Search for the Supersymmetric Partner to the Top Quark Using Recoils Against Strong Initial State Radiation

The ATLAS experiment at Large Hadron Collider (LHC) searches for experimental evidence of beyond the standard model physics at the TeV scale. As we collect more data at the LHC we continue to extend our sensitivity to these new phenomena, probing for the existence of increasingly massive particles. Despite this progress there are still regions of parameter space where constraints remain weak. One common region where we lack sensitivity is when the new BSM particle has a very small mass splitting between it and its decay products. The BSM particle then has little energy left over to give momenta to its decay products and the low momenta decay products are difficult to experimentally detect. These regions of small mass splitting are called compressed regions. We are able to gain sensitivity to these difficult regions by searching for new particles produced in conjunction with hard initial state radiation (ISR). The hard ISR boosts the new particle’s decay products and gives them momentum. This thesis covers the search for the supersymmetric partner to the top quark, called the stop, in the region when the stop and its decay products are nearly degenerate in mass. No searches prior to 2016 were sensitive to this region. We were able to exclude stops up to a mass of 600 GeV in this region using 36.07 fb-1 of sqrt(s) = 13 TeV LHC proton-proton collision data. This data was collected by the ATLAS experiment during 2015 and 2016. We introduce a new and more accurate technique for identifying whole ISR systems composed of multiple ISR jets as part of this analysis. The methods demonstrated in this analysis are completely generalizable and can be used in many other BSM searches and precision SM model measurements of the ISR pT spectrum.