Neutron Electric Dipole Moment Experiment Jen-Chieh Peng • Physics of neutron EDM • Proposal for a new neutron EDM experiment at SNS (Spallation Neutron Source) • Results of R&D and future prospect University of Illinois at Urbana-Champaign International Workshop on “High Energy Physics in the LHC Era” Valparaiso, Chile, December 11-15, 2006
Neutron Electric Dipole Moment Non-zero dn violates both P and T symmetry Under a parity operation: Under a time-reversal operation:
Physics Motivation for Neutron EDM Measurement • Time Reversal Violation • CP Violation (in the light-quark baryon sector) • Physics Beyond the Standard Model • Standard Model predicts dn~ 10-31 e•cm • Super Symmetric Models predict dn ≤ 10-25 e•cm • Baryon Asymmetry of universe • Require CP violation beyond the SM
History of Neutron EDM Measurements Current neutron EDM upper limit: < 3.0 x 10-26 e•cm (90% C.L.)
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.
Neutron EDM Experiments (d = 10-26 e•cm, E = 10 KV/cm => 10-7 Hz shift ) Ramsey’s Separated Oscillatory Field Method 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
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 (velocity < 8 m/s) can be stored in bottles (until they decay). • Gravitational energy is ~ 10-7 ev per meter • UCN can be produced with cold-moderator (tail of the Maxwell distribution)
Neutron EDM Experiment with Ultra Cold Neutrons ILL Measurement • Use 199Hg co-magnetometer to sample the variation of B-field in the UCN storage cell • Limited by low UCN flux of ~ 5 UCN/cm3 A higher UCN flux can be obtained by using the “superthermal” down-scattering process in superfluid He
UCN Production in Superfluid 4He Incident cold neutron with momentum of 0.7 A-1 (10-3 ev) can excite a phonon in 4He and become an UCN (Golub and Pendlebury)
Kinematics of n - 4He Scattering E(Q) is the phonon dispersion relation • 200nev (typical wall potential) For 1 mev neutron beam, σ(UCN)/σ(tot) ~ 10-3 for 200 nev wall potential θ is neutron’s scattering angle Mono-energetic cold neutron beam with ΔKi/Ki ~ 2%
UCN Production in Superfluid 4He Magnetic Trapping of UCN (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 ( Based on the idea originated by R. Golub and S. Lamoreaux in 1994 ) Collaborating institutes: Arizona State, UC Berkeley, Caltech, Duke, Hahn-Meitner, UIUC, Indiana, Kentucky, Leiden, LANL, MIT, NCSU, ORNL, Simon-Fraser, Tennessee, Yale
How to measure the precession of UCN in the Superfluid 4He bottle? • Add polarized 3He to the bottle • n – 3He absorption is strongly spin-dependent
Neutron EDM Measurement Cycle • Fill cells with superfluid 4He containing polarized 3He • Produce polarized UCNs with polarized 1mev neutron beam • Flip n and 3He spin by 90o using a π/2 RF coil • Precess UCN and 3He in a uniform B field (~10mG) and a strong E field (~50KV/cm). (ν(3He) ~ 33 Hz, ν(n) ~ 30 Hz) • Detect scintillation light from the reaction n + 3He p + t • Empty the cells and change E field direction and repeat the measurement
Two oscillatory signals SQUID signal Scintillation signal
Status of SNS neutron EDM • Many feasibility studies and measurements (2003-2006 R&D) • CD-0 approval by DOE: 11/2005 • Construction Possible: FY07-FY10 • Cost: 15-18 M$ • CD-1 approval anticipated at end of 2006 • Collaboration prepared to begin construction in FY07
Position 3He Distributions in Superfluid 4He Dilution Refrigerator at LANSCE Flight Path 11a Target Cell 3He Neutron Beam 4He T = 330 mK Preliminary Physica B329-333, 236 (2003)
3He Diffusion Coefficient in 4He Europhysics Letters 58, 781 (2002); Phys. Rev. Lett. 93, 105302 (2004)
Polarized 3He Atomic Beam Source 3He RGA detector Spin flip region Injection nozzle 1 K cold head Analyzer quadrupole Polarizer quadrupole Produce polarized 3He with 99.5% polarization at a flux of 2×1014/sec and a mean velocity of 100 m/sec
Dressed Spin in Neutron EDM • Neutrons and 3He naturally precess at different frequencies (different g factors) • Applying an RF field perpendicular to the constant B field, the effective g factors of neutrons and 3He will be modified (dressed spin effect) • At a critical dressing field, the effective g factors of neutron and 3He can be made identical !
Critical dressing of neutrons and 3He Crossing points equalize neutron and 3He g factors: Reduce the danger of B0 instability between measurements Effective dressed g factors: neutron 3He
Los Alamos Polarized 3He Source 3He RGA detector Spin flip region Injection nozzle 1 K cold head Analyzer quadrupole Polarizer quadrupole 3He Spin dressing experiment 36 in B0static RGA Polarizer Analyzer Ramsey coils B1 dressing
Polarized 3He source at LANL Mapping the dressing field Cold head analyzer Quad separator Solenoid RGA source Spin-flip coils and dressing coils added inside the solenoid.
Observation of 3He dressed-spin effect Esler, Peng and Lamoreaux, Preprint (2006)
Polarized 3He relaxation time measurements T1 > 3000 seconds in 1.9K superfluid 4He Acrylic cell coated with dTPB H. Gao, R. McKeown, et al, arXiv:Physics/0603176 Additional test is being done at 600mK
High voltage tests Goal is 50 kV/cm 90 kV/cm is reached for normal state helium. 30 kV/cm is reached below the λ-point 200 liter LHe. Voltage is amplified with a variable capacitor J. Long et al., arXiv:physics/0603231
SNS at ORNL 1.4 MW Spallation Source (1GeV proton, 1.4mA) First proton beam was delivered in April 2006
SNS Target Hall p beam FNPB construction underway Cold beam available ~2007 UCN line via LHe ~2009 FNPB-Fundamental Neutron Physics Beamline
FNPB Beamline Double monochrometer Selects 8.9 neutrons for UCN via LHe
Neutron EDM Detector Conceptual Design Report is being prepared
n-EDM Sensitivity vs Time 2000 2010 EDM @ SNS dn<1x10-28 e-cm
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 3He 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 FY2007.