Measurement of the production cross section for W- and Z-bosons in association with jets in ATLAS
aa r X i v : . [ h e p - e x ] J un Measurement of the production cross section forW– and Z–bosons in association with jets inATLAS
Stefan Ask(on behalf of the ATLAS collaboration)
Cavendish Laboratory, University of Cambridge, Cambridge, UK.
Abstract.
We report on the measurements of inclusive W +jets and Z +jets cross sections in proton–proton collisions at √ s = H T which is the scalar sum of the p T in the event. Measure-ments are also presented of the ratios of cross sections. The measured cross sections are compared todifferent particle–level predictions, based on perturbative QCD, where the measured W +3jet crosssection is for the first time compared with next–to–leading order calculations. Keywords:
LHC, ATLAS, QCD, W, Z, jets
PACS:
INTRODUCTION
The experimentally clean signatures of W – and Z –bosons make the measurement ofthese processes in association with jets well suited to test perturbative QCD at the LHC.The processes allow for comparisons of multi–jet production with predictions eitherfrom the parton shower approach or from exact multi–parton matrix elements ( ME )matched with parton showers. In addition, full next–to–leading order ( NLO ) calculationsare also available for comparison with many of the results. The W / Z processes also differfrom pure QCD multi–jet processes with respect to the scale of the hard interaction, dueto the large mass of the electroweak gauge bosons.Measurements of W / Z +jets are also important to control backgrounds to other mea-surements at the LHC. In the Standard Model context, one example is the top quarkcross section measurements, where W +jet is often the dominant background. Also sev-eral beyond the standard model searches, such as the zero lepton SUSY search, sufferfrom irreducible background from either W +jets or Z +jets, or both.Here we report on the ATLAS Z +jets and W +jets cross section measurements [1, 2]based on data recorded during 2010. The analyses include the electron and muon decaychannels and are based on an integrated luminosity of 33 pb − . ATLAS ANALYSIS
The ATLAS detector systems were all fully operational during this data taking periodand the detector acceptance considered was approximately determined by the follow-ng constraints. Electrons were used within the inner detector acceptance ( | h | < . | h | < . E T ( p T ) was required to be larger than 20 GeV,in order to be well inside the highly efficient plateau of the triggers. Jets were recon-structed inside the main calorimeters ( | h | < .
2) and missing transverse energy wasbased on the full calorimeter acceptance ( | h | < . • Electrons : E T >
20 GeV; | h | < . excluding . < | h | < . . • Muons : p T >
20 GeV; | h | < . . • Jets ( anti − k T , R = . ) : p T > or
30 GeV; | y or h | < . D R ( ℓ, jet ) > . . • W selection : N ℓ = E missT >
25 GeV; m T >
40 GeV . • Z selection : N ℓ =
2; 66 < m ℓℓ <
116 GeV . Note that here the jet p T requirement, as well as the rapidity variable, differs between the W (20 GeV) and Z analysis (30 GeV). The D R ( ℓ, jet ) criteria refers to the leptons and allselected jets and the Z selection also require the two leptons to be of opposite charge. Theresults were then corrected for detector effects, using a bin–by–bin unfolding method,and compared with theory ( MC ) predictions inside the same kinematic region. For thetheory results, jets were reconstructed using the same algorithm based on all final stateparticles with a lifetime larger than 10 ps, except the leptons from the W / Z decays.Lepton momenta also included any photons radiated within D R < . Z and W events in both the electron and muon channels. Re-garding the background for these measurements, the background coming from QCD wasestimated based on a data–driven method whereas the electroweak and top backgroundswere estimated from MC. The background contamination of the selected Z +jets sampleswas of the order of 1% for the muon channel, as well as 5% for the electron channel. Inthe W +jets samples, the background was in the order of 10%. The main source of uncer-tainty in these measurements comes from the jet energy scale, which contributes withapproximately 10%, followed by pile–up corrections ( ∼ ∼ CROSS SECTION MEASUREMENTS
