Performance of electron and photon reconstruction in Run 2 with the CMS experiment

Link to CERN CDS: https://cds.cern.ch/record/2725004

Introduction

• In this note, results obtained with the Legacy 2017 dataset are presented and compared to results using a previous reconstruction and calibration of the same 2017 data, referred to as End-of-Year.
• The results from the Legacy 2017 dataset are also compared to the data recorded in 2016 and 2018, and reconstructed using initial or End-of-Year calibrations.
• The figures presented in this note comprise:
  • Data-to-MC scale factors (SF) for electron identification (ID)
  • Relative ECAL energy resolution
  • Absolute ECAL timing resolution

Outline

• 2017 Legacy vs End-of-Year ReReco
  • Electron identification efficiency and scale factors
• Full Run 2
  • Electron identification efficiency and scale factors
  • Mass resolution with Z→ee events
  • Timing resolution with Z→ee events

Electron Identification Efficiency - event selection

• Data: recorded with single-electron trigger in 2016, 2017 and 2018.
• Simulation: Drell-Yan events simulated using the Madgraph generator with MLM matching and the NNPDF3.1 NLO PDF set. Residual backgrounds have not been estimated.

Tag electron selection

• pT(e) > 35 GeV
• |η(e)| < 2.17
• Tight identification criteria
• Trigger matched

• pT(e) > 10 GeV
• |η(e)| < 2.5
• Very loose pre-selection criteria
• Additional identification requirement for passing-probe electrons

Event selection

• 60 < m(ee) < 120 GeV

Electron MVA Identification 2017 Legacy vs EOY ReReco

• Electron identification efficiency in data (top) and data-to-MC efficiency ratios (bottom), for the multivariate (Boosted Decision Trees) identification working point of 80% signal efficiency, excluding the isolation criteria.
• The efficiency is measured with the tag-and-probe method and shown in one pseudorapidity range as a function of the transverse momentum of the electron.
• An improvement of up to 10%, in the ECAL endcap region, is observed with respect to the End-of-Year calibration.

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Electron Cut Based Identification for full Run 2

• Electron identification efficiency in data (top) and data-to-MC efficiency ratios (bottom), for the cut based identification working point of 90% of signal efficiency.
• The efficiency is measured with the tag-and-probe method and shown in one pseudorapidity range as a function of the transverse momentum of the electron.
• The same identification criteria are applied on the three data taking periods of Run 2. A good level of agreement in terms of data efficiencies and Data/MC scale factors is observed.

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Z→ee invariant mass resolution vs η for full Run 2

• Z→ee events are selected with two electrons having transverse momenta pT>32 and 20 GeV and passing loose cut based ID. The data were collected with a double e/gamma trigger requiring threshold of 30 and 18 GeV, respectively on the pT of the online electrons.
• The electron ECAL energy scale and resolution have been corrected to ensure good data/MC agreement at the Z peak.
• The Z mass peak is fitted with a Breit-Wigner convoluted with a Crystal-Ball function to obtain the mass resolution. The fit range is 80 to 100 GeV.
• The relative resolution of the reconstructed invariant mass of the Z-boson as a function of the pseudo-rapidity (η) of the electron. The event selection is described above.
• The electrons in this figure are low bremsstrahlung where low bremsstrahlung is defined as electrons with high fraction (at least 96%) of shower energy contained within 3x3 crystals around the most energetic one.
• The results for 2016 and 2018 are reconstructed with End-of-Year calibration while for 2017 both Legacy and End-of-Year calibrations are shown.

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ECAL timing resolution with Z→ee events vs energy for full Run 2

• Intrinsic ECAL time resolution measured starting from the time difference between electrons from Z using LHC Run 2 data.

• Study similar to a previous publication where only 2016 and 2017 data were shown in https://cds.cern.ch/record/2682203/files/DP2019_021.pdf
• Z→ee events are selected with two electrons in the ECAL barrel (|η| < 1.4442) having transverse momenta 20<pT<120 GeV and passing very loose cut based ID.
• The two electron energies are required to be within 20%. The Z mass is required to be between 60 and 150 GeV.
• The time of the electron corresponds to the time of the cluster seed crystal. When comparing the time of the two electrons the time-of-flight differences due to the primary vertex positions are corrected.

The resolution is measured as a function of the effective crystal energy given by:

The resolution is estimated from a gaussian fit in bins of Eeff, taking the core of the distribution. The fit is in the range mean±2RMS.

The resolution of time difference between the times of the seed crystals of the clusters of the two electrons from Z->ee decays, as a function of the effective energy in the ECAL Barrel for 2016, Legacy 2017 and 2018 data combined together. A global timing resolution of the order of 200 ps for energies above 40 GeV is measured.

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-- SwagataMukherjee - 2020-07-22