Measurements of the high momentum jet production in protonproton collisions at √s = 7TeV and 8TeV with the CMS experiment at the LHC are summarized. The CMS experimental program in this domain is, on the one hand, oriented toward the observables which may help to constrain the QCD parameters: strong coupling constant and evolution and proton PDF. On the other hand, a large effort is dedicated to the understanding of the conditions under which the soft

gluon radiation plays an important role in the LHC physics. Different MC parton showering models are studied and measurements are provided to improve them

Gouzevitch, M. G.

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Gouzevitch, M.

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Jet physics at CMS

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DOI 10.1393/ncc/i2014-11621-xColloquia: LaThuile13IL NUOVO CIMENTO Vol. 36 C, N. 6 Novembre-Dicembre 2013Jet physics at CMSM. G. GouzevitchUniversite´ Claude Bernard-Lyon I - Lyon, Francericevuto il 20 Giugno 2013; approvato l’1 Luglio 2013Summary. — Measurements of the high momentum jet production in proton-proton collisions at√s = 7TeV and 8TeV with the CMS experiment at the LHCare summarized. The CMS experimental program in this domain is, on the one hand,oriented toward the observables which may help to constrain the QCD parameters:strong coupling constant and evolution and proton PDF. On the other hand, alarge effort is dedicated to the understanding of the conditions under which the softgluon radiation plays an important role in the LHC physics. Different MC partonshowering models are studied and measurements are provided to improve them.PACS 13.87.-a – Jets in large-Q2 scattering.PACS 12.38.-t – Chromodynamics, quantum.1. – IntroductionDuring the last data taking campaign from 2010 to 2012 the CMS detector collecteda large data sample in the proton-proton collisions provided by the LHC at the centreof mass of√s = 7TeV and√s = 8TeV. Those data was used to explore the quan-tum chromodynamics (QCD) properties at different momentum scales and kinematicregimes. In this report we would concentrate on the production of the jets with the alarge transverse momentum (pT > 30–50GeV). The physics program of the previousgeneration of colliders (ee like LEP, ep like HERA or pp¯ like Tevatron) have shown thatin this regime the next-to-leading-order (NLO) QCD calculations, supplemented withnon-perturbative corrections and the factorisation theorem, are usually able to describethe production cross sections. The effect of the soft gluon emissions was usually ob-served to be sub-leading with respect to the NLO (or NNLO) corrections, provided thatthe observables under consideration was infrared and collinear safe.After a brief presentation of the experimental aspects of the jet reconstruction inCMS we would consider the two major aspects of the QCD program at CMS. First wewould present the inclusive jet observables and study the conditions under which theclassical fixed order approach performs well. Then we would concentrate on the multi-jet production with multiples scales and large rapidity gaps between the hard jets wherethe soft gluon effects are expected to be important.c© Societa` Italiana di Fisica 9798 M. G. GOUZEVITCH2. – Jet reconstructionThe description of the CMS detector can be found elsewhere [1]. From the pointof view of the jet reconstruction the main features of the CMS apparatus are: afully silicon based tracker immersed into an uniform magnetic field of 4T along thez-direction. It allows to measure a charged particle momentum with a high precisionfrom O(0.5GeV) up to O(0.5TeV) in the region in pseudorapidity η < 2.5. The PbW0electromagnetic calorimeter shows an excellent performance in measuring the energy ofelectromagnetic particles for the energy range going from O(1GeV) and up to O(1TeV).It is complemented with the brass/scintillator hadronic calorimeter up to η < 3. Theregion 3.3 < |η| < 5.0 is covered by an iron/quartz-fiber hadron forward calorimeter.Finally one shall notice that in general the response of the calorimetry to the hadronsexhibits significant non-linearities.The Particle Flow technique [2] (PF) optimise the performances of this setup. Thecharged hadrons, electrons and muons are reconstructed by combining the tracks (trackerand muon chambers) with the calibrated calorimetric clusters. The photons are builtfrom the remaining electromagnetic clusters. Finally remaining hadronic clusters areconsidered as neutral hadrons. In summary 90% of the energy of a jet, which is carriedby the charged hadrons and photons, is reconstructed by linear and