Electric Dipole Moment of Neutron and Neutrinos JenChieh

Electric Dipole Moment of Neutron and Neutrinos Jen-Chieh Peng University of Illinois at Urbana-Champaign Workshop on Future PRC-U. S. Cooperation in High Energy Physics, IHEP, June 11 -18, 2006 • Physics of neutron EDM • Status of neutron EDM measurements • Proposal for a new neutron EDM experiment at SNS • Neutrino EDM 1

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

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 e μ n SM Prediction 10 -40 e • cm 10 -38 e • cm 10 -31 e • cm Experiment 10 -27 e • cm 10 -19 e • cm 10 -25 e • cm 3

SUSY Prediction of Neutron versus Electron EDM Barbieri et al. 4

History of Neutron EDM Measurements Current neutron EDM upper limit: < 6. 3 x 10 -26 e • cm (90% C. L. ) 5

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 6

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 potential is ~ 10 -7 ev per meter • UCN can be produced with cold-moderator (tail of the Maxwell distribution) 7

Neutron EDM Experiment with Ultra Cold Neutrons Most Recent 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 much higher UCN flux can be obtained by using the “down-scattering” process in superfluid 4 He 8

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 9

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) 10

A proposal for a new neutron EDM experiment ( Based on the idea originated by R. Golub and S. Lamoreaux in 1994 ) Collaborating institutes: UC Berkeley, Caltech, Duke, Hahn-Meitner, Harvard, Hungarian Academy of Sciences, UIUC, ILL, Indiana, Leiden, LANL, MIT, NIST, NCSU, UNM, ORNL, Simon. Fraser 11

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 J=0 J=1 σabs at v = 5 m/sec ~ 4. 8 x 106 barns ~0 12

Neutron EDM Measurement Cycle • • Fill cells with superfluid 4 He containing polarized 3 He Produce polarized UCNs with polarized 1 mev 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 (~10 m. G) and a strong E field (~50 KV/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 13

Two oscillatory signals SQUID signal Scintillation signal 14

Status of SNS neutron EDM • Many feasibility studies and measurements (2003 -2006 R&D) • CD-0 approval by DOE: 11/2005 – Construction Possible: FY 07 -FY 10 – Cost: 15 -18 M$ • CD-1 approval anticipated around 10/2006 • Collaboration prepared to begin construction in FY 07 15

3 He Distributions in Superfluid 4 He Dilution Refrigerator at LANSCE Flight Path 11 a Position Target Cell 3 He Neutron Beam T = 330 m. K 4 He Pr eli mi Physica B 329 -333, 236 (2003) na ry 16

Neutron Tomography of Impurity-Seeded Superfluid Helium Phys. Rev. Lett. 93, 105302 (2004) 17

Critical dressing of neutrons and 3 He Reduce the error caused by B 0 instability between measurements Dress field can modify neutron and 3 He g factors: Effective dressed g factors: B 1 3 He neutron 1. 19 0. 408 3. 86 1. 324 6. 77 3. 333 9. 72 4. 348 18

Los Alamos Polarized 3 He Source Spin flip region 3 He RGA detector Injection nozzle 1 K cold head Analyzer quadrupole Polarizer quadrupole 3 He Spin dressing experiment 36 in B 0 static Polarizer Ramsey coils RGA Analyzer B 1 dressing 19

Observation of 3 He dressed-spin effect Esler, Peng and Lamoreaux (2006) 20

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Polarized 3 He relaxation time measurements T 1 > 3000 seconds in 1. 9 K superfluid 4 He H. Gao, R. Mc. Keown, et al, ar. Xiv: Physics/0603176 22

UIUC Test Apparatus for Polarized 3 He Relaxation at 600 m. K Work carried out by UIUC and students from Hong Kong (CUHK) 23

SNS at ORNL 1. 4 MW Spallation Source 24

n EDM Experiment at SNS 25

n-EDM Sensitivity vs Time EDM @ SNS dn<1 x 10 -28 e-cm 2000 2010 26

Neutrino electric dipole moment • For Majorana neutrinos, CPT invariance ensures zero electric and magnetic dipole moments • For Dirac neutrinos, non-zero EDM is possible (CP-violation) Another dedicated neutrino experiment is required at Daya Bay to improve the sensitivity on the neutrino EDM 27

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 FY 2007. 28