deep-Inelastic scattering and the discovery of quarks Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Deep-inelastic lepton-proton scattering: First evidence for proton structure (SLAC, 70s)! Measured scattering cross-section constant as resolution scale 1/Q decreases.! Evidence for point-like constituents in the proton: the quarks If the proton had a different structure, a form factor F(Q) would be expected! Analogous to Rutherford’s discovery of the point-like atomic nucleus, while expecting Thomson’s Plum model
14 Deep-Inelastic scattering and the discovery of quarks Partonic xsec Parton Distributions Parton Distribution Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 ( 1/ Resolution Scale ) Deep-inelastic lepton-proton scattering: First evidence for proton structure (SLAC, 70s)! Measured scattering cross-section constant as resolution scale 1/Q decreases.! Evidence for point-like constituents in the proton: the quarks
15 QCD Factorization and PDFs QCD Factorization Theorem: separate the hadronic cross section into a perturbative, process dependent partonic cross section and non-perturbative, process independent Parton Distributions. In DIS we have:! ! ! ! The same Factorization Theorem allows to use the same universal PDFs to provide predict ions for proton-proton collisions at the LHC: Hadron-level cross section Parton-level cross-section Parton Distribution Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Hadron-level cross section (2) Parton Distributions Parton-level cross-section To make sense of LHC collisions, we need first of all to determine the parton distributions of the proton with good precision! PDF PDF PDF
16 Parton Distributions There is one independent PDF for each parton in the proton: u(x,Q2), d(x,Q2), g(x,Q2), …! A total of 13 PDFs, but heavy quark PDFs generated radiatively from gluon and light quarks! At Leading Order, PDFs understood as the probability of finding a parton of a given flavor that carries a fraction x of the total proton’s momentum! Once QCD corrections included, PDFs become scheme- dependent and have no probabilistic interpretation! Shape and normalization of PDFs are very different for each flavor, reflecting the different underlying dynamics that determine each PDF flavor! QCD imposes valence and momentum sum rules valid to all orders in perturbation theory! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Momentum Sum Rule Valence Sum Rules PDG Review 2014
17 Perturbative evolution equations The dependence of PDFs on Bjorken-x (momentum fraction) is determined by non-perturbative QCD dynamics, but that on the scale Q2 (resolution) is instead known from perturbative QCD: the DGLAP evolution equations! ! ! Once x-dependence q(x,Q2 0) extracted from data, pQCD determines PDFs at other scales q(x,Q2)! Evolution kernels have been computed up to next-to-next-to-leading order (NNLO):! ! ! Reasonable convergence of the perturbative expansion of PDFs up to NNLO! ! ! ! ! ! ! ! ! ! x -5 10 -4 10 -3 10 -2 10 -1 10 -2 0 2 4 6 8 10 12 ) 0 2 xg (x, Q LO NLO NNLO ) 0 2 xg (x, Q x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 ) 0 2 (x, Q3xT LO NLO NNLO ) 0 2 (x, Q3xT T3=u-dg NNPDF2.1 NNPDF2.1 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
18 Perturbative evolution equations The dependence of PDFs on Bjorken-x (momentum fraction) is determined by non-perturbative QCD dynamics, but that on the scale Q2 (resolution) is instead known from perturbative QCD: the DGLAP evolution equations! ! ! Once x-dependence q(x,Q2 0) extracted from data, pQCD determines PDFs at other scales q(x,Q2)! ! ! ! ! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Hadronic scale LHC scale Perturbative Evolution
Theory! Partonic cross-sections! DGLAP evolution! Heavy quark treatment! Fast interfaces to NLO calculations! NNLO QCD effects! QED and electroweak corrections! ! Data! Deep-inelastic scattering structure functions! DIS charm production! DIS Neutrino production! Jets at pp colliders! Drell-Yan (dilepton pair) production at pp colliders! Top quark data! Methodology! PDF parametrisation! PDF uncertainty estimation and error propagation to LHC cross- sections! Theoretical uncertainties! Minimisation strategy! ! Parton Distributions for the LHC! Higgs cross-sections! BSM New Physics searches! Precision Standard Model measurements Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Global PDF analysis Parton Distributions, and their associated uncertainties, need to be determined from a global analysis of hard-scattering data that requires as input the most updated theory calculations, precise and varied experimental data and robust statistical methodology
20 Experimental data in global PDF fits A global dataset covering a wide set of hard-scattering observables is required to constrain all possible PDF combinations in the whole range of Bjorken-x! For example, inclusive jets are sensitive to the large-x gluon, while HERA neutral current data pins down the small-x quarks! LHC data is introducing completely new observables to be used for PDF constraints! x dependence of PDFs: ! determined from data Q2 dependence of PDFs: ! determined by pQCD Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! The Neural Network Approach! to Parton Distributions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF
22 The NNPDF approach Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 The limitations of available PDF sets circa 2005, and the requirements of precision physics at the upcoming LHC, prompted us to develop a completely novel approach to PDF determination! PDF sets typically based on restrictive functional forms leading to strong theoretical bias! ! NNPDF solution: use artificial neural networks as universal unbiased interpolants! ! PDF sets often rely on the the Gaussian/linear approximation for error estimation and propagation! ! ! NNPDF solution: Use the Monte Carlo method to create a probability distribution in the space of PDFs! ! ! Traditional PDF analyses based on deterministic minimisation of the χ2 to reach convergence in the fit! NNPDF solution: Use Genetic Algorithms to be able to explore efficiently the vast parameter space Consisteny error propagation no LHC xsecs no Gaussian assumptions
23 Artificial Neural Networks Biological NeuralNets Artificial NeuralNets Artificial neural networks aimed to excel in the same domains as their biological counterparts: pattern recognition, forecasting, classification, .... where our evolution-driven biology outperforms traditional algorithms Neurons, axions, ! synapses, ... Inspired by biological brain models, Artificial Neural Networks are mathematical algorithms widely used in a wide range of applications, from high energy physics to targeted marketing and finance forecasting Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
24 Artificial Neural Networks % of customers contacted %ofpositiveanswers Example 1: Pattern recognition. During the Yugoslavian wars, the NATO used ANNs to recognise hidden military vehicles! A military aircraft is identified, despite being hidden below a commercial plane.! Many other applications of ANN in pattern recognition: OCR software, hand writing recognition, automated anti-plagiarism software, .....! Example 2: Marketing. A bank wants to offer a new credit card to their clients. Two possible strategies:! Contact all customers: slow and costly! Contact 5% of the customers, train a ANN with their input (sex, income, loans) and their ourput (yes/no) and use the information to contact only clients likely to accepy the offer! Cost-effective method to improve marketing performance Random client selection ANN based client selection Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
25 Artificial Neural Networks Artificial Neural Networks (ANNs) provide universal unbiased interpolants to parametrize PDFs at low input scales! ! The ANN class that we adopt are feed-forward multilayer neural networks (perceptrons)! ! ! ! ! In traditional PDF determinations, the input ansatz is a simple polynomial! ! ! The use of Artificial Neural Networks allows:! No theory bias introduced in the PDF determination by the choice of ad-hoc functional forms! The use of very flexible parametrizations for all PDFs - regardless of the dataset used. The NNPDF analysis allow for O(400) free parameters, to be compared with O(10-20) in traditional PDFs! Faithful extrapolation: PDF uncertainties blow up in regions with scarce experimental data! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
