Search for Time Reversal Violation in Neutron Decay

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Search for Time Reversal Violationin Neutron
Decay

• A Measurement of the Transverse Polarization of
  Electrons

G. Band, M. Becke, A. Bialekc, K. Bodeka, A.
Czarneckig, P. Goreldb, A. Kaczmareka, K.
Kirchb, St. Kistryna, A. Kozelac, M. Kuzniaka, A.
Lindrothe, O. Naviliat-Cuncicd, J. Pulutabe, N.
Severijnse, E. Stephanf, J. Zejmaa

• Jagiellonian University, Cracow, Poland
• Paul Scherrer Institute, Villigen, Switzerland
• Institute of Nuclear Physics, Cracow, Poland
• Laboratoire de Physique Corpusculaire, Caen,
  France
• Catholic University, Leuven, Belgium
• University of Silesia, Katowice, Poland
• University of Alberta, Edmonton, Canada

) Theory support ) Present contact address
Marcus.Beck1_at_gmx.net ) Present contact address
axel.lindroth_at_helpinghome.se
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Angular correlations in ß-decay
Angular distribution contain 4 T-odd observables
(lowest order)
T-invariance holds ? D, R, V, L 0 !!!
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Angular correlations in ß-decay
R-coefficient can be deduced from transversal
polarization of electrons emitted from polarized
nuclei (or neutrons)
D T-odd P-even R T-odd P-odd V T-odd
P-odd L T-odd P-even N T-even P-even
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Angular correlations in ß-decay
D and R are sensitive to distinct aspects of
T-violation
D is primarily sensitive to the relative phase
between V and A couplings.
R is sensitive to the linear combination of
imaginary parts of scalar and tensor couplings.
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T-violation in ß-decay

• T-violation in ß-decay may arise from
• semileptonic interaction (d?ue-?e)
• nonleptonic interactions
• SM-contributions for D- and R-correlations
• Mixing phase ?CKM gives contribution which is
  2nd order in weak interactions lt 10-10
• ? -term contributes through induced NN PVTV
  interactions lt 10-9
• Candidate models for scalar contributions (at
  tree-level) are
• Charged Higgs exchange
• Slepton exchange (R-parity violating super
  symmetric models)
• Leptoquark exchange
• The only candidate model for tree-level tensor
  contribution is
• Spin-zero leptoquark exchange.

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Measurements of triple correlations in ß-decay
provide direct, i.e. first-order access to
the T-violating part of the weak interaction
coupling constants
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R-correlation in neutron decay

• Transverse electron polarization component
  contained in the plane perpendicular to the
  parent polarization.

• Not measured for the decay of free neutron yet !

• Inserting specific matrix elements
  and defining

• one obtains finally

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Anticipated accuracy of the present experiment
?R (neutron) ? 5?10-3
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N-correlation

• Can be deduced from the transverse electron
  polarization component contained in the plane
  parallel to the parent polarization.
• Scales with the decay asymmetry A (? ? CA/CV)

• Self calibration tool for R-correlation
  measurement.
• Excellent cross check for systematic effects in
  R-correlation.

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Conclusion
Simultaneously measure both components of the
transverse polarization of electrons emitted in
neutron decay
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Mott polarimetry

• Mott scattering
• Analyzing power caused by spin-orbit force
• Parity and time reversal conserving
  (electromagnetic process)
• Sensitive exclusively to the transversal
  polarization

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FUNSPIN Polarized Cold Neutron Facility at PSI
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Mott polarimeter

• Challenges
• Weak and diffuse decay source
• Electron depolarization in multiple Coulomb
  scatterings
• Low energy electrons (lt783 keV)
• High background (n-capture)

• Solutions
• Tracking of electrons in low-mass, low-Z MWPCs
• Identification of Mott- scattering vertex.
• Frequent neutron spin flipping.
• foil-in and foil-out measurements.

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Mott-scattering V-track events
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Projection of vertices onto XY-plane
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Projection of the Mott-scattering vertices onto
Pb-foil planes
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Projection of vertices onto X-axis
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Data analysis

• Idea
• Express event rate distributions as functions of
  the azimuthal angle ? making use of reconstructed
  (event-by-event) angles.
• Finite geometry and unknown efficiency-acceptance
  will be absorbed in the experimental factors
  evaluated with high precision.

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Data analysis

• Scenario I
• Efficiency and acceptance do not change with
  neutron spin flip

• Asymmetry parameter A for correction is taken
  from another, high precision, dedicated experiment

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Monte-Carlo simulation
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Experiment

• First phase of data taking - completed
• Analyzed
• 40000 events (10 of full data set)
• Analysis of systematic effects still in progress
• PRELIMINARY RESULT

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Experimental geometry factors
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Data analysis

• Scenario II
• Make use of symmetry of the detecting system

• Calculate super-ratio

• Now the correction is of the order

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Data analysis

• Scenario II
• PRELIMINARY RESULT

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Conclusions

• N ? 0 ? transversal polarization of electrons
  from ?-decay experimentally confirmed (for the
  first time !)
• Mott-polarimeter has expected effective analyzing
  power (18)
• Size and sign of measured N-parameter agree with
  expectations !
• Errors are dominated by statistics
• Analysis of full data set (500000 events) - in
  progress
• Plans Collect 1000000 events in 2006

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MWPCs, scintillators and electronics
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V-track events on-line display
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Experimental setup
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Energy calibration
Single track events
V-track events
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Energy distribution
V-track events
Single-track events
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1-st order FSI contribution
In the SM
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Theoretical uncertainty of RFSI

• Vogel Werner NP 404 (1983) 345 corrected
  for ? ?RFSI(neutron) ? 10-5
• A. Czarnecki with new theory input parameters,
  one can reach ? ?RFSI(neutron) ? 5?10-6

Discovery potential or exclusion power (4
standard deviations) of the R-parameter in the
free neutron decay with present FSI theory is
Rn ? 2?10-5 Im(CSCS) 1.2?Im(CTCT) ? 10-4