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Top physics and data analysis Cunfeng Feng 2015/7/16 1 Motivation for Studying Top What really distinguishes it is the huge mass, roughly 40x larger than bottom. top is special in some way. a special role for top within the Standard model itself. Top is only fermion for which the coupling to the Higgs is important: it is a laboratory in which we can study EWSB. 2015/7/16 2 Top pair production Quark-antiquark annihilation: TeV:85% LHC:~0% Gluon fusion: TeV:15% LHC:~100% 2015/7/16 at the LHC About 90% of the rate is due to gluongluon collisions, quark-antiquark collisions give the remaining 10%. 3 Top pair cross section Top pair cross section at 1.96 TeV is 6.7 pb At 12 TeV is 850 pb At the LHC more than 8 million pairs of top per year at low luminosiy 2015/7/16 4 Top decay Since the top lifetime top ~ 1/ M3top~10 -25 sec qcd ~ Λ-1 ~10 -23 sec the top quark does not hadronize. It decays as a free quark! Because Vtb=~1 tWb in 99.8%. We always have two b-jets in the final state 2015/7/16 5 Decay Branch ratio e-e (1/81) mu-mu (1/81) tau-tau (1/81) e -mu (2/81) e -tau (2/81) mu-tau (2/81) e+jets (12/81) mu+jets(12/81) tau+jets(12/81) jets 2015/7/16 (36/81) 6 observable • • • • • • Top mass Top charge W-helicity in top decays ttbar resonances Single top Spin corelation 2015/7/16 7 Top identification in lepton+jets channel criteria • An isolated lepton with PT>20 GeV （ tight lepton quality requirements） • missing energy ETmiss>20 GeV. • At least four jets, with a cone size of DR =0.4, with PT>40 GeV and |η|<2.5 • One or two jets be tagged as b-jets. 2015/7/16 8 W jets • non b-tagged jets • |mjj- mW |<20 GeV. • W boson associated with 1, 2, 3, and 4 or more jets with ET > 15 GeV. • Signal events are mostly concentrated in the third and fourth jet multiplicity bins, • the first and the second bins, dominated by W boson production in association with jets, are used for a cross check of background normalization. • There is still some small • contamination from multijet QCD events. relies on the fact that non-W events have smaller probability to pass tight lepton quality requirements. 2015/7/16 9 Tagging b-jets After traveling ~1mm from the primary vertex (PV) b-quarks decay into a jet of lighter particles. Charged products from b-quark decay ionize silicon sensors, leaving dot-like hits. Dots are connected and form a track corresponding to a particle’s path. b-quark d0 PV Jet is tagged as a b-jet if it contains several tracks not coming from the primary vertex. b-tagging algorithms developed by D0 are CSIP and SVT. 2015/7/16 10 Pulse Clusters of ionization Dot-like hits 1 ADC cos q q MIP Particle crossing silicon sensor MIP q 2015/7/16 11 Background in l+jets channel • Main background :W(lν)+jets • Only a small fraction of these jets are bjets • b-jet tagging reject it efficiently. 2015/7/16 12 Combination criteria • The combination of the jet pair jj with the btagged jet yields a combinatoric ambiguity. • one tagged b-jet the events : opening angle of the b-jet with the W is smaller than with the lepton. • two b-tagged jets: the b-jet with the highest PT combined with the jet pair jj(W). background is well under control, with a signal to background ratio S/B~65. 2015/7/16 13 what to do next step? 2015/7/16 14 Rome data • https://uimon.cern.ch/twiki/bin/view/Atlas/R omeGetFilesWiki • How to use AOD? 2015/7/16 15 Castor • graphical user interface on top of the CERN Castor file system • Need AFS account • https://uimon.cern.ch/twiki/bin/view/Atlas/C astorBrowser 2015/7/16 16 Distributed Analysis • Distributed Analysis, which is distinct from distributed/grid production. • https://uimon.cern.ch/twiki/bin/view/Atlas/D istributedAnalysisReviewFinalReport 2015/7/16 17 ATLAS mirror? • Easy to set up environment fellowing the guidance of Work Book (https://uimon.cern.ch/twiki/bin/view/Atlas/ WorkBook) • Need new storages and PCs 2015/7/16 18