Jet production cross section and jet properties in pp collisions at 7 TeV centre-of-mass energy with the ATLAS detector

Abstract

Jet cross sections have been measured with the ATLAS detector for the first time in proton-proton collisions at center-of-mass energy of t TeV. The measurement uses an integrated luminosity of 17 nb-1 recorded at the Large Hadron Collider. The anti-kt algorithm is used to identify jets, with jet resolution parameters R=0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidity. Dijet cross sections are presented as functions of dijet mass and angle chi. The experimental results are compared to the expectations based on next-to-leading order QCD. Comment: 3 pages, 4 figures, Proceedings of HCP2010, Toronto

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arXiv:1010.0871v1 [hep-ex] 5 Oct 2010
Jet production cross section and jet properties in pp collisions at
√s = 7 TeV with the ATLAS detector
Zinonos Zinonas
Universit` a di Pisa & INFN Sezione di Pisa, Italia
Jet cross sections have been measured with the ATLAS detector for the first time in proton-proton collisions at
center-of-mass energy of 7 TeV. The measurement uses an integrated luminosity of 17 nb−1recorded at the Large
Hadron Collider. The anti-ktalgorithm is used to identify jets, with jet resolution parameters R = 0.6. The dominant
uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse
momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and
rapidity. Dijet cross sections are presented as functions of dijet mass and angle χ. The experimental results are
compared to the expectations based on next-to-leading order QCD.
1. INTRODUCTION
In proton-proton (pp) collisions produced by the Large Hadron Collider (LHC) at high center-of-mass energy (√s),
the production of jets is the dominant high transverse momentum (pT) process.
Inclusive jet and dijet cross sections have been measured with the ATLAS detector [1] for first time at an energy
more than 3 times higher than that of the measurements done at Tevatron. The measurements presented here are
performed using the data collected between 30 March and 5 June 2010, when the LHC delivered pp collisions at
√s = 7 TeV. The data correspond to an integrated luminosity of 17 ± 2 nb−1and have negligible impact from
pile-up events. The integrated luminosities are calculated run-by-run using the Van der Meer calibration technique
[1].
The measured cross sections extend up to a jet pTof 600 GeV/c and dijet masses of 2 TeV/c2. Leading logarithmic
(LO) parton-shower Monte Carlo (MC) programs provide a reasonable description of the shapes of the measured
cross section distributions and of the energy flow in the proximity of the jets. All measurements are well described
by next-to-leading order (NLO) perturbative calculations corrected for non-perturbative QCD effects.
2. ANALYSIS
Jets are identified using the infrared-safe anti-kT algorithm [1] with size parameter R = 0.6, and using three-
dimensional calorimeter clusters as input with full 4-momenta recombination. Jet energies are calibrated using a
MC-derived pT and η-dependent calibration scheme [1]. Afterwards, jet 4-momenta are corrected for the primary
vertex position and for detector effects back to the hadron level.
Inclusive single-jet double-differential cross sections are measured as a function of the jet pT in slices of absolute
rapidity (|y|). Dijet cross sections are measured as a function of the dijet mass in bins of |y|max= max(|y1|, |y2|)
and of the dijet angle χ = exp|y1− y2| ≃1+cos θ∗
frame.
It is also important that the energy flow around the jet cone is well understood and described by MC models. The
energy and momentum flow within jets can be expressed in terms of the differential jet shape defined as the fraction,
ρ, of the jet momentum contained within a ring of thickness ∆r = 0.1 at a radius r =
jet axis, divided by ∆r. This variable offers a useful indication of the sensitivity of the reconstructed jets to the
hadron fragmentation and underlying event.
Pythia 6.4 event generator, with MSTW L0∗ parton density function and a set of parameters (ATLAS MC09) tuned
to describe the exisiting minimum bias and underlying event data [1], was used to simulate jets in pp collisions at
√s = 7 TeV. These samples are used for the baseline comparisons, the study of the systematic uncertainties and
1−cosθ∗, where θ∗is the polar scattering angle in the dijet center-of-mass
?(∆η)2+ (∆φ)2around the
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for data corrections. The jet algorithm is run on calorimeter clusters assuming that the event vertex is at the origin.
The jet 4-momentum is then recalculated with respect to the z-position of the primary vertex. After calibration, all
events are required to have at least one jet within the kinematic region pT > 60 GeV/c, |y| < 2.8. Any subleading
jets should have pT > 30 GeV/c. Additional quality criteria are also applied to ensure that jets are not produced
by single noisy calorimeter cells nor problematic detector regions [1]. Events are required to have at least one vertex
consistent with the beamspot position, with at least five tracks connected to it, and with |z| < 10 cm. The z
vertex distribution in the MC simulation is reweighted to agree with that in data. Background contributions from
non-pp-collision sources were evaluated using unpaired and empty bunches and found to be negligible [1].
Two types of triggers are used to select events in this analysis. A minimum bias trigger, which requires a single
minimum bias counter over threshold, was operated early in the data-taking period. It was used to trigger approxi-
mately 2% of the integrated luminosity of the data sample analyzed. The lowest threshold jet trigger, uses a 0.4×0.4
window size in η − φ space at the first level of the trigger system and requires a jet with pT > 5 GeV/c at the
electromagnetic scale. Both triggers are found to be fully efficient for selecting events with at least one jet with
pT> 60 GeV/c and |y| < 2.8.
The data correction for all trigger and detector efficiencies and resolutions, other than the energy scale correc-
tion already applied, is performed in a single step using a bin-by-bin migration function evaluated using validated
