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

CHAPTER 4: Reconstructing and Selecting Physics Event 57Exactly One ElectronSince one of the signal signatures is the presence of exactly one isolated electron in thefinal state, cutting on this property of the event will reduce the huge amount of backgroundsspecially QCD multijet events where no prompt electron is usually produced.The selected electron is required to pass some more tighter cuts than the ones alreadyapplied at the pre-selection step. A detailed set of cuts that the selected electron isasked to pass, is listed below.• The selected electron for each event, if there exists one, is required to pass atighter cut on p T namely p T > 30 GeV . Also, it is asked not to be reconstructedin the transition region between the ECAL barrel and the ECAL endcap.As a result, if the η of the supercluster of a candidate electron fulfills1.4442 < |η supercluster | < 1.5660, then it is failed to be labeled as selected.• Another powerfull observable that is widely used to distinguish between promptelectrons coming out of the hard scattering and those which are produced in jetstogether with lots of hadrons, is the isolation variable. The isolation value, asthe name says, is a measure of how much an electron is isolated with respect tothe other activity in the event and defined as the sum of the energy deposits in acone with a specific radius around the electron. Instead of an absolute isolationvalue, a relative isolation variable is mostly used which is defined asRelIso = caloIsoECAL+HCAL + trackIso,ETewhere caloIso ECAL+HCAL and trackIso refer to the absolute observed transversemomentum or energy obtained in a cone of 0.3 around the electron axis in thecalorimeter and the tracker, respectively. The distribution of the relative isolationvariable of all reconstructed electrons for those events that already passed theprimary vertex cut, is plotted in Figure 4.8. The distribution of the relativeisolation variable of the electron with the highest p T in the event is also shownin Figure 4.9. An electron is selected if its relative isolation value is below 0.1.• In order to be selected, the electron candidate is asked to pass the identificationcuts. The parameters used for electron identification aim to make a differencebetween the shape of the showering of a reconstructed electron compared to ajet. They are briefly reviewed below. The cut values are chosen so that an identificationefficiency equal to 70% can be reached [98].– h/e which is defined as the ratio of the energy deposited in the HCAL towerjust behind the electromagnetic seed cluster over the energy of the seed cluster.The selected electron is required to have an h/e value less than 0.025.