DOI 10.1393/ncc/i2016-16330-x
Colloquia: La Thuile 2016
QCD Measurements at ATLAS
Z. Hubacekon behalf of the ATLAS Collaboration
Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering Prague, Czech Republic
received 26 July 2016
Summary. — This paper presents recent QCD related measurements from the ATLAS Experiment at the LHC at CERN. The results on the total inelastic cross-section, charged particle production, jet production, photon production, and W -,
Z-bosons productions are briefly summarized. The measurments are performed at
different center-of-mass energies√s = 7, 8, and 13 TeV. The measured cross-sections
are generally found to be in agreement with the expectations from the Standard Model within the estimated uncertainties.
1. – Introduction
Generally, the physics results of proton-proton, proton-lead and lead-lead collisions at the Large Hadron Collider (LHC) [1] can be roughly divided into searches for new unknown physics and precision measurements, which improve our understanding of the so-called Standard Model (SM) of particle physics. This letter describes the latest physics results of the ATLAS Experiment [2] related to the production of primary charged par-ticles, hadronic jets, photons, and W -, Z-bosons. Closely related results of diboson production [3] and four-jet production [4] are also discussed within this proceedings series. The summary of cross-section measurements by the ATLAS Collaboration [5] as a function of center-of-mass energy is shown for selected processes in fig. 1.
A measurement of the inelastic cross-section [6] is performed using scintillator coun-ters mounted in front of the forward calorimecoun-ters with 63 μb−1 of data from proton-proton collisions at √s = 13 TeV. The full inelastic cross-section is determined to be
73.1± 0.9 (exp .) ± 6.6 ( lum.) ± 3.8 (extr.) mb, where (exp.) indicates the experimental uncertainties, (lum.) the luminosity uncertainty, and (extr.) the model-dependent un-certainties on the extrapolation from the fiducial region. The measured value as shown in fig. 2 is about one standard deviation below most current theoretical predictions. Inelas-tic interactions can be divided into nondiffractive or diffractive components, depending
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CERN on behalf of the ATLAS Collaboration
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5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 0 10 1 10 2 10 3 10 4 10 5 10 inelastic Pythia8 (LO) W → pp NNLO * γ / Z → pp NNLO t t → pp NNLO+NNLL tq → pp NLO+NNLL H → pp LHC-XS (NNLO) ZZ → pp NNLO inelastic , Nat. Commun. 2, 463 (2011) -1 b μ 7 TeV, 20 , ATLAS-CONF-2015-038 -1 b μ 13 TeV, 63 W → pp , PRD 85, 072004 (2012) -1 7 TeV, 36 pb , arXiv:1603.09222 -1 13 TeV, 81 pb * γ / Z → pp , PRD 85, 072004 (2012) -1 7 TeV, 36 pb , arXiv:1603.09222 -1 13 TeV, 81 pb t t → pp , Eur. Phys. J. C 74:3109 (2014) -1 7 TeV, 4.6 fb , Eur. Phys. J. C 74:3109 (2014) -1 8 TeV, 20.3 fb , ATLAS-CONF-2016-005 -1 13 TeV, 3.2 fb tq → pp , PRD 90, 112006 (2014) -1 7 TeV, 4.6 fb , ATLAS-CONF-2014-007 -1 8 TeV, 20.3 fb , ATLAS-CONF-2015-079 -1 13 TeV, 3.2 fb H → pp , Eur. Phys. J. C76 (2016) -1 7 TeV, 4.5 fb , Eur. Phys. J. C76 (2016) -1 8 TeV, 20.3 fb , ATLAS-CONF-2015-069 -1 13 TeV, 3.2 fb ZZ → pp , JHEP 03, 128 (2013) -1 7 TeV, 4.6 fb , ATLAS-CONF-2013-020 -1 8 TeV, 20.3 fb , PRL 116, 101801 (2016) -1 13 TeV, 3.2 fb Preliminary ATLAS Prediction MeasurementFig. 1. – Summary of cross-section measurements by ATLAS Collaboration [5] presented as a function of center-of-mass energy√s from 7 to 13 TeV for a few selected processes.
on whether or not one or both protons dissociate in the scattering. Nondiffractive com-ponent can be then split into soft and hard part depending on the momentum transfer involved.
