Sparkling beauty: a measurement of the bbγγ cross-section at 8TeV with ATLAS

In the Standard Model of elementary particle physics many particles gain their mass via the Higgs mechanism. It does this by spontaneously breaking the electroweak symmetry in the very early Universe. The discovery of the boson is not enough to show this mechanism is correct; for this the Higgs potential needs to be measured. In this thesis we first investigated the possibility of measuring at the LHC a crucial parameter that defines this potential; the Higgs trilinear coupling, λ3h. We found this to be highly unlikely, and therefore moved on to study the background to one of the more promising channels; pp -> hh -> bbyy. This choice was made because this channel strikes a nice balance between having a measurable cross-section and a clean signature. In addition there are large theory uncertainties on this final state making experimental guidance useful. Using the √s = 8 TeV data the ATLAS detector collected in 2012 at the LHC we analyzed the inclusive and differential production cross-section of pp -> bbyy. The theory predicts a very small cross-section for this channel, and we have only a few hundred such events in the whole dataset. The major backgrounds are formed by objects which are wrongly reconstructed by the detector; jets and electrons faking photons, and by jets which fail to be correctly identified as b-jets. We used data-driven methods to quantify each of these backgrounds and subtract them off the signal. We then unfolded the results and compared the measured cross-section to the one the theory predicts. We find a cross-section of 0.0267 pb which lies comfortable in between the leading and next-to-leading order theory predictions.