Summary of the H $\rightarrow$ $\gamma$$\gamma$ decay channel

 The H $\rightarrow$ $\gamma$$\gamma$ decay mode of a standard model Higgs boson is the most promising detection channel for a Higgs boson in the mass range 90 < m_H < 130 GeV. The signal of two isolated photons is clear but both the irreducible photon-photon background and the reducible jet-jet and jet-photon backgrounds are much larger than the signal.

It was shown that the crucial points for the detection are a good mass resolution to achieve as narrow a mass peak as possible and a high rejection power of QCD-jets faking photons.

The mass resolution can be improved with a separate calibration of the energy scale in the electromagnetic calorimeter for converted and non-converted photons giving rise to a 10% higher significance of a 100 GeV Higgs particle.

A determination of the primary vertex position along the beam axis improves the mass resolution through a better measurement of the angle between the two photons. Three methods to measure the position of the primary vertex was compared: pointing in the calorimeter, reconstruction of converted photons and global tracking where all charged tracks in the event are reconstructed. The significance of the Higgs signal improves by an additional 3-7% depending on the description of the underlying event. This theoretical uncertainty affecting the global tracking pointing method was reviewed and it was concluded that the results presented from a full detector simulation are pessimistic estimates.

After the standard rejection of QCD-jets in the calorimeters the reducible background consists mainly of jets where a single $\pi^{0}_{}$ carry the main part of the energy. The role of the Inner Detector in the rejection of these $\pi^{0}_{}$ events was analysed. At high luminosity and with a photon efficiency of 90% the overall rejection factor of $\pi^{0}_{}$ events with transverse momentum of 50 GeV is 3.2 using the presampler in the calorimeter alone, while a combined method using the identification of conversions in the Inner Detector gives a rejection factor of 3.5.

A pessimistic estimate of the total significance of a Higgs signal shows it will be possible to have a clear identification within one year at high luminosity for a Higgs mass above 105 GeV. The improved QCD-jet rejection from the combined $\pi^{0}_{}$ rejection will improve the absolute significance by less than 1%; more importantly it reduces the uncertainty from the many unknown factors in the rate and rejection of the QCD-jets.

The ATLAS detector was shown to be robust for the detection of the H $\rightarrow$ $\gamma$$\gamma$ decay. If either the performance of the calorimeter pointing is reduced or the difference between the underlying events for the gluon fusion Higgs production and the minimum bias events are less than expected, the Higgs signal efficiency will only be marginally effected.