Search for Exotic Higgs Boson Decays $H\to AA\to 4\gamma $ with Events Containing Two Merged Diphotons in Proton-Proton Collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$

We present the first direct search for exotic Higgs boson decays $H\to \mathrm{AA}$, $A\to \gamma \gamma $ in events with two photonlike objects. The hypothetical particle $A$ is a low-mass spin-0 particle decaying promptly to a merged diphoton reconstructed as a single photonlike object. We analyze the data collected by the CMS experiment at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$ corresponding to an integrated luminosity of $136\text{}\text{}{\mathrm{fb}}^{-1}$. No excess above the estimated background is found. We set upper limits on the branching fraction $B\left(H\to AA\to 4\gamma \right)$ of $\left(0.9–3.3\right)×{10}^{-3}$ at 95% confidence level for masses of $A$ in the range 0.1–1.2 GeV.