Top quark production and properties in 7 TeV proton-proton collisions

• Applicant: Professor Dr. Markus Cristinziani
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2006 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 30143759

• Final Report Year: 2015

Final Report Abstract

Despite being an elementary particle, the top quark has a very large mass, similar to that of a gold atom. As a consequence the top quark decays immediately after production, even before it can form compound hadrons, like any other quark would do. In proton-proton collisions at the Large Hadron Collider (LHC) top quarks are produced in abundance and can therefore be studied in detail for the first time. After a first phase of preparation for data taking and data analysis, collisions at the LHC were recorded at energies of 7 TeV (2010-2011) and 8 TeV (2012) by the ATLAS detector. Top quarks are predominantly produced in tt̄ pairs (a top quark and an anti-top quark). Each top quark decays almost always in a W boson and a b quark. The W boson can decay either ‘leptonically’ (a charged and a neutral lepton) or ‘hadronically’ (a pair of lighter quarks). Depending on the W boson decays the tt̄ events are classified in the following three channels: the dilepton, the single lepton or the all hadronic channels. As member of the ATLAS collaboration the Emmy Noether group in Bonn studied top quark pair decays in the dilepton channel. The particular interest in this channel is a very good signal-to-background ratio, the main disadvantage being the presence of two undetectable neutrinos. In this channel we measured the production cross-section with increasing precision as more and more data became available. Precise predictions of Quantum Chromodynamics, the theory of the strong interaction, could thus be verified. A first measurement of single-top production in the t-channel has also been performed. The top quark mass is traditionally measured in the single-lepton channel. We performed the first measurement of the top quark mass in the dilepton channel in ATLAS. The uncertainty obtained today is very comparable with the uncertainty in the single-lepton channel. With reconstructed tt̄ events the calibration of the b-tagging efficiency could be extended to larger transverse jet momenta. We pioneered these efforts in ATLAS, in collaboration with other institutes in Germany. With tt̄ dilepton events we also searched for discrepancies in the prediction that top quarks almost exclusively decay to a W boson and a b quark. Data, however, show no indication of such a hypothetical discrepancy. Also searches for new physics effects that would enhance events with two same-sign leptons or three leptons were negative and allowed to constrain SUSY or exotic models. Finally we set up the TOPLHCWG with the aim of combining top quark observables as measured by ATLAS and CMS. First results include the combination of the top quark mass, tt̄ cross-section and the W helicity from top quark decays. In conclusion first very precise measurement of the production and properties of top quarks at the Large Hadron Collider at 7 TeV were performed in the dilepton channel. Fundamental parameters of the theory, like the top quark mass have been measured and no discrepancy from the expectations of the Standard Model of particle physics have been observed.

Publications

• Search for top pair candidate events in ATLAS at √s = 7 TeV, ATLAS-CONF-2010-063
  ATLAS Collaboration
  
• Measurement of the top quark pair production cross section in dilepton final states with ATLAS, ATL-PHYS- PROC-2011-162, Proceedings of PLHC 2011
  T. Abajyan (Poghosyan) on behalf of the ATLAS Collaboration
  
• Measurement of the top quark pair production cross-section with ATLAS in pp collisions at √s = 7 TeV, EPJC 71 (2011) 1577
  ATLAS Collaboration
  
• Recent results on top physics from ATLAS, ATL-PHYS-PROC-2011-044, Proceedings of the XLVIth Recontres de Moriond (QCD)
  M. Cristinziani on behalf of the ATLAS Collaboration
  
• Top physics with 0.70 – 1.08 fb–1 of pp collisions with the ATLAS detector at the LHC, ATL-PHYS-PROC-2011-271, Proceedings of Lepton Photon 2011
  M. Cristinziani on behalf of the ATLAS Collaboration
  
• b-tagging calibration using tt̄ events with the ATLAS experiment, ATL-PHYS-PROC-2012-063, Proceedings of Rencontres de Moriond (EWK) 2012
  A. Leyko on behalf of the ATLAS Collaboration
  
• Combination of ATLAS and CMS results on the mass of the top quark using up to 4.9 fb–1 of data, ATLAS-CONF-2012-095, CMS PAS TOP-12-001
  ATLAS and CMS Collaborations
  
• Measurement of the b-jet tagging efficiency using top quark pair events with ATLAS data, ATL-PHYS-PROC-2012-281, Proceedings of ICHEP
  A. Leyko on behalf of the ATLAS Collaboration
  
• Measurement of the cross section for top-quark pair production in pp collisions at √s = 7 TeV with the ATLAS detector using final states with two high-pT leptons, JHEP 1205 (2012) 059
  ATLAS Collaboration
  
• Measurement of the top quark pair production cross section in pp collisions at √s = 7 TeV in dilepton final states with ATLAS, Phys. Lett. B 707 (2012) 459
  ATLAS Collaboration
  
• Search for FCNC in top-quark events in ATLAS, ATL-PHYS-PROC-2012-249, Proceedings of ICHEP
  M. Cristinziani on behalf of the ATLAS Collaboration
  
• Top-quark mass measurement in the eµ channel using the mT2 variable at ATLAS, ATLAS-CONF-2012-082
  ATLAS Collaboration