Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

More documents

Recommendations

Info

64 CHAPTER 4: Reconstructing and Selecting Physics EventThe multiplicity of the selected jets per event which already passed the Muon Vetocut, is shown in Figure 4.16. Those events with more than or equal to four selectedjets, are accepted for further analysis.#Events410310tt+jets semi-ele (1019.3 entries)tt+jets other (221.7 entries)t+jets, (all channels) (146.0 entries)Z+jets (1039.5 entries)W+jets (5308.2 entries)Multi-jets (12129.1 entries)210101-1100 1 2 3 4 5 6 7 8 9Nr. of Selected JetsFigure 4.16: Multiplicity of the selected jets per event which pass the Muon Veto cut.At Least One B-Tagged JetAs there are two b jets produced in the final state of the signal topology, it can beused as a discriminator between signal and backgrounds. In order to identify b quarkjets, various b-tagging algorithms have been defined within the CMS collaboration. Asimple one is called the “Track Counting High Efficiency” algorithm [99] which exploitsthe long lifetime of the b mesons produced in the hadronization process of the b quarks.Because of this property, the tracks which are associated to a b quark jet, are usuallydisplaced with respect to the primary vertex.The algorithm to tag b jets starts by selecting good quality tracks and associatingthem to a jet by a matching procedure. In a first step, the track impact parameter IPis computed by the algorithm, which is defined as the distance between the primaryvertex and the linearised track in the point of minimum distance between the trackand the jet axis. This is followed by the calculation of the track impact parametersignificance IP S , which is defined as the impact parameter divided by the resolution ofthe impact parameter σ IP , hence IP S = IPσ IP. In the next step, the tracks associatedto a jet are ordered in decreasing IP S . Finally, the “Track Counting High Efficiency”algorithm returns the IP S assigned to the second track in the list of ordered tracks asthe b-tagging discriminant. The distribution of the b-tag discriminator value returnedby this algorithms, is shown in Figure 4.17. The events with at least four selected jetsare taken into account.Due to the fact that the efficiency of b-tagging is not high, and in order not to loosetoo many signal events, the event is asked to have at least one b-tagged jet amongthe four selected jets. A jet is defined as a b jet if the tagging value returned by the“Track Counting High Efficiency” algorithm exceeds 4. The multiplicity of b-taggedjets among the four leading jets for those events that have at least four selected jets is
CHAPTER 4: Reconstructing and Selecting Physics Event 65#Events310210tt+jets semi-ele (1581.3 entries)tt+jets other (191.9 entries)t+jets, (all channels) (74.2 entries)Z+jets (105.9 entries)W+jets (939.5 entries)Multi-jets (1439.5 entries)101-110-10 -5 0 5 10 15 20 25 30 35 40B-Tag ValueFigure 4.17: The distribution of the b-tagging discriminator of the four leading jets,calculated by the “Track Counting High Efficiency” algorithm, for the events whichhave at least four selected jets.shown in Figure 4.18.#Events310210tt+jets semi-ele (405.9 entries)tt+jets other (50.5 entries)t+jets, (all channels) (19.1 entries)Z+jets (28.1 entries)W+jets (241.0 entries)Multi-jets (379.3 entries)101-110-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Nr. of B-Tagged JetsFigure 4.18: Multiplicity of the b-tagged jets, which is defined as a jet with a b-taggingvalue returned by the “Track Counting High Efficiency” algorithm exceeding 4, for theevents which have at least four selected jets.Most of the W+jets backgrounds as well as the QCD multijet events do not containa b-tagged jet and are suppressed when requiring the event to have at least one b jetcandidate.A summary of all selection cuts and the number of events surviving each step for varioussimulated signal and background samples is listed in Table 4.6.Having applied the above-mentioned selection cuts, one would get a signal overbackground ratio of about 2.5. Hence the final sample of selected events is dominatedby signal events and the purity of selecting the signal events can be further improvedby applying additional cuts which are described in more detail in the next chapter.
Page 1:
Vrije Universiteit BrusselFaculteit
Page 5 and 6:
IntroductionThe Standard Model of p
Page 7 and 8:
Chapter 1The Standard Model of Part
Page 9 and 10:
CHAPTER 1: The Standard Model of Pa
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
Page 17 and 18: CHAPTER 1: The Standard Model of Pa
Page 19 and 20: Chapter 2The CMS experiment at the
Page 21 and 22: CHAPTER 2: The CMS experiment at th
Page 33 and 34: Chapter 3Object ReconstructionThe e
Page 35 and 36: CHAPTER 3: Object Reconstruction 31
Page 51 and 52: Chapter 4Reconstructing and Selecti
Page 53 and 54: CHAPTER 4: Reconstructing and Selec
Page 67: CHAPTER 4: Reconstructing and Selec
Page 71 and 72: Chapter 5Estimating of the Jet Ener
Page 73 and 74: CHAPTER 5: Estimating of the Jet En
Page 91 and 92: 2χ / ndf18.76 / 7p0 9.492e-08 ±9.
Page 119 and 120:
CHAPTER 5: Estimating of the Jet En
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
Chapter 6ConclusionThe Standard Mod
Page 133 and 134:
CHAPTER 6: Conclusion 129The method
Page 135 and 136:
Bibliography[1] M. E. Peskin and D.
Page 137 and 138:
BIBLIOGRAPHY 133[40] The ALICE Coll
Page 139 and 140:
BIBLIOGRAPHY 135[76] The CMS Collab
Page 141 and 142:
SummaryJets appear in the final sta
show all

Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

Create successful ePaper yourself

Delete template?

Save as template?