ATLAS-CONF-2011-001 03February2011
ATLAS NOTE
ATLAS-CONF-2011-001 February 3, 2011
Measurement of the production cross section for Z/γ
∗in association with jets in pp collisions at √
s = 7 TeV with the ATLAS Detector
ATLAS Collaboration
Abstract
We report preliminary results on the production of jets of particles in association with a Z/γ∗ boson in the final state, in proton-proton collisions at√
s=7 TeV with the ATLAS detector, based on an integrated luminosity of 1.3 pb−1. Inclusive jet cross sections in Z/γ∗ events, with Z/γ∗ decaying into electron or muon pairs, are measured as a function of jet transverse momentum and jet multiplicity, for jets with transverse momentum pjetT >20 GeV and jet pseudorapidity in the region|ηjet|<2.8. The measurements are compared to next-to- leading order perturbative QCD calculations, and to predictions from different Monte Carlo generators implementing leading-order matrix elements supplemented by parton showers.
1 Introduction
The study of the production of jets of particles in association with a Z/γ∗ boson in proton-proton col- lisions at√
s=7 TeV provides a stringent test of perturbative quantum chromodynamics (pQCD). The proper understanding of events containing QCD jets in association with a Z/γ∗ boson in the Standard Model (SM) is a fundamental element of the LHC physics program of searches for new physics, for which events with such final states constitute irreducible backgrounds. These SM background contribu- tions are estimated using Monte Carlo (MC) predictions that include leading-order (LO) matrix elements supplemented by parton showers, which a priori are affected by large scale uncertainties and need to be tuned and validated using data.
This note presents measurements of jet production in events with a Z/γ∗ boson in the final state, using the first 1.3 pb−1 of data collected by the ATLAS experiment. Events are selected with a Z/γ∗ decaying into electrons or muons, and the measurements are corrected for detector effects. Inclusive jet differential cross sections as a function of jet transverse momentum1), pjetT, and total cross sections as a function of jet multiplicity, Njet, are measured in well-defined kinematic regions for the leptons and jets in the final state. The data are compared to next-to-leading order (NLO) pQCD predictions [1], including non-perturbative contributions, and to predictions from several MC programs.
2 The ATLAS detector
The ATLAS detector [2] covers almost the whole solid angle around the collision point with layers of tracking detectors, calorimeters and muon chambers. The ATLAS inner detector (ID) has full coverage in φ and covers the pseudorapidity range |η|<2.5. It consists of a silicon pixel detector, a silicon microstrip detector (SCT), and a transition radiation tracker (TRT) all immersed in a 2 Tesla magnetic field.
High-granularity liquid-argon (LAr) electromagnetic sampling calorimeters, with excellent energy and position resolution, cover the pseudorapidity range|η|<3.2. The hadronic calorimetry in the range
|η|< 1.7 is provided by a scintillator-tile calorimeter, which is separated into a large barrel and two smaller extended barrel cylinders, one on either side of the central barrel. In the end caps (|η|>1.5), LAr hadronic calorimeters match the outer|η|limits of the end cap electromagnetic calorimeters. The LAr forward calorimeters provide both electromagnetic and hadronic energy measurements, and extend the coverage to|η|<4.9.
The muon spectrometer is based on the magnetic deflection of muon tracks in the large superconduct- ing air-core toroid magnets, instrumented with separate trigger and high-precision tracking chambers. A system of three large air-core toroids, a barrel and two end caps, generates the magnetic field for the muon spectrometer in the pseudorapidity range of|η|<2.7. Over most of theη-range, a precision mea- surement of the track coordinates in the principal bending direction of the magnetic field is provided by Monitored Drift Tubes. At large pseudorapidities, Cathode Strip Chambers with higher granularity are used in the innermost plane over 2.0<|η|<2.7. The muon trigger system, which covers the pseudo- rapidity range|η|<2.4, consists of Resistive Plate Chambers in the barrel (|η|<1.05) and Thin Gap Chambers in the end cap regions (1.05<|η|<2.4), with a small overlap in the|η|=1.05 region.
1)The ATLAS reference system is a Cartesian right-handed coordinate system, with the nominal collision point at the origin.
The anti-clockwise beam direction defines the positive z-axis, while the positive x-axis is defined as pointing from the collision point to the centre of the LHC ring and the positive y-axis points upwards. The azimuthal angleφis measured around the beam axis, and the polar angleθis measured with respect to the z-axis. The pseudorapidity is defined asη=−ln(tan(θ/2)). The rapidity is defined as y=0.5×ln[(E+pz)/(E−pz)], where E denotes the energy and pzis the component of the momentum along the beam direction.
