Fatjet Signatures of Quintuplet Fermions at the LHC

TL;DR

This work analyzes a beyond-Standard-Model scenario with a neutral fermion quintuplet Σ_R and a scalar quadruplet Φ_4, which generate neutrino masses via tree- and loop-level processes. At the LHC, pair production of Σ^{++}Σ^{--} followed by scalar-mediated decays yields highly boosted W and Z bosons that manifest as fatjets, enabling distinctive multilepton plus fatjet final states. A detailed MG5_aMC@NLO+Pythia+Delphes study targets two channels, 2L2F_j and 3L1F_j, using jet-substructure observables and a three-stage selection to suppress SM backgrounds, with significance evaluated under background uncertainties via the Cowan formula. The results show strong discovery potential for both channels, especially in the low-mass benchmark, with >5σ significance achievable under realistic luminosities and background uncertainties, highlighting fatjet techniques as a powerful probe of TeV-scale multiplet-driven neutrino-mass models at the LHC and HL-LHC.

Abstract

This paper explores a simplified extension of the standard model featuring a neutral fermion quintuplet and a scalar quadruplet, which together generate neutrino masses through tree and loop level mechanisms. The quintuplet fermions decay into standard model gauge bosons via the scalars, producing unique collider signatures at the LHC characterized by multilepton and multijet final states. The study focuses on the pair production of quintuplet fermions in the 700-1200 GeV mass range, where their decays produce highly boosted W and Z bosons identifiable as fatjets. Emphasis is placed on the production and decay of doubly charged fermions due to their higher cross section. Advanced jet substructure and kinematic techniques are applied to enhance sensitivity by reducing standard model backgrounds. A detailed analysis of signal significance is performed in the two lepton, two fatjet and three lepton, one fatjet channels for different masses of the fermion and the scalars, optimizing selection cuts to maximize signal efficiency over standard model backgrounds. The study found that both channels exhibit excellent performance, with significance exceeding $5σ$ under realistic conditions including a 50\% background uncertainty at integrated luminosity up to 3000 fb$^{-1}$.

Figures (8)

• Figure 1: Feynman diagrams of the doubly charged fermion pair production at the LHC, where $q$ stands for $u,d,c,s,b$ quarks.
• Figure 2: Jet mass (left) and $p_{\mathrm{T}}$ (right) of the leading fatjet for two different jet grooming algorithms: soft drop and jet pruning. The low-mass benchmark point is used here.
• Figure 3: The $\tau_{21}$ and $\tau_{32}$ variables (left) for the leading fatjets in the low-mass benchmark point and the relative mass $\rho$ (right) for the leading fatjets in the low-mass and high-mass benchmark points.
• Figure 4: The $p_{\mathrm{T}}$ of the leading and subleading lepton (left), $p_{\mathrm{T}}$ of the leading and subleading fatjet (middle), and $p_{\mathrm{T}}^{\mathrm{miss}}$ (right) are shown for the low-mass and high-mass benchmark points. The distributions in each figure are normalized to have unit area.
• Figure 5: The distributions of $L_{\mathrm{T}}+p_{\mathrm{T}}^{\mathrm{miss}},S_{\mathrm{T}}$ (top row) and $m_{min}, JMF$ (bottom row) are shown for signal and background processes after the S1 selections are imposed in the $\mathrm{2L2F_j}$ final state.