Probing Boosted Light Scalars in the Type-I 2HDM

In the Type-I two-Higgs Doublet Model (2HDM), the additional scalars may be light ($\lesssim 100$ GeV) without conflicting with experimental constraints from LHC searches or from flavour observables. So far, the studies of light scalars at the LHC have been limited to exploring non-standard decays of the Standard Model (SM) Higgs boson or via $b\bar b$ associated production followed by leptonic decays of the light scalar. A light scalar in Type-I 2HDM can evade these search strategies due to its potentially tiny coupling to the SM Higgs boson and its suppressed coupling to quarks. In this work, we have studied electroweak production of a light scalar ($h$) in association with heavy pseudoscalar $A$ or charged Higgs $H^\pm$, which further decays into $h$, resulting in a multi-$h$ final state, where $h$ is boosted due to its lightness. The decay of the boosted $h$ into $b\bar b$ can be reconstructed within a fat-jet containing a pair of $b$-subjets. We find that tagging such a `boosted double-$b$ fat-jet ($J_{bb}$)' signature in association with a SM gauge boson provides an excellent probe of the Type-I 2HDM for hierarchical scalar spectra. Using multiple light mass $M_h$ benchmarks, we demonstrate that such analysis can explore a large region of the parameter space, with the $2\sigma$ exclusion reach for the heavy scalars extending up to $\sim 540$ GeV ($\sim$ 365 GeV) at the HL-LHC with 3000 fb$^{-1}$ (LHC with 300 fb$^{-1}$) luminosity for light scalar masses in the range $30$--$70$ GeV. Furthermore, we show that significant sensitivity and even resonance reconstruction can be achieved within a model-independent framework, highlighting the robustness of this search strategy.