Multi-photon decays of the Higgs boson at the LHC

Abstract

Many new physics scenarios predict multi-photon Higgs resonances. One such scenario is the dark axion portal model. The primary decay chain that we study is the Higgs to dark photon ($γ_D$) pairs that subsequently decay into a photon ($γ$) and an axion-like particle ($a$). The axion-like particles then decay into photon pairs. Hence, the signal is a six-photon Higgs decay: $h\rightarrow γ_D\,γ_D\rightarrow 2\,γ2\,a\rightarrow 6γ$. However, depending on the relevant kinematics, the photons can become well-collimated and appear as photon-jets (multiple photons that appear as a single photon in the detector) or $ξ$-jets (non-isolated multi-photon signals that do not pass the isolation criterion). These effects cause the true six-photon resonance to appear as other multi-photon signals, such as two and four photons. We classify the mass regions where two, four, and six-photon resonances dominate. The four-photon signal is particularly interesting. These events mainly occur when the photons from the axion-like particles are collimated into photon-jets. The decay of the dark photon is then $γ_D\rightarrow γa\rightarrow γ+γ$-jet, which is an apparent violation of the Landau-Yang theorem. We show that current measurements of $h\rightarrow 2γ$ and searches for $h\rightarrow 4γ$ at the Large Hadron Collider (LHC) can limit ${\rm BR}(h\rightarrow γ_Dγ_D)\lesssim 10^{-3}$. This model also motivates new searches for Higgs decays into six isolated photons or $ξ$-jets at the LHC. While there are currently no dedicated searches, we show that many of the Higgs to six isolated photons or $ξ$-jet events could pass two or three-photon triggers. That is, new physics could be found by reanalyzing existing data. These multi-photon signals provide excellent footing to explore new physics at the LHC and beyond.