Electroweak multi-Higgs production: A smoking gun for the Type-I 2HDM

Extending the Higgs sector of the Standard Model (SM) by just one additional Higgs doublet field leads to the 2-Higgs Doublet Model (2HDM). In the Type-I $Z_2$-symmetric limit of the 2HDM, all the five new physical Higgs states can be fairly light, $\mathcal{O}(100)$\,GeV or less, without being in conflict with current data from the direct Higgs boson searches and the $B$-physics measurements. In this article, we establish that the new neutral as well charged Higgs bosons in this model can all be simultaneously observable in the multi-$b$ final state. The statistical significance of the signature for each of these Higgs states, resulting from the Electro-Weak (EW) production of their pairs, can exceed 5$\sigma$ at the 13\,TeV High-Luminosity Large Hadron Collider (HL-LHC) with 3000\,fb$^{-1}$ integrated luminosity. Since the parameter space configurations where this is achievable are precluded in the other, more extensively pursued, 2HDM Types, an experimental validation of our findings would be a clear indication that the true underlying Higgs sector in nature is the Type-I 2HDM.