Search for exclusive Higgs and $Z$ boson decays to $\omega\gamma$ and Higgs boson decays to $K^{*}\gamma$ with the ATLAS detector

TL;DR

This paper reports on searches for rare Higgs and Z boson decays to mesons and photons using ATLAS data, setting new upper limits that constrain new physics beyond the Standard Model.

Contribution

First search for Higgs and Z boson decays to specific mesons and photons at the LHC, providing the most stringent limits to date.

Findings

Upper limits on branching fractions are set: ${ m B}(H ightarrow ext{omega}\gamma)<5.5 imes 10^{-4}$

Upper limits on branching fractions are set: ${ m B}(H ightarrow K^{*}\gamma)<2.2 imes 10^{-4}$

Upper limit on ${ m B}(Z ightarrow ext{omega}\gamma)$ improves previous bounds by two orders of magnitude.

Abstract

Searches for the exclusive decays of the Higgs boson to an $\omega$ meson and a photon or a $K^{*}$ meson and a photon can probe flavour-conserving and flavour-violating Higgs boson couplings to light quarks, respectively. Searches for these decays, along with the analogous $Z$ boson decay to an $\omega$ meson and a photon, are performed with a $pp$ collision data sample corresponding to integrated luminosities of up to 134 fb$^{-1}$ collected at $\sqrt{s}=13$ TeV with the ATLAS detector at the CERN Large Hadron Collider. The obtained 95% confidence-level upper limits on the respective branching fractions are ${\cal B}(H\rightarrow\omega\gamma)< 5.5\times 10^{-4}$, ${\cal B}(H\rightarrow K^{*}\gamma)< 2.2\times10^{-4}$ and ${\cal B}(Z\rightarrow \omega\gamma)<3.9\times 10^{-6}$. The limits for $H\rightarrow \omega\gamma$ and $Z\rightarrow \omega\gamma$ are 370 times and 140 times the Standard Model expected values, respectively. The result for $Z\rightarrow \omega\gamma$ corresponds to a two-orders-of-magnitude improvement over the limit obtained by the DELPHI experiment at LEP.