Search for new phenomena in events with at least three photons collected in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

TL;DR

This paper reports a search for new phenomena involving events with at least three photons in proton-proton collisions at 8 TeV, finding results consistent with the Standard Model and setting limits on various hypothetical particles and decay processes.

Contribution

It provides the first limits on several rare decay processes involving three photons and extends previous bounds on the Z boson decay to three photons.

Findings

No significant excess observed over Standard Model predictions.

Set new upper limits on Higgs decay to four photons via pseudoscalars.

Established the most stringent limit on Z→3γ decay to date.

Abstract

Results of a search for new phenomena in events with at least three photons are reported. Data from proton--proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95% confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay $Z\to 3\gamma$, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles ($a$), limits are found to be $\sigma \times {\text {BR}}(h \rightarrow aa) \times {\text {BR}}(a \rightarrow \gamma \gamma)^{2} < 10^{-3} \sigma_{\text{SM}}$ for 10 GeV $< m_{a} <$ 62 GeV. Limits are also presented for Higgs boson-like scalars ($H$) for $m_{H} > $ 125 GeV, and for a $Z'$ decaying to three photons via $Z' \rightarrow a+\gamma \rightarrow 3\gamma$. Additionally, the observed limit on the branching ratio of the $Z$ boson decay to three photons is found to be BR$(Z \rightarrow 3\gamma) < 2.2 \times 10^{-6}$, a result five times stronger than the previous result from LEP.