1 Neutron Electric Dipole Moment Experiment Jen-Chieh Peng International Workshop on “High Energy Physics in the LHC Era” Valparaiso, Chile, December 11-15, 2006 University of Illinois at Urbana-Champaign Physics of neutron EDM Proposal for a new neutron EDM experiment at SNS (Spallation Neutron Source) Results of R&D and future prospect

2 Neutron Electric Dipole Moment Non-zero d n violates both P and T symmetry Under a parity operation:Under a time-reversal operation:

3 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 d n ~ ecm –Super Symmetric Models predict d n ≤ ecm Baryon Asymmetry of universe –Require CP violation beyond the SM SM Prediction Experiment e ecm 2× ecm μ ecm 1× ecm n ecm 3× ecm

4 History of Neutron EDM Measurements Current neutron EDM upper limit: < 3.0 x ecm (90% C.L.)

5 List of Neutron EDM Experiments B = 1mG => 3 Hz neutron precession freq. Ex. Type (m/cm)E (kV/cm)B (Gauss)Coh. Time (s)EDM (e.cm)year Scattering < 3 x Beam Mag. Res < 4x Beam Mag. Res < 7 x Bragg Reflection < 8 x Beam Mag. Res < 3 x Beam Mag. Res < 1 x Beam Mag. Res < 5 x Beam Mag. Res < 1 x Beam Mag. Res < 3 x UCN Mag. Res.< < 1.6 x UCN Mag. Res.< < 6 x UCN Mag. Res.< < 8 x UCN Mag. Res.< < 2.6 x UCN Mag. Res.< < 12 x UCN Mag. Res.< < 9.7 x UCN Mag. Res.< < 6.3 x d = ecm, E = 10 KV/cm => Hz shift in precession freq.

6 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 = ecm, E = 10 KV/cm => Hz shift )

7 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 ~ ev per meter UCN can be produced with cold-moderator (tail of the Maxwell distribution)

8 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 flux of ~ 5 UCN/cm 3 A higher UCN flux can be obtained by using the “superthermal” down-scattering process in superfluid He ILL Measurement

9 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)

10 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) ~ for 200 nev wall potential Mono-energetic cold neutron beam with ΔK i /K i ~ 2%

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

12 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 )

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

14 Neutron EDM Measurement Cycle Fill cells with superfluid 4 He containing polarized 3 He Produce polarized UCNs with polarized 1mev neutron beam Flip n and 3 He spin by 90 o using a π/2 RF coil 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 Empty the cells and change E field direction and repeat the measurement

15 Two oscillatory signals SQUID signal Scintillation signal

16 Status of SNS neutron EDM Many feasibility studies and measurements ( R&D) CD-0 approval by DOE: 11/2005 –Construction Possible: FY07-FY10 –Cost: M$ CD-1 approval anticipated at end of 2006 Collaboration prepared to begin construction in FY07

17 3 He Distributions in Superfluid 4 He Neutron Beam Position 4 He Target Cell 3 He Preliminary T = 330 mK Dilution Refrigerator at LANSCE Flight Path 11a Physica B , 236 (2003)

18 3 He Diffusion Coefficient in 4 He Europhysics Letters 58, 781 (2002); Phys. Rev. Lett. 93, (2004)

19 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

20 Dressed Spin in Neutron EDM Neutrons and 3 He 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 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 !

21 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

22 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

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

24 Observation of 3 He dressed-spin effect Esler, Peng and Lamoreaux, Preprint (2006)

25 Polarized 3 He relaxation time measurements H. Gao, R. McKeown, et al, arXiv:Physics/ T 1 > 3000 seconds in 1.9K superfluid 4 He Acrylic cell coated with dTPB Additional test is being done at 600mK

26 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/

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

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

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

30 Neutron EDM Detector Conceptual Design Report is being prepared

31 n-EDM Sensitivity vs Time d n <1x e-cm SNS

32 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 FY2007.