Searching for doubly charged Higgs bosons using same-sign hadronic tau final states - A focused study on the charge-flip background of hadronic taus

Sammanfattning: The search for doubly charged Higgs bosons (H±±) aims at resolving the mass generation mechanism of neutrinos in the Standard Model. The current search by the ATLAS Same-Sign Dilepton group uses the decay product of H±± to a pair of same-sign hadronic τ-leptons, which is an extension of their previous search using same-sign dielectrons and dimuons. This thesis focuses on the study of one of the backgrounds, known as the charge-flip background, as a result of the misidentification of the charge of hadronic τ-leptons by the ATLAS detector. To estimate this background, three methods are used with the data-driven and tag-and-probe method applied to MC simulated samples and the template fit method applied to detector data collected from 2015 to 2017 by the ATLAS detector at √s = 13 TeV with a total integrated luminosity of 79.8 fb−1. The charge-flip rate of hadronic τ-leptons rises from around 0.3% to 2.2% with the increase of pseudorapidity from 0 to 2.5, while the charge-flip rate for detector ranges from 0.4% to 3.8%. This leads to a successful calculation of the scale factor equalling to 1.52±0.42.