Feldman-Cousins calculation for final KamLAND-Zen 800 analysis.

KamLAND-Zen 800’s analysis for neutrinoless double beta decay (0νββ) decay has yielded some significant improvements, providing the most stringent limits on the nature of the neutrino. As no signal has been observed, we report an upper limit on the 0νββ decay rate for Xe-136 at 90% confidence level. Using Wilk’s theorem approximation, we derived a lower limit on the half-life of 3.8 x 10²⁶ years. However, Wilk’s theorem can generate improper coverage due to the proximity to the unphysical region (a negative Majorana mass), thus a full Feldman-Cousins calculations was needed for the accurate statistical treatment. This approach yielded a lower limit on the half-life of 4.3 x 10²⁶ years. This talk aims to discuss the methodology that KLZ800 used to perform the Feldman-Cousins calculation within a frequentist framework, in the context of rare 0νββ decay rate search.