The New Search for a Neutron EDM at the SNS Jen-Chieh Peng University of Illinois at Urbana-Champaign The Third International Symposium on LEPTON MOMENTS, Cape Cod, June 19-22, 2006 Physics of neutron EDM Proposal for a new neutron EDM experiment at SNS (Spallation Neutron Source) Results of R&D and future prospect 1
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 ~ 10-31 e cm Super Symmetric Models predict d n 10-25 e cm Baryon Asymmetry of universe Require CP violation beyond the SM SM Prediction Experiment e 10-38 e cm 2 10-27 e cm μ 10-35 e cm 1 10-19 e cm n 10-31 e cm 3 10-25 e cm 2
History of Neutron EDM Measurements Current neutron EDM upper limit: < 3.0 x 10-26 e cm (90% C.L.) 3
Neutron EDM Experiments r r Neutron precession frequency will shift by Δ ω = 2 d E/ h (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 4
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) 5
Neutron EDM Experiment with Ultra Cold Neutrons ILL Measurement 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 6
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) 7
Kinematics of n - 4 He Scattering 2 h K 2m 2 i = h 2 K 2m 2 f r + E( Q); Q = K E(Q) is the phonon dispersion relation i r K f 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 ΔK i /K i ~ 2% 8
UCN Production in Superfluid 4 He Magnetic Trapping of UCN (Nature 403 (2000) 62) 0.2 a Acrylic lightguide 0.1 Magnet form Racetrack coil Trapping region Solenoid 0.0 Cupronickel tube -0.1 b 0.1 Beam stop 10 cm Count rate (s 1 ) 0.0-0.1 0.1 c TPB-coated acrylic tube 0.0 Neutron shielding Collimator -0.1 d 560 ± 160 UCNs trapped per cycle (observed) 0.1 480 ± 100 UCNs trapped per cycle (predicted) 0.0-0.1 0 1000 2000 Time (s) 9 3000
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 10
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 3 n + He p + t + 764 KeV Total spin σ abs at v = 5m/sec J = 0 ~ 4.8 x 10 6 barns J = 1 ~ 0 11
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 1 1 Nt () = Ne { + [1 PPcos( ω t+ φ)]} Γ tott 3 τ n r β τ3 Empty the cells and change E field direction and repeat the measurement 12
Two oscillatory signals 3 1) Scintillation light from p with 2) SQUID signal from the precession of n+ He + t ω = [2( μ μ ) B ± 2 d E] / h 3 3He n 0 n He with ω = [2 μ B ]/ h 3He 0 15 Time (sec) 603.4 603.6 603.8 604 604.2 604.4 604.6 10 Amplitude 5 0-5 -10 SQUID signal Scintillation signal -15 13
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 around 10/2006 Collaboration prepared to begin construction in FY07 14
3 He Distributions in Superfluid 4 He Dilution Refrigerator at LANSCE Flight Path 11a Position Target Cell 3 He Neutron Beam 4 He 40000 T = 330 mk Preliminary 35000 Beam FWHM = 0.26 cm 30000 25000 20000 15000 10000 n-3he Normalized Counts 5000 0-6.00-4.00-2.00 0.00 2.00 4.00 6.00 Position (cm) Physica B329-333, 236 (2003) 15
3 He Diffusion Coefficient in 4 He n-3he Captures 160000 140000 120000 Heater Off 0.84 K Heater 27.1 mw 1.08K 175k -Li Glass 100000 Counts 80000 60000 40000 20000 0-3 -2-1 0 1 2 3 Position (cm) Europhysics Letters 58, 781 (2002); Phys. Rev. Lett. 93, 105302 (2004) 16
Polarized 3 He Atomic Beam Source Injection nozzle Spin flip region 3 He RGA detector 1 K cold head Analyzer quadrupole Polarizer quadrupole Produce polarized 3 He with 99.5% polarization at a flux of 2 10 14 /sec and a mean velocity of 100 m/sec 17
Los Alamos Polarized 3 He Source Injection nozzle Spin flip region 3 He RGA detector 1 K cold head Polarizer quadrupole Analyzer quadrupole 3 He Spin dressing experiment 36 in B 0 static Polarizer Analyzer RGA Ramsey coils B 1 dressing 18
Polarized 3 He source at LANL Cold head Mapping the dressing field Quad separator analyzer Solenoid RGA source Spin-flip coils and dressing coils added inside the solenoid. 19
Critical dressing of neutrons and 3 He Reduce the error caused by B 0 instability between measurements Effective dressed g factors: gj Dress field can modify neutron and 3 He g factors: g neutron = g 3 He γ B γ 3B1 = gj ω ω n 1 n 0 3 0 J x = αj αx ( ) ( ) 0 c 0 c 3 He neutron x = γ ω B / n 1 x c B 1 1.19 0.408 3.86 1.324 6.77 3.333 9.72 4.348 α =1.1127 20
Observation of 3 He dressed-spin effect 3He Larmor Frequency 3He Larmor Frequency [khz] 27.6 27.4 27.2 27.0 26.8 26.6 26.4 26.2 0 2 4 6 8 10 Dressing Coil Current [A] Esler, Peng and Lamoreaux (2006) 21
Polarized 3 He relaxation time measurements T 1 > 3000 seconds in 1.9K superfluid 4 He Acrylic cell coated with dtpb H. Gao, R. McKeown, et al, arxiv:physics/0603176 Additional test is being done at 600mK 22
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 23
SNS at ORNL 1.4 MW Spallation Source (1GeV proton, 1.4mA) First proton beam was delivered in April 2006 24
p beam SNS Target Hall FNPB-Fundamental Neutron Physics Beamline FNPB construction underway Cold beam available ~2007 UCN line via LHe ~2009 25
FNPB Beamline Double monochrometer Selects 8.9 neutrons for UCN via LHe o A 26
Neutron EDM Detector Dilution Fridge Int er me d ia te shield LHe Reser vo ir 4 K Shield Cryovessel Helium I nsu latio n Volum e Conceptual Design Report is being prepared 27
n-edm Sensitivity vs Time EDM @ SNS d n <1x10-28 e-cm 2000 2010 28
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. 29