Generate a large number of Monte Carlo replicas of the experimental data with the same underlying probability distribution! ! Perform a PDF determination on each of these MC replicas! The set of PDF replicas form a representation of the probability density in the space of parton distribution functions! PDF uncertainties can be propagated to physical cross sections using textbook statistics, no need of linear/gaussian assumptions! ! ! 26 PDF Uncertainties: The Monte Carlo Method PDF Uncertainty = Standard Deviation of MC sample Central PDF prediction = Expectation Value of MC sample stat errorsys errors random numberslumi error >>1 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
27 PDF Replica Neural Network Learning xg(x,Q2=2GeV2) x The minimisation of the data vs theory chi2 is performed using Genetic Algorithms! Each green curve corresponds to a gluon PDF Monte Carlo replica Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
28 Artificial Neural Networks vs. Polynomials Compare a benchmark PDF analysis where the same dataset is fitted with Artificial Neural Networks and with standard polynomials (everything else identical)! ANN avoid biasing the PDFs, faithful extrapolation at small-x (very few data, thus error blow up)! ! Polynomials Neural Networks PDF error PDF error Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 No Data No Data
29 Adding more data: better or worse?! In traditional Hessian PDF approach, unexpected behaviors might arise when data added! ! ! ! ! In the NNPDF approach, more data always leads to an improvement of PDF uncertainties! ! ! ! x 0.1 0.2 0.3 0.4 0.5 0.6 )[ref] 2 )[new]/g(x,Q 2 g(x,Q 0.4 0.6 0.8 1 1.2 1.4 1.6 2.1 HERA-only 2.1 Global = 0.119S Ratio to NNPDF2.1 NNLO HERA-only, 2.1 HERA-only 2.1 Global CT10 (Traditional approach) Less Data More Data Less Data New data Need more flexible PDFs! to fit data: increase # of params More directions in ! Hessian space to explore Larger ! PDF uncertainties? More Data NNPDF approach Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! PDFs at the LHC:! From Higgs and SM to BSM searches Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
31 Higgs Boson Characterisation The study of the Higgs boson properties is a cornerstone of the LHC program. All production cross sections require accurate knowledge of PDFs! gg fusion, ttH: gluon luminosity! vector-boson fusion: quark-quark luminosity! associated production with W/Z: quark-antiquark luminosity! For many crucial channels, QCD uncertainties, including PDFs, hatched areas, limit the accuracy of Higgs coupling extraction form LHC data Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
32 Parton Distributions with LHC data Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 A major breakthrough in the recent years has been the inclusion of LHC data into global PDF fits! PDF constraints from a wide variety of LHC processes have been studied, many of which for first time Isolated photon LHC data constraints gluons at medium-x: ! relevant for Higgs production in gluon fusion x -4 10 -3 10 -2 10 -1 10 RelErr[xs] 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 , ratio to NNPDF2.1 Collider Only2 W= M 2 Q NNPDF2.1 Collider-only -1 NNPDF2.1 Collider-only + Wc Data L = 5 fb , ratio to NNPDF2.1 Collider Only2 W= M 2 Q W production in association with charm quarks ! provides direct access to the proton strangeness x 0.1 0.2 0.3 0.4 0.5 0.6 ) 2 (x,Q (ref) )/g 2 (x,Q (new) g 0.7 0.8 0.9 1 1.1 1.2 1.3 NNPDF2.3 NNPDF2.3 + Top Data 2 = 100 GeV2 Q = 0.118SαRatio to NNPDF2.3 NNLO, NNPDF2.3 NNPDF2.3 + Top Data 2 = 100 GeV2 Q Large-x gluon from top quark data Large-x gluon from inclusive Jet production NNPDF 14 J.R. 11 D’Enterria, J.R. 12
33 Top quarks as gluon luminometers The recent NNLO top quark cross section make top data the only LHC observable that is both directly sensitive to the gluon PDF and can be included consistently in a NNLO global analysis! The precise 7 and 8 TeV LHC data can be used to discriminate between PDF sets and to reduce the PDF uncertainties on the poorly known large-x gluon The improved large-x gluon leads to more accurate theory predictions for BSM searches Juan Rojo LHCP2013, Barcelona, 22/04/2013 High mass Graviton Tail of the invariant ! tt mass distribution Czakon, Mangano, Mitov, J.R. 13
! QED and electroweak corrections are essential for precision LHC phenomenology: W and Z production, W mass determination, WW boson pair production, TeV scale jet and top quark pair production, searches for new W’, Z’ bosons! Consistent inclusion of electroweak effects require PDFs with QED corrections and a photon PDF ! NNPDF2.3 QED: first-ever determination of the photon PDF from LHC data! Neglecting photon-initiated contributions leads to systematically underestimating theory errors in crucial BSM search channels! ! ! 34 PDFs with QED corrections ( GeV )llM 500 1000 1500 2000 2500 3000 3500 /dM(fb/GeV)[ref]/dM(fb/GeV)/dd 0 0.5 1 1.5 2 2.5 /Z production @ LHC 8 TeV * gamma BornqNNPDF2.3 QED, q ) QEDNNPDF2.3 QED, full O( ) QEDMRST04 QED, full O( /Z production @ LHC 8 TeV * gamma ( GeV )cut WW M 200 400 600 800 1000 1200 (fb) 0 0.5 1 1.5 2 2.5 WW production @ LHC 8 TeV qNNPDF2.3 QED, q NNPDF2.3 QED, MRST04 QED, WW production @ LHC 8 TeV High-Mass Drell-Yan High-Mass WW prod Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF 13
35 Closure Testing of Parton Distributions PDF uncertainties have been often criticised by a potential lack of statistical interpretation! In the recent NNPDF3.0 paper, we performed a systematic closure tests analysis based on pseudo-data, and verified that PDF uncertainties show a statistically robust behaviour! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF 14
36 Closure Testing of Parton Distributions Level 0 Closure Tests: Pseudo data = Theory! Central values of input PDF reproduced with arbitrary accuracy! PDF uncertainties on the fitted data points can become arbitrarily small Level 2 Closure Tests: Pseudo data with “exp” fluctuations! Reproduced χ2 of input PDF - both total and individual experiments! Fitted PDF central values fluctuate around input values by the same amount as expected from the size of the PDF uncertainties -> PDF errors indeed correspond to 68% Confidence Levels Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
37 PDFs and Monte Carlo generators PDFs are an essential ingredient for the tuning of soft and semi-hard physics in LO Monte Carlo event generators like Pythia8, Herwig++ or Sherpa! Most updated tune of Pythia8, the Monash 2013 Tune, is based on the NNPDF2.3LO set! The harder small-x gluon in NNPDF2.3LO is essential to improve the description of the LHC forward data! Next step: specific tunes for NLO Monte Carlo event generators! ! ! ! ! ! ! Skands, Carrazza, J.R. 14 Small-x behaviour of gluon determines soft and semi-hard physics at the LHC Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
38 Precision tests of the Factorisation Theorem Momentum Integral 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 NNPDF2.1 LO* NNPDF2.1 NLO* NNPDF2.1 NNLO* Perturbative QCD requires that the momentum integral should be unity to all orders! ! ! ! Is it possible to determine the value of the momentum integral from the global PDF analysis, rather than imposing it? Check in LO*, NLO* and NNLO* fits without setting M=1! ! ! ! Experimental data beautifully confirms the pQCD expectation! Extremely non trivial test of the global analysis framework and the factorization hypotheses! Very good convergence of the QCD perturbative expansion! ! !Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF, 11
39 PDF comparisons made easy: APFEL-Web Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 http://apfel.mi.infn.it/ Comparing different PDF sets is now really easy thanks to the new APFEL-Web online PDF plotter! Just log in, select the PDF sets that you want to compare, the plotting settings, and have fun!! Bertone, Carrazza, J.R. 13
40 Cross section Ratios between 7, 8 and 14 TeV! The staged increase of the LHC beam energy provides a new class of interesting observables: cross section ratios for different beam energies! ! ! These ratios can be computed with very high precision due to the large degree of correlation of theoretical uncertainties at different energies! Experimentally these ratios can also be measured accurately since many systematics, like luminosity or jet energy scale, cancel partially in the ratios! These ratios allow stringent precision tests of the SM, like PDF discrimination! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Mangano, J. R., 12
