HCAL ECAL
4.6 Electrons
The CMS particle reconstruction process produces electron candidates from pairs of tracks and ECAL superclusters whose position and energies match well.
Effective AreaAeff Isolation Barrel Endcaps Tracker 0.0167 0.032
ECAL 0.183 0.090
HCAL 0.062 0.180
Table 4.1: Effective areas Aeff for the pileup correction of the photon isolation.
Due to their low mass, electrons radiate bremsstrahlung photons in the tracker material significantly more often than other charged particles. The default CTF track reconstruction algoritm assumes a Gaussian distribution for the fractional energy loss in the tracker material — not a suitable assumption for electrons. Their energy loss is better described by a distribution based on the Bethe-Heitler formula [108]. There- fore, the electron reconstruction uses tracks from a special collection produced with the same algorithm as the default tracks but with a different configuration that takes this into account. This is the so-calledGaussian-sum filter (GSF) technique [109,110].
It approximates the correct energy loss distribution by a sum of Gaussian distribu- tions rather than a single Gaussian distribution. It also uses ECAL superclusters withET above a certain threshold to seed the tracks. Figures 4.10 and 4.11 show the electron reconstruction efficiency for the 2011A run period in the barrel and endcaps, respectively. Similarly, Figures 4.12 and 4.13 show it for the 2011B run period.
We use electron identification variables designed to select highly energetic prompt electrons from Z decays with high efficiency, and simultaneously suppresses back- ground electron candidates due to prompt charged hadrons, non-prompt electrons from early photon conversions, non-prompt electrons from hadron decays possibly embedded in jets, and random coincidences of unrelated tracks and ECAL deposits.
The identification variables can be organized into four groups: (i) ECAL superclus- ter variables, (ii) track-cluster compatibility variables, and (iii) conversion rejection variables [111].
Similarly to photons, for electron identification, we use the following variables defined above for ECAL superclusters in Section 4.4:
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011A, ECAL Barrel CMS Preliminary 1
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011A, ECAL Barrel CMS Simulation 1
Figure 4.10: Electron reconstruction efficiency in the barrel for the 2011A run pe- riod [107]. Top: data, bottom: simulation.
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011A, ECAL Endcap CMS Preliminary 1
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011B, ECAL Endcap CMS Simulation 1
Figure 4.11: Electron reconstruction efficiency in the endcaps for the 2011A run period [107]. Top: data, bottom: simulation.
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011B, ECAL Barrel CMS Preliminary 1
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011B, ECAL Barrel CMS Simulation 1
Figure 4.12: Electron reconstruction efficiency in the barrel for the 2011B run pe- riod [107]. Top: data, bottom: simulation.
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011B, ECAL Endcap CMS Preliminary 1
Vertex Multiplicity
2 4 6 8 10 12
(GeV)
TElectron p
20 30 40 50 60 70 80 90 100
0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 Run 2011B, ECAL Endcap CMS Simulation 1
Figure 4.13: Electron reconstruction efficiency in the endcaps for the 2011B run period [107]. Top: data, bottom: simulation.
Shower width σiηiη, and
Hadronic-over-electromagnetic ratio H/E.
These variables are useful for suppressing background due to hadrons and electrons embedded in jets.
We use the following electron identification variables describing the spatial com- patibility of the GSF track and the matching supercluster:
Track-cluster compatibility ∆ηin, the difference in theη coordinates between the supercluster position and the expected electron incidence extrapolated from the innermost track momentum measurement.
Track-cluster compatibility ∆φin, the difference in theφcoordinates between the supercluster position and the expected electron incidence extrapolated from the innermost track momentum measurement.
These variables are useful in suppressing background due to random coincidences of unrelated tracks and ECAL deposits.
We use the following electron identification variables to suppress electrons origi- nating from early conversions of primary prompt photons:
Number of missing hits nmiss, the number of missing hits before the first valid hit of the electron track.
Conversion opening angle cot ∆θ, the cotangent of the opening angle between the electron track and a track corresponding to a potential conversion partner of opposite charge.
Conversion track distance dconv, the transverse distance between the electron track and a track corresponding to a potential conversion partner at the point were the two tracks become parallel.
In addition to these electron identification variables, we use the following isolation variables to further suppress electrons and background from jets and hadron decays:
Electron tracker isolation ITRKe , the scalar sum of the transverse momenta of tracks originating from the same primary vertex, withpT >0.7 GeV, within an annulus of 0.015<∆R <0.3 around the electron momentum direction, exclud- ing a rectangular region of 0.03×0.3 in ∆η×∆φ centered on the electron to protect the isolation sum from the inclusion of the electron track and conversion tracks of eventual bremsstrahlung photons [112, 113].
Electron ECAL isolation IECALe , the scalar sum of transverse energy of ECAL crystal deposits within a 3∆η0 <∆R <0.3 cone around the supercluster ECAL location, excluding a rectangular strip of 3∆η0×0.3 in ∆η×∆φto protect the isolation sum from the inclusion of energy deposits due bremsstrahlung showers and secondary conversions. Here, ∆η0 is the size of a single crystal in units of pseudorapidityη. In the ECAL barrel, only deposits with energies E >80 MeV are included in the sum. In the ECAL endcaps, only deposits with transverse energiesET >100 MeV are included [114, 115].
Electron HCAL isolation IHCALe , the scalar sum of the transverse energy of HCAL tower deposits within a solid cone of ∆R < 0.3 around the ECAL supercluster location.
Electron combined relative isolation Icombrel, e = (ITRKe +IECALe +IHCALe )/pT, the sum of the tracker, ECAL and HCAL isolations divided by the electron trans- verse momentum.
Similarly to photons, to minimize the impact of the pileup energy deposits on the isolation, we correct the combined isolation by subtracting the average energy deposited by the pileup using eq. (4.5). Here, the effective area is given byAeff =πR2 with R= 0.3.