On the Robustness of type-II Seesaw Collider Searches

Abstract

Electroweak triplet Higgs sector extensions are well-motivated scenarios to address lepton flavour observations. These models can also be strongly constrained by combining precise, indirect low-energy measurements with direct searches for exotic, doubly charged Higgs bosons. Together, these searches set competitive constraints on the type-II seesaw mechanism. In this work, we consider extensions of the type-II seesaw, specifically through the lens of a modified collider phenomenology. Surveying motivated extensions, we map out changes in expected correlations, focusing on the modified production and decay phenomenology of exotic Higgs particles. This enables us to assess the robustness of the type-II seesaw collider constraints against extended new-physics contributions that modify standard sensitivity expectations and projections.

Figures (7)

• Figure 1: Representative Feynman diagrams illustrating the pair production of $\Delta^{\pm\pm}$ in vanilla type-II seesaw and in presence of different EFT operators. The left-most panel corresponds to the vanilla type-II seesaw, while the colored blobs in other panels indicate contributions from $\mathcal{O}_{G\Delta}$ (red), $\mathcal{O}_{Q\Delta\mathcal{D}}^{(1)}$ (green), and $\mathcal{O}_{u\Delta\mathcal{D}}^{(1)}$ (cyan).
• Figure 2: (a) Production cross section of $pp \rightarrow \Delta^{++} \Delta^{--}$ for vanilla type-II and different EFT operator contributions. (b) Branching ratios of different decay modes of $\Delta^{\pm \pm}$ as a function of $M_{\Delta^{\pm \pm}}$.
• Figure 3: Decay topologies of $\Delta^{\pm \pm}$ into leptonic, bosonic, mixed gauge--leptonic, and cascade final states. Panels (c) and (d) arise from effective operator, represented by black blobs.
• Figure 4: Distributions of the leading same-sign dilepton invariant mass, $m(\ell^{\pm}, \ell^{\prime \pm})_{\text{lead}}$, for the scenarios S1, S2, and S3. The results are shown for two benchmark masses, $M_{\Delta^{\pm\pm}} = 900~\text{GeV}$ and $1100~\text{GeV}$, across the signal regions SR2L, SR3L, and SR4L.
• Figure 5: Exclusion limits for scenarios S1, S2, and S3, shown in Fig. \ref{['fig:cs1']}, Fig. \ref{['fig:cs2']}, and Fig. \ref{['fig:cs3']}, respectively. The black solid and dashed curves indicate the observed and expected 95% CL upper limits, respectively, while the red solid curve shows the leading-order theoretical prediction. The green and yellow bands represent the $\pm1\sigma$ and $\pm2\sigma$ uncertainty intervals around the expected limit.