Search for sterile neutrinos decaying into pions at the LHC

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Abstract

We study the possibility to observe sterile neutrinos with masses in the range $5\text{ }\text{ }\mathrm{GeV}<{m}_{N}<20\text{ }\text{ }\mathrm{GeV}$ at the LHC, using the exclusive semileptonic modes involving pions, namely $W\ensuremath{\rightarrow}\ensuremath{\ell}N\ensuremath{\rightarrow}n\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\ell}$ ($n=1,2,3$). 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 $\ensuremath{\ell}\ensuremath{\ell}jj$ 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 $1{0}^{9}\text{ }\text{ }W$ 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, $|{U}_{\ensuremath{\ell}N}{|}^{2}\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Moreover, by studying the equal sign and opposite sign dileptons, the Majorana or Dirac character of the sterile neutrino may be revealed.

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