Search for heavy neutral leptons in $π^+$ decays to positrons

TL;DR

This study reports a search for heavy neutral leptons produced in in-flight π^+ decays to positrons, using NA62 data from 2017–2024. The analysis employs a data-driven missing-mass technique over 63 heavy neutral lepton mass hypotheses in the 95–126 MeV/$c^2$ range, and defines a single-event sensitivity to relate observed events to the mixing parameter $|U_{e4}|^2$ via the kinematic factor $ρ(m_N)$. The results establish 90% CL upper limits on $|U_{e4}|^2$ at the level of ~10^{-8}$ for lifetimes longer than 50 ns, competitive with prior stopped-pion searches such as PIENU, thereby constraining the νMSM parameter space. The work demonstrates NA62’s capability to probe leptonic decays with high precision and sets the stage for improved sensitivity with future data.

Abstract

A search for heavy neutral lepton ($N$) production in $π^+\to e^+ N$ in-flight decays using data collected by the NA62 experiment at CERN in 2017-2024 is reported. Upper limits for the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-8}$ for heavy neutral leptons with mass in the range 95-126 $MeV/c^2$ and lifetime exceeding 50 ns.

Figures (5)

• Figure 1: Schematic side view of the NA62 detector during the 2017--2018 data taking period.
• Figure 2: Squared missing mass distributions assuming the beam particle is a $\pi^+$ (left) and a $K^+$ (right). Pairs of vertical arrows indicate the squared missing mass ranges used for the selection of $\pi^+\to e^+\nu_e$ decays and HNL search region (left), and for the selection of $K^+\to e^+\nu_e$ decays (right). Kaon and pion decays to muons contribute via muon decays in flight.
• Figure 3: Squared missing mass resolution $\sigma_{m^2}$ (left) and acceptance $A_{N}$ (right) evaluated from simulations as functions of HNL mass. The boundaries of the HNL search region are indicated by the vertical arrows.
• Figure 4: Left: single event sensitivities ${\cal B}_{\rm SES}(\pi^+\to e^+N)$ and $|U_{e4}|_{\rm SES}^2$ as functions of the HNL mass, evaluated according to Eq. \ref{['eq:ses']}. Right: number of observed events $N_{\rm obs}$, observed and expected upper limits at 90 % CL for the number of $\pi^{+}\rightarrow e^{+} N$ events, and the expected $\pm$1$\sigma$ and $\pm$2$\sigma$ bands in the background-only hypothesis for each HNL mass value considered.
• Figure 5: Left: upper limits obtained by NA62 at 90 % CL for $|U_{e4}|^2$ and upper limits established by the PIENU experiment in a wider mass range PIENU:2017wbj. Right: summary of upper limits at 90 % CL for $|U_{e4}|^{2}$ and $|U_{\mu4}|^{2}$ obtained from HNL production searches at NA62 in kaon (2016--2018 data) NA62:2020mcvNA62:2021bji and pion (2016--2024 data) decays.