Search for Higgs boson decays into pairs of light (pseudo)scalar particles in the $\gamma\gamma jj$ final state in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

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This Letter presents a search for exotic decays of the Higgs boson to a pair of new (pseudo)scalar particles, $H\to aa$, where the $a$ particle has a mass in the range 20-60 GeV, and where one of the $a$ bosons decays into a pair of photons and the other to a pair of gluons.

• The search is performed in event samples enhanced in vector-boson fusion Higgs boson production by requiring two jets with large invariant mass in addition to the Higgs boson candidate decay products.
• The analysis is based on the full dataset of $pp$ collisions at $\sqrt{s}= 13$ TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36.7 fb$^{-1}$.
• The data are in agreement with the Standard Model predictions and an upper limit at the 95% confidence level is placed on the production cross section times the branching ratio for the decay $H\to aa\to \gamma\gamma gg$.