MC samples. For each measured distribution, final-state-particle cross section using Pythia MC09 is evaluated in
the relevant bins, along with the equivalent distributions as observed in the simulated detector. The ratio of the
distributions provides a correction factor which is then applied to the data as seen in the detector.
The dominant source of systematic uncertainty is from the jet energy response. This scale uncertainty has been
determined to be below 10% over the whole kinematic range, and to be below 7% for central jets with pT> 60 GeV,
neglecting possible topological correlations between jets. This leads to an overall systematic uncertainty on the cross
sections of around 40% [1].
The measured jet cross sections, corrected to particle-level, are then compared to NLO perturbative QCD predic-
tions. The NLOJET++ program [1] is used for all predictions, with APPLgrid [1] being used for efficient evaluation
of uncertainties. Since the NLO calculations predict the value of partonic cross sections, which are not accessible,
they are corrected for the non-perturbative effects of hadronization and underlying event. Pythia MC09 is used to
derive the baseline correction factors, and Pythia Perugia0/Hard/Soft and Herwig+Jimmy [1] are used for system-
atic variations. The main sources of theoretical uncertainties are the choice of renormalization and factorization
scales, the parton distribution functions, αs(MZ) and the modeling of soft QCD effects (underlying event and parton
hadronization processes) [1]. The overall uncertainty is defined by the envelope of maximum variation around the
prediction obtained with Pythia MC09 [1].
3. RESULTS
Figure 1(a) shows the double-differential cross section as a function of pTin several different regions of y. The cross
section extends from pT= 60 GeV/c up to around pT = 600 GeV/c, and falls by more than four orders of magnitude
over this range. The error bars indicate the statistical uncertainty on the measurement, and the gray shaded band
indicates the quadratic sum of the systematic uncertainties, dominated by the jet energy scale uncertainty. The
theory uncertainty shown in orange is the quadratic sum of all uncertainties mentioned above. In all y regions, the
theoretical predictions are consistent with the data.
In Figure 1(b) the double-differential dijet cross section is shown as a function of the dijet mass m12, for different
bins of the maximum y of the two leading jets. The cross section falls rapidly with mass, but extends up to masses
of nearly 2 TeV/c2. Figure 1(c) shows the cross section as a function of the dijet angular variable χ for different
ranges of the dijet mass. Again, the theory is reasonably in agreement with the experimental results.
The jet shapes measured using calorimeter clusters, without applying any corrections for detector effects, are shown
in Figure 1(d). The jets simulated by Pythia 6 are slightly narrower than the jets in the data, while the Herwig 6
+ Jimmy and Herwig++ simulations provide a better description.
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[GeV]
T
p
100 200300400 500 600
dy [pb/GeV]
/ dp
σ
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d
T
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9
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10
Systematic Uncertainties
NLO-pQCD (CTEQ 6.6)+ Non pert. corr.
)
12
10
×
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≤
|
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2.8 (1 )
≤
|
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2.1 < |y
ATLAS Preliminary
=7 TeVs ,
-1
L dt=17 nb
∫
jets, R=0.6
anti-k
[GeV]
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m
2
10
×
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3
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2
[pb/GeV]
max
d|y|
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σ
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10
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10
7
10
9
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=7 TeV =7 TeV=7 TeV =7 TeV=7 TeVsssss , , , , ,
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1e2)
×
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max
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1e0)
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max
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ATLAS Preliminary
χ
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[pb/GeV]
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3
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=7 TeV =7 TeV=7 TeVsss , , ,
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dt=17 nb dt=17 nb dt=17 nb
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Data
Pythia 6
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> 60 GeV , |y| < 2.8
jet
T
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Inclusive jets, Calorimeter
Figure 1:
Right: Dijet double-differential cross section as a function of dijet mass, in the maximum y-bin between the two leading jets.
Bottom Left: Dijet double-differential cross section as a function of angular variable χ in different bins of the dijet mass.
Bottom Right: The uncorrected jet shape as a function of radius r, compared to MC simulations.
Top Left: Inclusive jet double-differential cross section as a function of jet pT in different regions of |y|. Top
4. CONCLUSIONS
Inclusive and dijet cross sections have been measured in pp collisions at√s = 7 TeV using the first data collected
by the ATLAS detector. LO parton-shower MC programs provide a reasonable description of the energy flow around
the jets, and the cross section distributions. The measurements are well described by NLO perturbative QCD
calculations corrected for non-perturbative QCD effects.
Acknowledgments
This work is partlially supported by the European Commission, through the ARTEMIS Research Training Network,
contract number MRTN-CT2006-035657.
References
1 ATLAS Collaboration, Measurement of jet production in proton-proton collisions at 7 TeV center-of-mass energy
with the ATLAS Detector, ATLAS-CONF-2010-050, arXiv:1009.5908v2 [hep-ex] and references therein.
Proceedings of HCP2010 – Toronto