LHC signals for singlet neutrinos from a natural warped seesaw mechanism. I

Abstract

Recently, it was shown in arXiv:1512.06742 that a straightforward implementation of the type I seesaw mechanism in a warped extra dimensional framework is in reality a {\em natural} realization of "inverse" seesaw, i.e., the Standard Model (SM) neutrino mass is dominantly generated by exchange of pseudo-Dirac {\em TeV}-mass SM singlet neutrinos. By the AdS/CFT correspondence, this scenario is {\em dual} to these singlet particles being composites of some new strong dynamics, along with the SM Higgs boson, with the rest of the SM particles being mostly elementary. We study signals from production of these heavy neutrinos at the Large Hadron Collider (LHC). We focus on the scenario where the strong sector has a global $SU(2)_{\rm L} \times SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry; such a left-right (LR) structure being motivated by consistency with the electroweak (EW) precision tests. The singlet neutrinos are charged under $SU(2)_{\rm R} \times U(1)_{\rm X}$ symmetry, thus can be produced from $W^{ \pm }_R$ exchange, as in four-dimensional (4D) LR symmetric models. However, the direct coupling of light quarks to $W^{ \pm }_R$ is negligible, due to $W^{ \pm }_R$ also being composite; nonetheless, a sizable coupling can be induced by mixings among the various types of $W^{ \pm }$ bosons. Furthermore, $W^{ \pm }_R$ decays dominantly into the singlet and {\em composite} partner of charged lepton. This heavy charged lepton, in turn, decays into SM lepton, {\em plus} $Z$/Higgs, thus the latter can be used for extra identification of the signal. For a benchmark scenario with $W^{ \pm }_R$ of mass 2 TeV and singlet neutrino of mass 750 GeV, we find that, in both the di-lepton + di-jet + Higgs and tri-lepton + Higgs channels, significant evidence can be seen at the LHC14 for an integrated luminosity of 300/fb and that even discovery is possible with slightly more luminosity.