precise componentsof the CMS detector, while only 10% carried by the neutral hadrons fully rely on theECAL/HCAL combination.The different particles candidates identified by the PF algorithm are fed into thejet reconstruction algorithm as implemented in the FASTJET package [3]. The anti-kTclustering algorithm [4] is used by the CMS Collaboration with two possible values of theradius parameter R = 0.5 and R = 0.7, denoted henceforth AK5 and AK7. To suppressnon-physical jets, i.e. jets resulting from noise in the ECAL and/or HCAL calorimeters,identification criteria based on the PF candidates content was applied.The difference between the measured momentum of the jet and the true momentumat the particle level was estimated by the simulation. It appears to be of the order of5–10% slowly vanishing with the jet momentum [5]. The transverse momentum of thereconstructed jet is corrected in average for the detection effects and as for the extraenergy clustered into jets from additional proton-proton interactions within the sameor neighbouring bunch crossings (in-time and out-of-time pileup) [5]. The uncertainty(called JES), associated to the calibration procedure is typically of the order of 1–3% de-pending on the data-taking period and jet kinematics [5]. This is usually the dominatingsource of the uncertainty on the jet measurements.In addition to the average transverse momentum correction all the measured crosssections provided by the CMS experiment are unfolded for the detector resolution effects.They can be therefore directly compared to the prediction at the particles level.The CMS detector records events using a two-level trigger system consisting of ahardware-based level-1 (L1) trigger and a software-based high-level trigger (HLT). Allthe QCD analyses uses online reconstructed jets to select their events. The most widelyused class of jet trigger is the single jet trigger requesting at least one jet with online pTabove a threshold ranging from 30GeV up to 370GeV. The triggers was progressivelyprescaled together with the rise of the instantaneous luminosity provided by the LHC.3. – Inclusive production of high pT jetsThe measurement of the inclusive jet and di-jet productions is a bread and buttermeasurement to validate the QCD predictions in a hadronic collider. At CMS it wasJET PHYSICS AT CMS 99 (GeV)TJet p200 300 1000 2000d|y| (pb/GeV)T/dpσ2 d-510-11031071011101310 )4 10×|y| < 0.5 (  )3 10×0.5 < |y| < 1.0 (  )2 10×1.0 < |y| < 1.5 (  )1 10×1.5 < |y| < 2.0 (  )0 10×2.0 < |y| < 2.5 ( CMS = 7 TeVs-1L = 5.0 fb R = 0.7Tanti-kT= pFμ=Rμ NP Corr.⊗NNPDF2.1 (GeV)TJet p200 300 400 1000 2000Ratio to NNPDF2.10.60.811.21.41.6|y| < 0.5DataCT10HERA1.5MSTW2008ABKM09CMS = 7 TeVs-1L = 5.0 fb R = 0.7Tanti-kExp. UncertaintyTheo. Uncertainty (GeV)TJet p200 300 400 500 600Ratio to NNPDF2.10.511.522.532.0 < |y| < 2.5DataCT10HERA1.5MSTW2008ABKM09CMS = 7 TeVs-1L = 5.0 fb R = 0.7Tanti-kExp. UncertaintyTheo. UncertaintyFig. 1. – Inclusive jets cross sections measured at√s = 7TeV with R = 0.7.performed at√s = 7TeV with two different jet radius parameters AK5 [6,7], and AK7 [8]and at 8TeV for the inclusive jets cross section with AK7 [9]. This new measurementof the inclusive jets cross section extends up to pT = 2TeV the reach in transversemomentum. The√s = 7TeV is shown on fig. 1 compared to the NLO+NP predictionswith the NNPDF2.1 proton structure function (PDF) [10].In general the measurements with R = 0.7 are fairly well described by the fixed orderpredictions in all the rapidity regions. The differences are covered on the one hand by theexperimental uncertainties dominated by the JES and on the other hand by the theoryuncertainties. Those ones are driven by the scale dependence of the NLO predictions,and by the uncertainty on the PDF parametrization. This consideration also applies tothe results at 7TeV: inclusive and dijet cross sections.Given the good description agreement with the fixed order predictions in that energyregime, the CMS Collaboration exploited the ratio of the 3-jet to 2-jet cross sections at7TeV to extract the strong coupling constantαS(MZ) = 0.1148± 0.0014(exp .)± 0.0018(PDF) + 0.0050− 0.0000(scale)(1)100 M. G. GOUZEVITCHQ (GeV)10210 310(Q)Sα0.060.080.10.120.140.160.180.20.220.24 +0.0055-0.0023)=0.1148Z(MSα :32CMS R32CMS RD0 inclusive jetsD0 angular correlationH1ZEUSFig. 