The obtained number of events were then used to measure the differential cross sectiontimes branching ratio with respect to a number of different quantities. All the results cor-respond to inclusive measurements, corrected for detector effects, within the kinematicregion defined by the event selection above.The differential cross section with respect of the number of selected jets was measuredboth using W and Z events. The absolute cross section and the ratio of cross sections The following definition was used, D R = p D h + D f . ) [ pb ] j e t N ‡ ) + - e + e fi * ( g ( Z / s ATLAS
Preliminary -1 L dt = 33 pb (cid:242) jets, R = 0.4, t anti-k > 30 GeV jetsT p ) + jets - e + e fi *( g Z/ = 7 TeV)sData 2010 (AlpgenSherpaPythiaMCFM D a t a / N L O = 7 TeV)sData 2010 (theoretical uncertainties D a t a / M C Data 2010 / AlpgenData 2010 / SherpaNNLO normalization jet N ‡ D a t a / M C
100 200 300 [ pb / G e V ] T / dp s d -6 -5 -4 -3 -2 -1 + jets n e fi W =7 TeVsData 2010, ALPGENSHERPABLACKHAT-SHERPAMCFM -1 Ldt=33 pb (cid:242)
ATLAS
Preliminary j e t s ‡ W + - j e t s , x ‡ W + - j e t s , x ‡ W + - j e t s , x ‡ W +
100 200 300 T heo r y / D a t a ‡ W + [GeV] T First Jet p100 200 300 T heo r y / D a t a ‡ W +
FIGURE 1.
Cross section for Z → ee as function of the number of jets (left). Cross section for W → e n as function of p T of the leading jet (right). from events with N jets over N − Z → ee cross section as a function ofnumber of selected jets. The measured values show a good agreement with the NLOpredictions, here represented by results obtained by MCFM. The results are also ingood agreement with expectations from the multi–parton ME programs (A LPGEN andS
HERPA ), which have been normalized to the inclusive NNLO cross sections obtainedby the FEWZ program. The results do on the other hand show poor agreement with theLO plus parton shower results (P
YTHIA ) for events with more than one jet. This is dueto the combination of a not–properly–covered phase space ( Z s are not produced by theparton shower) together with using the parton shower approach for the hard jets. Theresults are shown together with the corresponding ratios between the results obtainedfrom data over predictions from the MC programs MCFM, A LPGEN and S
HERPA .Figure 1 (right) shows the differential cross section with respect to the leading jet p T for W → e n . The measurement is performed separately for events with 1 to 4 jets.The results from the W +jets measurements are also compared against NLO predictionsfrom B LACKHAT –S HERPA , where W +3jets predictions at NLO is compared with LHCdata for the first time. The results are shown together with the corresponding ratiosbetween results from MC over data, for events with 1 and 2 selected jets. Again a goodagreement was found between the measurement and the MC predictions. The differentialcross sections were also measured with respect to the p T of the other selected jets in the j e t s ) [ / G e V ] ‡ ) + - m + mfi * ( g ( Z / s / T / dp s d -4 -3 -2 j e t s ) [ / G e V ] ‡ ) + - m + mfi * ( g ( Z / s / T / dp s d -4 -3 -2 = 7 TeV)sData 2010 (Alpgen Sherpa MCFM )+jets - m + m fi *( g Z/Preliminary
ATLAS -1 L dt = 33 pb (cid:242) jets, R = 0.4, t anti-k > 30 GeV jetsT p D a t a / N L O D a t a / N L O = 7 TeV)sData 2010 (theoretical uncertainties D a t a / M C D a t a / M C Data 2010 / Alpgen Data 2010 / Sherpa (2nd leading jet) [GeV] jetT p30 40 50 60 70 80 90 D a t a / M C
200 400 600 [ pb / G e V ] T / d H s d -6 -5 -4 -3 -2 -1 + jets nmfi W =7 TeVsData 2010, ALPGENSHERPABLACKHAT-SHERPA -1 Ldt=33 pb (cid:242)
ATLAS
Preliminary j e t s ‡ W + - j e t s , x ‡ W + - j e t s , x ‡ W + - j e t s , x ‡ W +
200 400 600 T heo r y / D a t a ‡ W + [GeV] T H200 400 600 T heo r y / D a t a ‡ W +
FIGURE 2.