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

We present the first direct search for exotic Higgs boson decays $H\to \mathcal{AA}$, $\mathcal{A}\to \gamma \gamma$ in events with two photonlike objects. The hypothetical particle $\mathcal{A}$ 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 $\sqrt{s}=13\mathrm{TeV}$ corresponding to an integrated luminosity of $136{\mathrm{fb}}^{-1}$. No excess above the estimated background is found. We set upper limits on the branching fraction $\mathcal{B}(H\to \mathcal{A}\mathcal{A}\to 4\gamma )$ of $(0.9–3.3)\times {10}^{-3}$ at 95% confidence level for masses of $\mathcal{A}$ in the range 0.1–1.2 GeV.

Figure 1

Mass distributions from selected events in data. Center: the $2\mathrm{D}\text{−}{m}_{\mathrm{\Gamma }}$ distribution for data events in the $m_H\text{−}\mathrm{SR}$. The red dashed lines indicate the $m_{\mathcal{A}}\text{−}\mathrm{SR}$ boundaries. The contours of simulated $H\to \mathcal{A}\mathcal{A}\to 4\gamma$ events for $m_{\mathcal{A}}=0.4\mathrm{GeV}$ are plotted for 75% (solid contour) and 50% (dotted contour) of the distribution maximum. The corresponding $m_{{\mathrm{\Gamma }}_1}$ (left) and $m_{{\mathrm{\Gamma }}_2}$ (right) projections for the overlap of the $m_H\text{−}\mathrm{SR}$ and $m_{\mathcal{A}}\text{−}\mathrm{SR}$ are also shown. The data distributions (black points) are plotted against the total predicted background distributions (blue curves). The statistical uncertainties in the former are negligible and the total uncertainties in the latter are barely visible as green bands. The spectra of simulated $H\to \mathcal{A}\mathcal{A}\to 4\gamma$ events for $m_{\mathcal{A}}=0.1$ (purple dashed curve), 0.4 (gray dotted curve), and 1.0 GeV (orange dash-dotted curve) are also provided. They are each normalized to the value of $\mathcal{B}(H\to \mathcal{A}\mathcal{A}\to 4\gamma )$ that is expected to be excluded by the background model (described under the $C{L}_s$ criterion in our results) times ${10}^3$. The black points in the lower panels of the left and right plots give the ratios of the data to the predicted background distributions. The vertical bars represent the statistical uncertainties in the former, and the green bands represent the total uncertainty in the latter.

Figure 2

Observed (black solid curve with points) and median expected (blue dashed curve) 95% C.L. upper limit on $\mathcal{B}(H\to \mathcal{A}\mathcal{A}\to 4\gamma )$ as a function of $m_{\mathcal{A}}$ for prompt $\mathcal{A}$ decays. The 68% (green band) and 95% (yellow band) CIs are plotted around the expected limit. The 95% C.L. upper limit from the CMS measurement [1] of $\mathcal{B}(H\to \gamma \gamma )$ is also shown (red band, where the width represents the uncertainty in the measurement).