Search for Neutron Electric Dipole Moment

Search for Neutron Electric Dipole Moment Physics of neutron EDM Proposal for a new neutron EDM experiment at the SNS (Spallation Neutron Source) Results of R&D and future prospect Jen-Chieh Peng Purdue University March 12, 2009 University of Illinois at Urbana-Champaign

EDM violates parity Dirac’s magnetic monopole can generate EDM Neutron scattering sets an upper limit of 3 x 10 -18 cm A dedicated experiment for neutron EDM is underway

Neutron Electric Dipole Moment EDM has to be pointing parallel to the spin direction Mirror S + - d S - + d

Observation of parity violation in 60 Co beta-decay

No evidence for neutron EDM ! Upper limit set at 5 x 10 -20 e • cm “ The absence of an electric dipole moment in our neutron experiment and the forced postponement of our 60 Co experiments were the greatest disappointments in my research career. But by then I had realized that research scientists have both good and bad luck and productive scientists do not allow the bad luck to discourage them from further research ” N. Ramsey, 1998

EDM violates time-reversal symmetry CPT invariance implies EDM violates CP Time-reversal No new results on neutron EDM measurement between 1957 and 1964 S + - d S - + d

History of Neutron EDM Measurements Current neutron EDM upper limit: < 3.0 x 10 -26 e•cm (90% C.L.) Still no evidence for neutron EDM

Physics Motivation for New Neutron EDM Measurements CP Violation (in the light-quark baryon sector) Physics Beyond the Standard Model Standard Model predicts d n ~ 10 -31 e•cm Super Symmetric Models predict d n ≤ 10 -25 e•cm Baryon Asymmetry of universe Require CP violation beyond the SM 3×10 -26 e•cm 10 -31 e•cm n 1×10 -19 e•cm 10 -35 e•cm μ 2×10 -27 e•cm 10 -38 e•cm e Experiment SM Prediction

EDM measurement principle B 0 E <S z > = + h/2 <S z > = - h/2 h  (0) = -2 μ .B h  (  )= 2 (- μ .B + d n .E) h  (  )= 2(- μ .B - d n .E ) B 0 B 0 E d n defined +ve  ↑↑ -  ↑↓ = Δ  = 4 d n . E / h (slides from Karamath)

Neutron EDM Experiments Limitations: • Short duration for observing the precession • Systematic error due to motional magnetic field (v x E) Both can be improved by using ultra-cold neutrons Ramsey’s Separated Oscillatory Field Method (d = 10 -26 e•cm, E = 10 KV/cm -> 10 -7 Hz shift )

Ultra-Cold Neutrons (UCN) First suggested by Fermi Many material provides a repulsive potential of ~ 100 nev (10 -7 ev) for neutrons Ultra-cold neutrons (E < 10 -7 ev, v < 8 m/s) can be stored in bottles (until they decay). Gravitational energy is ~ 10 -7 ev per meter (can store UCN in a bottle without a lid) UCN can be produced with cold-moderator (tail of the Maxwell distribution)

Neutron EDM Experiment with Ultra Cold Neutrons • Use 199 Hg co-magnetometer to sample the variation of B-field in the UCN storage cell • Limited by low UCN density of ~ 5 UCN/cm 3 How can one obtain a higher UCN flux? Measurement at Institute Laue-Langevin

UCN Production in Superfluid 4 He Incident cold neutron with momentum of 0.7 A -1 (~10 -3 ev) can excite a phonon in 4 He and become an UCN (Golub and Pendlebury) neutron Superfluid 4 He UCN phonon (~1 mev)

UCN Production in Superfluid 4 He Magnetic Trapping of UCN at NIST (Nature 403 (2000) 62) 560 ± 160 UCNs trapped per cycle (observed) 480 ± 100 UCNs trapped per cycle (predicted)

A proposal for a new neutron EDM experiment Collaborating institutes: Arizona State, UC Berkeley, Caltech, Duke, Hahn-Meitner, UIUC, Indiana, Kentucky, Leiden, LANL, MIT, NCSU, ORNL, Simon-Fraser, Tennessee, Yale ( Based on the idea originated by R. Golub and S. Lamoreaux in 1994 )