2. – Soft QCD results
The strong interaction is described in the framework of quantum chromodynamics (QCD). Inclusive charged-particle measurements in proton-proton collisions provide in-sight into the strong interaction in the low-energy where perturbative QCD (pQCD) methods cannot be used. Interactions at these momentum transfer scales are typically described by QCD-inspired models implemented in the Monte Carlo (MC) event genera-tors. Primary-charged-particle multiplicities measured at√s = 13 TeV [7] as a function
of a track transverse momentum, pT, and pseudorapidity, η, in events with at least one charged track with transverse momentum over 500 MeV are shown in fig. 3 overlaid with predictions of four different MC generators. The average primary-charged-particle multiplicities at central pseudorapidity are compared to other existing measurements at different center-of-mass energies in fig. 4 [8].
3. – Jets and photons
At high momentum transfers, collimated sprays of particles, jets, are produced in the hadron-hadron collisions. At these energy scales, the perturbative QCD can be applied and the jet measurements serve as tests of the higher-order calculations, usually corrected for non-perturbative and electroweak effects. Additionally, the cross-section
measurements constrain the parton distribution functions (PDFs) of the proton and can be used to extract the value of the strong coupling constant at high energy scales.
The unfolded inclusive production cross-section of jets [9] reconstructed with the anti-kTjet clustering algorithm with R = 0.4 at
√
s = 13 TeV in the central part of the
detector with the prediction of next-to-leading order QCD calculation from NLOJET++ using the CT10 PDFs is shown in fig. 5 (left). The dominant systematic uncertainty arises from the jet-energy calibration.
Prompt photon production provides another handle for testing perturbative QCD predictions. Directly produced photons are colorless probes of the hard-scattering process which are directly sensitive to the gluon content of the proton and can thus constrain the PDFs. In the isolated prompt photon cross-section measurement [10], all photons not coming from hadron decays are considered prompt. They include direct photons coming from the hard subprocess and fragmentation photons coming from the fragmentation of a high pTparton. Photons are considered isolated, if the energy carried by partons within a cone centered around the photon is less than a certain criterion (“isolation cut”). The inclusive isolation prompt photon cross-section at √s = 8 TeV in four pseudorapidity
regions is shown in fig. 5 (right) together with the next-to-leading order prediction from JetPhoxusing the CT10 PDFs. The total cross-sections are about 20% higher in data than those predicted by JetPhox, but are consistent within the uncertainties.
4. – W - and Z-boson production
The measurement of electroweak vector-boson production [11] tests our understanding of both QCD and electroweak (EW) processes. Theoretical predictions are available up to next-to-next-to-leading order accuracy in QCD and include EW corrections at NLO
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Fig. 2. – The inelastic cross-section measurement with the ATLAS Detector. The new measure-ment [6] is at√s = 13 TeV.
] -2 [ GeV T p d η / d ch N 2 ) d T pπ 1/(2 ev N 1/ 11 − 10 10 − 10 9 − 10 8 − 10 7 − 10 6 − 10 5 − 10 4 − 10 3 − 10 2 − 10 1 − 10 1 10 2 10 3 10 Data PYTHIA 8 A2 PYTHIA 8 Monash EPOS LHC QGSJET II-04 | < 2.5 η | > 500 MeV, T p 1, ≥ ch n > 300 ps τ = 13 TeV s ATLAS [GeV] T p 1 10 MC / Data 0.5 1 1.5 η / d ch N d⋅ ev N 1/ 1 1.5 2 2.5 3 3.5 4 Data PYTHIA 8 A2 PYTHIA 8 Monash EPOS LHC QGSJET II-04 | < 2.5 η | > 500 MeV, T p 1, ≥ ch n > 300 ps τ = 13 TeV s ATLAS η 2.5 − −2−1.5− 0.51− 0 0.5 1 1.5 2 2.5 MC / Data 0.9 1 1.1 1.2
Fig. 3. – Primary-charged-particle multiplicities as a function of transverse momentum, pT,
(left) and pseudorapidity, η, (right) in events with nch≥ 1, pT> 500 MeV and|η| < 2.5. The
dots represent the data and the curves show the predictions from different MC models. Figures from [7].
