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 ± 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 0.1 − 0.5 (0.05 − 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 0.8 (0.4). 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.

Leptonic anomalous magnetic moments in $ν$SMEFT

“…As these limits, much like those from s-wave unitarity in [95,96], are much weaker than experimental bounds, we neglect them for the remainder of this study. We now consider the sensitivity of ATLAS and CMS to direct production of ν 4 pairs through dimension-four and -six interactions.…”

“…the "large" mixing / "large" cutoff (lower curves), which is dominated by physics at d = 4, we find rates are about 10× smaller than the first scenario. At √ s = 13 TeV and with L ≈ 36 fb −1 of data, searches by the CMS experiment are presently the most constraining for m 4 ≈ 100 − 1000 GeV due to their inclusion of the W γ channel [91,92], though the sensitivity of ATLAS is projected to be comparable [94,96]. In its trilepton analysis [91], which is sensitive to (pseudo)Dirac and Majorana neutrinos, the CMS experiment reports upper limits on |U µ4 | 2 at the 95% CL.…”

Sterile neutrinos in $Λ_b^0\to (Λ_c^+,p^+)\ell^-_1\ell^-_2\ell^+_3ν$ decays

“…But, if the sterile neutrino masses are within a few hundreds of MeV to a few GeV, then they can go on-shell resulting in appreciable decay rates [25,26]. A number of searches of Majorana neutrino mediated LNV decays are underway at the LHC and Belle-II which has renewed its interests in the theoretical aspects including in hadronic decays , τ -lepton decays [60][61][62][63][64][65], and different scattering processes [66][67][68][69][70][71][72][73][74][75][76]. Experimentally, the B − → π + µ − µ − decay has been searched by the LHCb [77] and the K − → π + µ − µ − decay has been searched by the NA48/2 Collaborations [78] and these measurements give the stringent constraints on the heavy-to-light mixing elements.…”

CP violation with GeV-scale Majorana neutrino in $Λ_b\to(Λ_c^+, p^+)π^+μ^-μ^-$ decays

“…But, if the Majorana mass is within a few hundred MeV to a few GeV then the decay rates can be within the sensitivity reach of future experiments [34,35]. Due to ongoing searches of LNV processes at flavor factories including the LHC and Belle-II, there have been theoretical interests in the LNV decays of hadrons , τ -lepton decays [67][68][69][70][71], and in different scattering processes [72][73][74][75][76][77][78][79][80][81]. The LHCb has searched for the process B − → π + µ − µ − [82] and the NA48/2 has searched for [83] and these experiments provide stringent constraints on the heavy to light mixing matrix elements.…”