Indirect search for new physics affecting the B0(s)!¹u¹u decay with the ATLAS experiment

<p dir="ltr">This thesis focuses on searches for New Physics using the rare B0 (s) !¹Å¹¡ decay in data collected by the ATLAS experiment. The small amplitudes of the B0 (s) !¹Å¹¡ decays are precisely predicted in the StandardModel (SM) and receive sizable contributions in various beyond-SM theories. The presented work consists of the first ATLAS measurement of the B0 s ! ¹Å¹¡ effective lifetime using 26.3 fb¡1of LHC proton–proton collision data collected in 2015–2016. The backgrounds are subtracted using the sPlot technique. The resulting proper decay time distribution of 58§13 signal candidates is fit with simulated signal templates parametrized as a function of the B0 s effective lifetime. Statistical uncertainties are evaluated through the Neyman construction. The measurement results in the B0 s !¹Å¹¡ effective lifetime value 0.99Å0.42 ¡0.07 (stat.)§ 0.17(syst.) ps, with the dominant systematic uncertainty arising fromthe discrepancies between the data and simulations. The result is consistent with the SMprediction. Developments towards the B0 (s) ! ¹Å¹¡ branching ratio and effective lifetime measurements on the 2015–2018 ATLAS dataset are presented. Sensitivity studies for both measurements are performed, suggesting a large improvement in the statistical uncertainties with respect to the last published ATLAS results.</p>