accuracy. Only the electron and muon decay channels are considered in the following measurements. For the W -boson measurement, the fiducial phase space is defined by the lepton transverse momentum pl
T > 25 GeV and lepton pseudorapidity |ηl| < 2.5,
the neutrino transverse momentum pνT > 25 GeV and the W -boson transverse mass mT > 50 GeV. Similarly, the fiducial phase space for the Z-boson measurement is
pl
T > 25 GeV, |ηl| < 2.5 and 66 < mll < 116 GeV, where mll is the invariant mass
of the lepton pair. The dominant background contribution to the W -boson channel are the τ -lepton decay mode of the W -bosons, top-quark production and also the Z-boson dileptonic decay (the W -boson event selection requires exactly one identified electron or muon, but the second lepton from the Z-boson decay could be misidentified in the background process). Similarly, in the Z-boson channel the background is dominated by
t¯t production.
The ratios of the predicted to measured fiducial cross-sections for the combined elec-tron and muon channels for both W - and Z-boson production are shown in fig. 6 (left), while the ratios of electron- and muon- channel W - and Z-boson production testing the SM lepton universality are shown in fig. 6 (right).
Events with leptonically decaying Z-boson were also studied in order to improve the modeling of the underlying event [12]. In this measurement at a center-of-mass energy of 7 TeV, several observables sensitive to the underlying event were selected and event properties were analyzed in different ranges of the transverse momentum of the
Z-boson candidate pT(l+l−). At small pT(l+l−) values, events are expected to have a low jet activity, while at higher pT(l+l−), one or more hadronic jets are expected to be reasonably described by the pQCD.
The observables were calculated for primary charged particles with transverse mo-menta pT > 0.5 GeV and pseudorapidities |η| < 2.5. The following observables were
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1 ≥ ch n > 500 MeV, T p 2 ≥ ch n > 100 MeV, T p 6 ≥ ch n > 500 MeV, T p > 30 ps (extrapolated) τ ATLAS Data PYTHIA8 A2 PYTHIA8 Monash EPOS LHC QGSJET II-04Fig. 4. – The average primary-charged-particle multiplicity per unit of pseudorapidity at
η = 0 as a function of the center-of-mass energy [8]. Results are shown for the phase spaces
(pT> 500 MeV, nch≥ 1), (pT > 500 MeV, nch ≥ 6), and (pT > 100 MeV, nch ≥ 2). The data
are compared to various particle-level MC predictions. The results at√s = 8 and 13 TeV are
extrapolated to include strange baryons. The vertical error bars on the data represent the total uncertainty. [GeV] T p 2 10 × 4 2 10 × 5 2 10 × 6 2 10 × 7 2 10 × 8 [pb/GeV] y d T p /dσ 2 d -1 10 1 10 2 10 Preliminary ATLAS -1 13 TeV, 78 pb | < 0.5 y =0.4; | R jets, t anti-k uncertainties Systematic Non-pert. corr. × NLOJET++ (CT10) Data
Relative uncertainty of 9% in the integrated luminosity not included
[GeV] γ T E 30 40 100 200 300 1000 [pb/GeV] γ T / dEσ d -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 0) | < 0.6 (x 10 γ η | ≤ 0 Data 2012 -1 = 8 TeV, 20.2 fb s ATLAS CT10 HOX P ET NLO: J ) -2 | < 1.37 (x 10 γ η | ≤ 0.6 ) -4 | < 1.81 (x 10 γ η | ≤ 1.56 ) -6 | < 2.37 (x 10 γ η | ≤ 1.81
Fig. 5. – Inclusive jet cross-section at 13 TeV [9] (left) and inclusive isolated prompt photon cross-section at 8 TeV [10] (right). Both measurements are compared with the next-to-leading order pQCD predictions using CT10 PDFs.
