Studies of the Electron Neutrino Charged-current Interaction on $^{127}$I

Abstract

An inclusive measurement of the cross section of the electron neutrino charged-current interaction on $^{127}$I can probe the quenching of g$_{A}$, the axial-vector coupling constant, which affects the rate of neutrinoless double beta decays, and contribute to the design of next-generation solar neutrino detectors. At the Los Alamos Meson Production Facility (LAMPF), an exclusive measurement of the flux-averaged cross section was measured to be [2.84 $\pm$ 0.91 (stat) $\pm$ 0.25 (syst)] $\times\; 10^{\boldsymbol{-}40}$ cm$^2$ [1]. This measurement has a large statistical error and only counts the number of {$^{127}$Xe} in the bound state. To make a first measurement of the inclusive cross section with lower statistical uncertainties, a 185-kg NaI[Tl] prototype detector, NaI$\upnu$E-185, was deployed by the COHERENT collaboration at the Spallation Neutron Source. To study electron neutrino charged-current interaction detection efficiencies, simulations were performed. To explore new approaches of steady state background rejection, a convolutional neural network (CNN) classifier and a XGBoost based classifier were developed. The best 4-class model, tested with simulations, achieved a 50.2\% classification accuracy. To address the non-linearity of NaI[Tl] crystals at high energies, calibrations using Michel positrons from stopped muon decays were performed. The cross section measurement was done through probability density function (PDF) fitting. The flux-averaged total inclusive cross section, excluding forbidden transitions, was measured to be [9.2 + 2.1 $-$ 1.8 (stat+syst)] $\times\; 10^{\boldsymbol{-}40}$ cm$^2$, rejecting the null hypothesis by 5.8 $\sigma$.[1] J. R. Distel, B. T. Cleveland, K. Lande, C. K. Lee, P. S. Wildenhain, G. E. Allen, and R. L. Burman. Measurement of the cross section for the reaction $^{127}\mathrm{I}({\ensuremath{\nu}}_{e},{e}^{\ensuremath{-}})^{127}{\mathrm{Xe}}_{\textrm{bound states}}$ with neutrinos from the decay of stopped muons, Phys. Rev. C, 68:054613, Nov 2003.