The Cryogenic Neutron Electric Dipole Moment ExperimentJim Grozier
University of Sussex, UK
The Cryogenic Neutron Electric Dipole Moment Experiment
Jim Grozier
University of Sussex, UK
Overview
Why EDM? - Theoretical Background
The Method of Ramsey Resonance
Previous Experiments
Ultra-Cold Neutrons
The Cryogenic Experiment
EDMs and Time Reversal Symmetry
Picture a spherical particle spinning in a particular direction It just looks like the same particle turned upside down Imagine it running backwards in time Now, imagine it is “pear-shaped” Imagine it running backwards in time It no longer looks like the original particle turned upside down!
CPT Conservation
If T is violated, so is CP ....... Basic assumption:
CPT IS INVARIANT
CP violation: 1964
KS KL Long-lived neutral kaon KL 3 (CP = -1)
Short-lived neutral kaon KS 2 (CP = +1) ...but sometimes KL 2, so CP is violated. Conclusion: Nature distinguishes matter from antimatter.
e.g. KL - e+ e preferred over KL + e- e.
This is the only mechanism we know of
for explaining the baryon asymmetry
of the universe.
Measurement principle
B E mB Measure Larmor spin precession freq in
parallel & antiparallel B and E fields: ... or slower. dn makes precession
faster...
N S Four-layer mu-metal shield High voltage lead Quartz insulating cylinder Coil for 10 mG magnetic field Upper electrode Main storage cell Hg u.v. lamp PMT to detect Hg u.v. light Vacuum wall Mercury prepolarising cell Hg u.v. lamp RF coil to flip spins Magnet UCN polarising foil UCN guide changeover Ultracold neutrons
(UCN) UCN detector
EDM Limits & Theoretical Predictions
Factor ~10 per 8 years Experimental Limit on d (e m) 1960 1970 1980 1990 f ~ 1 Left-Right 10-34 10-22 10-24 10-26 10-32 Multi Higgs SUSY f ~ a/p Standard Model Electro- magnetic neutron: electron: 10-36 10-38 10-40 2000 10-22 10-32 Barr: Int. J. Mod Phys. A8 208 (1993)
Statistical limits
h / 2 E T N d = Factor Current Increase Total increase = x 80 (x200 with new beamline) Polarisation+detection = 0.75 x 1.5
Electric field: E = 106 V/m x 2.0
Precession period: T = 130 s x 1.8
Neutrons counted: N = 6 x 106 /day x 14.9
(with new beamline) x 2.6
The Neutron Mirror
n n n s s B vc depends on orientation of neutron spin, so can polarise by transmission.
>> interatomic spacing; neutrons see Fermi potential VF
Critical velocity for reflection:
½mvc2 = VF
Ultracold neutrons (UCN): v 6 m/s: total internal reflection possible.
UCN production in Liquid 4He
11 K:
l = h/p = 8.9 Å Landau-Feynman dispersion curve for 4He excitations Free neutron dispersion curve: kT = h2/2ml2 (= 2p/l) /1010m-1 Neutrons at 11 K downscatter by emission of phonon
Upscattering suppressed: Boltzmann factor e-E/kT means not many 11 K phonons present
Neutron Detection Rate
time (sec) Beam off Excess neutrons at low T due to UCN production & storage Neutron count vs. time
Institut Laue-Langevin, Grenoble
Tower 1 ... ... & Tower 2
Main Vessel, Tower 2 & DAQ Equipment
The Equipment: Cross-Section
0.
The Ramsey Cell
UCN Detection in Liquid Helium
Solid-state detectors developed for use in LHe
Thin surface film of 6LiF: n + 6Li a + 3H
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