Physics process Generator σ×Br (nb) Z/γ∗(→ll)+jets, l=e,µ,τ(mll>40 GeV, 0≤Nparton≤5) ALPGEN 1.07 (NNLO) Z/γ∗(→ll)+jets, l=e,µ (mll>60 GeV, 0≤Nparton≤4) SHERPA 0.99 (NNLO) Z/γ∗(→ll)+jets, l=e,µ (mll>40 GeV, ˆpt>10 GeV) PYTHIA 0.47 (LO) Z/γ∗(→ll)+jets, l=e,µ (mll>40 GeV, ˆpt>10 GeV) HERWIG+JIMMY 0.37 (LO)
W(→lνl)+jets, l=e,µ ALPGEN 10.46 (NNLO)
t ¯t (lepton + X final state) MC@NLO 0.16 (NLO)
WW+W Z+ZZ (2 leptons + X final state, 0≤Nparton≤3) ALPGEN 0.007 (NLO) Dijets (inclusive jets, electron filter ET>17 GeV) PYTHIA 97700 (LO) Dijets (b¯b+c ¯c, muon filter pT>15 GeV) PYTHIA 102.3 (LO) Table 1: Details of MC samples employed in the analyses. The third column tabulates cross section times branching ratio theoretical values used to normalize the different MC samples (see Section 3). The Z/γ∗+jets samples generated with ALPGEN and SHERPA are normalized using next-to-next-to-leading order pQCD predictions for inclusive Z/γ∗ production. PYTHIA and HERWIG+JIMMY Z/γ∗+jets samples are generated using leading-order pQCD matrix elements for 2→2 processes (q ¯q→Z/γ∗g and qg→Z/γ∗q) with a minimum ˆpt, where ˆpt is defined for two-body reactions in terms of Mandelstam variables as ˆpt=p
ˆt·u/ˆ s.ˆ
3 Monte Carlo simulation
Monte Carlo event samples are used to compute detector acceptance and reconstruction efficiencies, de- termine background contributions, unfold the measurements for detector effects, and estimate systematic uncertainties on the final results. Table 1 presents the list of MC samples employed.
Samples of simulated Z/γ∗(→e+e−)+jets and Z/γ∗(→µ+µ−)+jets events are generated using ALP- GEN v2.13 [3] interfaced to HERWIG v6.510 [4] for parton shower and fragmentation into particles, and to JIMMY v4.31 [5] to model underlying event (UE) contributions. Similar samples are generated using SHERPA [6]. In both cases, CTEQ61L [7] parton density functions (PDFs) are employed, and the samples are normalized to next-to-next-to-leading order (NNLO) pQCD inclusive Drell-Yan predic- tions as determined by the FEWZ [8] program using MSTW2008 PDFs. In addition, Z/γ∗+jets samples (q ¯q→Z/γ∗g and qg→Z/γ∗q processes) are produced using PYTHIA v6.421 [9] and HERWIG plus JIMMY with MRST2007LO∗ [10] PDFs. In the case of ALPGEN and PYTHIA samples, events are generated with the ATLAS-MC09 [11] tuned set of parameters to control underlying event (UE) contri- butions in the final state. PYTHIA samples with different sets of UE-related parameters (DW [12] and AMBT1 [13]), modified levels of initial- and final-state gluon radiation, or with UE contributions turned off, are also considered.
Background samples from W +jets and Z/γ∗(→τ+τ−)+jets final states, and dibosons (WW , W Z, ZZ) processes are generated using ALPGEN normalized to NNLO and NLO pQCD predictions, respec- tively. Simulated top-quark pair production samples are generated using MC@NLO [14] and CTEQ6.6 PDFs. Finally, dijet background contributions are simulated using PYTHIA-MC09 QCD samples with MRST2007LO∗PDFs, filtered at generator level to select events with the potential to give the signature of an electron with ET>17 GeV, or containing a muon with transverse momentum above 15 GeV in the final state. In the electron case, the dijet MC sample receives an additional×0.41 normalization factor, as determined from data using a W →eνcontrol sample with a loose electron selection [15].
The MC generated samples are passed through a full simulation [16] of the ATLAS detector and trigger, based on GEANT4 [17]. The simulated events are then reconstructed and analyzed with the same analysis chain as for the data, and the same trigger and event selection criteria.
