Search for Quadruplet Scalars using Boosted Decision Trees at the LHC

TL;DR

This paper investigates a Beyond the Standard Model scenario featuring a scalar quadruplet and a neutral fermion quintuplet, focusing on neutrino mass generation and distinctive collider signatures at the LHC. It analyzes production channels and decay patterns, distinguishing fermiophobic and fermiophilic regimes that depend on the mass ordering and Yukawa coupling, and performs a detailed HL-LHC study using a multivariate Boosted Decision Tree to isolate events with at least four leptons and two jets. The authors find discovery potential for scalar masses around 600–700 GeV in pair production and reach up to ~1 TeV in associated production, with mass reconstruction facilitated by kinematic variables; these results highlight promising prospects for testing cascade seesaw-inspired models at current and future colliders. Overall, the work demonstrates that multi-lepton final states combined with BDT discrimination provide a powerful approach to probe extended scalar sectors and heavy fermion multiplets in collider experiments.

Abstract

Beyond the Standard Model scenarios introduce additional scalar and fermion multiplets, which influence neutrino mass generation mechanisms and yield distinctive collider signatures. This work focuses on a particular scenario involving a neutral fermion quintuplet and a scalar quadruplet. The study examines the production and decay of the scalar quadruplet components at the LHC, emphasizing how their decay patterns, fermiophobic versus fermiophilic, depend on mass differences and Yukawa couplings with the fermion multiplets. A detailed collider analysis targeting final states with at least four leptons and two jets is conducted. The study also incorporates Standard Model backgrounds, leveraging multivariate techniques via Boosted Decision Trees. Results indicate discovery potential for scalar masses around 600-700 GeV and exclusion sensitivity extending beyond 1 TeV, highlighting the promising experimental signatures in the model and its role in probing new physics at colliders.

Figures (7)

• Figure 1: (left) Cross-sections for the doubly charged scalar pair production and associated production, (right) pair production cross-section of the singly charged scalar as a function of their mass at 14 TeV LHC.
• Figure 2: Branching ratios for the singly ($\phi^{\pm}$) and the doubly charged ($\phi^{\pm\pm}$) scalars considering three different values of the Yukawa coupling, as indicated in graphs. Here, $l=e,\mu,\tau$. The quartet and quintuplet masses are fixed at $M_\Phi=500$ GeV and $M_\Sigma=420$ GeV.
• Figure 3: Feynman diagrams for the pair production and associated production of the charged scalars at $pp$ collision and their decays in the fermiophobic scenario.
• Figure 4: Feynman diagrams for the pair production and associated production of the charged scalars at $pp$ collision and their decays in the fermiophilic scenario. The conjugate processes are also considered.
• Figure 5: Normalized distributions of the variables used in the pre-selection: Lepton multiplicity (Top left), jet multiplicity (Top middle), b-jet multiplicity (Top right), the invariant mass of the two opposite-sign lepton pairs, (Bottom left and Bottom right).