How to measure the precession of UCN in the superfluid 4 He bottle? Use polarized 3 He to detect the UCN precession n – 3 He absorption is strongly spin-dependent ~ 0 J = 1 ~ 4.8 x 10 6 barns J = 0 σ abs (at v = 5m/sec) Total spin

Neutron EDM Measurement in superfluid 4 He Fill cells with superfluid 4 He containing polarized 3 He Produce polarized UCNs with polarized 1mev neutron beam Precess UCN and 3 He in a uniform B field (~10mG) and a strong E field (~50KV/cm). ( ν ( 3 He) ~ 33 Hz, ν (n) ~ 30 Hz) Detect scintillation light from the reaction n + 3 He  p + t

Two oscillatory signals SQUID signal Scintillation signal

Status of SNS neutron EDM Many feasibility studies and measurements have been performed Conceptual Design approved: Feb 2007 Construction approval (expected Aug 2009) Cost: ~ 18 M$ Collaboration prepares to begin construction in FY10

3 He Distributions in Superfluid 4 He Neutron Beam 4 He Target Cell 3 He Preliminary T = 330 mK Dilution Refrigerator at LANSCE Flight Path 11a Phys. Rev. Lett. 93, 105302 (2004) Position

Polarized 3 He Atomic Beam Source 1 K cold head Injection nozzle Polarizer quadrupole Spin flip region Analyzer quadrupole 3 He RGA detector Produce polarized 3 He with 99.5% polarization at a flux of 2 ×10 14 /sec and a mean velocity of 100 m/sec

Dressed Spin in Neutron EDM Neutrons and 3 He naturally precess at different frequencies (different g factors) Applying a RF field (dressing field), B d Sin(  d t) , perpendicular to the constant B 0 field, the effective g factors of neutrons and 3 He will be modified (dressed spin effect) At a critical dressing field, the effective g factors of neutron and 3 He can be made identical! (As an alternative for SQUID magnetometer)

Critical dressing of neutrons and 3 He Crossing points equalize neutron and 3 He g factors: 3 He neutron Effective dressed g factors: Reduce the danger of B 0 instability between measurements 9.72 6.77 3.86 1.19

Los Alamos Polarized 3 He Source 1 K cold head Injection nozzle Polarizer quadrupole Spin flip region Analyzer quadrupole 3 He RGA detector B 1 dressing B 0 static Polarizer Analyzer RGA 36 in 3 He Spin dressing experiment Ramsey coils

Polarized 3 He source at LANL Mapping the dressing field source analyzer RGA Spin-flip coils and dressing coils used inside the solenoid. Cold head Quad separator Solenoid

Observation of 3 He dressed-spin effect Esler, Peng, Lamoreaux, et al. Nucl-ex/0703029 (2007)

Polarized 3 He relaxation time measurements H. Gao, R. McKeown, et al, arXiv:Physics/0603176 T 1 > 3000 seconds in 1.9K superfluid 4 He Acrylic cell coated with dTPB Test has also been done at 600 mK at UIUC

High voltage tests Goal is 50 kV/cm 200 liter LHe. Voltage is amplified with a variable capacitor 90 kV/cm is reached for normal state helium. 30 kV/cm is reached below the λ -point J. Long et al., arXiv:physics/0603231

SNS at ORNL First proton beam was delivered in April 2006 1.4 MW Spallation Source (1GeV proton, 1.4mA)

SNS Target Hall p beam FNPB-Fundamental Neutron Physics Beamline FNPB construction underway Cold beam available ~2007 UCN line via LHe ~2009

FNPB Beamline Double monochrometer Selects 8.9  neutrons for UCN via LHe

nEDM ground “breaking” Feb. 6, 2009 (Shovel ready)

n-EDM Sensitivity vs Time d n < 1x10 -28 e-cm EDM @ SNS 2000 2010

Summary Neutron EDM measurement addresses fundamental questions in physics (CP violation in light-quark baryons). A new neutron EDM experiment uses UCN production in superfluid helium and polarized 3 He as co-magnetometer and analyser. The goal of the proposed measurement is to improve the current neutron EDM sensitivity by two orders of magnitude. Many feasibility studies have been carried out. Construction is expected to start in FY2010.

