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CMS reconstruction and identification Part II CMS reconstruction and identification Part II A. Nikitenko Tau jetsTau jets Missing E T (briefly)Missing.

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Presentation on theme: "CMS reconstruction and identification Part II CMS reconstruction and identification Part II A. Nikitenko Tau jetsTau jets Missing E T (briefly)Missing."— Presentation transcript:

1 CMS reconstruction and identification Part II CMS reconstruction and identification Part II A. Nikitenko Tau jetsTau jets Missing E T (briefly)Missing E T (briefly) ElectronsElectrons B jets (briefly)B jets (briefly) Muons (briefly)Muons (briefly)

2 Tau jet reconstruction and identification

3 Tau properties relevant to  -jet reco and id and tagging methods c  =  m, m  =1.78 GeV/c 2 Br(  ->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  -jet reco with calorimeter Cone of R = 0.4 looks as optimal: > 98 % of  -jet energy content worse resolution for smaller cone

5  and phi calorimeter jet resolution:  ->hadrons + 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: p isol =  E T em (r<0.40) –  E T em (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 –p T of the Leading Track (i.e. highest p T track) must exceed cut p T LT –Leading Track must be found inside the Matching cone R M : calo jet - leading track matching cone R S : signal cone around leading track R i : isolation cone (around jet axis or leading track) p T i : cut on p T of tracks in the isolation cone  z : 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)  R between the jet axes and the leading track in  jets. It justifies the choice of the matching cone. Max  R between the leading track and all the other tracks in  jets. It justifies the choise of the signal cone. matching cone RMRM signal cone RSRS

10 Isolation: tau jets and QCD jets efficiency p T LT > 6 GeV/c, R M = 0.1 R i = , p T i > 1 GeV |  z| < 2mm - 8 hits per track - norm.  2 < 10 Single Tau QCD jets

11 All the following tagging methods are applied to jets which passed the Tracker Isolation R S = 0.07, R i =0.4, R M =0.1 p T LT >10 GeV/c, p T i >1 GeV/c |  z| 10 GeV/c, p T i >1 GeV/c |  z| <2mm 1 || 3 Tracks inside the signal cone

12 Tagging with IP significance for one prong (1 track) taus 3D2D

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 Tau mass tagging (I): track plus ecal clusters with  R(cl-trk) > 0.1 p T : GeV/c p T : GeV/c p T : GeV/cp T : GeV/c

16 Tau mass tagging (II) The reconstructed mass must be lower than2.5 GeV/c 2. The reconstructed mass must be lower than 2.5 GeV/c 2. The signal efficiency hardly dependens on the p T, Bkg. rejection larger for higher p T jets Bkg. rejection with mass tagging is strongly Bkg. rejection with mass tagging is strongly correlated with ECAL isolation at HLT correlated with ECAL isolation at HLT

17 Electron rejections: cut on hottest HCAL tower in jet

18 Tau jet energy scale: Tau jet energy scale: Effect of  0 s and e/  on the energy scale and resolution  =0.87  =0.21  =0.89  =0.11  =0.89  =0.11  =0.78  =0.20 E T reco in this plot is E T of calorimeter jet with cone 0.4

19 Tau Jet energy scale E T reco /E T MC ratio as a function of E T MC of  jets before and after corrections - thresholds on calo tower input used: E T = 0.5 GeV, E=0.8 GeV - jet cone size: 0.4

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

21 Missing E T (MET) reconstruction

22 Jets + calo towers

23 Jets only

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

25 Reminder on A->  ->2j mass reconstruction 1 2 E miss  jet2  jet1 E 1 x  jet1 + E 2 x  jet2 = E TX miss E 1 y  jet1 + E 2 y  jet2 = E TY miss x  jet = sin(   jet ) cos(   jet ) y  jet = sin(   jet ) sin(   jet ) E  = E  jet + E Negative E solutions due to E T miss measurement error : E T  jet1 E T  jet2 E T miss E 1 < 0 E 1 & E 2 < 0 E 2 < 0 Higgs boson mass can not be reconstructed if E  jet + E < 0 Collinear approximation : m  << p T  

