Top Quark Reconstruction at LHCb

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By Henry Brown Top Quark Reconstruction at LHCb Henry Brown, LHCb, IOP 10/04/13 1
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Top Quark Reconstruction at LHCb. By Henry Brown. Henry Brown, LHCb, IOP 10/04/13. 1. Introduction to Top. Still the heaviest known fundamental particle Decays before any hadronization Best chance to see ‘bare’ quark Pair production (4 π ) at 7 TeV = 163 pb - PowerPoint PPT Presentation

Transcript of Top Quark Reconstruction at LHCb

Top Quark Reconstruction at LHCb

By Henry BrownTop Quark Reconstruction at LHCbHenry Brown, LHCb, IOP 10/04/131

Still the heaviest known fundamental particleDecays before any hadronizationBest chance to see bare quarkPair production (4) at 7 TeV = 163 pb At 8 TeV, this increases to 234 + 12 pbA forward region search has yet to be performed at the LHCSensitive to BSM physics

Introduction to Top


Henry Brown, LHCb, IOP 10/04/132Top Asymmetry [R.Gaulds IOP talk of 9/4/13]2t b W+ is dominant because of the CKM term |Vtb|2. This process occurs in approximately 99.8% of top decaysThe decay modes refer to the decays of the W, e.g. all jets refers to W qq, so 6-jet final stateTop Decay Modes

Henry Brown, LHCb, IOP 10/04/133 into 4LHCb Detector

Beam 1Beam 2Collision pointAcceptanceForward arm spectrometer with unique acceptanceVELO offers high precision vertexing and tracking Henry Brown, LHCb, IOP 10/04/1342 90%Minv(e) > 15 GeV/c2l IP < 35 mDOCA(ll) < 35 mNeural Network b-tag 0.9SelectionBackgroundsBackgrounds considered:Z Z to ( faking e)WWZ ee (e faking )ZWQCDJets are reconstructed using the Anti-KT algorithm, PT>5 GeVTracks+neutralsHenry Brown, LHCb, IOP 10/04/138Normalized by NLO cross sectionsTrigger on single high PT (>15 GeV/c)

Plots are for 8 TeV samplesPreselection

LHCb UnofficialSignal Purity 0.2%Henry Brown, LHCb, IOP 10/04/139e PT > 5 GeV/cIP (l)< 35 m PT > 15 GeV/cMinv > 15 GeV/c2l PT > 15 GeV/cl isolation > 90%Minv(e) > 15 GeV/c2l IP < 35 mDOCA(ll) < 35 mNeural Network b-tag 0.9Lepton CutsHenry Brown, LHCb, IOP 10/04/1310Lepton Cuts

LHCb UnofficialSignal Purity 9%Henry Brown, LHCb, IOP 10/04/1311b-tagged

LHCb UnofficialSignal Purity 70%Henry Brown, LHCb, IOP 10/04/1312ProcessEvents (in 2.01 fb-1)Diboson2.6 + 0.1Z1.6 + 0.5Top pair9.6 + 0.1Data (2fb-1 8 TeV sample)12b-tagged ExpectationHenry Brown, LHCb, IOP 10/04/1313Event Display

ElectronMuonTracks tagged to B-jete PT = 25 GeV/c PT = 25 GeV/cMinv = 48.5 GeV/c2b-jet PT = 25 GeV/cHenry Brown, LHCb, IOP 10/04/1314First measurements of top production in the forward region underway at LHCbCurrent data samples at LHC phase 1 highly statistically limitedA top cross section in the forward region is interestingAt 14 TeV (LHCb Upgrade), 15fb-1 (approx) will have a statistical error comparable to theoretical cross section errors50fb-1 = ~10000 events70% final state purity, 20% from diboson, 10% from Z12 candidates found in 2012 dataCross section measurement being produced

SummaryHenry Brown, LHCb, IOP 10/04/1315Control regionsFull selection before b-tagCheck data hypothesis is consistent with Z, diboson, and same sign contributionsSame sign full selectionCheck same sign after b-tag is consistent with just ZW hypothesisHenry Brown, LHCb, IOP 10/04/1316Fiducial cross-section calculated by producing large MC setsUsing POWHEG16,000,000 evts for 7 TeV24,000,000 evts for 8 TeVUse 3 different PDFs (NNPDF2.2, MSTW2008, CT10nlo)Look at rate of e +b quark in acceptance of LHCbNormalize to theoretical cross-sectionsb- quark acceptance systematic to be determined

Fiducial cross-sections (Backup)Henry Brown, LHCb, IOP 10/04/1317A summary of the possible final states in LHCbs acceptance per fb-1Notes: l refers to or e, e.g. l+l- can be , e, eebany refers to any b, rather than the b from the same top as the lTop Quark at LHCb

On going efforts in this channel too, early stagesHenry Brown, LHCb, IOP 10/04/1318Variables:Impact Parameter - 32IP Error - 34IP Significance - 59PT 3.4 - 11Precuts:PT > 500 MeV0.08 mm < IP < 3 mmIP Sig. > 0.5Track Net Variables

LHCb UnofficialHenry Brown, LHCb, IOP 10/04/1319Variables:Maximum track net output - 120 Jet PT - 191 Maximum Track PT 10.8 Average track IP 40.4 Sum of track PT 59.6 Signed IP Tag Sum 34.5 Signed IP Tag 2nd track 21.4 Signed IP Tag 3rd track 12.6 Jet Net Variables

LHCb UnofficialHenry Brown, LHCb, IOP 10/04/1320Purity vs Output (JetNet)

LHCb UnofficialHenry Brown, LHCb, IOP 10/04/1321Different preferential directions for top quark vs antitopTop quark is preferentially emitted in the direction of the incoming quarkAsymmetries have been observed in the top quark sector at CDF and D-0 [arXiv:1211.6028]

Top Asymmetry

Note: gluon fusion does not contribute to asymmetryHenry Brown, LHCb, IOP 10/04/1322