41 Cross section Ratios between 7, 8 and 14 TeV! The staged increase of the LHC beam energy provides a new class of interesting observables: cross section ratios for different beam energies! ! ! These ratios can be computed with very high precision due to the large degree of correlation of theoretical uncertainties at different energies! Experimentally these ratios can also be measured accurately since many systematics, like luminosity or jet energy scale, cancel partially in the ratios! These ratios allow stringent precision tests of the SM, like PDF discrimination! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Mangano, J. R., 12 ATLAS: Gluon from 7 TeV / 2.76 TeV jet xsecs CMS: Drell-Yan 8 TeV / 7 TeV ratio
42 Cross section Ratios between 7, 8 and 14 TeV! If SM theory systematics under control, these cross section ratios can provide an improved sensitivity to New Physics than absolute cross sections! ! The visibility of a BSM contribution in the evolution with energy of the cross section requires that it evolves in Q2 differently from the SM contribution! ! ( GeV )Xm 500 1000 1500 2000 2500 3000 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 ]qq / Lgg [ L 1 / s2 s 8 TeV over 7 TeV 14 TeV over 8 TeV Example: a gluon-gluon initiated BSM ! contribution to high-mass Z production.! The cross section ratio enhanced by: With greatly reduced experimental ! and theoretical uncertainties But theory systematics, mostly PDFs, need to be known accurately! for this new approach to show its full potential Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
43 Determination of Standard Model parameters Accurate PDFs are required for precision determination of fundamental Standard Model parameters in processes involving initial state hadrons! These include, among many others, the strong coupling constant αS, the W boson mass, the effective lepton mixing angle, CKM matrix elements, ....! The unbiased nature of the NNPDF approach approach to faithfully disentangle PDF uncertainties from other parametric uncertainties. One example in neutrino DIS:! ! ! ! ! ! ! CKM matrix element Vcs can be determined from ! neutrino DIS data - but large uncertainties from strange PDF! ! NNPDF analysis manages to obtain the most accurate ever determination of Vcs from a single process:! ! Vcs = 1.04 ±0.06 (PDG average)! Vcs = 0.96 ±0.07 (NNPDF from NuTeV data)! ! ! The same analysis shows that the strangeness asymmetry in the proton has just the right size to cancel the NuteV anomaly sin2θW NNPDF, arXiv:0906.1958Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
44 Determination of Standard Model parameters Accurate PDFs are required for precision determination of fundamental Standard Model parameters in processes involving initial state hadrons! The strong coupling constant αS can be determined from a global PDF analysis, mostly from scaling violations in Deep-Inelastic Scattering and in inclusive jet production! The NNPDF result is the most accurate determination of αS from a QCD global fit, and nicely consistent with the latest PDG average, to which is one of the dominant contributions! In the pipeline: αS determinations from LHC data at the higher scales ever probed! ! ! ! ! ! PDG 2012 PDG average from PDF fits PDG 2013 average NNPDF2.1 NNLO Consistency check of the global PDF framework: the distributions of pulls for αS fitted to individual experiments follows a Gaussian distribution! ! ! ! NNPDF, 11 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
! ! ! ! ! ! Polarised PDFs:! Unraveling the origin of the proton spin Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
Up to know in this talk everything was valid for unpolarised protons! The polarised case is also extremely interesting: unique windows on the spin structure of the proton.! How the proton spin is distributed among its constituents is a crucial issue for our understanding of non-perturbative QCD and confinement Polarised Parton Distributions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Foreword f (x, Q2 ) = f )! (x, Q2 ) f ) (x, Q2 ) How do quarks (including sea quarks) and gluons carry the proton spin S(µ) = 1 2 = X f D P; S| ˆJz f (µ)|P; S E = 1 2 Z 1 0 dx ⌃(x, µ) + Z 1 0 dx g(x, µ) + Lz All quantities depend on factorization scheme and scale µ Spin decomposition is not unique (Y. Hatta, S1) Very little of the proton spin is carried by quarks Z 1 0 dx ⌃ = Z 1 0 dx X q=u,d,s ( q + ¯q) ⇠ 30% Quark and gluon longitudinal contributions () longitudinal spin-dependent PDFs Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 2 / 25 Probes of nucleon helicity structure Guiding principle: factorization e.g. DIS d = X q,¯q,g f (x, Q2 )⌦d ˆ ⇤f (xP, ↵s (Q2 )) d ˆ ⇤f = 1X n=0 ⇣ ↵s 4⇡ ⌘n d ˆ (n) ⇤f Reaction Partonic subprocess PDF probed x Q2 [GeV2 ] `± {p, d, n} ! `± X ⇤ q ! q q + ¯q 0.003 . x . 0.8 1 . Q2 . 70 g `± {p, d} ! `± hX ⇤ q ! q u ¯u 0.005 . x . 0.5 1 . Q2 . 60d ¯d g `± {p, d} ! `± DX ⇤ g ! c¯c g 0.06 . x . 0.2 ⇠ 10 !p !p ! jet(s)X gg ! qg g 0.05 . x . 0.2 30 . p2 T . 800 qg ! qg !p p ! W ± X uL ¯dR ! W + u ¯u 0.05 . x . 0.4 ⇠ M2 WdL¯uR ! W d ¯d !p !p ! ⇡X gg ! qg g 0.05 . x . 0.4 1 . p2 T . 200 qg ! qg Di↵erent processes constrain di↵erent PDFs, factorization is successful Quarks? Gluons? Angular Mom? First measurements with polarised DIS (80s) showed that quark contribution much smaller than expected (proton spin crisis)! With the availability of polarised hadronic and semi-inclusive data, global polarised PDF fits possible! The NNPDF framework has also been applied here: NNPDFpol sets Nocera, SPIN14
Contribution of gluon polarisation to the proton spin has been of the big unknowns in the last 30 years! The analysis of recent RHIC polarised jet data in the NNPDFpol and DSSV frameworks has provided first ever evidence for positive (non-zero) polarisation of the gluon in the proton! Importance of this important result recognised by the popular media, i.e. Scientific American Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 2) pp collisions at RHIC: jet and ⇡ production effects on g distribution x -3 10 -2 10 -1 10 1 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 NNPDFpol1.1 =12 χ∆DSSV08 positivity bound )2 =10 GeV2 g(x,Q∆x [arXiv:1406.5539] NEW FIT DSSV* DSSV incl. 90% C.L. variations Q 2 = 10 GeV 2 RHIC x range x∆g x -0.2 -0.1 0 0.1 0.2 10 -3 10 -2 10 -1 1 [arXiv:1404.4293] (GeV/c)T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 | < 0.5η| STAR 2009 Jet+X→p+p =200 GeVs (GeV/c) T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 STAR BB10 DSSV LSS10p LSS10 NNPDF | < 1η0.5 < | 6.5% scale uncertainty± from polarization not shown [arXiv:1405.5134] [GeV/c] T p 5 6 7 8 9 10 11 12 LLA -0.1 -0.05 0 0.05 0.1 0.15 g = g∆GRSV g = std∆GRSV g = -g∆GRSV DSSV 0 πSTAR + X0 π→p+p = 200 GeVs < 2.0η0.8 < 6% Scale Uncertainty [arXiv:1309.1800] Longitudinal double-spin asymmetry ALL = ++ + ++ + + features at RHIC, hx1,2i ' 2pTp s e ⌘/2 ⇡ [0.05, 0.2] qg and gg initiated subprocesses dominate (for most of the RHIC kinematics) ALL sensitive to gluon polarization cross sections are well described at NLO in pQCD measurements STAR (mainly jets) (Z. Chang & C. Dilk) PHENIX (⇡ production) (A. Manion & I. Yoon) much more to come from ongoing RHIC run ! gaining precision ! di-jet measurements Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 12 / 25 2) pp collisions at RHIC: jet and ⇡ production (GeV/c)T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 | < 0.5η| STAR 2009 Jet+X→p+p =200 GeVs (GeV/c) T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 STAR BB10 DSSV LSS10p LSS10 NNPDF | < 1η0.5 < | 6.5% scale uncertainty± from polarization not shown [arXiv:1405.5134] [GeV/c] T p 5 6 7 8 9 10 11 12 LLA -0.1 -0.05 0 0.05 0.1 0.15 g = g∆GRSV g = std∆GRSV g = -g∆GRSV DSSV 0 πSTAR + X0 π→p+p = 200 GeVs < 2.0η0.8 < 6% Scale Uncertainty [arXiv:1309.1800] Longitudinal double-spin asymmetry ALL = ++ + ++ + + features at RHIC, hx1,2i ' 2pTp s e ⌘/2 ⇡ [0.05, 0.2] qg and gg initiated subprocesses dominate (for most of the RHIC kinematics) ALL sensitive to gluon polarization cross sections are well described at NLO in pQCD measurements STAR (mainly jets) (Z. Chang & C. Dilk) PHENIX (⇡ production) (A. Manion & I. Yoon) much more to come from ongoing RHIC run ! gaining precision ! di-jet measurements Unraveling the gluon polarisation Total contribution of gluons to proton spin still unknown since large uncertainties at small-x from lack of data: need an Electron-Ion Collider NNPDFpol 14
