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Search for Time Reversal Violation in Polarized Neutron Decay (emiT)


The "emiT" experiment searches for - or will set an improved upper bound on - the time-reversal asymmetry term in neutron beta decay. It does so by measuring electron-proton coincidence events from the decay of polarized neutrons. An asymmetry in coincidence pairs is formed as a function of the direction of the neutron spin. A measurement of a nonzero asymmetry would be an unambiguous indication of time-reversal violation.


The performance of the detector during the 2003 run was dramatically improved over its first run in 1997. The measured electron-proton coincidence rate is a factor of 10 higher than in this first run. In addition, the signal-to-background ratio is two orders of magnitude higher. These improvements were primarily due to better proton detectors, greatly reduced high voltage-induced backgrounds, and improved electronics. Since the experiment was expected to be statistics limited, the majority of the running time was devoted to reducing the statistical uncertainty on the asymmetry.

Since the last run we have identified a number of systematic effects related to the acceleration and focusing of the protons that allows them to be efficiently detected.This has necessitated the development of a detailed Monte Carlo. Much of the last year has been devoted to validating this Monte Carlo and finalizing a systematic error budget.  Estimates of all systematic effects not related to proton detection have been shown to be smaller that the statistical sensitivity of the experiment and the Monte Carlo has been shown to perform exceedingly well.We are completing an assessment of the sole remaining systematic correction and stand poised to un-blind the data.

In all, the new data set is approximately 25 times larger than the 1997 run. We anticipate the completion of data analysis in early FY11 with a value that is a factor of nearly 4 better than the current limit. This result will represent the most sensitive test of T-violation in beta decay. It is well established that new sources of CP (and T) violation are required by the observed baryon asymmetry of the universe. However, CP violation has been observed so far only in the decays of neutral kaons and B mesons (recently evidence for the implied T violation in the neutral kaon system has also been reported). These effects are consistent with a phase in the Standard Model quark mixing matrix and thus do not explain the baryon asymmetry. The emiT experiment searches for new sources of CP violation whose signature would be a T-odd correlation in the decay of free neutrons.

Graph Caption: (Top) Schematic of the emiT apparatus. The beta detectors are plastic scintillator, the proton detectors consist of arrays of surface barrier diode detectors. (Bottom) Histogram of coincidence events. The peak is due to neutron decay.
Search for Time Reversal Violation in Polarized Neutron Decay

Lead Organizational Unit:



Neutron Physics Group

Hans (Pieter) Mumm

Jeffrey S. Nico

NIST Associates:

John F. Wilkerson
University of North Carolina

Timothy E. Chupp
University of Michigan