4 Event selection
The data samples considered in this note were collected with ATLAS tracking detectors, calorimeters, muon chambers, and magnetic fields fully operational, and correspond to a total integrated luminosity of about 1.3 pb−1.
In the case of the Z/γ∗(→e+e−)analysis, events are selected online using a first-level trigger logic that requires the presence of at least one electromagnetic cluster in the calorimeter with transverse energy above 14 GeV in the region|η|<2.5. The events are then selected to have exactly two opposite charge reconstructed electrons2)with transverse energy ETe >20 GeV, pseudorapidity in the range|ηe|<2.47 (where the transition region between calorimeter sections 1.37<|ηe|<1.52 has been excluded), and a dilepton invariant mass in the range 71<Me+e−<111 GeV.
The Z/γ∗(→µ+µ−)sample is collected online using a first-level trigger logic that requires the pres- ence of a muon candidate reconstructed in the muon spectrometer consistent with having originated from the interaction region, with pT>10 GeV and|η|<2.4. The muon candidates are associated with track segments reconstructed in the inner detectors which, combined with the muon spectrometer information, define the final muon track. Combined muon tracks with pµT >20 GeV and|ηµ|<2.4 are selected. A number of quality requirements are applied to the muon candidates: the associated inner detector track segment is required to have at least 2, 6, and 1 hits in the pixel, SCT and TRT detectors, respectively, where the TRT requirement is only applied for tracks within|η|<2.0; the difference between the track pT, as determined using muon stations or inner detectors separately, pMST and pIDT respectively, is required to fulfill|pMST −pIDT |<0.5pIDT and pMST >10 GeV ; the muon impact parameters with respect to the re- constructed primary vertex (see below) are required to be<0.1 mm and<10 mm in the r−φ and r−z planes, respectively; the scalar sum of the transverse momenta of the tracks in a cone (η−φ space) of radius 0.2 around the muon candidate is required to be less than 1.8 GeV. Finally, events are selected with exactly two opposite-charge muons and an invariant mass 71<Mµ+µ−<111 GeV.
In both analyses, events are required to have a reconstructed primary vertex with z-position within 15 cm of the nominal interaction point and with at least 3 tracks associated to it, which suppresses beam- related background contributions and cosmic rays. The final selected dilepton samples contain a total of 316 and 429 events for the electron and muon channels, respectively.
5 Jet reconstruction
Jets are defined using the anti-kt jet algorithm [18] with the distance parameter set to R=0.4. Energy depositions in calorimeter clusters are employed as input to the jet algorithm in data and MC simulated events 3). The same algorithm is applied to final state particles in the MC generated events to define jets at the particle level4). The measured jet angular variables,ηjet andφjet are reconstructed with no significant shift and a resolution better than 0.05, which improves as the jet transverse momentum, pjetT , increases. The measured jet pjetT is corrected to the true jet energy [19] using an average correction, computed as a function of jet transverse momentum and pseudorapidity, and extracted from inclusive jet MC samples. In this analysis, events are required to have at least one jet with corrected transverse momentum pjetT >20 GeV and pseudorapidity|ηjet|<2.8, and well separated from the final state leptons from the Z/γ∗ decay. Jets within a cone of radius 0.5 around any selected lepton are not considered.
Finally, the contribution from jets originating from additional proton-proton interactions in the same bunch crossing (pile-up) in data is suppressed using the tracking information within the jets. A track
2)For a detailed description of the electron selection requirements see [15].
3)The jet reconstruction uses(x,y,z) = (0,0,0)as a reference to calculate the direction and pTof the input jet constituents.
4)The final state in the MC generators is defined using all particles with lifetime above 10−11s, after excluding the muons from the Z/γ∗decay.
is associated with a jet if it lies inside a cone of radius 0.4 around the jet direction. For each jet, the jet vertex fraction (JVF) is defined as the ratio of the scalar sum of the pT of the tracks inside a jet and pointing to the primary vertex, and the total scalar sum of the tracks associated to the jet. Jets with a JVF value below 0.75 are rejected if they are within the pseudorapidity region covered by the ATLAS ID and have tracks associated. For the data sample considered in this analysis, the JVF requirement removes up to 4% of the jets at low pjetT and translates into a net 1.5% to 3% reduction of events as the jet multiplicity increases. The final sample for Z/γ∗(→e+e−)+jets contains 82, 26, 9, and 2 events with at least one, two, three, and four jets in the final state, respectively. Similarly, the Z/γ∗(→µ+µ−)+jets sample contains 110, 31, 8, and 2 events with at least one, two, three, and four jets in the final state, respectively.