Inclusive DIS data does not allow to separate polarised quarks from antiquarks! Recent data on polarised semi-inclusive DIS and hadronic W production allow this separation for first time! Stringent constraints on non-perturbative models of the proton The polarised quark sea Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1) pp collisions at RHIC: W ± production ηlepton -2 -1 0 1 2 -0.5 0 0.5 = 2% error2 χ/ 2 χ∆DSSV08 L0 - W + W ν+± e→± W→+pp =510 GeVs < 50 GeV e T 25 < E LA Rel lumi syst 3.4% beam pol scale uncertainty not shown + W - W STAR Data CL=68% DSSV08 RHICBOS DSSV08 CHE NLO LSS10 CHE NLO [arXiv:1404.6880] |ηlepton | 0 0.5 1 -0.5 0 0.5 + W - W ν+±e→± W→p+p =510 GeVs < 50 GeV e T 25 < E LLA DSSV08 CHE NLO STAR Data CL=68% 6.5% beam pol scale uncertainty not shown [arXiv:1404.6880] Longitudinal single- and double-spin asymmetries AL = + + + ALL = ++ + ++ + + features quark/antiquark separation at Q ⇠ MW no need of fragmentation functions at RHIC, hx1,2i ' MWp s e ⌘l /2 ⇡ [0.04, 0.4] for W +, d ! d and d ! u non-trivial positivity bound [arXiv:1104.2920] 1 ± ALL(yW ) > |AL(yW ) ± AL( yW )| no access to strangeness (W ± + c required) measurements STAR + PHENIX (Z. Jinlong & F. Giordano) much more to come from ongoing RHIC run Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 9 / 25 RHIC: W ± production metry ¯u ¯d -1 10 1 [PoS(DIS2014)204] x -3 10 -2 10 -1 10 1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 )d∆-u∆x( NNPDFpol1.1 =12 χ∆DSSV08 2 =10 GeV2 Q )ρ-πMC ( )ρMC ( )σ-πMC ( PB (ansatz) CQSM ST [PoS(DIS2014)204] Model Refs. d/u d/ u u/u d/d An 1 Ap 1 SU(6) [41] 1/2 1/4 2/3 1/3 0 5/9 RCQM [43] 0 0 1 1/3 1 1 QHD ( 1/2) [44] 1/5 1/5 1 1 1 1 QHD ( ⇢) [44] 0 0 1 1/3 1 1 NJL [45] 0.20 0.06 0.80 0.25 0.35 0.77 DSE (realistic) [46] 0.28 0.11 0.65 0.26 0.17 0.59 DSE (contact) [46] 0.18 0.07 0.88 0.33 0.34 0.88 pQCD [50] 1/5 1/5 1 1 1 1 NNPDF (x = 0.7) [15, 18] 0.22 ± 0.04 0.07 ± 0.12 0.07 ± 0.05 0.19 ± 0.34 0.41 ± 0.31 0.75 ± 0.07 NNPDF (x = 0.8) [15, 18] 0.18 ± 0.09 0.12 ± 0.23 0.70 ± 0.13 0.34 ± 0.67 0.57 ± 0.61 0.75 ± 0.12 NNPDF (x = 0.9) [15, 18] 0.06 ± 0.49 0.51 ± 0.69 0.61 ± 0.48 0.85 ± 6.55 0.36 ± 0.61 0.74 ± 0.34 Table 2: A collection of several model expectations for various ratios of polarized/unpolarized PDFs and spin-dependent neutron and proton asymmetries, An 1 and Ap 1, at x ! 0. The NNPDF prediction, obtained using unpolarized NNPDF2.3 [18] and polarized NNPDFpol1.1 [15] parton sets, is shown at Q2 = 4 GeV2 for di↵erent values of x. x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 )2 =4 GeV2 (x,Q+ /u+ u∆ Avakian et al. Statistical LSS (BBS) NJL Su(6)-breaking NNPDF =12 χ∆DSSV08 x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -1.5 -1 -0.5 0 0.5 1 1.5 2 )2 =4 GeV2 (x,Q+ /d+ d∆ Avakian et al. Statistical LSS (BBS) NJL Su(6)-breaking NNPDF =12 χ∆DSSV08 Nocera 14 NNPDFpol 14
Parton Distributions are an essential ingredient for LHC phenomenology! Accurate PDFs are required for precision SM measurements, Higgs characterisation and New Physics searches! The determination of fundamental SM parameters like the W mass or αS from LHC data also greatly benefit from improved PDFs! PDFs are also a basic component for Monte Carlo event generators! The NNPDF approach provides parton distributions based on a robust, unbiased methodology, the most updated theoretical information and all the relevant hard scattering data including LHC data! Near future developments in NNPDF include ! Inclusion of more HERA and LHC data! Tailored PDFs for NLO Monte Carlo event generators at the LHC! PDFs with threshold and high energy resummation, impact on Higgs and top physics! PDFs with and electroweak effects (both in evolution and in partonic xsecs)! PDFs with Intrinsic Charm Summary Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
50 ! ! ! ! ! ! ! ! ! ! Going Beyond: PDFs at a 100 TeV collider Juan Rojo SMatLHC14, Madrid, 09/04/2014 ! ! Growing consensus that the next big machine more suitable to explore the energy frontier should be a 100 TeV hadron collider, possibly with also e+e- and ep operation modes! The phenomenology of PDFs at such extreme energies is very rich: top quark PDFs, electroweak effects on PDFs and W/Z boson PDFs, ultra-low-x physics, BFKL dynamics, BSM physics with polarized PDFs, ...., lots of fun!! First studies being now performed in the context of the CERN FCC working group! ! ! ! ! ! BSM physics with! polarized PDFs!
51 ! ! ! ! ! ! ! ! ! ! Going Beyond: PDFs at a 100 TeV collider Juan Rojo SMatLHC14, Madrid, 09/04/2014 ! ! Growing consensus that the next big machine more suitable to explore the energy frontier should be a 100 TeV hadron collider, possibly with also e+e- and ep operation modes! The phenomenology of PDFs at such extreme energies is very rich: top quark PDFs, electroweak effects on PDFs and W/Z boson PDFs, ultra-low-x physics, BFKL dynamics, BSM physics with polarized PDFs, ...., lots of fun!! First studies being now performed in the context of the CERN FCC working group! ! ! ! ! ! BSM physics with! polarized PDFs! Thanks for your attention!

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The Structure of the Proton in the Higgs Boson Era

  • 1. Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 ! The Structure of the Proton ! in the Higgs Boson Era Juan Rojo! STFC Rutherford Fellow! Rudolf Peierls Center for Theoretical Physics! University of Oxford! ! ! Theory Seminar ! NIKHEF, 22/01/2015
  • 2. ! ! ! ! ! ! Exploring the high-energy frontier:! The Large Hadron Collider Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 3. ! ! ! ! ! ! The Higgs Boson is the most important discovery in particle physics in 25 years ! The Higgs completes the extremely successful Standard Model of particle physics, but at the same time opens a number of crucial questions for the field that we need to address! The LHC will play a crucial role in exploring the energy frontier in the next 20 years Particle Physics in the headlines Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 4. ! ! ! ! ! ! Standard Model of Particle Physics Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1" Higgs"boson"and"EWSB" !  Is"mH"natural"or"fine8tuned"?" """if"natural:"what"new"physics/symmetry"?" !  does"it"regularize"the"divergent"WLWL"cross8secEon" """"""at"high"M(WLWL)"?"Or"is"there"a"new"dynamics"?" !  elementary"or"composite"Higgs"?" !  is"it"alone"or"are"there"other"Higgs"bosons"?" !  origin"of"couplings"to"fermions""" !  coupling"to"dark"maKer"?"" !  does"it"violate"CP"?" !  cosmological"EW"phase"transiEon"" Neutrinos:" !  ν"masses"and"and"their"origin" !  what"is"the"role"of"H(125)"?""" !  Majorana"or"Dirac"?" !  CP"violaEon"" !  addiEonal"species"""sterile"ν"?" Dark"maKer:" !  composiEon:"WIMP,"sterile"neutrinos,"" """""axions,"other"hidden"sector"parEcles,".." !  one"type"or"more"?"" !  only"gravitaEonal"or"other"interacEons"?" The"two"epochs"of"Universe’s"accelerated"expansion:" !  primordial:"is"inflaEon"correct"?"" """""which"(scalar)"fields?"role"of"quantum"gravity?""" !  today:"dark"energy"(why"is"Λ"so"small?)"or" """""modificaEon"of"gravity"theory"?" Quarks"and"leptons:" !  why"3"families"?" !  masses"and"mixing" !  CP"violaEon"in"the"lepton"sector" !  maKer"and"anEmaKer"asymmetry" !  baryon"and"charged"lepton"" """""number"violaEon"" Physics"at"the"highest"E8scales:" !  how"is"gravity"connected"with"the"other"forces"?" !  do"forces"unify"at"high"energy"?" Outstanding**Ques-ons*in*Par-cle*Physics*circa%2014* …"there"has"never"been"a"beKer"Eme"to"be"a"parEcle"physicist!" List from Ian Shipsey Many of these crucial questions can be addressed at the Large Hadron Collider
  • 5. ! ! ! ! ! ! Standard Model of Particle Physics Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1" Higgs"boson"and"EWSB" !  Is"mH"natural"or"fine8tuned"?" """if"natural:"what"new"physics/symmetry"?" !  does"it"regularize"the"divergent"WLWL"cross8secEon" """"""at"high"M(WLWL)"?"Or"is"there"a"new"dynamics"?" !  elementary"or"composite"Higgs"?" !  is"it"alone"or"are"there"other"Higgs"bosons"?" !  origin"of"couplings"to"fermions""" !  coupling"to"dark"maKer"?"" !  does"it"violate"CP"?" !  cosmological"EW"phase"transiEon"" Neutrinos:" !  ν"masses"and"and"their"origin" !  what"is"the"role"of"H(125)"?""" !  Majorana"or"Dirac"?" !  CP"violaEon"" !  addiEonal"species"""sterile"ν"?" Dark"maKer:" !  composiEon:"WIMP,"sterile"neutrinos,"" """""axions,"other"hidden"sector"parEcles,".." !  one"type"or"more"?"" !  only"gravitaEonal"or"other"interacEons"?" The"two"epochs"of"Universe’s"accelerated"expansion:" !  primordial:"is"inflaEon"correct"?"" """""which"(scalar)"fields?"role"of"quantum"gravity?""" !  today:"dark"energy"(why"is"Λ"so"small?)"or" """""modificaEon"of"gravity"theory"?" Quarks"and"leptons:" !  why"3"families"?" !  masses"and"mixing" !  CP"violaEon"in"the"lepton"sector" !  maKer"and"anEmaKer"asymmetry" !  baryon"and"charged"lepton"" """""number"violaEon"" Physics"at"the"highest"E8scales:" !  how"is"gravity"connected"with"the"other"forces"?" !  do"forces"unify"at"high"energy"?" Outstanding**Ques-ons*in*Par-cle*Physics*circa%2014* …"there"has"never"been"a"beKer"Eme"to"be"a"parEcle"physicist!" List from Ian Shipsey For the next 20 years, LHC will be at the forefront of the exploration of the high-energy frontier
