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Experience with muons in analysis : W’ search

Experience with muons in analysis : W’ search. Carmen Diez Pardos CIEMAT Madrid Muon Barrel Workshop Physics sesion 25/02/2011. Outline. Introduction Muons in W´ searches Analysis concept Efficiency determination in W´ (and W xsec measurement) Background determination Systematics

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Experience with muons in analysis : W’ search

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  1. Experiencewith muons in analysis:W’ search Carmen Diez Pardos CIEMAT Madrid Muon Barrel Workshop Physics sesion 25/02/2011

  2. Outline • Introduction • Muons in W´ searches • Analysis concept • Efficiency determination in W´ (and W xsec measurement) • Background determination • Systematics • Results • Outlook C. Diez Pardos (CIEMAT)

  3. 1. Introduction: aboutthistalk • Presentanexample of a physicschannelfocusingontheaspectsrelatedto muons, as efficiencymeasurements, reconstruction and systematics • The Wʹ→μν Search at the LHC • Jump in collision energy (from TeVatron to LHC) single most important factor in search for new heavy particles • (Relatively) easy analysis • High-pT (unprescaled) single muon trigger • Reconstruction of MT by combing high-pTmuon, MET • Practically background-free channel •  Study muon efficiencies, effect of muon momentum resolution and scale, at the moment without high pT muons from collisions An analysis that can lead to one of the first LHC discoveries! C. Diez Pardos (CIEMAT)

  4. Wʹ→μν Topology and Limits • Spectacular signature • One high-pT muon in event (“straight track”) • Plus, “nothing else” Event with Max MT 1 Muon with ~550 GeV Experimental limits from Tevatron in the channel W‘ → ev: m(W‘)>1.1 TeV [CDF, published] CMS limit from electron channel m(W‘)>1.36 TeV C. Diez Pardos (CIEMAT)

  5. Analysisstrategy • Identify high-pT (unprescaled) single muon trigger • Follow Exotica Muon group recommendation for muon selection (Backup slides) • Reconstruction of MT by combing high-pT muon, MET • Determine efficiencies: Efficiencies used for our signal MC (the BG is obtained from data) and to crosscheck if we understand our BG from data/MC comparisons • Determine BG from pp collisions (fit in MT sideband extrapolating to signal region) and cosmics • Comparison between data and MC MT distributions: No excess seen in data  Set a limit Samples: DATA: 36/pbNov4ReReco, MC BG: Fall10 38X samples C. Diez Pardos (CIEMAT)

  6. 1. Object reconstruction: • MUONS: Extensive studies of high-pT muon reconstructor • Global, tracker-only, TPFMS, cocktail, picky, TMR, DYT: Using „cocktail“ as recommended by Exotica Muon group • For this analysis tails of the pT distribution matter • Using both MC and cosmics/CRAFT (see also Z`→  note) MET reconstruction: For the dominant background MET is driven by muon-pT measurement: hadronic component less relevant for tail of MET (MT) distribution C. Diez Pardos (CIEMAT)

  7. 2. Efficiencymeasurement • Why are theyimportant? • For BG – comparison data/MC toextractsystematics, differencescouldindicatethesource of disagreement • Forthesignal: usedtoextractthelimit (orthexsec!) • Thereare no high-pt muons in collisiondata, in ordernottorelyjuston MC we use • A data drivenmethod: Tag and ProbeusingZmumuevents (AN-2010/233): nottheofficial TP package, differentbackgroundsubtraction, no analyticaldescription of the Z->mumushape • Fortriggerefficiencies, alsoJetMettriggered data (gettohighervalues of pT) • Theagreement at mediumpTbetween data and MC isalsoseen at highpTwithcosmic muon data. C. Diez Pardos (CIEMAT)

  8. Single muon id. efficiency: T&P methodSelection of a Z->mumu sample • Preselection of tracks with kinematics cut pt >25. GeV and |eta| < 2.1 • which combined with an opposite charged track give an invariant mass in • the range 60. < Mμμ < 120. GeV. • Match track-muon is done with the track reference in the muon object. • Tag muons must fullfill the following selection: (Exotica, VBTF recommendation) • Must be Global and Tracker Muons • Combined isolation < 0.15 in a cone R < 0.3 • Quality cuts related to the track: χ2 < 10/ndof, d0<0.02 cm, number of valid muon and pixel hits >0, number of valid tracker hits >10, number of matching segments >1, valid muon hits >0 • Muon matched to a L3 muon: • hltSingleMu9L3Filtered9 (RunA + Run B until 147196) hltSingleMu15L3Filtered15 (till run 149442) • The other track is considered as a probe to study the efficiency. C. Diez Pardos (CIEMAT)

