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Physical Review C

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Background: Time-reversal-invariance violation, or equivalently CP violation, may explain the observed cosmological baryon asymmetry as well as indicate physics beyond the Standard Model. In the decay of polarized neutrons, the triple correlation D⟨⃗Jn⟩/Jn⋅(⃗βe׈pν) is a parity-even, time-reversal-odd observable that is uniquely sensitive to the relative phase of the axial-vector amplitude with respect to the vector amplitude. The triple correlation is also sensitive to possible contributions from scalar and tensor amplitudes. Final-state effects contribute to D at the level of 10−5 and can be calculated with a precision of 1% or better.

Purpose: We have improved the sensitivity to T-odd, P-even interactions in nuclear β decay.

Methods: We measured proton-electron coincidences from decays of longitudinally polarized neutrons with a highly symmetric detector array designed to cancel the time-reversal-even, parity-odd Standard-Model contributions to polarized neutron decay. Over 300 million proton-electron coincidence events were used to extract D and study systematic effects in a blind analysis.

Results: We find D=[−0.94±1.89(stat)±0.97(sys)]×10−4. This differs from the result of our recent paper [Phys. Rev. Lett. 107, 102301 (2011)] due to refinement of corrections for background and backscattering.

Conclusions: This is the most sensitive measurement of D in nuclear β decay. Our result can be interpreted as a measurement of the phase of the ratio of the axial-vector and vector coupling constants (CA/CV=|λ|eiφAV) with φAV=180.012∘±0.028∘ (68% confidence level). This result can also be used to constrain time-reversal-violating scalar and tensor interactions that arise in certain extensions to the Standard Model such as leptoquarks.


This document is the publisher's version of an article published in:

Physical Review C., vol. 86, no. 3 (2012): 035505. doi: 10.1103/PhysRevC.86.035505

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Nuclear Commons