Search for sterile neutrinos decaying into pions at the LHC

Claudio O Dib,C S Kim,Nicolás A Neill,Xing-Bo Yuan

doi:10.1103/physrevd.97.035022

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.

Highlights

Neutrino oscillation data [1,2,3] demonstrates that at least two active neutrinos are massive, which provides strong indication of new physics [4,5,6,7] beyond the minimal standard model
The two pion and three pion modes require extrapolations of form factors to large time-like q2, 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 lljj modes or trilepton modes and impossible to explore in rare meson decays
In this work we have proposed a sterile neutrino search for masses mN in the range 5–20 GeV, which is a region where neither rare meson decays (B, D, etc.) nor pp → lljj; lllν modes at the LHC are sensitive to the presence of such neutrinos

Summary

INTRODUCTION

Neutrino oscillation data [1,2,3] demonstrates that at least two active neutrinos are massive, which provides strong indication of new physics [4,5,6,7] beyond the minimal standard model. The exclusive single-pion mode N → lπ is suppressed with respect to the inclusive process, modeled by the production of an open quark pair N → lqq 0, by a factor f2π/m2N, where fπ is the pion decay constant. This suppression is clearly milder for lower masses of the sterile neutrino. We focus on neutrino masses within the window from 5 GeV to 20 GeV, using the exclusive processes Wþ → lþN followed by the pionic decays N → π−lþ, N → π0π−lþ and N → π−π−πþlþ.

PIONIC DECAYS OF A STERILE NEUTRINO

NUMERICAL ANALYSIS

Findings

CONCLUSIONS