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

CHAPTER 5: Estimating of the Jet Energy Scale Calibration Factor 117The next observable of the event which is interesting to check, is the distribution ofthe missing transverse energy ETmiss . Since the source of ETmiss in the e+jets t¯t events,is the neutrino which appears in the decay product of the W boson, then it is expectedto obtain a clear peak around 40 GeV, as it is clearly seen from Figure 5.27.#Events605040tt+jets semi-ele (146.9 entries)tt+jets other (23.4 entries)SingleTop+jets (7.5 entries)Z+jets (3.5 entries)W+jets (15.1 entries)QCD (4.6 entries)Data (36.1 pb-1) (176.0 entries)3020100 20 40 60 80 100 120 140 160 180 200missSelected Events E T(GeV)Figure 5.27: The distribution of the missing transverse energy ETmiss . The collisiondata events are overimposed to the simulation for comparison purpose.It can be concluded from Figure 5.27 that the distribution of the missing transverseenergy in the collision data is comparable with the one in the simulated data.Although the four-vector of the W boson can not be fully reconstructed due to theunknown z component of the neutrino momentum, it is still possible to reconstructanother variable that contains the information of the mass of the W boson. Thetransverse mass of the W boson, M T , is defined as√M T = 2p e T Emiss T(1 − cos φ eν ),where e denotes the selected electron and φ eν is the azimuthal angle between theselected electron and the missing transverse energy which is measured in the transverseplane. The distribution of the M T is shown in Figure 5.28.By definition, if the longitudinal component of the momentum of the W boson isequal to zero, which means the W boson is produced in the transverse plane, then thetransverse mass of the W boson, as defined above, becomes the invariant mass of theW boson. In other words, the mass of the W boson is the upper limit for the transversemass variable, M T < 80.4. As it can be seen from Figure 5.28, there is a clear peakaround the mass of the W boson. Also again a good agreement between the collisiondata and the simulation can be observed.There is another important observable, called M3, that can be used to compare thebehaviour of the data versus simulation. The M3 variable can also be used as anestimator for the mass of the reconstructed top quark. Before applying the MVAmethod, which makes it possible to label the selected jets and associate them to thehadronic quarks coming from the hadronic decay of the top quark t → Wb → q¯qb, itis still possible to construct an estimator that can be a measure of the invariant mass