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CMS reconstruction and identification Part II A. Nikitenko

CMS reconstruction and identification Part II A. Nikitenko. Tau jets Missing E T (briefly) Electrons B jets (briefly) Muons (briefly). Tau jet reconstruction and identification. Tau properties relevant to t -jet reco and id and tagging methods. c t = 87.11 m m, m t =1.78 GeV/c 2

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CMS reconstruction and identification Part II A. Nikitenko

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  1. CMS reconstruction and identificationPart IIA. Nikitenko • Tau jets • Missing ET (briefly) • Electrons • B jets (briefly) • Muons (briefly)

  2. Tau jet reconstruction and identification

  3. Tau properties relevant to t-jet reco and id and tagging methods • ct = 87.11 mm, mt=1.78 GeV/c2 • Br(t->hadr)=0.65; one prong hadronic decays – 77% • Tau tagging methods in CMS : • ECAL isolation; used only at HLT so far • Tracker isolation is the basic requirement • Set of other methods after tracker isolation • Tagging with impact parameter • Tagging with flight path (vertex tagging) • Tagging with jet mass using tracker and ECAL

  4. Choice of cone size for t-jet reco with calorimeter Cone of R = 0.4 looks as optimal: > 98 % of t-jet energy content worse resolution for smaller cone

  5. h and phi calorimeter jet resolution:t->hadrons + n Field effect is visible for soft tau-jets

  6. Tau identification with calorimeter:ECAL isolation; used at HLT so far. Cut on e.m. isolation parameter: pisol = SETem(r<0.40) – SETem(r<0.13)

  7. Ecal isolation: signal and jet efficiencies vs cut on ECAL isolation parameter

  8. Tracker isolation • Isolation based on the number of tracks inside the isolation cone (default is 0). • Only good tracks are considered: • Associated to the Primary Vertex • pT of the Leading Track (i.e. highest pT track) must exceed cut pTLT • Leading Track must be found inside the Matching cone • RM : calo jet - leading track matching cone • RS: signal cone around leading track • Ri : isolation cone (around jet axis or leading track) • pTi: cut on pT of tracks in the isolation cone • Dz : cut on the distance between z ip of the leading track and z ip of other tracks counted in isolation

  9. Motivations for the choice of the parameters:(single tau events have been used) matching cone signal cone RM RS DR between the jet axes and the leading track in t jets. It justifies the choice of the matching cone. Max DR between the leading track and all the other tracks in t jets. It justifies the choise of the signal cone.

  10. pTLT > 6 GeV/c, RM = 0.1 Ri = 0.2-0.5, pTi > 1 GeV |Dz| < 2mm - 8 hits per track - norm. c2 < 10 Isolation:tau jets and QCD jets efficiency Single Tau QCD jets

  11. All the following tagging methods are applied to jets which passed the Tracker Isolation:RS = 0.07, Ri=0.4, RM=0.1pTLT>10 GeV/c, pTi>1 GeV/c |Dz| <2mm1 || 3 Tracks inside the signal cone

  12. Tagging with IP significance for one prong (1 track) taus 2D 3D

  13. Tagging with decay length (I) (3 prong MC taus are considered) First pixel layer ~ 40 mm; fake vertices for hadr tau jets

  14. Tagging with decay length (II) Tau jet vs QCD jet efficiency, when the cut on the significance of the decay length is varied Decay length in the Transverse plane must be < 35 mm

  15. pT: 30-50 GeV/c pT: 50-70 GeV/c pT: 80-110 GeV/c pT: 130-150 GeV/c Tau mass tagging (I):track plus ecal clusters with DR(cl-trk) > 0.1

  16. Tau mass tagging (II) The reconstructed mass must be lower than2.5 GeV/c2. The signal efficiency hardly dependens on the pT, Bkg. rejection larger for higher pT jets Bkg. rejection with mass tagging is strongly correlated with ECAL isolation at HLT

  17. Electron rejections: cut on hottest HCAL tower in jet

  18. Tau jet energy scale: Effect of p0s and e/p on the energy scale and resolution m=0.87 s/m=0.21 m=0.89 s/m=0.11 m=0.89 s/m=0.11 m=0.78 s/m=0.20 ETreco in this plot is ET of calorimeter jet with cone 0.4

  19. Tau Jet energy scale ETreco/ETMC ratio as a function of ETMC of t jets before and after corrections - thresholds on calo tower input used: ET = 0.5 GeV, E=0.8 GeV - jet cone size: 0.4

  20. Can we use “t- like” QCD jets from g+jet events for t-jet calibration ? QCD “tau like” jets: tracker and calorimeter isolation; Preliminary Hoped that these two curves will be very similar. Difference still to be understood

  21. Missing ET (MET) reconstruction

  22. Jets + calo towers

  23. Jets only

  24. Type 1 ETmissA. Nikitenko, S. Kunori, R. Kinnunen, CMS Note 2001/040. Used in PTDR 2006 ETx(y)miss = ETx(y)towers + S(ETx(y)jet (corrected) – ETx(y)jet(raw)) PTDR; Inclusive tt~ : improvement in MET resolution and scale Type 1 MET Raw MET Type 1 MET Raw MET