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Figure 1: Measured dilepton invariant mass in Z/γ∗(→e+e−)(left) and Z/γ∗(→µ+µ−)(right) events with at least one jet with pjetT >20 GeV and|ηjet|<2.8 in the final state. The data are shown in a wider dilepton mass region than the one selected, and compared to MC predictions for signal (ALPGEN) and background processes.
6 Lepton reconstruction
Due to the limited statistics of the data sample, reliable estimates from data of the trigger and the offline reconstruction efficiencies for the leptons are not always possible. The estimates are therefore based on MC simulated samples, validated with data.
For the Z/γ∗(→e+e−)channel, the trigger and offline electron reconstruction efficiencies for single electrons are estimated using W(→eν)events [20] in data and compared to MC predictions, indicating a good agreement between data and simulation. In the kinematic range considered in the analysis, the trigger and offline reconstruction efficiencies per electron are above 99% and 96%, respectively, and do not present any significant dependence on the jet multiplicity or on the electron-jet separation in the final state.
In the Z/γ∗(→µ+µ−)analysis, the trigger and reconstruction efficiencies for muons are estimated in situ using the data. The measured single muon trigger efficiency varies between 80% in the barrel (|ηµ|<1.05) and 86% in the end caps (1.05<|ηµ|<2.4), limited mainly by the trigger chamber
acceptance. The offline muon reconstruction efficiency is about 87% and approximately independent of pTµ,ηµ, and the jet kinematics. The MC simulation overestimates the trigger efficiency for forward muons (|ηµ|>1.05) by approximately 10% compared to that in data, and correction factors are applied in the analysis to take it into account. Similarly, the MC simulation overestimates the offline muon reconstruction efficiency by approximately 7%5). In this case, the difference between data and MC efficiencies is conservatively taken as one of the systematic uncertainties.
7 Background estimation
The background contribution to Z/γ∗(→e+e−)+jets and Z/γ∗(→µ+µ−)+jets analyses from SM pro- cesses is estimated using MC simulated samples, as discussed in Section 3. As a cross-check, an attempt was made to estimate the dijet background from data, and the results were broadly consistent with the MC predictions. A conservative 100% uncertainty on the predicted jet background is adopted in each analysis as part of the systematic uncertainties (see below). In the electron channel, the total background increases from 5% to 11% as the jet multiplicity increases, and it is dominated by jet processes, followed by contributions from t ¯t and diboson production at large jet multiplicities. In the muon channel, the SM background contribution, as estimated from simulation, increases from 1% to 5% as the jet multiplic- ity increases, dominated by t ¯t and diboson processes. Figure 1 shows the measured dilepton invariant mass for both electron and muon channels compared to MC predictions for signal and background, in events with at least one jet in the final state. The measured uncorrected inclusive jet multiplicity in Z/γ∗(→e+e−)events is presented in Fig. 2, together with the uncorrected inclusive jet pjetT spectrum, the pjetT distribution of the leading jet in events with at least one jet, and the pjetT distribution of the second-leading jet in events with at least two jets in the final state. Similarly, the uncorrected distribu- tions in Z/γ∗(→µ+µ−)events are shown in Fig. 3. The data yields are reasonably well described by the simulation in all cases.
8 Unfolding and systematic uncertainties
The measurements are unfolded for detector effects back to the particle level using a bin-by-bin unfold- ing procedure, based on MC simulated samples, that corrects for acceptance and resolution effects and accounts for the efficiency of the Z/γ∗selection. At the particle level, the lepton kinematics in the MC generated samples is defined such that it includes the contributions from the radiated photons within a cone of radius 0.1 around the lepton direction. The ALPGEN samples for Z/γ∗+jets processes provide a satisfactory description of both lepton and jet distributions in data and are employed to compute the unfolding factors. For each observable, the bin-by-bin unfolding factors are defined as the ratio between the simulated distribution, after all selection criteria are applied, and the corresponding distribution at the particle level defined in a limited fiducial region and kinematics for the generated leptons and jets (see Sections 4 and 5). The unfolding factors multiply the measured uncorrected distributions to obtain the final result. In the case of the inclusive jet multiplicity, the unfolding factors are about 1.4 for the elec- tron channel and 1.2 for the muon channel, and show a rather mild dependence with Njet. Similarly, the unfolding factors applied to the measured pjetT distributions increase from 1.38 to 1.45 as pjetT increases in the electron channel, and from 1.18 to 1.25 in the muon channel.
