Search for a Higgs boson decaying to four photons through light CP-odd scalar coupling using \lumifull of $7~\mathrm{TeV}$ $pp$ collision data taken with ATLAS detector at the LHC

In some extensions of the Standard Model, such as MCHM or NMSSM, the Higgs boson can couple to light CP-odd scalar particles. This document presents a study of a possible contribution to a Higgs boson signal in the $H\to\gamma\gamma$ channel due to $H\to a a$ decays, where $a$ is a light CP-odd scalar particle that decays predominantly into two photons before reaching the electromagnetic calorimeter. Although this process yields four photon final states, we show that for $a$ in the mass range of 100-400 MeV, the two photons from each $a$ decay are often collimated so that they may be identified as a single photon. A large fraction of these events would contribute to an effective $H\rightarrow\gamma\gamma$ signal. By following a selection similar to that of the $H\to\gamma\gamma$ analysis, we set experimental limits for a generic class of Standard Model extensions having Higgs boson couplings to light scalar particles, assuming the $a$ decays before reaching the electromagnetic calorimeter. The 95$\%$ confidence-level exclusion limits are provided on the Higgs boson production cross section times $H\to a a \to 4 \gamma$ branching fraction in the range of $110\gev < m_H < 150\GeV$ and for the particle $a$ in the mass range from $100\mev$ to $400\MeV$.