Observation of Higgs boson production in association with a top quark pair (ttH) is presented, based on the analysis of up to 79.8 fb 1 of proton-proton collision data produced at 13 TeV center-of-mass energy by the Large Hadron Collider and recorded with the ATLAS detector. The results for Higgs boson decay channels of bb, diphoton, WW, ditau, and ZZ are combined. The observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the ttH searches using a dataset corresponding to integrated luminosities of 4.5 and 20.3 fb 1 at 7 and 8 TeV, respectively, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total ttH production cross section at 13 TeV is measured to be 670 ± 90 (stat.) +110/-100 (syst.) fb, while the Standard Model prediction is 507+35 50 fb. CERN issued a press release for both the ATLAS and CMS ttH results on June 4th 2018. Moreover, this talk will give detailed description of the new ttH (H→gamma gamma) analysis. Applying advanced machine learning package XGBoost BDT, exceptional performance is achieved by using object level kinematic variables from jets, leptons, and photons to separate the ttH signal from the background.

报告人简介：Zhou Chen attended Peking University during 2006-2010 and received his Bachelor degree in Physics. His undergraduate thesis is on resistive plate chamber (RPC) detectors, under the supervision of Prof. Yanlin Ye. He got his PhD degree in Physics at Duke University, under the direction of Prof. Mark Kruse. During his PhD time (2010-2016), he was involved in both the ATLAS and CDF experiments. At ATLAS, he was working on ttV (V=W/Z) measurement (PhD thesis topic) and supersymmetry search, as well as the silicon strip detector upgrade. At CDF, he contributed to search for Upsilon+V (V=W/Z) production. In 2016, he moved to University of Wisconsin as a postdoc research associate, and continued working on the ATLAS experiment under the supervision of Prof. Sau Lan Wu. In the past 3 years, he has been involved in measurement of the Standard Model Higgs properties, search for double-Higgs production, search for dark matter, search for high-mass resonances, as well as the silicon pixel detector upgrade. For the recent ATLAS ttH observation, he made contribution to the ttH (H->gamgam) analysis and the combination of all ttH analyses.