A Search for Mirror Neutron Oscillation Using the nEDM Apparatus at PSI: Details about computing the mean time between collisions and extraction of the limit on the n-n' oscillation time

Abstract

Baryon number violation is a key ingredient of baryogenesis. Since the famous parity violation paper of Lee and Yang, it has been hypothesized that there could also be a parity conjugated copy, in the context of the weak interaction, of the standard model particles. The existence of such a mirror universe has specific testable implications, especially in the domain of neutral particle oscillation, viz. the baryon number violating neutron to mirror-neutron (n-n')oscillation. It was shown that such n-n' oscillations could happen rapidly with an oscillation time as small as a second. Consequently, there were many experiments which searched for n-n' oscillation, but which have so far reported finding no evidence of n-n' oscillation. Even though the experiments report having found no evidence of n-n' oscillation, reanalysis of some of these results have identified three particular anomalies which could point to the detection of n-n' oscillation. All but one of these efforts were conducted at the Institut Laue-Langevin in Grenoble, France, with the most recent search having been conducted at the Paul Scherrer Institute in Villigen, Switzerland. The results from this latest effort were announced in Phys. Lett. B 812, 135993 (2021). This search for n-n' oscillation involved measuring the number of neutrons after storage for a time period under the influence of zero magnetic field or a finite magnetic field. The neutron counting measurables were combined with the mean time between collisions of the stored Ultra Cold Neutrons (UCNs) to yield constraints on the n-n' oscillation time constant. In this paper, additional details regarding the analysis, i.e. the method of estimating mean time between collisions, as well as the numerical technique of extracting the constraint upon the n-n' oscillation time constant from the neutron counting measurables, is described. We obtained <t_f>(t^*_s=180s)=0.0628(27)s, <t_f>(t^*_s=380s)=0.0756(30)s, and (<t^2_f>/<t_f>)(t^*_s=180s)=0.104(10)s, (<t^2_f>/<t_f>)(t^*_s=380s)=0.120(10)s.