Search for sterile neutrinos decaying into pions at the LHC

TL;DR

This paper explores the potential to detect sterile neutrinos with masses between 5 and 20 GeV at the LHC through exclusive semileptonic pion modes, proposing methods to overcome experimental challenges and improve current bounds.

Contribution

It introduces a novel approach to search for sterile neutrinos in a difficult mass range using exclusive pion decay modes and form factor extrapolations, with potential to significantly enhance detection sensitivity.

Findings

Potential to discover sterile neutrinos or set new bounds on mixing angles.

Use of form factor models to accurately predict decay modes.

Vertex displacement helps reduce background noise.

Abstract

We study the possibility to observe sterile neutrinos with masses in the range between 5 GeV and 20 GeV at the LHC, using the exclusive semileptonic modes involving pions, namely W to lepton + N to n pions + lepton+lepton (n = 1, 2, 3). The two pion and three pion modes require extrapolations of form factors to large time-like $q^2$, which we do using vector dominance models as well as light front holographic QCD, with remarkable agreement. This mass region is difficult to explore with inclusive dilepton+dijet modes or trilepton modes and impossible to explore in rare meson decays. While particle identification is a real challenge in these modes, vertex displacement due to the long living neutrino in the above mass range can greatly help reduce backgrounds. Assuming a sample of $10^9$ W bosons at the end of the LHC Run 2, these modes could discover a sterile neutrino in the above mass range or improve the current bounds on the heavy-to-light lepton mixings by an order of magnitude, $U_{l N}^2 \sim 2 \times 10^{-6}$. Moreover, by studying the equal sign and opposite sign dileptons, the Majorana or Dirac character of the sterile neutrino may be revealed.