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Jet energy scale and resolution in the CMS
experiment in pp collisions at 8 TeV
To cite this article: V. Khachatryan et al 2017 JINST 12 P02014
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This content was downloaded from IP address 217.21.43.91 on 09/04/2021 at 13:59
2017 JINST 12 P02014
Published by IOP Publishing for Sissa Medialab
Received: July 13, 2016
Revised: January 22, 2017
Accepted: January 29, 2017
Published: February 22, 2017
Jet energy scale and resolution in the CMS experiment in
pp collisions at 8 TeV
The CMS collaboration
E-mail:
cms-publication-committee-chair@cern.ch
Abstract: Improved jet energy scale corrections, based on a data sample corresponding to an
integrated luminosity of 19.7 fb
−1
collected by the CMS experiment in proton-proton collisions at
a center-of-mass energy of 8 TeV, are presented. The corrections as a function of pseudorapidity
η and transverse momentum p
T
are extracted from data and simulated events combining several
channels and methods. They account successively for the effects of pileup, uniformity of the detector
response, and residual data-simulation jet energy scale differences. Further corrections, depending
on the jet flavor and distance parameter (jet size) R, are also presented. The jet energy resolution
is measured in data and simulated events and is studied as a function of pileup, jet size, and jet
flavor. Typical jet energy resolutions at the central rapidities are 15–20% at 30 GeV, about 10% at
100 GeV, and 5% at 1 TeV. The studies exploit events with dijet topology, as well as photon+jet,
Z+jet and multijet events. Several new techniques are used to account for the various sources of
jet energy scale corrections, and a full set of uncertainties, and their cor relations, are provided.The
final uncertainties on the jet energy scale are below 3% across the phase space considered by most
analyses (p
T
> 30 GeV and |η| < 5.0). In the barrel region (|η| < 1.3) an uncertainty below 1% for
p
T
> 30 GeV is reached, when excluding the jet flavor uncertainties, which are provided separately
for different jet flavors. A new benchmark for jet energy scale determination at hadron colliders is
achieved with 0.32% uncertainty for jets with p
T
of the order of 165–330 GeV, and |η| < 0 .8.
Keywords: Large detector-systems performance; Performance of High Energy Physics Detectors
ArXiv ePrint:
1607.03663
© CERN 2017 for the benefit of the CMS collaboration, published under the terms of the
Creative Commons Attribution 3.0 License by IOP Publishing Ltd and Sissa Medialab
srl. Any further distribution of this work must maintain attribution to the author(s) and the published
article’s title, journal citation and DOI.
doi:
10.1088/1748-0221/12/02/P02014
2017 JINST 12 P02014
Contents
1 Introduction
1
2 The CMS detector and event reconstruction 4
2.1 Jet reconstruction 5
3 Event samples and selection criteria 6
3.1 Simulated samples 6
3.2 Data sets and event selection 6
4 Pileup offset corrections 8
4.1 Pileup observables 9
4.2 Pileup mitigation 10
4.3 Hybrid jet area method 12
4.4 Pileup offset correction uncertainties 16
4.5 Summary of pileup offset corrections 18
5 Simulated response corrections 20
5.1 Corrections versus η and p
T
21
5.2 Dependence on the jet size 21
5.3 Detector simulation uncertainties 22
5.4 Jet energy corrections propagation to missing transverse momentum 23
5.5 Summary of simulated response corrections 24
6 Residual corrections for data 25
6.1 Relative η-dependent corrections 26
6.2 Relative correction uncertainties 30
6.3 Absolute corrections 32
6.4 Global fit of absolute corrections 38
6.5 Absolute correction uncertainties 40
6.6 Summary of residual corrections 42
7 Jet flavor corrections 43
7.1 Jet flavor definitions 43
7.2 Simulated flavor corrections 43
7.3 Flavor uncertainties 46
7.4 Z+b balance 49
8 Jet p
T
resolution 50
8.1 Methods 51
8.2 Simulated particle-level resolution 52
8.3 Dijet asymmetry 54
– i –
2017 JINST 12 P02014
8.4 The γ+jet balance 58
9 Systematic uncertainties 60
9.1 Uncertainties in 7 TeV analyses 65
10 The PF jet composition 66
11 Conclusions 69
The CMS collaboration 75
1 Introduction
The state-of-the-art techniques used in the CMS experiment at the CERN LHC for jet energy
scale (JES) and jet energy resolution (JER) calibration are presented, based on a data sample
corresponding to an integrated luminosity of 19.7 fb
−1
collected in proton-proton collisions at a
center-of-mass energy of 8 TeV. Jets are the experimental signatures of energetic quarks and gluons
produced in high-energy processes. Like all experimentally-reconstructed objects, jets need to be
calibrated in order to have the correct energy scale: this is the aim of the jet energy corrections
(JEC). The detailed understanding of both the energy scale and the transverse momentum resolution
of the jets is of crucial importance for many physics analyses, and a leading component of their
associated systematic uncertainties. Improvements made in understanding the JES in the recent
years have resulted in very precise measurements of, e.g., the inclusive jet cross section [
1–5], and
the top quark mass [
6–9]. The JES uncertainties presented here propagate to uncertainties of 2–4%
in the jet cross sections in the central region, and of ±0.35 GeV in the top-quark mass determination.