  • 6. 6 Juan Rojo PDF@CMS Kick-off Workshop, CERN, 07/05/2012
  • 7. The inner life of the protons! The Large Hadron Collider collides proton, but these are not fundamental particles: really what the LHC is doing is colliding quarks and gluons! The distribution of momentum that the quarks and gluons carry is quantified by the Parton Distribution Functions (PDFs), determined by non-perturbative dynamics: cannot be computed from first principles and need to be extracted from experimental data! An accurate determination of PDFs is of paramount importance to be able to do precision physics at hadronic colliders as the LHC! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 8. LHC collisions in a nutshell Drawing by K. Hamilton Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 9. Drawing by K. Hamilton Initial State:! Parton Distribution Functions (PDFs) Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 LHC collisions in a nutshell The accurate determination of the Parton Distribution Functions of the proton is a vital ingredient for LHC phenomenology
  • 10. QCD: The Toolbox for Discoveries at the LHC ! ! ! ! ! ! Campbell, ICHEP12 My key message • The days of “guaranteed” discoveries or of no-lose theorems in particle physics are over, at least for the time being .... • .... but the big questions of our field remain wild open (hierarchy problem, flavour, neutrinos, DM, BAU, .... ) • This simply implies that, more than for the past 30 years, future HEP’s progress is to be driven by experimental exploration, possibly renouncing/reviewing deeply rooted theoretical bias • This has become particularly apparent in the DM-related sessions: • Direct detection experiments and astrophysics are challenging the theoretical DM folklore as much as the LHC is challenging the theoretical folklore about the hierarchy problem. • But great opportunities lie ahead, and the current challenges are simply hardening theorists’ ingenuity, creativity and skills 3 Mangano, Aspen14 Improving our quantitative understanding of the Standard Model is essential in this new era for HEP, where we need to hunt, unbiased, for answers to the big questions of our field! Now, more than ever, sharpening our QCD tools could be the key for new discoveries at the LHC Prime example: extraction of Higgs couplings from LHC data soon to be limited by QCD uncertainties! ! More accurate determination of PDFs ! ! Improved indirect sensitivity to New Physics via deviations of Higgs couplings from SM expectations Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 11. Parton Distributions and LHC phenomenology ! ! ! ! ! ! 2) Very large PDF uncertainties (>100%) for new heavy particle production Supersymmetric QCD 1) PDFs fundamental limit for Higgs boson characterization in terms of couplings 3) PDFs dominant systematic for precision measurements, like W boson mass, that test internal consistency of the Standard Model Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 12. ! ! ! ! ! ! The inner life of protons :! Parton Distribution Functions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 13. 13 Deep-Inelastic scattering and the discovery of quarks Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Deep-inelastic lepton-proton scattering: First evidence for proton structure (SLAC, 70s)! Measured scattering cross-section constant as resolution scale 1/Q decreases.! Evidence for point-like constituents in the proton: the quarks If the proton had a different structure, a form factor F(Q) would be expected! Analogous to Rutherford’s discovery of the point-like atomic nucleus, while expecting Thomson’s Plum model
  • 14. 14 Deep-Inelastic scattering and the discovery of quarks Partonic xsec Parton Distributions Parton Distribution Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 ( 1/ Resolution Scale ) Deep-inelastic lepton-proton scattering: First evidence for proton structure (SLAC, 70s)! Measured scattering cross-section constant as resolution scale 1/Q decreases.! Evidence for point-like constituents in the proton: the quarks
  • 15. 15 QCD Factorization and PDFs QCD Factorization Theorem: separate the hadronic cross section into a perturbative, process dependent partonic cross section and non-perturbative, process independent Parton Distributions. In DIS we have:! ! ! ! The same Factorization Theorem allows to use the same universal PDFs to provide predict ions for proton-proton collisions at the LHC: Hadron-level cross section Parton-level cross-section Parton Distribution Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Hadron-level cross section (2) Parton Distributions Parton-level cross-section To make sense of LHC collisions, we need first of all to determine the parton distributions of the proton with good precision! PDF PDF PDF
  • 16. 16 Parton Distributions There is one independent PDF for each parton in the proton: u(x,Q2), d(x,Q2), g(x,Q2), …! A total of 13 PDFs, but heavy quark PDFs generated radiatively from gluon and light quarks! At Leading Order, PDFs understood as the probability of finding a parton of a given flavor that carries a fraction x of the total proton’s momentum! Once QCD corrections included, PDFs become scheme- dependent and have no probabilistic interpretation! Shape and normalization of PDFs are very different for each flavor, reflecting the different underlying dynamics that determine each PDF flavor! QCD imposes valence and momentum sum rules valid to all orders in perturbation theory! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Momentum Sum Rule Valence Sum Rules PDG Review 2014
  • 17. 17 Perturbative evolution equations The dependence of PDFs on Bjorken-x (momentum fraction) is determined by non-perturbative QCD dynamics, but that on the scale Q2 (resolution) is instead known from perturbative QCD: the DGLAP evolution equations! ! ! Once x-dependence q(x,Q2 0) extracted from data, pQCD determines PDFs at other scales q(x,Q2)! Evolution kernels have been computed up to next-to-next-to-leading order (NNLO):! ! ! Reasonable convergence of the perturbative expansion of PDFs up to NNLO! ! ! ! ! ! ! ! ! ! x -5 10 -4 10 -3 10 -2 10 -1 10 -2 0 2 4 6 8 10 12 ) 0 2 xg (x, Q LO NLO NNLO ) 0 2 xg (x, Q x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 ) 0 2 (x, Q3xT LO NLO NNLO ) 0 2 (x, Q3xT T3=u-dg NNPDF2.1 NNPDF2.1 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 18. 18 Perturbative evolution equations The dependence of PDFs on Bjorken-x (momentum fraction) is determined by non-perturbative QCD dynamics, but that on the scale Q2 (resolution) is instead known from perturbative QCD: the DGLAP evolution equations! ! ! Once x-dependence q(x,Q2 0) extracted from data, pQCD determines PDFs at other scales q(x,Q2)! ! ! ! ! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Hadronic scale LHC scale Perturbative Evolution
  • 19. Theory! Partonic cross-sections! DGLAP evolution! Heavy quark treatment! Fast interfaces to NLO calculations! NNLO QCD effects! QED and electroweak corrections! ! Data! Deep-inelastic scattering structure functions! DIS charm production! DIS Neutrino production! Jets at pp colliders! Drell-Yan (dilepton pair) production at pp colliders! Top quark data! Methodology! PDF parametrisation! PDF uncertainty estimation and error propagation to LHC cross- sections! Theoretical uncertainties! Minimisation strategy! ! Parton Distributions for the LHC! Higgs cross-sections! BSM New Physics searches! Precision Standard Model measurements Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Global PDF analysis Parton Distributions, and their associated uncertainties, need to be determined from a global analysis of hard-scattering data that requires as input the most updated theory calculations, precise and varied experimental data and robust statistical methodology
  • 20. 20 Experimental data in global PDF fits A global dataset covering a wide set of hard-scattering observables is required to constrain all possible PDF combinations in the whole range of Bjorken-x! For example, inclusive jets are sensitive to the large-x gluon, while HERA neutral current data pins down the small-x quarks! LHC data is introducing completely new observables to be used for PDF constraints! x dependence of PDFs: ! determined from data Q2 dependence of PDFs: ! determined by pQCD Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 21. ! ! ! ! ! ! The Neural Network Approach! to Parton Distributions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF
  • 22. 22 The NNPDF approach Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 The limitations of available PDF sets circa 2005, and the requirements of precision physics at the upcoming LHC, prompted us to develop a completely novel approach to PDF determination! PDF sets typically based on restrictive functional forms leading to strong theoretical bias! ! NNPDF solution: use artificial neural networks as universal unbiased interpolants! ! PDF sets often rely on the the Gaussian/linear approximation for error estimation and propagation! ! ! NNPDF solution: Use the Monte Carlo method to create a probability distribution in the space of PDFs! ! ! Traditional PDF analyses based on deterministic minimisation of the χ2 to reach convergence in the fit! NNPDF solution: Use Genetic Algorithms to be able to explore efficiently the vast parameter space Consisteny error propagation no LHC xsecs no Gaussian assumptions
  • 23. 23 Artificial Neural Networks Biological NeuralNets Artificial NeuralNets Artificial neural networks aimed to excel in the same domains as their biological counterparts: pattern recognition, forecasting, classification, .... where our evolution-driven biology outperforms traditional algorithms Neurons, axions, ! synapses, ... Inspired by biological brain models, Artificial Neural Networks are mathematical algorithms widely used in a wide range of applications, from high energy physics to targeted marketing and finance forecasting Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 24. 24 Artificial Neural Networks % of customers contacted %ofpositiveanswers Example 1: Pattern recognition. During the Yugoslavian wars, the NATO used ANNs to recognise hidden military vehicles! A military aircraft is identified, despite being hidden below a commercial plane.! Many other applications of ANN in pattern recognition: OCR software, hand writing recognition, automated anti-plagiarism software, .....! Example 2: Marketing. A bank wants to offer a new credit card to their clients. Two possible strategies:! Contact all customers: slow and costly! Contact 5% of the customers, train a ANN with their input (sex, income, loans) and their ourput (yes/no) and use the information to contact only clients likely to accepy the offer! Cost-effective method to improve marketing performance Random client selection ANN based client selection Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 25. 25 Artificial Neural Networks Artificial Neural Networks (ANNs) provide universal unbiased interpolants to parametrize PDFs at low input scales! ! The ANN class that we adopt are feed-forward multilayer neural networks (perceptrons)! ! ! ! ! In traditional PDF determinations, the input ansatz is a simple polynomial! ! ! The use of Artificial Neural Networks allows:! No theory bias introduced in the PDF determination by the choice of ad-hoc functional forms! The use of very flexible parametrizations for all PDFs - regardless of the dataset used. The NNPDF analysis allow for O(400) free parameters, to be compared with O(10-20) in traditional PDFs! Faithful extrapolation: PDF uncertainties blow up in regions with scarce experimental data! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 26. Generate a large number of Monte Carlo replicas of the experimental data with the same underlying probability distribution! ! Perform a PDF determination on each of these MC replicas! The set of PDF replicas form a representation of the probability density in the space of parton distribution functions! PDF uncertainties can be propagated to physical cross sections using textbook statistics, no need of linear/gaussian assumptions! ! ! 26 PDF Uncertainties: The Monte Carlo Method PDF Uncertainty = Standard Deviation of MC sample Central PDF prediction = Expectation Value of MC sample stat errorsys errors random numberslumi error >>1 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 27. 27 PDF Replica Neural Network Learning xg(x,Q2=2GeV2) x The minimisation of the data vs theory chi2 is performed using Genetic Algorithms! Each green curve corresponds to a gluon PDF Monte Carlo replica Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 28. 28 Artificial Neural Networks vs. Polynomials Compare a benchmark PDF analysis where the same dataset is fitted with Artificial Neural Networks and with standard polynomials (everything else identical)! ANN avoid biasing the PDFs, faithful extrapolation at small-x (very few data, thus error blow up)! ! Polynomials Neural Networks PDF error PDF error Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 No Data No Data
  • 29. 29 Adding more data: better or worse?! In traditional Hessian PDF approach, unexpected behaviors might arise when data added! ! ! ! ! In the NNPDF approach, more data always leads to an improvement of PDF uncertainties! ! ! ! x 0.1 0.2 0.3 0.4 0.5 0.6 )[ref] 2 )[new]/g(x,Q 2 g(x,Q 0.4 0.6 0.8 1 1.2 1.4 1.6 2.1 HERA-only 2.1 Global = 0.119S Ratio to NNPDF2.1 NNLO HERA-only, 2.1 HERA-only 2.1 Global CT10 (Traditional approach) Less Data More Data Less Data New data Need more flexible PDFs! to fit data: increase # of params More directions in ! Hessian space to explore Larger ! PDF uncertainties? More Data NNPDF approach Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 30. ! ! ! ! ! ! PDFs at the LHC:! From Higgs and SM to BSM searches Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 31. 31 Higgs Boson Characterisation The study of the Higgs boson properties is a cornerstone of the LHC program. All production cross sections require accurate knowledge of PDFs! gg fusion, ttH: gluon luminosity! vector-boson fusion: quark-quark luminosity! associated production with W/Z: quark-antiquark luminosity! For many crucial channels, QCD uncertainties, including PDFs, hatched areas, limit the accuracy of Higgs coupling extraction form LHC data Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 32. 32 Parton Distributions with LHC data Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 A major breakthrough in the recent years has been the inclusion of LHC data into global PDF fits! PDF constraints from a wide variety of LHC processes have been studied, many of which for first time Isolated photon LHC data constraints gluons at medium-x: ! relevant for Higgs production in gluon fusion x -4 10 -3 10 -2 10 -1 10 RelErr[xs] 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 , ratio to NNPDF2.1 Collider Only2 W= M 2 Q NNPDF2.1 Collider-only -1 NNPDF2.1 Collider-only + Wc Data L = 5 fb , ratio to NNPDF2.1 Collider Only2 W= M 2 Q W production in association with charm quarks ! provides direct access to the proton strangeness x 0.1 0.2 0.3 0.4 0.5 0.6 ) 2 (x,Q (ref) )/g 2 (x,Q (new) g 0.7 0.8 0.9 1 1.1 1.2 1.3 NNPDF2.3 NNPDF2.3 + Top Data 2 = 100 GeV2 Q = 0.118SαRatio to NNPDF2.3 NNLO, NNPDF2.3 NNPDF2.3 + Top Data 2 = 100 GeV2 Q Large-x gluon from top quark data Large-x gluon from inclusive Jet production NNPDF 14 J.R. 11 D’Enterria, J.R. 12
  • 33. 33 Top quarks as gluon luminometers The recent NNLO top quark cross section make top data the only LHC observable that is both directly sensitive to the gluon PDF and can be included consistently in a NNLO global analysis! The precise 7 and 8 TeV LHC data can be used to discriminate between PDF sets and to reduce the PDF uncertainties on the poorly known large-x gluon The improved large-x gluon leads to more accurate theory predictions for BSM searches Juan Rojo LHCP2013, Barcelona, 22/04/2013 High mass Graviton Tail of the invariant ! tt mass distribution Czakon, Mangano, Mitov, J.R. 13
  • 34. ! QED and electroweak corrections are essential for precision LHC phenomenology: W and Z production, W mass determination, WW boson pair production, TeV scale jet and top quark pair production, searches for new W’, Z’ bosons! Consistent inclusion of electroweak effects require PDFs with QED corrections and a photon PDF ! NNPDF2.3 QED: first-ever determination of the photon PDF from LHC data! Neglecting photon-initiated contributions leads to systematically underestimating theory errors in crucial BSM search channels! ! ! 34 PDFs with QED corrections ( GeV )llM 500 1000 1500 2000 2500 3000 3500 /dM(fb/GeV)[ref]/dM(fb/GeV)/dd 0 0.5 1 1.5 2 2.5 /Z production @ LHC 8 TeV * gamma BornqNNPDF2.3 QED, q ) QEDNNPDF2.3 QED, full O( ) QEDMRST04 QED, full O( /Z production @ LHC 8 TeV * gamma ( GeV )cut WW M 200 400 600 800 1000 1200 (fb) 0 0.5 1 1.5 2 2.5 WW production @ LHC 8 TeV qNNPDF2.3 QED, q NNPDF2.3 QED, MRST04 QED, WW production @ LHC 8 TeV High-Mass Drell-Yan High-Mass WW prod Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF 13