26 E T miss and M  reco in A->2  - Type 1 MET corrections, M  reconstruction requires E 1, E 2 > 0 Efficiency of M  reco Dashed – raw MET Solid – Type 1 MET Dotted – raw MET for MC MET < 40 GeV mean = 1.01 mean = 1.01 for Type 1 MET for Type 1 MET M A =500 GeV/c 2

27 Electron reconstruction

28 For single photons or electrons (no conversions, no “brem”)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)Local containment (impact position) Cracks/gaps between ECAL modulesCracks/gaps between ECAL modules Leakage from front of ECAL : ~ 0.7% variation over barrel ecalLeakage from front of ECAL : ~ 0.7% variation over barrel ecal

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

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

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

32 Tuning of clustering parameters for off-line (soft) electrons Threshold onThreshold on supercluster seed supercluster seed 4 GeV -> 1 GeV 4 GeV -> 1 GeV Increase road in Increase road in  to recover more to recover more brem photons 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 ,  )Trajectory building uses Bethe Heitler modeling of the electron energy losses (different from charged ,  ) Track fit with GaussianTrack fit with Gaussian Sum Filter Sum Filter (not Kalman Filter as for ,  ) (not Kalman Filter as for ,  )

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 f brem =(p in -p out )/p in Electrons can be classified based on f brem, E SC /p in, number of clusters in supercluster and supercluster- track matching in  p T e = 10 GeV

36

37 Electron classification

38 Electron energy with ECAL: energy corrections

39 Electron energy with track-cluster combination Use E when E/p >  E/p or for E > 15 GeV when E/p 15 GeV when E/p < 1-2  E/p Use p when E < 15 GeV and E/p < 1-2  E/p Use weighted mean of E and p (w i ~ 1/  mes ) when |E/p-1|<2  E/p

40 Isolated Electron selection (CMS IN 2005/028) Isolated Electron selection (CMS IN 2005/028) Require : A Super Cluster with a (Kalman) track matched to it (  R track-SC <0.15) E t SC >20 GeV and |  |<2 E hcal /E ecal <0.05 |  track -  SC corr |<0.005 |  track prop -  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 p t >0.9GeV, |z track -z elec |<0.4 and |d track -d elec |<0.1 Iso=  p t track /E t SC <0.05 Electron id is still under development: see another (quite similar) set of selections in CMS Note 2006/040

41 Fake electrons/muons from the W+jet background p t Wgen interval  NNLO X BR 7.6nb6.3nb3.1nb0.5nb N jets generated N (jet  e) electron sel N (jet   ) muon sel.2011 electron reduction factor · · · ·10 -4 muon reduction factor 2.4·10 -5 <1.5· · · : events where a jet gives a Super Cluster (with E t >20GeV, |  |<2) together with a matched track 2 : event where a jet gives a GlobalRecMuon with E t >20GeV, |  |<2 3: efficiency for electrons is 80 %, for electrons passed HLT – 90 %

42 B-jet tagging

43 B tagging algorithms used in CMS Track counting b tagging with impact parameter (IP)Track counting b tagging with impact parameter (IP) Probability b-tagging with IPProbability b-tagging with IP Combined secondary vertex btaggingCombined secondary vertex btagging Lepton b taggingLepton b tagging Tracks used for b-tagging algos: - associated with calo jet,  R < at least 8 rec hits in tracker - at least 2 hits in pixel detector - p T > 1 GeV/c -  2 /ndf of the track fit < 10 - IPT < 2 mm (remove V0, conversions interactions) b-jet c-jet u,d,s,g jets IP/  IP for N=2

44 Algorithm performance c g u,d,s g c Track counting combined secondary vertex Jets of p T = GeV/c and |  | < 1.4

45 Mistagging efficiency vs p T and  |  | < 2.4 p T = GeV/c Combined secondary vertex tagging for a fixed b-tagging efficiency 50% c g u,d,s c g

46 Muon reconstruction

47 “Work/data flow” for muon reconstruction

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

49 Muon momentum resolution and muon isolation Muon p resolution  = 0.5  = 1.5 Muon isolation with calorimeter and tracker

50 THE END

51 BACKUP

52 Sources of the inefficiency of the tracker isolation algorithm for tau jets Efficiency for the cut on the number of tracks inside the signal cone


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