Diphotons from Tetraphotons in the Decay of a 125 GeV Higgs at the LHC

TL;DR

This paper explores how Higgs decays into light pseudoscalars that mimic photons could explain the observed excess in diphoton signals at the LHC, proposing a novel decay mechanism and its experimental implications.

Contribution

It introduces a model where Higgs decays into collimated photon pairs from pseudoscalars can enhance the diphoton signal, providing a new interpretation of the excess.

Findings

Pseudoscalar decays can mimic single photons at the LHC.

The mechanism can increase the observed Higgs to diphoton rate.

Constraints on pseudoscalar properties are derived.

Abstract

Recently the ATLAS and CMS experiments have presented data hinting at the presence of a Higgs boson at $m_h\simeq125$ GeV. The best-fit $h\rightarrow\gamma\gamma$ rate averaged over the two experiments is approximately $2.1\pm0.5$ times the Standard Model prediction. We study the possibility that the excess relative to the Standard Model is due to $h\rightarrow aa$ decays, where $a$ is a light pseudoscalar that decays predominantly into $\gamma\gamma$. Although this process yields $4\gamma$ final states, if the pseudoscalar has a mass of the order tens of MeV, the two photons from each $a$ decay can be so highly collimated that they may be identified as a single photon. Some fraction of the events then contribute to an effective $h\rightarrow\gamma\gamma$ signal. We study the constraints on the parameter space where the net $h\rightarrow\gamma\gamma$ rate is enhanced over the Standard Model by this mechanism and describe some simple models that give rise to the pseudoscalar-photon interaction. Further tests and prospects for searches in the near future are discussed.