Towards a measurement of the tHq process in channels with hadronic tau lepton decays

Abstract

This thesis presents the first study to find events of the rare tHq process with ATLAS data in channels with hadronically decaying &#964;. A baseline framework is developed to select as many signal events as possible in three channels: 1 e/&#956; + 2 &#964;(had), 2 e/&#956; (OS) + 1 &#964;(had) and 2 e/&#956; (SS) + 1 &#964;(had). Within this framework a full reconstruction of tHq events is performed which leads to a H mass reconstruction. A binned profile likelihood fit with Asimov data and real data is done for the three channels separately and in the combination of all three channels. The expected and observed SM result of the combined fit is: <br /> &#956;(tHq,expected)=1 (+7.4) (-6.7) <br /> &#956;(tHq,observed)=5.4 (+7.8) (-7.1) <br /> This result is fully compatible with the SM. A limit on the tHq cross-section is set to 20 times the SM expectation (15.5 expected). <br /> The BSM scenario of an inverted H to t coupling with respect to the H to W coupling is disfavored at the 2.8&#963; level. A significant improvement of the result is expected when more data is available. However, additional improvements on the result are expected if the size of the simulated datasets is increased. Other fields of improvements are a better fake estimation and estimation of uncertainties related to tt&#772; modelling. <br /> The presented result is compatible with the previous results. No study on tHq has been published quoting the exact same final states as used in this thesis that the result could be directly compared to. The enlarged uncertainty with respect to the published results (additionally) including other channels is expected, since three challenging final states are targeted.