  9. 2.1 Trigger Efficiencies • Mixture of triggers in the data, with raising pT threshold • Always use lowest-threshold un-prescaled single-muon trigger: HLT_Mu9, HLT_Mu11, HLT_Mu15 • All trigger threshold lower than the pT>25 GeV requirement • Trigger efficiencies determined with tag-and-probe Flat for pT>25 GeV Run 2010B flat trigger efficiency is used throughout the analysis C. Diez Pardos (CIEMAT)

  10. 2.1 Trigger Efficiencies Trigger efficiencies in data are determined with a complementary method (1.5% difference): using jet triggered samples with offline muons, can reach higher values of pT In data no muons with pT>240 GeV while muons from potential W‘ signal are O(>=500 GeV) Run2010A 0.854+-0.010 Run2010B 0.914+-0.010 Efficiency from Data ~ 92% Run2010B 0.913+-0.010 Extrapolation for high pT needed, done with MC → reasonable agreement with the data driven studies for low pT C. Diez Pardos (CIEMAT)

  11. 2.2 Data: T&P Determined Efficiencies εMu Muon reconstruction εSELSelection εISOIsolation εTRK Reconstruction inner track Excellent agreement between MC-predicted and data-derived (T&P) efficiencies (largest discrepancy coming from trigger: 4% assigned as systematic uncertainty) Efficiencies are only used for our signal MC, the background is obtained from data C. Diez Pardos (CIEMAT)

  12. 2.2 Data: T&P Determined Efficiencies Data Isolation Tracking MC Selection Reconstruction C. Diez Pardos (CIEMAT)

  13. Other Efficiencies to High pT C. Diez Pardos (CIEMAT)

  14. (An analysis with signal…) Efficiencies for W cross-section measurement • Same method used for the W->munu cross-section measurement with all 2010 data (already pre-approved) • Advantage of extracting the efficiencies on a sample of muons kinematically very similar Data MC • Theselectionisslightlydifferent (d0< 0.2 cm iso<0.1) C. Diez Pardos (CIEMAT)

  15. W cross-section measurement • The ratio is roughly flat in pT, the correction is applied in eta bins • Good agreement between data and MC • The correction factor is incorporated to the cross-section measurement, as a correction to Aw*eff factors C. Diez Pardos (CIEMAT)

  16. (Back to W’) 4. Systematic uncertainties Signal Focusonsystematicsrelatedto muon Background Uncertainty on the number of background eventsdue to statistical uncertainty of the sideband fit itself C. Diez Pardos (CIEMAT)

  17. Systematic in MT due to momentum scale and resolution • Method: 'Distort' the muon momentum by certain amount, recreate MET with the new momentum and recalculate MT. • Assign the relative difference between # of events with 'standard' MT and 'distorted MT”> 600 GeV as a systematic • Muon distorted like: • Momentum scale pt'=pt+a0*pt^2 • Momentum resolution 1/pt' = 1/pt + C*Gaussian(0,1) Updatedstudieswithall data, effectsalsodependon eta, phi C. Diez Pardos (CIEMAT)

  18. Systematic in MT due to momentum scale and resolution • Realistic values for the resolution and momentum scale from data (results available at the time of doing the analysis, most conservative values taken, it doesn´t affect significatively the analysis) • MuscleFit (CMS AN-2010/059), SIDRA (CMS AN-2010/059) studies for Z->mumu – intermediate pT (a0 = 0.0039/40GeV, C = 0.14/TeV) • Cosmic studies: alignment, end point analysis (pt' = pt + ( 0.0031 +/-0.0005)*pt, C = -0.047/TeV) (CMS PAS TRK-10-004, http://indico.cern.ch/conferenceDisplay.py?confId=101503) • For W cross-section this systematics (with the latest results from muon momentum resolution and scale) is estimated to be <0.1% Results Updatedstudieswithalldata: http://indico.cern.ch/getFile.py/access?contribId=5&resId=0&materialId=slides&confId=127861 Cosmics are theonlywaytohaveinformationabouthigh-pT(TeV) muons C. Diez Pardos (CIEMAT)