Why do molecules have EDM (without violating parity)? Consider a diatomic polar molecule. The only possible orientation of the EDM is along the molecular axis, but the rotation (spin) is directed perpendicular to the axis. For polyatomic molecules (like NH 3 ), the +k and –k (k is the spin projection) are degenerate states with opposite sign of EDM. The superposition of these two states would give zero EDM.

Neutron EDM in Standard Model One and two-loop contributions are zero. Three-loop contribution is ~10 -34 e•cm a) Contributions from single quark’s EDM: 1) Electroweak Process b) Contributions from diquark interactions: ( hep-ph/0008248) d n ~ 10 -32 e•cm

Neutron EDM in Standard Model Θ term’s contribution to the neutron EDM : Θ term in the QCD Lagrangian : 2) Strong Interaction Spontaneously broken Pecci-Quinn symmetry? No evidence of a pseudoscalar axion! d n < 10 -25 e•cm -> | θ | < 3 x 10 -10

SUSY Prediction of Neutron versus Electron EDM Barbieri et al.

List of Neutron EDM Experiments B = 1mG => 3 Hz neutron precession freq. d = 10 -26 e•cm, E = 10 KV/cm => 10 -7 Hz shift in precession freq. 1999 < 6.3 x 10 -26 120-150 0.01 4.5 <6.9 UCN Mag. Res. 1992 < 9.7 x 10 -26 70-100 0.018 12-15 <6.9 UCN Mag. Res. 1990 < 12 x 10 -26 70 0.01 16 <6.9 UCN Mag. Res. 1986 < 2.6 x 10 -25 50-55 0.025 12-15 <6.9 UCN Mag. Res. 1984 < 8 x 10 -25 60-80 0.01 10 <6.9 UCN Mag. Res. 1981 < 6 x 10 -25 5 0.025 20 <6.9 UCN Mag. Res. 1980 < 1.6 x 10 -24 5 0.028 25 <6.9 UCN Mag. Res. 1977 < 3 x 10 -24 0.0125 17 100 154 Beam Mag. Res. 1973 < 1 x 10 -23 0.012 14 120 154 Beam Mag. Res. 1969 < 5 x 10 -23 0.015 17 120 115 Beam Mag. Res. 1969 < 1 x 10 -21 0.0009 1.5 50 2200 Beam Mag. Res. 1968 < 3 x 10 -22 0.00625 9 140 130 Beam Mag. Res. 1967 < 8 x 10 -22 10 -7 -- 10 9 2200 Bragg Reflection 1967 < 7 x 10 -22 0.014 9 140 60 Beam Mag. Res. 1957 < 4x 10 -20 0.00077 150 71.6 2050 Beam Mag. Res. 1950 < 3 x 10 -18 10 -20 -- 10 25 2200 Scattering year EDM (e.cm) Coh. Time (s) B (Gauss) E (kV/cm) <v>(m/cm) Ex. Type

Kinematics of n - 4 He Scattering E(Q) is the phonon dispersion relation 200nev (typical wall potential) θ is neutron’s scattering angle For 1 mev neutron beam, σ (UCN)/ σ (tot) ~ 10 -3 for 200 nev wall potential Mono-energetic cold neutron beam with Δ K i /K i ~ 2%

UIUC Test Apparatus for Polarized 3 He Relaxation at 600 mK

SQUIDs M. Espy, A. Matlachov ~100 cm 2 superconducting pickup coil Flux = 2 x 10 -16 Tm 2 = 0.1  0 Noise = 4 m  0 /Hz 1/2 at 10  Hz ~ T 1/2 2.5 m  0 /Hz 1/2

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