  25. Reminder on A->tt->2j mass reconstruction Collinear approximation : mt << pTt : En1 xt jet1 + En2 xt jet2 = ETXmiss En1 yt jet1 + En2 yt jet2 = ETYmiss xt jet = sin(qt jet) cos(ft jet) yt jet = sin(qt jet) sin(ft jet) Et = Etjet + En Emiss tjet1 tjet2 n1 n2 Negative En solutions due to ETmiss measurement error : ETtjet2 ETt jet1 ETmiss En2 < 0 En1 & En2 < 0 En1 < 0 Higgs boson mass can not be reconstructed if Et jet + En < 0

  26. ETmiss and Mtt reco in A->2t - Type 1 MET corrections, Mtt reconstruction requires En1, En2 > 0 Dashed – raw MET Solid – Type 1 MET Dotted – raw MET for MC MET < 40 GeV mean = 1.01 for Type 1 MET MA=500 GeV/c2 Efficiency of Mtt reco

  27. Electron reconstruction

  28. For single photons or electrons (no conversions, no “brem”) • 94% energy containment in 3x3 crystals • 97% energy containment in 5x5 crystals • Corrections needed for • Local containment (impact position) • Cracks/gaps between ECAL modules • Leakage from front of ECAL : ~ 0.7% variation over barrel ecal

  29. Tracker material and strong magnetic fiels 4T lead to photon conversions, radiation of g’s and spread in f of the energy reached calorimeter=> need to build ECAL clusters and “superclusters” extended in f

  30. Island clustering. Works currently in the ecal endcap Improvement of electron energy reco in ecal Super cluster building: sum up clusters in f Cluster building

  31. Hybrid clustering. Works currently in the ecal barrel. at HLT !

  32. Tuning of clustering parameters for off-line (soft) electrons • Threshold on supercluster seed 4 GeV -> 1 GeV • Increase road in f to recover more brem photons

  33. Electron reco in tracker (I) Search for the tracking seeds in the pixel detector is driven by ecal supercluster energy and position More relaxed off-line cuts to search for the pixel hits comparable with “ecal predictions”

  34. Electron reco in tracker (II) • Trajectory building uses Bethe Heitler modeling of the electron energy losses (different from charged p, m) • Track fit with Gaussian Sum Filter (not Kalman Filter as for p, m)

  35. Electron reco in tracker (III) • With eGSF number of reco hits per track in increased, thus allowing to measure electron momentum at outermost tracker layer and therefore measure the brem. fraction fbrem=(pin-pout)/pin • Electrons can be classified based on fbrem, ESC/pin, number of clusters in supercluster and supercluster-track matching in f pTe = 10 GeV

  36. Electron classification

  37. Electron energy with ECAL: energy corrections

  38. Electron energy with track-cluster combination Use E when E/p > 1 + 2sE/p or for E > 15 GeV when E/p < 1-2sE/p Use p when E < 15 GeV and E/p < 1-2sE/p Use weighted mean of E and p (wi ~ 1/smes) when |E/p-1|<2sE/p

  39. Isolated Electron selection (CMS IN 2005/028) • Require: • A Super Cluster with a (Kalman) track matched to it (Rtrack-SC<0.15) • EtSC >20 GeV and ||<2 • Ehcal/Eecal<0.05 • |track -SCcorr|<0.005 • |trackprop-SC |<0.02 • E/p>0.8 • |1/E-1/p|<0.02 • Isolation • Select all tracks in a 0.2 cone around the Super Cluster with • pt>0.9GeV, |ztrack-zelec|<0.4 and |dtrack-delec|<0.1 • Iso=pttrack/EtSC<0.05 Electron id is still under development: see another (quite similar) set of selections in CMS Note 2006/040

  40. Fake electrons/muons from the W+jet background 1: events where a jet gives a Super Cluster (with Et>20GeV, ||<2) together with a matched track 2: event where a jet gives a GlobalRecMuon with Et>20GeV, ||<2 3: efficiency for electrons is 80 %, for electrons passed HLT – 90 %

  41. B-jet tagging

  42. B tagging algorithms used in CMS • Track counting b tagging with impact parameter (IP) • Probability b-tagging with IP • Combined secondary vertex btagging • Lepton b tagging IP/sIP for N=2 Tracks used for b-tagging algos: - associated with calo jet, DR < 0.4 - at least 8 rec hits in tracker - at least 2 hits in pixel detector - pT > 1 GeV/c - c2/ndf of the track fit < 10 - IPT < 2 mm (remove V0, conversions interactions) b-jet c-jet u,d,s,g jets

  43. Algorithm performance Jets of pT = 50-80 GeV/c and |h| < 1.4 Track counting combined secondary vertex c c g g u,d,s u,d,s

  44. Mistagging efficiency vs pT and h Combined secondary vertex tagging for a fixed b-tagging efficiency 50% c c g g u,d,s u,d,s |h| < 2.4 pT = 50-80 GeV/c

  45. Muon reconstruction

  46. “Work/data flow” for muon reconstruction

  47. Reconstruction efficiency Global reconstruction (muon system plus tracker) Local reconstruction (muon system only)

  48. Muon momentum resolution and muon isolation Muon isolation with calorimeter and tracker Muon p resolution h = 0.5 h = 1.5

  49. THE END

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