A study of systematic uncertainties on the measured cross sections is carried out (see Table 2). Fig- ures 4 and 5 show, separately for electron and muon channels, the contribution of the different sources of
5)This is mainly attributed to details of the alignment of the muon chambers in data, that are not fully implemented in the current MC simulation.
systematic uncertainty to two of the measurements: the inclusive cross section as a function of Njetand the inclusive jet cross section as a function of pjetT , in events with at least one jet in the final state.
• The measured jet energies are varied between 7% and 8%, depending on pjetT andηjet, to account for the absolute jet energy scale (JES) uncertainty, as determined in inclusive jet studies [19].
An additional 5% uncertainty on the measured jet energies, independent of pjetT , is considered to account for the different quark- and gluon-jet relative population in jet and Z/γ∗+jets final states, leading to a different average calorimeter response. This results in an uncertainty on the measured cross sections that increases from 10% to 20% as Njetand pjetT increase, and constitutes the dominant source of systematic uncertainty.
• A 14% uncertainty on the jet energy resolution (JER) [19] translates into a 5% to 8% uncertainty on the cross section as Njetincreases, and an uncertainty of about 8% at low pjetT.
• The effect of the JVF requirement on the measured distributions is conservatively taken as an additional contribution to the total systematic uncertainty. This introduces an uncertainty on the measured cross sections of 1.5% to 3% as Njetincreases, and a 4% uncertainty for pjetT <30 GeV.
• The 100% uncertainty on the estimated jet background translates into a 4% to 6% uncertainty on the measured cross sections in the electron channel. In addition, the background contributions from top quark, W +jets, Z/γ∗(→τ+τ−)+jets, and diboson production processes are varied by 6%, 5%, 5%, and 5%, respectively, to account for the uncertainty on the cross sections of the different MC samples. This translates into a less than 1% uncertainty in the measured cross sections. In the Z/γ∗(→µ+µ−)+jets measurements, the impact from the background uncertainties is negligible.
• The unfolding factors are re-computed using SHERPA instead of ALPGEN to account for possible dependencies on the parton shower, underlying event, and fragmentation models implemented in the MC samples. This introduces an uncertainty on the measured cross sections that increases from 2% to 3% with increasing Njet, and from 2% to 6% with increasing pjetT .
• A 4.9% uncertainty on the electron reconstruction efficiency [15] translates into a 10% uncertainty in the measured Z/γ∗(→e+e−)+jets cross sections, independent of Njet, pjetT , andηjet. The uncer- tainty on the measured cross sections due to the determination of the electron trigger efficiency is negligible.
• A 7% uncertainty on the muon reconstruction efficiency [15] translates into a 12% uncertainty in the measured Z/γ∗(→µ+µ−)+jets cross sections, independent of Njet, pjetT , and ηjet. Other uncertainties, related to the muon trigger efficiency, the muon momentum scale, and the muon momentum resolution translate into a less than 1% uncertainty on the measured cross sections.
The different sources of systematic uncertainty are added in quadrature to the statistical uncertainty to obtain the total uncertainty. The total systematic uncertainty increases from 16% to 26% as Njet
increases; and from 22% at low and very large pjetT, to 18% at intermediate pjetT . Finally, an additional 11% uncertainty on the total integrated luminosity [21] is also taken into account.
9 Next-to-leading order pQCD predictions
Inclusive NLO pQCD predictions for Z/γ∗(→e+e−)+jets and Z/γ∗(→µ+µ−)+jets production, with up to two jets in the final state, are computed using the MCFM program [1]. CTEQ6.6 PDFs [7] are
e channel
Source range uncertainty on cross section (%)
Jet energy scale 7% to 8%, depending on pjetT andηjet⊕5% 10% to 20%
Jet energy resolution 14% per jet 8% to 2%
Pile-up removal 4% in first pjetT bin 4% at pjetT <30 GeV
QCD background 100% uncertainty 4% to 6%
t ¯t, Z/W +jets, dibosons 6%, 5%, 5% on normalization 1%
Lepton reconstruction 4.9% independent on Njetand pjetT 10%
Unfolding using SHERPA instead of ALPGEN 2% to 6%
µchannel
Source range uncertainty on cross section (%)
Jet energy scale 7% to 8%, depending on pjetT andηjet⊕5% 10% to 20%
Jet energy resolution 14% per jet 8% to 2%
Pile-up removal 4% in first pjetT bin 4% at pjetT <30 GeV
QCD background 100% uncertainty <1%
t ¯t, Z/W +jets, dibosons 6%, 5%, 5% on normalization <1%
Lepton reconstruction 7% independent on Njetand pjetT 12%
Unfolding using SHERPA instead of ALPGEN 2% to 6%
Table 2: Summary of the systematic uncertainties in the cross sections. The uncertainties are shown only for Njet≥1 and as a range with increasing pjetT .
