Majorana Neutrinos in Same-Sign $W^{\pm }{W}^{\pm }$ Scattering at the LHC: Breaking the TeV Barrier

Abstract

We revisit the sensitivity to non-resonant, heavy Majorana neutrinos $N$ in same-sign $W^{\pm }{W}^{\pm }$ scattering at the $\sqrt{s}=13$ TeV LHC and its high-luminosity upgrade. As a benchmark scenario, we work in the context of the Phenomenological Type I Seesaw model, relying on a simulation up to next-to-leading order in QCD with parton shower matching. After extensively studying the phenomenology of the $pp\to {\mu }^{\pm }{\mu }^{\pm }jj$ process at the amplitude and differential levels, we design a simple collider analysis with remarkable signal-background separation power. At 95\% confidence level we find that the squared muon-heavy neutrino mixing element $|V_{\mu N}{|}^2$ can be probed down to about $0.06-0.3(0.03-0.1)$ for $m_N=1-10\mathrm{TeV}$ with $\mathcal{L}=300$ fb${}^{-1}(3$ ab${}^{-1})$. For heavier masses of $m_N=20\mathrm{TeV}$, we report sensitivity for $|V_{\mu N}{|}^2\gtrsim 0.5(0.3)$. The $W^{\pm }{W}^{\pm }$ scattering channel can greatly extend the mass range covered by current LHC searches for heavy Majorana neutrinos and particularly adds invaluable sensitivity above a few hundred GeV. We comment on areas where the analysis can be improved as well as on the applicability to other tests of neutrino mass models.

Comments: Journal version: results unchanged, only minor revisions w.r.t. v1. 31 pages (including three appendices), nine figures (23 png and pdf files), eight tables