  19. Final results: MT Distribution & Limit setting Search strategy & limit setting: good agreement data-MC, no excess seen→set an exclusion limit Apply search window for high MT region and compare #expected - #observed events Use a sliding search window to optimize the limit as done for electron channel Search window Combined exclusion m(W‘)<1.58 TeV Present Tevatron limits m(W‘)<1TeV [D0 publ.], 1.1TeV [CDF, to be pulished] C. Diez Pardos (CIEMAT)

  20. Outlook • Search for W´ mu performed with 36/pb • No excess seen in data  set exclusion on W´-mass with SM-like couplings of m(W‘)<1.39 TeV, combined with electron exclusion: m(W‘)<1.58 TeV • Trigger, reconstruction and selection efficiencies studied in data (up to pT~200 GeV) with tag-and-probe and extrapolated to higher pT • Cosmics are still our only high pt muons... • Used cosmic data to cross-check the muon momentum resolution, scale from Z->mumu studies • Next year challenge: start dealing with real high pT muons from collisions C. Diez Pardos (CIEMAT)

  21. Back up C. Diez Pardos (CIEMAT)

  22. Samples: Data and MC Im thinking about removing the tables or at least some columns,and summarise it in a couple of lines DATA: 36/pb Nov4ReReco MC BG: Fall10 38X samples MC Signal C. Diez Pardos (CIEMAT)

  23. (Back to W’) 3. Background determination Method • Get total pp background from MT data spectrum • Choose a region with low signal contamination (~1%) • Fit sideband, extrapolate to high MT tail to predict # of (pp) background events in signal region • Method proven to work in MC • Background in MC is sum of all SM contributions (but mainly: W→μν) • Cosmics not described by this method Sideband Extrapolation region C. Diez Pardos (CIEMAT)

  24. 3.1 Cosmic Background • Goal: determine potential cosmics contamination, not described by sideband fit • Cosmic contamination more relevant for tail of MT distribution pp cosmics Cosmic contamination for d0 < 0.02 cm C. Diez Pardos (CIEMAT)

  25. Table for d0 Optimization C. Diez Pardos (CIEMAT)

  26. Backup: Search Window for C&C Cut-and-count method C. Diez Pardos (CIEMAT)

  27. Select a muon: Trigger selection: HLT_Mu9, HLT_Mu11, HLT_Mu15 Muon identification and quality cuts: Follow Exotica Muon group recommondation Tracker & global muon ≥ 11 silicon tracker hits ≥ 1 pixel hit χ² /Ndof < 10 Transverse impact parameter d0 < 0.02 cm(order of magnitude tighter than VBTF recommondation) At least 2 matched muon segments Acceptance η < 2.1 Exactly 1 global muon with pT > 25 GeV Relative combined isolation in a cone (ΔR < 0.3) < 0.15 Dedicated kinematic selection 0.4 < pT / MET < 1.5 Δ(φμ,υ) > 2.5 Selection C. Diez Pardos (CIEMAT)

  28. More on T&P method C. Diez Pardos (CIEMAT)

  29. More on T&P method C. Diez Pardos (CIEMAT)

  30. More on T&P method C. Diez Pardos (CIEMAT)

  31. Outlook • Search for W´ mu performed with 36/pb • No excess seen in data  set exclusion on W´-mass with SM-like couplings of m(W‘)<1.39 TeV, combined with electron channel  combined limit m(W‘)>1.58 TeV • Trigger, reconstruction and selection efficiencies studied in data (up to pT~200 GeV) with tag-and-probe and extrapolated to higher pT • Cosmics are still our only high pt muons... • Main background Standard Model W mu, others contribute <10% in region high MT. Data-driven method for background: using sideband-fit to determine total background in signal region • Check the effect of muon reco, etc • Next year: start dealing with real high pT muons  CHALLENGE, NEED FOR COSMICS??? C. Diez Pardos (CIEMAT)

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