In this study, we explore the phenomenological signatures associated with a light fermiophobic Higgs boson, $h_{\mathrm{f}}$, within the type-I two-Higgs-doublet model at the HL-LHC. Our meticulous parameter scan illuminates an intriguing mass range for $m_{h_{\mathrm{f}}}$, spanning $[1,10]\mathrm{GeV}$. This mass range owes its viability to substantial parameter points, largely due to the inherent challenges of detecting the soft decay products of $h_{\mathrm{f}}$ at contemporary high-energy colliders. Given that this light $h_{\mathrm{f}}$ ensures $\mathrm{Br}(h_{\mathrm{f}}\to \gamma \gamma )\simeq 1$, $\mathrm{Br}(H^{\pm }\to h_{\mathrm{f}}{W}^{\pm })\simeq 1$, and $M_{H^{\pm }}\lesssim 330\mathrm{GeV}$, we propose a golden discovery channel: $pp\to h_{\mathrm{f}}{H}^{\pm }\to \gamma \gamma \gamma \gamma {\ell }^{\pm }\nu$, where ${\ell }^{\pm }$ includes $e^{\pm }$ and ${\mu }^{\pm }$. However, a significant obstacle arises as the two photons from the $h_{\mathrm{f}}$ decay mostly merge into a single jet due to their proximity within $\mathrm{\Delta }R<0.4$. 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 $3{\mathrm{ab}}^{-1}$. Furthermore, our approach facilitates accurate mass reconstructions for both $m_{h_{\mathrm{f}}}$ and $M_{H^{\pm }}$. Notably, in the intricate scenarios with heavy charged Higgs bosons, our application of machine learning techniques provides a significant boost in significance.

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• Extensions of Higgs sector
• Particle decays
• Particle detection signatures