We report on potential for measurement of W and Z boson production
accompanied by jets at the CMS experiment. Of particular interest are jet
multiplicity and Pt distributions. The 10/pb to 100/pb datasets expected in the
startup year of operation of LHC are likely to already provide information
beyond the reach of the Tevatron collider both in jet multiplicity and Pt
range. We are especially interested in understanding the ratios of W+jets to
Z+jets distributions by comparing them to next-to-leading order Monte Carlo
generators, as these processes present a formidable background for searches of
new physics phenomena.Comment: Poster session at ICHEP08, Philadelphia, USA, July 2008. 3 pages,
  LaTeX, 1 jpg figure. Updated with the new latex template for ICHEP0

De Mattia, Marco

English

arXiv

We report on potential for measurement of W and Z boson production accompanied by jets at the CMS experiment. Of particular interest are jet multiplicity and Pt distributions. The 10/pb to 100/pb datasets expected in the startup year of operation of LHC are likely to already provide information beyond the reach of the Tevatron collider both in jet multiplicity and Pt range. We are especially interested in understanding the ratios of W+jets to Z+jets distributions by comparing them to next-to-leading order Monte Carlo generators, as these processes present a formidable background for searches of new physics phenomena.We report on potential for measurement of W and Z boson production accompanied by jets at the CMS experiment. Of particular interest are jet multiplicity and Pt distributions. The 10/pb to 100/pb datasets expected in the startup year of operation of LHC are likely to already provide information beyond the reach of the Tevatron collider both in jet multiplicity and Pt range. We are especially interested in understanding the ratios of W+jets to Z+jets distributions by comparing them to next-to-leading order Monte Carlo generators, as these processes present a formidable background for searches of new physics phenomena