studied: the charged-particle multiplicity, the scalar sum of transverse momenta of se-lected charged particles, the beam thrust, the transverse thrust, the sphericity and the
F-parameter. Only the charged particle multiplicity and the scalar sum of transverse
mo-menta of selected charged particles are presented in this paper in fig. 7. The distributions are shown for four ranges of the transverse momenta of the Z-boson candidate.
fid) meas σ/ pred σ( 0.85 0.9 0.95 1 1.05 1.1 1.15 ATLAS -1 13 TeV, 81 pb
(inner uncert.: PDF only)
Z ± W + W -W experimental uncertainties ⊕ luminosity experimental uncertainties MMHT14nnlo68CL NNPDF3.0 CT14nnlo ABM12 ATLAS-epWZ12nnlo HERAPDF2.0nnlo -μ + μ → Z σ / -e + e → Z σ = Z R 0.85 0.9 0.95 1 1.05 νμ → ± W σ / ν e → ± W σ = W R 0.95 1 1.05 ATLAS -1 13 TeV, 81 pb 68% CL ellipse area Data PDG average W R PDG average Z R Standard Model
Fig. 6. – Ratio of the predicted to measured fiducial cross-section for the combined electron and muon channels using various PDFs. The inner (outer) band corresponds to the experimental uncertainty without (with) the luminosity uncertainty. The inner error bar of the predictions represents the PDF uncertainty while the outer error bar includes the sum in quadrature of all other systematic uncertainties (left) and ratio of the electron- and muon-channel W - and
Z-boson production fiducial cross-sections, compared to the expected values of the SM and
previous experimental verifications of lepton universality for on-shell W - and Z-bosons, shown as PDG average bands (right). Figures from [11].
Fig. 7. – Distributions of the event-shape variables from [12]: charged-particle multiplicity Nch
(left) and summed transverse momenta ΣpT(right) measured in Z→ e+e−events for the
differ-ent ranges of the transverse momdiffer-entum of the e+e−system, pT(e+e−) (open circles: 0–6 GeV,
open triangles: 6–12 GeV, open boxes: 12–25 GeV, open diamonds: ≥ 25 GeV). Nev denotes
the number of events passing the analysis cuts. The bands show the sum in quadrature of the statistical and all systematic uncertainties.
The distributions are compared to different MC generators Pythia8, Herwig7 and Sherpa in fig. 8. Typically, all three generators provide predictions that are in better agreement with the data at high Z-boson transverse momenta than at low Z-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event. The Sherpa event generator shows larger deviations from the measured observables than Pythia8 and Herwig7.
Fig. 8. – Distribution of charged-particle multiplicity, Nch, for Z→ e+e−with statistical (error
bars) and total systematic (band) uncertainties for the two pT(e+e−) ranges 0–6 GeV (left) and ≥ 25 GeV (right) compared to the predictions from the MC generators Pythia8 (full line),
Sherpa(dashed line), and Herwig7 (dashed-dotted line). The top plots show the observable and the bottom plots show the ratio of the MC simulation to the data. Figures taken from [12].
5. – Summary and conclusions
This paper has presented recent QCD related measurements from the ATLAS Ex-periment at different center-of-mass energies of the LHC at CERN. Monte Carlo gener-ators tuned in lower energy regimes describes the latest 13 TeV data well. Generally, a reasonable agreement between theory and experiment is observed. The latest ATLAS public results can be found at https://twiki.cern.ch/twiki/bin/view/ AtlasPublic/StandardModelPublicResults.
∗ ∗ ∗
The author gratefully acknowledges the financial support from the MSMT of the Czech Republic under the Grant No. RVO 68407700.
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