employed and renormalization and factorization scales are set toµ =HT/2, where HTis defined event- by-event as the scalar sum of the pTof all the particles and partons in the final state. The anti-ktalgorithm with R=0.4 is used to reconstruct jets at the parton level.
Systematic uncertainties on the predictions related to PDF uncertainties are computed using the Hes- sian method [22]. For the total cross sections, they increase from 2.6% to 3% with increasing Njet, and decrease from 2.8% to 2.2% for the inclusive jet differential cross sections as pjetT increases. They are defined as 68% C.L. uncertainties, and include in quadrature uncertainties related to the variation of the input value for αs(MZ). Variations of the renormalization and factorization scales by a factor of two (half) reduce (increase) the predicted cross sections by 2.5% to 3% as Njetincreases, and change by 2%
to 6% the predicted inclusive jet differential cross sections as pjetT increases.
The theoretical predictions are corrected for QED radiation effects. The correction factors are deter- mined using ALPGEN MC samples with and without photon radiation in the final state, and including in the definition of the lepton four-momentum the photons within a cone of radius 0.1 around the lepton direction. The correction factors are about 4% for the electron channel and 3% for the muon channel, and do not present a significant Njetdependence.
Finally, the theoretical predictions include parton-to-hadron correction factors that approximately ac- count for non-perturbative contributions from the underlying event and fragmentation into particles. In each measurement, the correction factor is estimated using PYTHIA MC samples, as the ratio between the nominal distribution and the one obtained by turning off both the interactions between proton rem- nants and the string fragmentation in the MC samples. In addition, the non-perturbative corrections are computed using HERWIG+JIMMY and different PYTHIA MC samples with modified parton shower and UE settings, as discussed in Section 3. The non-perturbative corrections reduce the theoretical pre- dictions by about 4% and exhibit a moderate Njetand pjetT dependence.
10 Results
The measurements presented in this note are corrected for detector effects and refer to particle level jets identified using the anti-kt algorithm with R=0.4, for jets with pjetT >20 GeV and|ηjet|<2.8. The results are defined in a limited kinematic range for the Z/γ∗ decay products. In the electron channel, the measured cross sections refer to the region: 71<Me+e− <111 GeV, ETe>20 GeV, 0<|ηe|<1.37 or 1.52<|ηe|<2.47, and∆R(jet−electron)>0.5. Similarly, in the muon case the measurements are presented in the region: 71<Mµ+µ−<111 GeV, pµT >20 GeV,|ηµ|<2.4, and∆R(jet−muon)>0.5.
The data are compared to the predictions from the different MC event generators implementing Z/γ∗(→ e+e−)+jets and Z/γ∗(→µ+µ−)+jets production, as discussed in Section 3, as well as to NLO pQCD predictions, as discussed in Section 9.
Figure 6 presents the measured cross sections as functions of the inclusive jet multiplicity (≥Njet) for Z/γ∗(→e+e−)and Z/γ∗(→µ+µ−)analyses, in events with up to at least three jets in the final state. As expected, the measured cross sections decrease with increasing jet multiplicity. The data are described by the predictions from ALPGEN, SHERPA and NLO pQCD (the latter only available for the inclusive production of at least one jet and two jets in the final state) within the relatively large uncertainties of the measurements. In the case of PYTHIA, the LO pQCD (2→2 processes) MC predictions are multiplied by a factor 1.18, as determined from data and extracted from the average between electron and muon results in the ≥1 jet bin in Fig. 6. This brings the PYTHIA predictions close to the data. However, for larger Njet, and despite the additional normalization applied, PYTHIA predictions underestimate the measured cross sections.
The measured ratio of cross sections for different jet multiplicitiesσNjet/σNjet−1is shown in Fig. 7, compared to the different theoretical predictions. The data indicate that the cross sections decrease by 70% to 80% with the requirement of each additional jet in the final state. The measurements are described by the ALPGEN, SHERPA and NLO pQCD predictions. PYTHIA predictions underestimate the measured ratios.
