Probing light fermiophobic Higgs boson via diphoton jets at the HL-LHC

In this study, we explore the phenomenological signatures associated with a light fermiophobic Higgs boson, h(f), within the type-I two-Higgs-doublet model at the HL-LHC. Our meticulous parameter scan illuminates an intriguing mass range for m(hf), spanning [1, 10] GeV. This mass range owes its viability to substantial parameter points, largely due to the inherent challenges of detecting the soft decay products of hf at contemporary high-energy colliders. Given that this light h(f) ensures Br(h(f) -> gamma gamma) similar or equal to 1, Br(H-+/- -> h(f)W(+/-)) similar or equal to 1, and M-H +/- less than or similar to 330 GeV, we propose a golden discovery channel: pp -> h(f)H(+/-) -> gamma gamma gamma gamma(l+nu), where l(+) includes e(+/-) and mu(+/-). However, a significant obstacle arises as the two photons from the h(f) decay mostly merge into a single jet due to their proximity within Delta R < 0.4. This results in a final state characterized by two jets, rather than four isolated photons, thus intensifying the QCD backgrounds. To tackle this, we devise a strategy within DELPHES to identify jets with two leading subparticles as photons, termed diphoton jets. Our thorough detector-level simulations across 18 benchmark points predominantly show signal significances exceeding the 5 sigma threshold at an integrated luminosity of 3 ab(-1). Furthermore, our approach facilitates accurate mass reconstructions for both m(hf) and M-H +/-. Notably, in the intricate scenarios with heavy charged Higgs bosons, our application of machine learning techniques provides a significant boost in significance.