  • 35. 35 Closure Testing of Parton Distributions PDF uncertainties have been often criticised by a potential lack of statistical interpretation! In the recent NNPDF3.0 paper, we performed a systematic closure tests analysis based on pseudo-data, and verified that PDF uncertainties show a statistically robust behaviour! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF 14
  • 36. 36 Closure Testing of Parton Distributions Level 0 Closure Tests: Pseudo data = Theory! Central values of input PDF reproduced with arbitrary accuracy! PDF uncertainties on the fitted data points can become arbitrarily small Level 2 Closure Tests: Pseudo data with “exp” fluctuations! Reproduced χ2 of input PDF - both total and individual experiments! Fitted PDF central values fluctuate around input values by the same amount as expected from the size of the PDF uncertainties -> PDF errors indeed correspond to 68% Confidence Levels Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 37. 37 PDFs and Monte Carlo generators PDFs are an essential ingredient for the tuning of soft and semi-hard physics in LO Monte Carlo event generators like Pythia8, Herwig++ or Sherpa! Most updated tune of Pythia8, the Monash 2013 Tune, is based on the NNPDF2.3LO set! The harder small-x gluon in NNPDF2.3LO is essential to improve the description of the LHC forward data! Next step: specific tunes for NLO Monte Carlo event generators! ! ! ! ! ! ! Skands, Carrazza, J.R. 14 Small-x behaviour of gluon determines soft and semi-hard physics at the LHC Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 38. 38 Precision tests of the Factorisation Theorem Momentum Integral 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 NNPDF2.1 LO* NNPDF2.1 NLO* NNPDF2.1 NNLO* Perturbative QCD requires that the momentum integral should be unity to all orders! ! ! ! Is it possible to determine the value of the momentum integral from the global PDF analysis, rather than imposing it? Check in LO*, NLO* and NNLO* fits without setting M=1! ! ! ! Experimental data beautifully confirms the pQCD expectation! Extremely non trivial test of the global analysis framework and the factorization hypotheses! Very good convergence of the QCD perturbative expansion! ! !Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 NNPDF, 11
  • 39. 39 PDF comparisons made easy: APFEL-Web Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 http://apfel.mi.infn.it/ Comparing different PDF sets is now really easy thanks to the new APFEL-Web online PDF plotter! Just log in, select the PDF sets that you want to compare, the plotting settings, and have fun!! Bertone, Carrazza, J.R. 13
  • 40. 40 Cross section Ratios between 7, 8 and 14 TeV! The staged increase of the LHC beam energy provides a new class of interesting observables: cross section ratios for different beam energies! ! ! These ratios can be computed with very high precision due to the large degree of correlation of theoretical uncertainties at different energies! Experimentally these ratios can also be measured accurately since many systematics, like luminosity or jet energy scale, cancel partially in the ratios! These ratios allow stringent precision tests of the SM, like PDF discrimination! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Mangano, J. R., 12
  • 41. 41 Cross section Ratios between 7, 8 and 14 TeV! The staged increase of the LHC beam energy provides a new class of interesting observables: cross section ratios for different beam energies! ! ! These ratios can be computed with very high precision due to the large degree of correlation of theoretical uncertainties at different energies! Experimentally these ratios can also be measured accurately since many systematics, like luminosity or jet energy scale, cancel partially in the ratios! These ratios allow stringent precision tests of the SM, like PDF discrimination! ! ! ! ! ! ! ! ! Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Mangano, J. R., 12 ATLAS: Gluon from 7 TeV / 2.76 TeV jet xsecs CMS: Drell-Yan 8 TeV / 7 TeV ratio
  • 42. 42 Cross section Ratios between 7, 8 and 14 TeV! If SM theory systematics under control, these cross section ratios can provide an improved sensitivity to New Physics than absolute cross sections! ! The visibility of a BSM contribution in the evolution with energy of the cross section requires that it evolves in Q2 differently from the SM contribution! ! ( GeV )Xm 500 1000 1500 2000 2500 3000 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 ]qq / Lgg [ L 1 / s2 s 8 TeV over 7 TeV 14 TeV over 8 TeV Example: a gluon-gluon initiated BSM ! contribution to high-mass Z production.! The cross section ratio enhanced by: With greatly reduced experimental ! and theoretical uncertainties But theory systematics, mostly PDFs, need to be known accurately! for this new approach to show its full potential Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 43. 43 Determination of Standard Model parameters Accurate PDFs are required for precision determination of fundamental Standard Model parameters in processes involving initial state hadrons! These include, among many others, the strong coupling constant αS, the W boson mass, the effective lepton mixing angle, CKM matrix elements, ....! The unbiased nature of the NNPDF approach approach to faithfully disentangle PDF uncertainties from other parametric uncertainties. One example in neutrino DIS:! ! ! ! ! ! ! CKM matrix element Vcs can be determined from ! neutrino DIS data - but large uncertainties from strange PDF! ! NNPDF analysis manages to obtain the most accurate ever determination of Vcs from a single process:! ! Vcs = 1.04 ±0.06 (PDG average)! Vcs = 0.96 ±0.07 (NNPDF from NuTeV data)! ! ! The same analysis shows that the strangeness asymmetry in the proton has just the right size to cancel the NuteV anomaly sin2θW NNPDF, arXiv:0906.1958Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 44. 44 Determination of Standard Model parameters Accurate PDFs are required for precision determination of fundamental Standard Model parameters in processes involving initial state hadrons! The strong coupling constant αS can be determined from a global PDF analysis, mostly from scaling violations in Deep-Inelastic Scattering and in inclusive jet production! The NNPDF result is the most accurate determination of αS from a QCD global fit, and nicely consistent with the latest PDG average, to which is one of the dominant contributions! In the pipeline: αS determinations from LHC data at the higher scales ever probed! ! ! ! ! ! PDG 2012 PDG average from PDF fits PDG 2013 average NNPDF2.1 NNLO Consistency check of the global PDF framework: the distributions of pulls for αS fitted to individual experiments follows a Gaussian distribution! ! ! ! NNPDF, 11 Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 45. ! ! ! ! ! ! Polarised PDFs:! Unraveling the origin of the proton spin Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 46. Up to know in this talk everything was valid for unpolarised protons! The polarised case is also extremely interesting: unique windows on the spin structure of the proton.! How the proton spin is distributed among its constituents is a crucial issue for our understanding of non-perturbative QCD and confinement Polarised Parton Distributions Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 Foreword f (x, Q2 ) = f )! (x, Q2 ) f ) (x, Q2 ) How do quarks (including sea quarks) and gluons carry the proton spin S(µ) = 1 2 = X f D P; S| ˆJz f (µ)|P; S E = 1 2 Z 1 0 dx ⌃(x, µ) + Z 1 0 dx g(x, µ) + Lz All quantities depend on factorization scheme and scale µ Spin decomposition is not unique (Y. Hatta, S1) Very little of the proton spin is carried by quarks Z 1 0 dx ⌃ = Z 1 0 dx X q=u,d,s ( q + ¯q) ⇠ 30% Quark and gluon longitudinal contributions () longitudinal spin-dependent PDFs Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 2 / 25 Probes of nucleon helicity structure Guiding principle: factorization e.g. DIS d = X q,¯q,g f (x, Q2 )⌦d ˆ ⇤f (xP, ↵s (Q2 )) d ˆ ⇤f = 1X n=0 ⇣ ↵s 4⇡ ⌘n d ˆ (n) ⇤f Reaction Partonic subprocess PDF probed x Q2 [GeV2 ] `± {p, d, n} ! `± X ⇤ q ! q q + ¯q 0.003 . x . 0.8 1 . Q2 . 70 g `± {p, d} ! `± hX ⇤ q ! q u ¯u 0.005 . x . 0.5 1 . Q2 . 60d ¯d g `± {p, d} ! `± DX ⇤ g ! c¯c g 0.06 . x . 0.2 ⇠ 10 !p !p ! jet(s)X gg ! qg g 0.05 . x . 0.2 30 . p2 T . 800 qg ! qg !p p ! W ± X uL ¯dR ! W + u ¯u 0.05 . x . 0.4 ⇠ M2 WdL¯uR ! W d ¯d !p !p ! ⇡X gg ! qg g 0.05 . x . 0.4 1 . p2 T . 200 qg ! qg Di↵erent processes constrain di↵erent PDFs, factorization is successful Quarks? Gluons? Angular Mom? First measurements with polarised DIS (80s) showed that quark contribution much smaller than expected (proton spin crisis)! With the availability of polarised hadronic and semi-inclusive data, global polarised PDF fits possible! The NNPDF framework has also been applied here: NNPDFpol sets Nocera, SPIN14
