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

Link to CERN CDS:

  • 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
  • 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
Probe electron selection
  • 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.

PDF Version

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.

PDF Version

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.

PDF Version

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
  • 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 timing resolution is given by:


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.

PDF Version

-- SwagataMukherjee - 2020-07-22

Topic attachments
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PDFpdf Picture1.pdf r1 manage 40.8 K 2020-07-22 - 12:04 LiviaSoffi  
PNGpng Picture1.png r1 manage 156.7 K 2020-07-22 - 12:04 LiviaSoffi  
PDFpdf SF_comparison_3YEARS.pdf r1 manage 17.6 K 2020-07-22 - 12:05 LiviaSoffi  
PNGpng SF_comparison_3YEARS.png r1 manage 28.0 K 2020-07-22 - 12:05 LiviaSoffi  
PNGpng SF_comparison_EOY_vs_UL.png r1 manage 24.3 K 2020-07-22 - 12:05 LiviaSoffi  
PDFpdf SF_comparison_EOY_vs_UL_.pdf r1 manage 16.5 K 2020-07-22 - 12:05 LiviaSoffi  
PDFpdf Timing.pdf r1 manage 43.2 K 2020-07-22 - 12:15 LiviaSoffi  
PNGpng Timing.png r1 manage 199.1 K 2020-07-22 - 12:15 LiviaSoffi  
PNGpng effectiveEnergy.png r1 manage 26.5 K 2020-07-22 - 11:37 SwagataMukherjee  
PNGpng timeResolution.png r1 manage 30.4 K 2020-07-22 - 11:36 SwagataMukherjee  
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