The following section describes the data and MC samples used to develop electron identifica- tion criteria.
5.2.1
Electron data samples using the tag-and-probe method (2012)
To study electrons for the purposes of developing an identification menu, an unbiased selection
of electrons is required. Unbiased electrons are obtained usingZ boson events decaying to two
5. Electron Identification 71
in detail in Chapter 6, events are collected using the single-electron trigger. Z boson candi-
date events are found by selecting pairs of same-flavor, opposite-charge electrons: a triggered “tag” electron passing tight identification and isolation requirements, and a second, “probe” reconstructed electron candidate object passing basic track quality criteria cuts. (These events also include the subset in which the probe also passes the tag criteria, and in this case both electrons are counted as probes.) The tag-probe pair is required to be within 10 GeV of the
PDG Z mass. Below a probepT of 20 GeV, the tag is required to originate from the barrel
(|η|<1.37). The set of electron probes is an unbiased, relatively clean source of electrons, at
the reconstructed electron candidate level.
For the construction of PDFs in 2012, events were collected using the e24vhi medium1 and
e60 medium triggers in 20.3 fb−1of 8 TeV data. To further reduce the amount of background in
this sample, a loose calorimeter isolation cut is applied to the probe electrons: PET/ET<0.5
in a cone of ∆R= 0.3. The effect of this cut is small in the high-ETregion, but significantly
reduces background contamination for electrons belowpT= 20 GeV. In principle, this isolation
cut is correlated with other calorimeter variables; however, the cut value is more than 99% efficient in all bins, so the bias on the PDFs is minimal and has been shown to be negligible
using MC. Additional steps to reduce background contamination in low-pTelectron candidates
are discussed in Section 5.3.4.2.
5.2.2
Background data samples using supporting triggers (2012)
The current formulation of the electron likelihood takes the simplistic approach of using a single set of PDFs to represent all sources of electron background, despite the fact that there are multiple types of electron background, each with their own distinct set of PDFs. (In the classification literature this is referred to as a one-against-one approach, as opposed to a one-against-all classifier.) In the one-against-one case, the exact mixture of background
5. Electron Identification 72
types (composition) of the PDFs affects the performance of the likelihood discriminant. As an example, if two background types have distinct sets of discriminating variables with equivalent discriminating power, and if their PDFs are combined in a single-background approach, then the type that is more represented in the PDFs will be rejected at a higher rate than the less represented type.
For the 2012 electron likelihood, the inclusive background is modeled using 20.3 fb−1 of
8 TeV data, collected using the electron and photon supporting triggers e5 etcut, e11 etcut,
g20 etcut, and g24 etcut. The number represents the ET threshold (in GeV); “g” triggers
are photon triggers requiring only a reconstructed trigger cluster, and “e” triggers require a
track matched within a loose ∆R window. To remove contamination by prompt electrons
from Z boson production, a background candidate is rejected if it forms an invariant mass
within 50 GeV of the PDGZmass with any other electron candidate in the event. To remove
prompt electrons from W boson production, events with Emiss
T > 25 GeV are rejected, and
background candidates withMT >40 GeV are rejected. Basic track quality criteria, matching
the criteria required of the signal electron sample, is also applied on the offline reconstructed object. The offline reconstructed electron candidate that matches the trigger object firing
the event within ∆R <0.15 is used as the background candidate. The composition of this
sample is predicted by MC to be roughly 80-85% LF hadrons, 15-20% conversions, and∼1%
background electrons from HF decays.
5.2.3
Signal and background MC samples (2015)
In preparation for the 2015 data taking period, a new electron likelihood was necessary to adapt to the expected changes in conditions, including updates to core reconstruction algo- rithms; a newly-installed IBL and its corresponding tracking improvements; new gas conditions in the TRT; and a 25 ns LHC bunch spacing instead of 50 ns, causing changes to calorimeter
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responses. Since a version of the likelihood was required for the start of data taking, the 2015 electron likelihood would need to be constructed using PDFs obtained from MC.
For this purpose, a sample ofZ→eeMC is used to obtain signal PDFs for electrons with
pT>15 GeV. Electrons are selected using the tag-and-probe method, and by requiring that
the probe electron is matched to a true electron using the MC truth record. To obtain unbiased
electrons below 15 GeV, a sample ofJ/ψ→eeis used as a source, with electrons identified by
finding two reconstructed electrons whose invariant mass satisfies 2.8< mee <3.3 GeV that
both match to true electrons from theJ/ψin the MC truth record. Furthermore, to suppress
highly collinear J/ψ electron pairs in which the electrons interfere with each other’s shower
shape variables, selected electrons must be a distance ∆R >0.1 away from any other electron
candidate.
For background, PDFs are obtained using simulation of 2→2 QCD processes, including
multijet,qg→qγ,qq¯→gγ, electroweak and top production processes. The MC is filtered at
truth level to enrich the sample in electron backgrounds: events are kept in which particles
in the event (excluding neutrinos and muons) deposit>17 GeV of energy into a square area
η×φ= 0.1×0.1, mimicking the highly localized energy deposits characteristic of electrons.
The filter increases the number of high-pT electron backgrounds; background objects with
energy below 17 GeV are also abundant in this sample. The electron background constitutes objects in this sample that are reconstructed as electrons and that are not matched to a true electron in the truth record.
After obtaining signal and background samples from MC, differences between MC and data variable distributions must be corrected. These differences are from imperfect detector modeling in the MC, and are discussed in Section 5.3.4.3.
5. Electron Identification 74