Extraction of neutrino mixing parameters from experiments with multiple identical detectors

TL;DR

This paper analyzes the statistical methods used in neutrino oscillation experiments, focusing on the sensitivity of mixing parameter extraction to the non-uniqueness of minimization results, and compares different minimization approaches.

Contribution

It introduces a detailed investigation of the impact of multiple minimization solutions on neutrino mixing parameter extraction, including all parameters and alternative fitting methods.

Findings

Results for mixing angle and normalization factor agree with original reports.

Multiple minimization solutions exist, affecting parameter estimates.

Fitting with fewer parameters yields consistent results.

Abstract

The statistical method used in the analyses of measurements of neutrino oscillation mixing angle $\theta_{13}$ by the Daya Bay Collaboration is based on variational minimization of a $\chi^2$ function defined in terms of quantities of interest and pull factors which are introduced to deal with effects of systematic uncertainties. For both experiments, the number of parameters that need to be determined is great than the number of available data points (20 vs 6 for the Daya Bay and 12 vs 2 for the RENO). While the results for the mixing angle and the normalization factor were reported, results for the other parameters (pull factors) were omitted in their publications. There exist multiple sets of parameters from the minimization of the $\chi^2$ function. We investigate the sensitivity of the extracted mixing angle on this non-uniqueness of minimization results for the Daya Bay data using two methods of minimization. We report results for all parameters, including those of physics interest and pull factors. The obtained results for the mixing angle and the normalization factor are in agreement with those reported by the Daya Bay Collaboration. Furthermore, we present plots of confidence level contours in the space of the mixing angle and normalization factor. We also present results from fittings using a reduced $\chi^2$ function with fewer parameters than the one employed by the Daya Bay Collaboration.