Transcript Document

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
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Neutron Electric Dipole Moment
qn    ( x)d 3 x  (0.4  1.1) 10 21 e


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d n   x ( x)d x  d n sˆ
Non-zero dn violates both P and T symmetry
Consider the energy dn  E
Under a parity operation:
sˆ  sˆ ,


E  E
 
 
dn  E  dn  E
Under a time-reversal operation:
sˆ  sˆ ,


E E
 
 
dn  E  dn  E
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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
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SUSY Prediction of Neutron versus
Electron EDM
Barbieri et al.
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History of Neutron EDM Measurements
Current neutron EDM upper limit: < 6.3 x 10-26 e•cm (90% C.L.)
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Neutron EDM Experiments
Neutron precession frequency will shift by   2d  E /
(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
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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)
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Neutron EDM Experiment with
Ultra Cold Neutrons
Most Recent 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 much higher UCN flux can be obtained by using the
“down-scattering” process in superfluid 4He
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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
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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)
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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
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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
n  He  p  t  764KeV
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Total spin
J=0
J=1
σabs at v = 5m/sec
~ 4.8 x 106 barns
~0
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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
N (t )  Ne
tot t
{
1


1
3
[1  P3 Pn cos(r t   )]}
• Empty the cells and change E field direction and repeat the
measurement
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Two oscillatory signals
1) Scintillation light from n  3 He  p  t with   [( 3He  n ) B0  2dn E ] /
2) SQUID signal from the precession of 3 He with   [ 3He B0 ] /
Time (sec)
603.4
15
603.6
603.8
604
604.2
604.4
604.6
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Amplitude
5
0
-5
-10
SQUID
signal
Scintillation
signal
-15
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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
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Distributions in Superfluid 4He
Dilution Refrigerator at
LANSCE Flight Path 11a
Position
Target
Cell
3He
4He
Neutron Beam
40000
T = 330 mK
35000
30000
25000
Beam FWHM = 0.26 cm
20000
15000
10000
5000
-6.00
-4.00
-2.00
0.00
2.00
4.00
0
6.00
Position (cm)
Physica B329-333, 236 (2003)
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n-3He Normalized Counts
3He
Neutron Tomography of Impurity-Seeded
Superfluid Helium
Phys. Rev. Lett. 93, 105302 (2004)
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Critical dressing of neutrons and 3He
Reduce the error caused by
B0 instability between
measurements
Effective dressed
g factors:
3He
neutron
x   n B1 / 
Dress field can modify neutron
and 3He g factors:
g neutron  g 3 He
  3 B1 
  n B1 
gn J 0 
  g3 J 0 







J 0  xc    J 0  xc 
xc
B1
1.19
0.408
3.86
1.324
6.77
3.333
9.72
4.348
  1.1127
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Los Alamos Polarized 3He Source
Spin flip region
3He
RGA
detector
Injection nozzle
1 K cold head
Analyzer
quadrupole
Polarizer
quadrupole
3He
Spin dressing experiment
36 in
B0 static
Polarizer
Ramsey
coils
RGA
Analyzer
B1 dressing
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Observation of 3He 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
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Dressing Coil Current [A]
Esler, Peng and Lamoreaux (2006)
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Polarized 3He relaxation time measurements
T1 > 3000 seconds
in 1.9K superfluid
4He
H. Gao, R. McKeown, et al,
arXiv:Physics/0603176
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UIUC Test Apparatus for Polarized
3He Relaxation at 600 mK
Work carried out
by UIUC and
students from
Hong Kong
(CUHK)
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SNS at ORNL
1.4 MW Spallation Source
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n EDM Experiment at SNS
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n-EDM Sensitivity vs Time
EDM @ SNS
dn<1x10-28 e-cm
2000
2010
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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)
Bounds on neutrino EDM ( d )
From   width
| d | 5.2  10 17 e  cm
From  -e scattering
2
2
d
2
2   1  T / E
 (|  |  | d | )
dT
me2
T
| d e | 2  10 21 e  cm (MUNU, TEXONO)
| d  | 1.4 10 20 e  cm (LSND)
| d | 7.8  1018 e  cm (DONUT)
From cosmology
| d | 2.5  10 22 e  cm (PL 128B (1983) 431)
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
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.
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