Studies of π + → e + + invisible topologies in the NA62 experiment

Abstract

A search for Heavy Neutral Leptons (HNL) in the decay π+ → e+N is presented using the data set collected by the NA62 experiment in the period 2017-2024. The analysis scans 63 distinct HNL mass hypotheses in the range 95–126 MeV/c2, assuming a lifetime exceeding 50 ns. The observed data are consistent with the background-only hypothesis across the entire mass range, resulting in upper limits on the mixing matrix element |Ue4|2 at the 90% confidence level of the order of 10−8. The same analysis framework was applied to perform studies of the Rπ ratio (Γ(πe2)/Γ(πμ2)) using the common event sample, where both decay modes are identified via their characteristic decay signatures. This work provides a statistical benchmark of the measurement precision; the computed ratio, considering only statistical uncertainties, establishes the intrinsic statistical resolution of the dataset pending future systematic evaluation.