The inclusive jet differential cross section dσ/d pjetT as a function of pjetT is presented in Fig. 8, for both Z/γ∗(→e+e−) and Z/γ∗(→ µ+µ−) analyses, in events with at least one jet in the final state.
The cross sections are divided by the corresponding inclusive Z/γ∗ cross section, σ(Z/γ∗(→e+e−)) andσ(Z/γ∗(→µ+µ−)), measured in the same kinematic region for the leptons, with the aim of can- celling systematic uncertainties related to lepton identification and the luminosity. The measured differ- ential cross sections decrease by almost two orders of magnitude as pjetT increases between 20 GeV and 120 GeV. The data are reasonably well described by ALPGEN, SHERPA and MCFM NLO pQCD pre- dictions for pjetT >30 GeV. At very low pjetT , the theoretical predictions are slightly below the measured cross sections. Similar conclusions are extracted from Fig. 9, where the differential cross sections are presented as a function of the leading-jet pjetT .
Finally, Fig. 10 shows the measured differential cross sections dσ/d pjetT , for electron and muon channels, as a function of pjetT of the second leading jet, divided by the inclusive Z/γ∗cross section, in events with at least two jets with pjetT >20 GeV and|ηjet|<2.8 in the final state. The measured cross sections are affected by the limited data statistics, decrease with increasing pjetT , and are well described by ALPGEN, SHERPA and NLO pQCD predictions.
11 Summary
In summary, we report first preliminary results for inclusive jet production in Z/γ∗(→e+e−)and Z/γ∗(→ µ+µ−)events in proton-proton collisions at√
s=7 TeV, using the ATLAS detector and corresponding to a total integrated luminosity of 1.3 pb−1. Jets are defined using the anti-kt algorithm with R=0.4 and the measurements are performed for jets in the region pjetT >20 GeV and|ηjet|<2.8. The measured cross
sections are generally well described by NLO pQCD predictions including non-perturbative corrections, as well as by the predictions from LO matrix elements supplemented by parton showers, as implemented in the ALPGEN and SHERPA MC generators.
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Njet
≥
0 1 2 3
events / bin
10-2
10-1
1 10 102
103
104
Njet
≥
0 1 2 3
events / bin
10-2
10-1
1 10 102
103
104 ATLAS Preliminary L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7 TeV) s Data 2010 (
) + jets e-
e+
→ γ*(
Z/
QCD WW,WZ,ZZ
t t
) + jets ν
→ e W(
) + jets τ-
τ+
→ γ*(
Z/
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
entries / 1 GeV
10-3
10-2
10-1
1 10 102
103
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
entries / 1 GeV
10-3
10-2
10-1
1 10 102
103
ATLAS Preliminary L dt = 1.3 pb-1
∫
1 jet,
≥ γ*+
Z/
jets, R = 0.4 anti-kt
= 7 TeV) s Data 2010 (
) + jets e-
e+
→ γ*(
Z/
QCD WW,WZ,ZZ
t t
) + jets ν
→ e W(
) + jets τ-
τ+
→ γ*(
Z/
(leading jet) [GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
events / 1 GeV
10-3
10-2
10-1
1 10 102
(leading jet) [GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
events / 1 GeV
10-3
10-2
10-1
1 10 102
ATLAS Preliminary L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7 TeV) s Data 2010 (
) + jets e-
e+
→ γ*(
Z/
QCD WW,WZ,ZZ
t t
) + jets ν
→ e W(
) + jets τ-
τ+
→ γ*(
Z/
(2nd leading jet) [GeV]
jet
pT
20 30 40 50 60 70 80 90
events / 1 GeV
10-3
10-2
10-1
1 10 102
(2nd leading jet) [GeV]
jet
pT
20 30 40 50 60 70 80 90
events / 1 GeV
10-3
10-2
10-1
1 10 102
ATLAS Preliminary L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7 TeV) s Data 2010 (
) + jets e-
e+
→ γ*(
Z/
QCD WW,WZ,ZZ
t t
) + jets ν
→ e W(
) + jets τ-
τ+
→ γ*(
Z/
Figure 2: Uncorrected measured inclusive jet multiplicity, inclusive pjetT spectrum, leading-jet pjetT in events with at least one jet, and second-leading-jet pjetT in events with at least two jets in Z/γ∗(→e+e−)+jets final states (black dots). Only statistical uncertainties in data are shown. The data are compared to MC predictions for signal (ALPGEN) and SM background processes, as described in Section 3.
