Probing compressed triplet scalars with ISR jets and soft leptons at the LHC

TL;DR

This paper proposes a new LHC search strategy for compressed triplet scalars in the Type-II seesaw model, focusing on cascade decays with soft leptons and missing energy, to explore previously weakly constrained parameter space.

Contribution

It introduces a dedicated search method using ISR jets to detect cascade decays of triplet scalars in compressed mass regions at the LHC.

Findings

Discovery potential with 3000 fb^{-1} at 14 TeV LHC.

Effective suppression of background with cut-and-count analysis.

Highlights importance of targeted searches for cascade-dominated spectra.

Abstract

The Type-II seesaw model predicts doubly and singly charged scalars along with neutral Higgs states originating from an $SU(2)$ triplet. Current LHC searches by the ATLAS and CMS collaborations constrain these particles mainly under the assumption that the doubly charged scalar decays dominantly into same-sign dileptons or dibosons. However, when moderate mass splittings exist among the triplet scalars, cascade decays can dominate, suppressing these conventional search channels and leaving sizeable regions of parameter space weakly constrained. We study this compressed region characterized by $1~\text{GeV} \lesssim \Delta M \lesssim 30~\text{GeV}$ and triplet vev $v_t \sim 10^{-7} - 10^{-3}$ GeV. In this scenario, charged scalars predominantly undergo cascade decays, while neutral scalars decay invisibly into neutrinos, leading to final states with soft leptons and missing transverse energy. We propose a dedicated search strategy at the 14 TeV LHC exploiting a hard initial-state radiation jet to boost the scalar system. Using a cut-and-count analysis, we show that discovery-level sensitivity can be achieved in this previously unexplored region with an integrated luminosity of $3000~\mathrm{fb}^{-1}$. Our results signify the importance of dedicated searches targeting cascade-dominated and compressed mass spectrum for beyond the standard model scenarios with an $SU(2)$ multiplet.