  • 47. Contribution of gluon polarisation to the proton spin has been of the big unknowns in the last 30 years! The analysis of recent RHIC polarised jet data in the NNPDFpol and DSSV frameworks has provided first ever evidence for positive (non-zero) polarisation of the gluon in the proton! Importance of this important result recognised by the popular media, i.e. Scientific American Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 2) pp collisions at RHIC: jet and ⇡ production effects on g distribution x -3 10 -2 10 -1 10 1 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 NNPDFpol1.1 =12 χ∆DSSV08 positivity bound )2 =10 GeV2 g(x,Q∆x [arXiv:1406.5539] NEW FIT DSSV* DSSV incl. 90% C.L. variations Q 2 = 10 GeV 2 RHIC x range x∆g x -0.2 -0.1 0 0.1 0.2 10 -3 10 -2 10 -1 1 [arXiv:1404.4293] (GeV/c)T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 | < 0.5η| STAR 2009 Jet+X→p+p =200 GeVs (GeV/c) T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 STAR BB10 DSSV LSS10p LSS10 NNPDF | < 1η0.5 < | 6.5% scale uncertainty± from polarization not shown [arXiv:1405.5134] [GeV/c] T p 5 6 7 8 9 10 11 12 LLA -0.1 -0.05 0 0.05 0.1 0.15 g = g∆GRSV g = std∆GRSV g = -g∆GRSV DSSV 0 πSTAR + X0 π→p+p = 200 GeVs < 2.0η0.8 < 6% Scale Uncertainty [arXiv:1309.1800] Longitudinal double-spin asymmetry ALL = ++ + ++ + + features at RHIC, hx1,2i ' 2pTp s e ⌘/2 ⇡ [0.05, 0.2] qg and gg initiated subprocesses dominate (for most of the RHIC kinematics) ALL sensitive to gluon polarization cross sections are well described at NLO in pQCD measurements STAR (mainly jets) (Z. Chang & C. Dilk) PHENIX (⇡ production) (A. Manion & I. Yoon) much more to come from ongoing RHIC run ! gaining precision ! di-jet measurements Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 12 / 25 2) pp collisions at RHIC: jet and ⇡ production (GeV/c)T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 | < 0.5η| STAR 2009 Jet+X→p+p =200 GeVs (GeV/c) T Parton Jet p 5 10 15 20 25 30 35 LLA -0.01 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 STAR BB10 DSSV LSS10p LSS10 NNPDF | < 1η0.5 < | 6.5% scale uncertainty± from polarization not shown [arXiv:1405.5134] [GeV/c] T p 5 6 7 8 9 10 11 12 LLA -0.1 -0.05 0 0.05 0.1 0.15 g = g∆GRSV g = std∆GRSV g = -g∆GRSV DSSV 0 πSTAR + X0 π→p+p = 200 GeVs < 2.0η0.8 < 6% Scale Uncertainty [arXiv:1309.1800] Longitudinal double-spin asymmetry ALL = ++ + ++ + + features at RHIC, hx1,2i ' 2pTp s e ⌘/2 ⇡ [0.05, 0.2] qg and gg initiated subprocesses dominate (for most of the RHIC kinematics) ALL sensitive to gluon polarization cross sections are well described at NLO in pQCD measurements STAR (mainly jets) (Z. Chang & C. Dilk) PHENIX (⇡ production) (A. Manion & I. Yoon) much more to come from ongoing RHIC run ! gaining precision ! di-jet measurements Unraveling the gluon polarisation Total contribution of gluons to proton spin still unknown since large uncertainties at small-x from lack of data: need an Electron-Ion Collider NNPDFpol 14
  • 48. Inclusive DIS data does not allow to separate polarised quarks from antiquarks! Recent data on polarised semi-inclusive DIS and hadronic W production allow this separation for first time! Stringent constraints on non-perturbative models of the proton The polarised quark sea Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015 1) pp collisions at RHIC: W ± production ηlepton -2 -1 0 1 2 -0.5 0 0.5 = 2% error2 χ/ 2 χ∆DSSV08 L0 - W + W ν+± e→± W→+pp =510 GeVs < 50 GeV e T 25 < E LA Rel lumi syst 3.4% beam pol scale uncertainty not shown + W - W STAR Data CL=68% DSSV08 RHICBOS DSSV08 CHE NLO LSS10 CHE NLO [arXiv:1404.6880] |ηlepton | 0 0.5 1 -0.5 0 0.5 + W - W ν+±e→± W→p+p =510 GeVs < 50 GeV e T 25 < E LLA DSSV08 CHE NLO STAR Data CL=68% 6.5% beam pol scale uncertainty not shown [arXiv:1404.6880] Longitudinal single- and double-spin asymmetries AL = + + + ALL = ++ + ++ + + features quark/antiquark separation at Q ⇠ MW no need of fragmentation functions at RHIC, hx1,2i ' MWp s e ⌘l /2 ⇡ [0.04, 0.4] for W +, d ! d and d ! u non-trivial positivity bound [arXiv:1104.2920] 1 ± ALL(yW ) > |AL(yW ) ± AL( yW )| no access to strangeness (W ± + c required) measurements STAR + PHENIX (Z. Jinlong & F. Giordano) much more to come from ongoing RHIC run Emanuele R. Nocera (UNIGE) Helicity-dependent PDFs October 21, 2014 9 / 25 RHIC: W ± production metry ¯u ¯d -1 10 1 [PoS(DIS2014)204] x -3 10 -2 10 -1 10 1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0.12 )d∆-u∆x( NNPDFpol1.1 =12 χ∆DSSV08 2 =10 GeV2 Q )ρ-πMC ( )ρMC ( )σ-πMC ( PB (ansatz) CQSM ST [PoS(DIS2014)204] Model Refs. d/u d/ u u/u d/d An 1 Ap 1 SU(6) [41] 1/2 1/4 2/3 1/3 0 5/9 RCQM [43] 0 0 1 1/3 1 1 QHD ( 1/2) [44] 1/5 1/5 1 1 1 1 QHD ( ⇢) [44] 0 0 1 1/3 1 1 NJL [45] 0.20 0.06 0.80 0.25 0.35 0.77 DSE (realistic) [46] 0.28 0.11 0.65 0.26 0.17 0.59 DSE (contact) [46] 0.18 0.07 0.88 0.33 0.34 0.88 pQCD [50] 1/5 1/5 1 1 1 1 NNPDF (x = 0.7) [15, 18] 0.22 ± 0.04 0.07 ± 0.12 0.07 ± 0.05 0.19 ± 0.34 0.41 ± 0.31 0.75 ± 0.07 NNPDF (x = 0.8) [15, 18] 0.18 ± 0.09 0.12 ± 0.23 0.70 ± 0.13 0.34 ± 0.67 0.57 ± 0.61 0.75 ± 0.12 NNPDF (x = 0.9) [15, 18] 0.06 ± 0.49 0.51 ± 0.69 0.61 ± 0.48 0.85 ± 6.55 0.36 ± 0.61 0.74 ± 0.34 Table 2: A collection of several model expectations for various ratios of polarized/unpolarized PDFs and spin-dependent neutron and proton asymmetries, An 1 and Ap 1, at x ! 0. The NNPDF prediction, obtained using unpolarized NNPDF2.3 [18] and polarized NNPDFpol1.1 [15] parton sets, is shown at Q2 = 4 GeV2 for di↵erent values of x. x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 )2 =4 GeV2 (x,Q+ /u+ u∆ Avakian et al. Statistical LSS (BBS) NJL Su(6)-breaking NNPDF =12 χ∆DSSV08 x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -1.5 -1 -0.5 0 0.5 1 1.5 2 )2 =4 GeV2 (x,Q+ /d+ d∆ Avakian et al. Statistical LSS (BBS) NJL Su(6)-breaking NNPDF =12 χ∆DSSV08 Nocera 14 NNPDFpol 14
  • 49. Parton Distributions are an essential ingredient for LHC phenomenology! Accurate PDFs are required for precision SM measurements, Higgs characterisation and New Physics searches! The determination of fundamental SM parameters like the W mass or αS from LHC data also greatly benefit from improved PDFs! PDFs are also a basic component for Monte Carlo event generators! The NNPDF approach provides parton distributions based on a robust, unbiased methodology, the most updated theoretical information and all the relevant hard scattering data including LHC data! Near future developments in NNPDF include ! Inclusion of more HERA and LHC data! Tailored PDFs for NLO Monte Carlo event generators at the LHC! PDFs with threshold and high energy resummation, impact on Higgs and top physics! PDFs with and electroweak effects (both in evolution and in partonic xsecs)! PDFs with Intrinsic Charm Summary Juan Rojo NIKHEF Theory Seminar, Amsterdam, 22/01/2015
  • 50. 50 ! ! ! ! ! ! ! ! ! ! Going Beyond: PDFs at a 100 TeV collider Juan Rojo SMatLHC14, Madrid, 09/04/2014 ! ! Growing consensus that the next big machine more suitable to explore the energy frontier should be a 100 TeV hadron collider, possibly with also e+e- and ep operation modes! The phenomenology of PDFs at such extreme energies is very rich: top quark PDFs, electroweak effects on PDFs and W/Z boson PDFs, ultra-low-x physics, BFKL dynamics, BSM physics with polarized PDFs, ...., lots of fun!! First studies being now performed in the context of the CERN FCC working group! ! ! ! ! ! BSM physics with! polarized PDFs!
  • 51. 51 ! ! ! ! ! ! ! ! ! ! Going Beyond: PDFs at a 100 TeV collider Juan Rojo SMatLHC14, Madrid, 09/04/2014 ! ! Growing consensus that the next big machine more suitable to explore the energy frontier should be a 100 TeV hadron collider, possibly with also e+e- and ep operation modes! The phenomenology of PDFs at such extreme energies is very rich: top quark PDFs, electroweak effects on PDFs and W/Z boson PDFs, ultra-low-x physics, BFKL dynamics, BSM physics with polarized PDFs, ...., lots of fun!! First studies being now performed in the context of the CERN FCC working group! ! ! ! ! ! BSM physics with! polarized PDFs! Thanks for your attention!