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

We revisit the sensitivity to nonresonant, heavy Majorana neutrinos N in same-sign W±W± scattering at the 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→μ±μ±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 |VμN|2 can be probed down to about 0.06–0.3(0.03–0.1) for mN=1–10 TeV with L=300 fb-1(3 ab-1). For heavier masses of mN=20 TeV, we report sensitivity for |VμN|2≳0.5(0.3). The W±W± 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.