The jet reconstruction efficiency is determined in the Monte Carlo simulation by counting in how many cases a
calorime-Fig. 71 Fractional JES uncertainties as a function of pjetT for anti-kt
jets with R= 0.6 for the various η regions for the LCW+JES (full line) and the GCW+JES (dashed line) schemes. These are derived from a combination of the in situ techniques which are limited in the num-ber of available events at large pTjet. The fractional JES uncertainty
for EM+JES derived from single hadron response measurements and systematic Monte Carlo simulation variations is overlaid as shaded area for comparison. The η-intercalibration uncertainty is shown as open symbols for|η| > 0.8 for the EM+JES and for |η| > 1.2 for the LCW+JES and GCW+JES schemes
Fig. 72 Calorimeter jet reconstruction efficiency with respect to truth jets (a) and track jets (b, c, d) as a function of the truth jet (a) or the calorimeter jet (b, c, d) pTfor the three calibration schemes: EM+JES (b), GCW+JES (c) and LCW+JES (d). The lower part of the figure (a) shows ratio of the efficiency of the LCW+JES and the GCW+JES
cal-ibration schemes to that of the EM+JES calcal-ibration scheme. Only jets within|η| < 1.9 are considered. The ratio of data to Monte Carlo sim-ulation is also shown in the lower part of the figure for (b), (c) and (d).
The hatched area correspond to the systematic uncertainty obtained by variations in the in situ method
ter jet can be matched to a truth jet. Reconstructed jets are matched to truth jets, if their jet axes are within R < 0.4.
Figure 72a shows the jet reconstruction efficiency for anti-kt jets with R= 0.6 calibrated with the EM+JES, GCW+JES, and LCW+JES calibration schemes as a func-tion of the transverse momentum of the truth jet. The ef-ficiency stays above a value of 99 % for a truth jet trans-verse momentum of 20 GeV. The lower part of the fig-ure shows the ratio of the efficiency in the GCW+JES and LCW+JES calibration schemes to that obtained from the EM+JES scheme. Similar performance is found for all cali-bration schemes.
The small differences at low pTjet might be caused by the slightly better jet energy resolution obtained with the GCW+JES and the LCW+JES calibration schemes as com-pared to the EM+JES scheme.
16.2 Efficiency in situ validation
The ability of the Monte Carlo simulation to correctly repro-duce the jet reconstruction in the data is tested using track jets that provide an independent reference.
A tag-and-probe technique is used as described in the fol-lowing steps:
1. Only track jets with pT>5 GeV and|η| < 1.9 are con-sidered.
2. The track jet with the highest pTin the event is defined as the reference object.
3. The reference object is required to have pT>15 GeV.39
39Reference track jets with pT<15 GeV are not used, since they would result in a sample of biased probe track jets. In this case, mostly
4. The reference track jet is matched to a calorimeter jet with pjetT >7 GeV, if R(tag, calo jet) < 0.6.40
5. The probe track jet must be back-to-back to the reference jet in φ with|φ| ≥ 2.8 radian.
6. Events with additional track jets within |φ| ≥ 2.8 ra-dian are rejected.
7. The calorimeter reconstruction efficiency with respect to track jets is then defined as the fraction of probe track jets matched to a calorimeter jet using R(probe, calo jet) <
R (with R= 0.4 or R = 0.6) with respect to all probe track jets.
The jet reconstruction efficiency is measured in a sample of minimum bias events and is compared to a minimum bias Monte Carlo simulation. Due to the restriction of|η| < 1.9 on track jets, the measurement is only valid for calorimeter jets with|η| < 1.9 + R, where R = 0.4 or R = 0.6.
Figures 72b–d show the measured calorimeter recon-struction efficiency with respect to track jets as a function of the calorimeter jet transverse momentum for anti-kt jets with R= 0.6 calibrated with the EM+JES, GCW+JES, and LCW+JES calibration schemes.41 The reconstruction effi-ciency reaches a plateau close to 100 % at a jet transverse calorimeter momentum of about 25 GeV. The matching ef-ficiency in data (εData) and in Monte Carlo simulation (εMC) shows a good overall agreement except at low pjetT where the efficiency in data is slightly lower than in the Monte Carlo simulation. Similar performance is found for all calibration schemes.
The systematic uncertainties on the jet reconstruction ef-ficiency measured in situ are estimated by varying the fol-lowing event selection requirements for both data and Monte Carlo simulation: the opening angle|φ| between the refer-ence and the probe track jets, the R requirement between the tag track jet and the calorimeter jet and the probe track jet and the calorimeter jet.
The sensitivity in both data and Monte Carlo simu-lation to the azimuthal opening angle as well as to the
R(tag, calo jet) variation is small. However, the efficiency shows a sensitivity with respect to the R(probe jet, calo jet).
events where the probe track jet has fluctuated up in energy (such that it passes the 5 GeV threshold) would be kept. The 15 GeV cut has been determined by measuring the jet reconstruction efficiency relative to track jets as a function of the reference track jet pT. The measured efficiency for low probe track jet pTwas found to be dependent on the reference track jet pTwhen the latter is smaller than 15 GeV. The jet reconstruction efficiency is stable for a reference track jet pTgreater than 15 GeV.
40The less restrictive matching criterion with respect to previous sec-tions is motivated by the lower pT.
41Technically, the efficiency is first measured as a function of the track jet pT. Using the known relation between the average track jet and the average calorimeter jet pT, the track jet pT is then converted to the calorimeter jet pjetT.
The variation of εData/εMCfor these different parameters is shown in Fig.72. At high pjetT the statistical uncertainties after the cut variations lead to an enlarged uncertainty band.
The systematic uncertainty of the in situ determination is larger than the observed shift between data and Monte Carlo simulation. For pjetT <30 GeV a systematic uncertainty of 2 % for jets is assigned.