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

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 $1~\text{GeV} \lesssim ΔM \lesssim 30~\text{GeV}$ 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.

Figures (5)

• Figure 1: Decay phase diagram of $H^{\pm\pm}$ (left), $H^{\pm}$ (middle), and $H^{0}/A^{0}$ (right) with $m_{H^{\pm\pm}}$ = 200 GeV for the positive mass hierarchy scenario. The solid, dashed and dot-dashed contours represent 99%, 95%, and 50% branching ratios in different decay regions.
• Figure 2: Normalized differential distribution corresponds to missing transverse momentum $E^{miss}_{T}$ (left) and the leading transverse momentum of the jet $p_{T}(j_{1})$ for the signal and the total SM background. The solid blue line and the red dashed line represents the signal and the total SM background respectively.
• Figure 3: Normalized differential distribution corresponds to $m_{\tau\tau}$ variables for the signal and the total SM background. The solid blue line and the red dashed line represents the signal and the total SM background respectively.
• Figure 4: Normalized differential distribution corresponds to the leading transverse momentum of the jet $p_{T}(\ell_{1})$ (left) and the vector sum of lepton transverse momentum $\sum_{i= 1, 2}p_{T}(\ell_{i})$ for the signal and the total SM background. The solid blue line represents the total SM background, and the red dashed line represents the signal, respectively.
• Figure 5: Projected sensitivity in the $M_{\Delta} - \Delta M$ plane after applying the cut-based analysis of the signal and background distribution at the 14 TeV LHC. The black dashed and solid black curves correspond to 2$\sigma$ exclusion and 5$\sigma$ discovery contours, respectively, assuming an integrated luminosity of $\mathcal{L} = 3000~fb^{-1}$. The shaded region indicates the parameter space where signal significance exceeds the corresponding confidence level, with the red (blue) shaded region representing 2$\sigma$ (5$\sigma$) sensitivity reaches. The unshaded region can not be probed using the present analysis assumptions.