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

Summary

The "emiT" experiment searched for time-reversal symmetry violation term in neutron beta decay. It did 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, and would be a clear hint to the source of the matter antimatter asymmetry of the early universe.

Description

Top: A schematic of the emiT detector illustrating the alternating electron and proton detector segments. The shaded detectors show the example of the paired-rings used to minimize systematic effects by symmeterizing the detector. The cross section view illustrates, in a greatly exaggerated manner, the effect of the magnetic field on the particle trajectories. A P2E2 coincidence event is shown. Bottom: Log plot of SBD-scintillator coincidence data showing proton energy vs delay time. Events near a zero delay time are prompt coincidences due primarily to beam-related backgrounds.

The stars and galaxies that make up our universe can only form if, during the initial moments after the Big Bang, there existed a slight dominance of matter over antimatter. It has been understood for some time that such an imbalance can only be explained if three conditions are met; a departure from thermal equilibrium, a violation of baryon (i.e., three quark states) number, and a violation of charge-conjugation-parity (CP) symmetry in the physical laws that govern particle interactions (charge conjugation turns particle states into the corresponding anti-particle states and parity is an inversion of the coordinate system). Thus far, CP violation has been observed only in the K and B meson systems (i.e., quark-antiquark states) and can be entirely accounted for within the Standard Model of particle physics (SM). However, the extent of SM CP violation is many orders of magnitude too small to account for the known asymmetry in the context of Big Bang cosmology, so there is compelling reason to search for CP violation in other systems. As CP and time-reversal symmetry (T) violation can be theoretically related to one another, experimental limits on T-odd observables in nuclear beta decay place strict constraints on possible sources of new CP violation.

emiT searched for time-reversal symmetry violation term in neutron beta decay 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 and a clear hint to the source of the matter antimatter asymmetry of the early universe.

The measurement was carried out at the NIST Center for Neutron Research NG-6 beamline. The detector, shown schematically in the figure, was built to be highly symmetric and consisted of an octagonal array of four electron-detection planes and four proton-detection planes concentric with the longitudinally polarized beam. The beam, with a neutron capture fluence rate at the detector of 1.7x10⁸ cm-2 s-1, was polarized to > 91 % by a double-sided bender-type supermirror. A 0.56 mT (5.6 G) guide field maintained the polarization direction, while a current-sheet spin-flipper was used to periodically reverse the neutron spin direction, allowing for first order cancellation of detector efficiency variations. The octagonal geometry was chosen to maximize sensitivity to the time reversal violating correlation in the decay (D), and the highly symmetric arrangement allows for the approximate cancellation of systematic effects stemming from residual coupling to the relatively large spin-correlations in beta decay. Each of the four proton segments consisted of an array of 28 silicon surface-barrier diode detectors (SBDs). Because the maximum proton energy from the decay is very low, approximately 750 eV, protons were accelerated through grounded wire-mesh boxes and onto SBDs mounted within an electrode normally held between -25 kV and -32 kV. The beta detectors were slabs of plastic scintillator with sufficient thickness to stop electrons at the decay endpoint energy of 782 keV. Scintillation photons were detected by photomultiplier tubes (PMTs) at each end of the slab. Details of the apparatus can be found in [2].

The apparatus went on the beamline in spring of 2001 and completed data acquisition in December of 2003. The majority of the running time was devoted to reducing the statistical uncertainty, the limiting factor in the experiment. The performance of the detector was dramatically improved over the first run in 1997. The measured electron-proton coincidence rate was a factor of ten higher and the signal-to-background ratio was two orders of magnitude higher. These improvements were primarily due to better proton detectors, greatly reduced high voltage-induced backgrounds, and improved electronics. Partly as a result of the extremely high data quality, the collaboration discovered two sets of unanticipated systematic effects that involve the proton detection system and the interplay of the magnetic transport field with the shape of the neutron beam respectively. Fully understanding these systematic effects required the development of a detailed Monte Carlo of the complete apparatus. After correcting for all systematic effects, the collaboration reported a final result of D = [-0.94±1.89(stat) ±0.97(sys)]x10^-4 [3,4]. While consistent with the Standard Model, this result represents the most sensitive test of time reversal invariance in beta decay and can be used to constrain scalar and tensor currents as well as Standard Model extensions.

While this result included systematic effects at the 0.97x10^-4 level, the improved understanding of the detector system provides a clear path forward for significant advances in systematic control.  Combined with improvements in the available neutron beam at the NCNR, it is possible to improve limits on D by more than an order of magnitude.  Efforts are in progress exploring such an experiment. 

Useful references:

[1] L.J. Lising, et al., Physical Review C, 62, 055501 (2000)
[2] H.P. Mumm, et al., Rev. Sci. Instrum. 75, 5343, (2004)
[3] H.P. Mumm, et al., Phys. Rev. Lett. 107, 102301 (2011)
[4] T.E. Chupp, et al., Phys. Rev. C, 86, 035505 (2012)