The New Search for a Neutron EDM

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The New Search for a Neutron EDM
Pinghan Chu
University of Illinois at Urbana-Champaign Indiana
University Nuclear Physics Summer School 2006

• Physics motivations for neutron EDM measurements
• A new neutron EDM experiment
• Results from UIUC
• Dressing of polarized 3He at room temperature

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Electric Dipole Moment
E.M. Purcell and N. F. Ramsey (Phys. Rev. 78
1950)
Fortson, Sandars, Barr, Physics Today, June 2003
Non-zero dn violates both P and T
Under a parity operation
Under a time-reversal operation
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Why Neutron EDM(electric dipole moment) ?

• Physics Beyond the Standard Model
• Standard Model predicts dn 10-31 ecm
• Super Symmetric Models predict dn 10-25 ecm
• Baryon Asymmetry of universe
• Require CP violation beyond the SM

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History of Neutron EDM Measurements

• Neutron Scattering(1950)
• Interference between the neutron-nucleus and
  neutron-electron scattering.
• In-Flight Neutron Magnetic Resonance(1960-1970)
• The neutrons precess at the frequency
• Upon reversal of the electric field
  direction
• Ultra-Cold Neutron(1980-)

(d 10-26 ecm, E 10 KV/cm gt 10-7 Hz shift )
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Neutron EDM Experiments
PR.108,120(1957) 5 x 10-20 ecm
Ramseys Separated Oscillatory Field
Method(PR.78,696(1950))

• Limitations
• Short duration for observing the precession
• Systematic error due to motional magnetic
  field (v x E) and fluctuation of the magnetic
  field -gt dn v/c

Both can be improved by using ultra-cold neutrons
neutron EDM upper limit lt 3.0 x 10-26 ecm (90
C.L.) (by ILL)
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Neutron EDM Measurement Cycle

• Fill cells with superfluid 4He containing
  polarized 3He (comagnetometry)
• magnetic dipole moment of 3He n (11 larger)
• EDM of 3He 0 (due to the shielding from 2 bound
  electrons)
• Produce polarized UCNs with polarized 1mEv
  neutron beam
• Flip n and 3He spin by 90o using a p/2 RF coil
• Precess UCN and 3He in a uniform B field (10mG)
  and a strong E field (50KV/cm). (?(3He) 33 Hz,
  ?(n) 30 Hz)
• Detect scintillation light from the reaction n
  3He ? p t g (strongly spin-dependent)
• Empty the cells and change
• E field direction and repeat the
• measurement

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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 !

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Critical dressing of neutrons and 3He
Dress field can modify neutron and 3He g
factors
Reduce the error caused by B0 instability
between measurements
Effective dressed g factors
neutron
3He
Muskat et. al., PRL 58, 2047 (1987) Dubbers,
NIM A284 (1989)
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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
B0 static
RGA
Polarizer
Analyzer
Ramsey coils
B1 dressing
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Observation of 3He dressed-spin effect
Esler, Peng and Lamoreaux (2006)
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Experimental Setup _at_ UIUC
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Experimental Setup(NMR)
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New Setup
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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 analyzer.
• 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.