Nuclear Seminar - Lucia Keszeghová (20.4.2022)
Wednesday 20.4.2022 at 14:00, Lecture room F1/364
Mgr. Lucia Keszeghová:
Associated production of the top-quark pair and Z boson in proton-proton collisions at the ATLAS experiment
Abstract:
Associated production of the top-quark pair and Z boson (ttZ) is a rare process predicted by the Standard Model. Comparisons of the measured ttZ cross section values with the theoretical prediction test the accuracy of the Standard Model. Measurements of ttZ cross section are possible thanks to unprecedented energies and luminosities reached in proton-proton collisions at the LHC. So far, all the measured values of ttZ cross section are consistent with the Standard Model prediction at given centre-of-mass energy. The latest analysis published by the ATLAS experiment in August 2021 measured the inclusive ttZ cross section at 13 TeV to be 0.99 ± 0.05(stat) ± 0.08(syst) pb. This result is based on the full LHC Run II dataset with integrated luminosity of 139 fb−1. In the analysis trilepton and tetralepton ttZ decays channels were considered. Measured ttZ cross section is consistent with the Standard Model prediction of 0.86+0.07−0.08(scale)±0.03(PDF+αS) pb. More precise experimental value is vital for more accurate confrontation with the Standard Model prediction. To achieve higher precision in the ttZ cross section measurement, the full ATLAS Run II dataset is being reanalysed with the inclusion of dilepton decay channel and the use of enhanced analysis methods. The preliminary results confirm that the use of neural networks for separation of signal and background events improves the measurement precision. The seminar will be mostly focused on the refined analysis in the tetralepton channel.