The results in this paper are reported for jets reconstructed with the particle-flow (PF)
method [
10, 11] using the anti-k
T
algorithm [12] with distance parameter R = 0.5. The jet
energy corrections are calculated using a detailed Monte Carlo (MC) simulation of the detector,
and are then adjusted for data using a combination of several channels and data-driven methods.
The JEC successively correct for the offset energy coming from multiple proton-proton collisions
in the same and adjacent beam crossings (pileup), the detector response to hadrons, and residual
differences between data and MC simulation as a function of the jet pseudorapidity η and transverse
momentum p
T
. The jet p
T
is corrected up to the so-called particle-level jets clustered from stable
(decay length cτ > 1 cm) and visible (excluding neutr inos) final-state particles.
Corrections depending on jet flavor (for quarks: u and d, s, c and b; and for gluons) and jet
distance parameter R are also presented. The uncertainties affecting the JES determination are
discussed, and a full set of uncertainties and their correlations are provided. Figure
1 shows the
jet response at the various stages of JEC for jets (produced in quantum chromodynamics (QCD)
hard-scattering processes) measured at central pseudorapidities (|η| < 1.3): for each bin in p
T, ptcl
,
the jet response is defined as the average value of the ratio of measured jet p
T
to particle-level jet
p
T, ptcl
. The response is shown before any correction, after correcting for the effect of pileup, and
after all stages of corrections, that will be detailed in the following. Distributions corresponding
– 1 –
2017 JINST 12 P02014
(GeV)
T, pt cl
p
30
100
200
1000
2000
Response
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
(8 TeV)
CMS
Simulation
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
(GeV)
T, pt cl
p
30
100
200
1000
2000
Pileup-corrected response
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
(8 TeV)
CMS
Simulation
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
(GeV)
T, pt cl
p
30
100
200
1000
2000
Corrected response
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
(8 TeV)
CMS
Simulation
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
QCD Monte Carlo
R=0.5, PF+CHS
T
Anti-k
| < 1.3η|
= 0)µNo pileup (
< 10µ0 <
< 20µ10 <
< 30µ20 <
< 40µ30 <
Figure 1. Average value of the ratio of measured jet p
T
to particle-level jet p
T, ptcl
in QCD MC simulation, in
bins of p
T, ptcl
, at various stages of JEC: before any corrections (left), after pileup offset corrections (middle),
after all JEC (right). Here µ is the average number of pileup interactions per bunch crossing.
to different average numbers of pileup interactions per bunch crossing (µ) are shown separately, to
display the dependence of the response on the pileup.
The jet p
T
resolution, measured after applying JEC, is extracted in data and simulated events.
It is studied as a function of pileup, jet size R, and jet flavor. The effect of the presence of neutrinos
in the jets is also studied. The typical JER is 15–20% at 30 GeV, about 10% at 100 GeV, and 5% at
1 TeV at central rapidities.
The general principles behind the methods of extraction of the JES, and the reasons why the
JES obtained with the PF algorithm is different from unity, are discussed. The results and methods
are compared to previous CMS studies done for 7 TeV proton-proton collisions [
13]. Several new
techniques are introduced in this paper to account for p
T
-dependent pileup offset, out-of-time (OOT)
pileup, initial- and final-state radiation (ISR+FSR), and b-quark jet (b-jet) flavor response. We also
add the information from multijet balancing [
14] and introduce a new technique that uses it as part
of a global p
T
-dependent fit which constrains the uncertainties by using their correlations between
channels and methods.
Pileup collisions result in unwanted calorimetric energy depositions and extra tracks. The
charged-hadron subtraction (CHS, section
4.2) reduces these effects by removing tracks identified
as originating from pileup vertices. The results in this paper are reported for jets reconstructed with
and without CHS.
The JEC are extracted for jets with p
T
> 10 GeV and |η| < 5 .2, with uncertainties less than
or about 3% over the whole phase space. The minimum JES uncertainty of 0 .32% for jets with
165 < p
T
< 330 GeV and |η| < 0.8, when excluding sample-dependent uncertainties due to jet-
flavor response and time-dependent detector response variations, surpasses the precision of previous
JES measurements at the Tevatron [
15, 16] and the LHC [13, 17].
Outline of the paper and overview of the corrections
The CMS detector and reconstruction algorithms are briefly described in section
2. The data
and MC samples used throughout this document, together with the different selection criteria, are
detailed in section
3.
– 2 –