Njet
≥
0 1 2 3
events / bin
10-2
10-1
1 10 102
103
104
105
Njet
≥
0 1 2 3
events / bin
10-2
10-1
1 10 102
103
104
105
Preliminary ATLAS
L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7TeV) s Data 2010 (
) + jets µ-
µ+
→ γ*(
Z/
WW,ZZ,WZ t t
) + jets τ-
τ+
→ γ*(
Z/
QCD ) + jets ν µ
→ W(
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100 110 120
entries / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100 110 120
entries / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
Preliminary ATLAS
L dt = 1.3 pb-1
∫
1 jet,
≥ γ*+
Z/
jets, R = 0.4 anti-kt
= 7TeV) s Data 2010 (
) + jets µ-
µ+
→ γ*(
Z/
WW,ZZ,WZ t t
) + jets τ-
τ+
→ γ*(
Z/
QCD ) + jets ν µ
→ W(
(leading jet)[GeV]
jet
pT
20 30 40 50 60 70 80 90 100 110 120
events / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
(leading jet)[GeV]
jet
pT
20 30 40 50 60 70 80 90 100 110 120
events / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
Preliminary ATLAS
L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7TeV) s Data 2010 (
) + jets µ-
µ+
→ γ*(
Z/
WW,ZZ,WZ t t
) + jets τ-
τ+
→ γ*(
Z/
QCD ) + jets ν µ
→ W(
(2nd leading jet)[GeV]
jet
pT
20 30 40 50 60 70 80 90
events / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
(2nd leading jet)[GeV]
jet
pT
20 30 40 50 60 70 80 90
events / 1 GeV
10-4
10-3
10-2
10-1
1 10 102
103
104
Preliminary ATLAS
L dt = 1.3 pb-1
∫
jets, R = 0.4 anti-kt
= 7TeV) s Data 2010 (
) + jets µ-
µ+
→ γ*(
Z/
WW,ZZ,WZ t t
) + jets τ-
τ+
→ γ*(
Z/
QCD ) + jets ν µ
→ W(
Figure 3: Uncorrected measured inclusive jet multiplicity, inclusive pjetT spectrum, leading-jet pjetT in events with at least one jet, and second-leading-jet pjetT in events with at least two jets in Z/γ∗(→µ+µ−)+jets final states (black dots). Only statistical uncertainties in data are shown. The data are compared to MC predictions for signal (ALPGEN) and SM background processes, as described in Section 3.
Njet
≥
1 2 3
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Njet
≥
1 2 3
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
ATLAS Preliminary jets, R = 0.4 anti-kt
) + jets e-
e+
→ γ*(
Z/
Total JES, JER EL Reco Bkg, Unfolding, JVF
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100110120
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
ATLAS Preliminary 1 jet,
≥ γ*+
Z/
jets, R = 0.4 anti-kt
) + jets e-
e+
→ γ*(
Z/
Total JES, JER EL Reco Bkg, Unfolding, JVF
Figure 4: Impact with respect to the nominal result from the different sources of systematic uncertainty on the Z/γ∗(→e+e−)+jets analysis for: (left) the measured cross section as a function of inclusive jet multiplicity; (right) the inclusive pjetT production cross section, for events with at least one jet with pjetT >20 GeV and|ηjet|<2.8 in the final state.
Njet
≥
1 2 3
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Preliminary ATLAS
) + jets µ-
µ+
→ γ*(
Z/
jets, R = 0.4 anti-kt
Total JES,JER Muon (reco, trig resol, scale) Bkg, Unfolding, JVF
[GeV]
jet
pT
20 30 40 50 60 70 80 90 100 110120
systematic uncertainty
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
Preliminary ATLAS
) + jets µ-
µ+
→ γ*(
Z/
1 jet,
≥ γ*+
Z/
jets, R = 0.4 anti-kt
Total JES,JER Muon (reco, trig resol, scale) Bkg, Unfolding, JVF
Figure 5: Impact with respect to the nominal result from the different sources of systematic uncertainty on the Z/γ∗(→µ+µ−)+jets analysis for: (left) the measured cross section as a function of inclusive jet multiplicity;
(right) the inclusive pjetT production cross section, for events with at least one jet with pjetT >20 GeV and|ηjet|<2.8 in the final state.
