Measurement of the backgrounds to the $H\rightarrow\gamma\gamma$ search and reappraisal of its sensitivity with 37 pb$^{-1}$ of data recorded by the ATLAS detector

This note presents the first measurement of the background to the search for the Higgs boson in the diphoton channel with the ATLAS detector at a proton-proton center-of-mass energy of $\sqrt{s}$=7 TeV with an integrated luminosity of 37~pb$^{-1}$. After applying the event selection criteria for the $H\rightarrow\gamma\gamma$ search in the mass range from 100 to 150 GeV, 83 diphoton events have been selected with an integrated luminosity of 37 pb$^{-1}$. The individual contributions from diphoton, photon-jet, dijet and Drell-Yan events are determined with data-driven methods to be $68.6\pm 11.0\pm 3.4$, $10.6\pm 4.8\pm 4.1$, $1.2\pm 0.6\pm 0.5$ and $2.7\pm 0.2\pm 0.6$, respectively. This measurement provides an accuracy on the background to $H\rightarrow\gamma\gamma$ analysis which is better than the predictions based on theoretical models and simulation. With the projected sensitivity derived from this measurement, ATLAS expects to exclude with 1 fb$^{-1}$ of data a Higgs boson produced with a cross-section 3.2-4.2 times larger than the Standard Model prediction in the 110-140 GeV mass range.