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34th International Conference on High Energy Physics, Philadelphia, 2008Production of W and Z bosons accompanied by jets at LHC startupMarco De Mattia (for the CMS collaboration)INFN and University of Padova, ItalyWe report on potential for measurement of W and Z boson production accompanied by jets atthe CMS experiment. Of particular interest are jet multiplicity and PT distributions. The 10 to100 pb−1 datasets expected in the startup year of operation of LHC are likely to already provideinformation beyond the reach of the Tevatron collider both in jet multiplicity and PT range. We areespecially interested in understanding the ratios of W+jets to Z+jets distributions by comparingthem to next-to-leading order Monte Carlo generators, as these processes present a formidablebackground for searches of new physics phenomena.I. INTRODUCTIONDirect production of electroweak gauge bosons (W±and Z) in association with jets are important hard processes,that can be used for several studies. First of all, they allow to perform critical tests of perturbative quantumchromodynamic (QCD). The NLO cross sections have been calculated only up to W/Z+2jets and predictions for thehigher jet multiplicity final states are accessible through matrix element (+partonshower) computations and in factcan be considered as a prime testing ground for the accuracy of such predictions as well as for measurements of αS .These events can also be used to calibrate the detector, as the jet energy scale systematic uncertainty can be reducedusing Z + jet events. The cross section is smaller than for other calibration samples, but the kinematics of the Z bosoncan be reconstructed very precisely. W/Z + jets are also an important background for many new particle searches.At the LHC they have high cross sections, with respect e.g. to the Tevatron, and thus constitute the dominantbackground for a variety of new physics signatures (Higgs, Supersymmetry, ...), besides being a background for SMmeasurements. Ultimately, they are a discovery channel for new physics. Any production of new heavy particles withquantum numbers conserved by the strong interaction and EWK couplings is likely to contribute to signatures withone or more EWK gauge bosons; additional jets will always be present at some level from initial-state radiation, andmay also be created in cascade decays of new heavy particles or from the decay of associated heavy particles. Precisemeasurements of the W + jets and Z + jets channels provide a broad search in a number of possible signatures ofphysics beyond the SM.II. THEORY PREDICTIONSIn this section we will describe theoretical predictions for the production of W/Z+jets at the LHC for the differentjet multiplicities [3, 4].W+1jet : At the LHC the dominant mechanisms for W boson production in association with 1 jet are qg → Wqand qq¯ → Wg, with the first one being the most frequent. In the qg → Wq channel the W is produced abundantlyin the forward rapidity region because of the difference in the shape of the p.d.f.s of the incoming partons. The sameconsiderations apply also in the case of qq¯ →Wg since the q¯ is a sea quark. In both cases the jet tends to accompanythe W in the same rapidity hemisphere or to go in the central region.W+2jets: The four main subprocesses contributing at LO are: qg → Wqg, qq¯ → Wgg + Wqq¯, qq → Wqq andgg → Wqq¯. The first one is the dominant process at the LHC. For the qg → Wqg channel, similar considerationsas for the W+1jet apply. Due to the p.d.f.s the W boson is produced preferentially in the forward region, with theadditional jet favoring even more this configuration. One of the jets tends to follow the W in rapidity. The secondjet can be easily separated in rapidity from the W boson. In qq¯ → Wgg + Wqq¯ the kinematical mechanism is thesame as in qg →Wqg since the antiquark is a sea quark. The qq →Wqq channel is peculiar, since both partons arevalence quarks. To make one x large it tends to have the W and one jet slightly forward in rapidity, while to make1arXiv:0809.4901v2  [hep-ex]  9 Oct 200834th International Conference on High Energy Physics, Philadelphia, 2008the other x large it has the second jet well forward and opposite in rapidity. The gg → Wqq¯ channel is perfectlysymmetric and the W and the two jets are produced mostly in the central rapidity region. In most of the cases one ofthe jets tends to accompany the W in the same rapidity region, while the other jet can be central or in the oppositehemisphere.W+>2jets: There is not direct NLO computation of the cross section in this case. Monte Carlo programs ableto simulate this events at LO have been developed and they have proven to be consistent with Tevatron data [1].In particular, the LO predictions and simulation of higher order effects via partonshower generators are alreadysufficient to describe the data. It is one of the tasks of the LHC experiments to check whether the same still holdstrue or if NLO or NNLO approaches are needed.Z+Njets: At the Tevatron the dominant process for production of Z+jets is qq¯ → Z + jets where both partonsare valence quarks. At the LHC the dominant process is the same, but the antiquark comes now from the sea. Thismeans that regions with much smaller x of the p.d.f.s will be explored with Z+Njets events at the LHC. This channelis not expected to differ much from what described for W+jets and most of the considerations already done in thatcase apply also for Z+jets.III. MEASUREMENT AT LHC STARTUPFocus at startup will be on the measurement of the cross sections of W+jets and Z+jets and their ratios with asmany accompanying jets as allowed by the statistic. Direct measurements of this cross sections suffer from inherenttheoretical and experimental uncertainties associated with the definition and measurement (and hence counting)of jets. These effects can change the number of jets measured in a given event and, together with theoreticaluncertainties, mean that the measurement of a specific N-jet exclusive channel at the LHC will be completelydominated by systematic uncertainties. By considering the ratio R = (W + Njets)/(Z + Njets) most of theuncertainties will cancel (at first order) [2]. Furthermore, most BSM signals can produce deviations in the measuredratio either by decaying into actual Z or W bosons or by having a high missing tranverse energy ( 6Et) signature.Of course, ratios alone are not very useful to measure the characteristics of the process itself. These require directmeasurements on the selected samples.A. Event SelectionAt CMS W/Z+jets events will first be selected by the standard single electron and single muon triggers forthe W+jets and the standard double electron and double muon triggers for Z+jets. An oﬄine selection based onthe reconstructed leptons and 6Et will be used. Identified leptons with a minimum required transverse momentumare checked for both calorimetric and tracker isolation. This requirement is essential in reducing the huge QCDbackground. For W identification, a constraint on the transverse mass of the lepton- 6Et system is used. For Zidentification a tight cut on the reconstructed mass from the two opposite sign leptons is used. Jet energy correctionsare very important in this kind of analysis. Z+jets events provide the possibility to use reconstructed Z bosonmomentum to calibrate the jet energy. The same calibration constant set can be used for W+jets, because W+jetsand Z+jets have similar jet ET distributions due to the physics correlation between the two processes. Jet number inthe event are counted after jet energy corrections have been applied and a minimum jet transverse energy of about50 GeV is required. After all these selections the effective cross sections shown in figure 1 are found.B. First Measurements with 10/100 pb−1With 10pb−1, About 2600 (570) W+1jet (2jets) and 350 (60) Z+1jet (2jets) events are expected to be availableafter selections. This will allow to measure the cross sections and their ratio with 1 and possibly 2 accompanying jetsand to test algorithms that use and combine measurements from different detectors (calorimetry, tracker, ...) which234th International Conference on High Energy Physics, Philadelphia, 2008FIG. 1: Effective cross section for Z + jets and W +jets events. The effective cross section corresponds to the generated crosssection of a physics process with the overall acceptance factor, which includes detector trigger efficiency and oﬄine selectionefficiency.are currently being developed. With 100pb−1 measurements of the ratio of up to 4 jets and the direct measurementof the cross sections of W+jets and Z+jets are expected. Comparisons with the Monte Carlos will be carried on toassess their predictive capacity especially in the low PT shapes of the jet energy distributions. These are the mostproblematic regions and in the long term, once NLO effects are understood, and low PT shapes well reproduced,systematics can be assigned according to NLO vs NNLO comparisons. Data driven methods will be used whereverpossible for the estimation of backgrounds and efficiencies.After trigger and selection cuts the Z+Njets inclusive samples with up to 4 jets are expected to be at least oneorder of magnitude above backgrounds. W+jets events, on the other hand, will still have significant backgrounds.As the number of jets increases, the top background becomes more important; already with 3 jets the background iscomparable with the signal and with 4 jets it is even higher than the signal. Top contribution to W+jets events isaccentuated at the LHC, since the production rate increases by about a factor of 100 with respect to the Tevatron,while W and Z production increase by just a factor of 5. The reduction of the top background in the W+>2 jetschannels will be crucial and strategies based on jet characteristics are being developed and tested on Monte Carlosimulations.IV. CONCLUSIONSAmong the first measurements that will be carried out at the LHC by the CMS detector there will be the measure-ment of the cross sections of W+jets and Z+jets and their ratios versus the jet multiplicity. With the first 10pb−1 upto W/Z+2 jets are expected to be measured. With the first 100pb−1 up to W/Z+4jets measurements will be carriedout. Detailed studies of techniques for background subtraction and selection efficiency improvement are ongoing.Monte Carlo generators predictions (both LO and NLO) will be compared to the data with special attention to thelow PT shape of the jet energy distributions.[1] T. Aaltonen, et al, for the CDF Collaboration, arXiv:hep-ex/0711.4044v2: “Measurement of the cross section for W-bosonproduction in association with jets in ppbar collisions at sqrt(s) = 1.96 TeV”, 4 Feb 2008[2] E. Abouzaid, H. Frischar, arXiv:hep-ph/0303088v1: “The Ratio of W+Njets To Z/gamma+Njets As a Precision Test ofthe Standard Model”, 12 Mar 2003[3] J.R. Andersen, et al, arXiv:hep-ph/0105146v1: “W Boson Production with Associated Jets at Large Rapidities”, 15 May2001[4] J. M. Campbell et al, “Hard Interactions of quarks and gluons: a primer for LHC physics”, 2007 Rep. Prog. Phys.7089-1933

Production of W and Z bosons accompanied by jets at LHC startup

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