New physics searches at NA62

TL;DR

The NA62 experiment at CERN has conducted extensive searches for rare kaon and pion decays, setting stringent limits on new physics scenarios including dark photons, scalars, ALPs, and heavy neutral leptons, using data from 2016 to 2024.

Contribution

This work provides the first comprehensive constraints on various new physics models from kaon and pion decay searches at NA62, covering multiple decay channels and mass ranges.

Findings

Upper limits on $K^+ ightarrow ext{pi}^+X$ branching ratio at $10^{-11}$ level.

Constraints on dark photon, scalar, and ALP couplings.

Limits on heavy neutral lepton mixing matrix element $|U_{e4}|^2$ at $10^{-8}$ level for 95--126 MeV/$c^2$ mass.

Abstract

The NA62 experiment at CERN has collected a large sample of $K^+$ and $\pi^+$ decays in flight during Run 1 in 2016--2018 and the ongoing Run 2 which started in 2021. Searches for the decays $K^+\rightarrow\pi^+X$ and $\pi^+\rightarrow e^+N$ are presented using NA62 data collected in 2016--2022 and 2017--2024, respectively. Results are interpreted to constrain a range of new physics scenarios covering all four portal model scenarios. Upper limits on the $K^+\rightarrow\pi^+X$ branching ratio are established at the $10^{-11}$ level, providing constraints on dark photon, scalar and ALP couplings. From the search for heavy neutral lepton production in $\pi^+\rightarrow e^+N$ decays of beam pions, upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the $10^{-8}$ level over the heavy neutral lepton mass range 95--126~MeV/$c^2$.