Model Independent Search for Deviations from the Standard Model ...

4.3. Object Identification and Selection Cuts 39
Figure 4.1: Fake probability of track matching cut vs MET for (a) Central Calorimeter (b) Central
Calorimeter with spatial requirement only (no E T /p T ), taken from [29]
Besides its energy deposition, electromagnetic showers can be distinguished from hadronic
showers by comparing the longitudinal and lateral shower shape. Hadronic showers are
much broader and travel a long way in the calorimeter until they are completely absorbed
by the detector material. In order to quantify this, seven correlated observables are used
for shower shape analyses (see [30]): The four EM energy fractions in each layer, the total
EM energy, vertex z-position, and transverse shower width in ϕ. The covariance matrix
is calculated for each tower in η using Monte Carlo electrons. With H, the inverse of this
covariance matrix, a χ is determined which is a measure of how similar the shower is to
an electron shower.
2
This analysis focuses on isolated leptons in order to suppress QCD-background. Electrons
and muons are also the decay products of, for example, B-mesons in a hadron jet, or jets
can be misidentied as electrons. Leptons from QCD-events tend to be near the energy
deposit of the hadron jet, so the cluster isolation is an important factor:
f iso = E tot(R < 0.4) − E EM (R < 0.2)
E EM (R < 0.2)
< 0.15 . (4.2)
Here a second conus of R = 0.2 combining clusters is used. This means that only a small
fraction of additional energy is allowed outside the R = 0.2-cone of the electromagnetic
calorimeter possessing most of the energy from the primary EM shower. A nearby hadron
jet would lead to a much greater energy deposit in the larger cone, and the EM candidate
would not pass the isolation cut.
For electrons and positrons, an associated track candidate is required in order to separate
them from photons or QCD contamination. In the central region the trackmatch is dened
using the χ 2 [31]:
( ) δϕ 2 ( ) (
δz
2 ET
) 2
χ 2 p
= + + T
− 1
.
σ ϕ σ z σ ET /p T
(4.3)
In the above expression, δϕ and δz are the dierences between the extrapolated track
position and the EM cluster position at the third layer of the calorimeter (nest segmen-