2. – The strong coupling running as extracted by different collider experiments includingCMS.and test its running properties at the unprecedented energy scales up to 1TeV [11] asshown on fig. 2. The experimental precision is very competitive with other experiments.The general picture provided by the colliders experiments is that the predictions of theSU(3)c renormalization group equation are verified over 3 orders of magnitude in theenergy scale.When the radius parameter is reduced to R = 0.5 it is interesting to observe on fig. 3a tendency of the fixed order predictions to overestimate by one standard deviation themeasurements. The effect is growing when going from the central jets production to theforward jets production.Fig. 3. – Inclusive jets cross sections measured at√s = 7TeV with R = 0.5.JET PHYSICS AT CMS 101Some indication about the nature of this tendency appears when the effects of theparton showers are considered on top of the NLO calculations as implemented for examplein the POWHEG events generator [12]. The parton shower corrections KPS are definedas the ratio of the inclusive jet cross section with the parton shower switched on andoff [13, 14]. Typically KPS = 0.8–0.95 for the AK5 jets and KPS = 1.0–1.05 for AK7.The structure of the correction factors appears from an interplay between the final stateradiation (FSR) which reduces the NLO cross section at a given pT and the initial stateradiation (ISR). The effect of ISR consist mainly in shifting the longitudinal momentumof the incoming parton accessible for the scattering. This effect is more important in theforward region due to the asymmetry in the parton-parton collision.The effect of the parton longitudinal momentum shift in the PDFs is also observed inthe inclusive b-jet production measured by the CMS Collaboration [15]. It appears thatthe use of the PS correction factors is necessary on top of the fixed orders calculationsto describe the measurements. It is shown in that a use of the transverse momentumdependant PDFs like uPDFs avoid the necessity to use the PS correction [13].A direct test of the impact of the ISR and FSR components of the PS models wasperformed in the specific kinematic regions where the contribution of each of them is ex-pected to be enhanced. The impact of the ISR was studied through the azimuthal angulardecorrelation in the central di-jet events [16]. The variable of interest Δφjet1,jet2 = π/2for the LO di-jet production. It could reduces typically down to 2π/3 when a thirdjet is considered. A larger decorrelation corresponds essentially to the presence of amulti-jet emission in the event. The fixed order NLO calculations describes poorly theregion of Δφjet1,jet2 < 2π/3 as shown on fig. 4. At the same time the LO+PS modelssuch as PYTHIA6 [17] or MADGRAPH [18] performs fairly well. A simple exercise wasperformed in varying the amount of ISR in PYTHIA by changing the value of the kISR pa-rameter. The results provided on fig. 4 exhibit clearly the importance of this phenomenon.The impact of the FSR was studied by looking on the evolution of the collinear emis-sion in the jet cone using the jet shapes variables [19]. The different LO+PS generatorswas compared to the data measurements of the amount of transverse momentum of theAK7 jet laying outside of the cone with radius r = 0.3: 1 − ψ(r = 0.3). This variablequantifies the transverse size of the jet. The low momentum jets (pT ≈ 20GeV) appearsto be quite wide with 1−ψ(r = 0.3) ≈ 45% while when the momentum increases the jetbecomes more and more narrow. For example at pT ≈ 1TeV only 5% of the jet energylie outside of the central cone defined by r = 0.3. It appears that the PS model andthe soft radiation and underlying event tune plays an important role in the descriptionof the in-cone radiation. Nevertheless all the MC models describe within 20% this smallamount of radiation produced outside of the central cone. Therefore they are expectedto provide a good description of the FSR effect in the variables such as total jet pT .4. – Jet production in the highly asymmetric collisionsThe soft radiation effects are expected to show up when the collision is highly asym-metric (x1 → 1 and x2 → 0). In that case the angular phase-space available for the ISRis very large. A simple measurement of the very forward inclusive jets cross section inthe region between 3.2 < η < 4.7 [20] was performed with first√s = 7TeV. It showsa good agreement within the experimental uncertainties between the data and the fixedorder NLO calculations or LO+PS MC simulations. Some of them consider alternativesto the DGLAP evolution such as CASCADE MC [21] generator which includes partonradiation from QCD evolution in 1/x inspired by the CCFM evolution.102 M. G. GOUZEVITCH123CMS NLO QCD Predictions CTEQ 6.6maxT = pfμ = rμ > 300 GeVTmax p123 < 300 GeVTmax 200 < p123 < 200 GeVTmax 140 < p123 < 140 GeVTmax 110 < p123 < 110 GeVTmax 80 < p Scale Dependencefμ, rμ    PDF Uncertainty  Non-Pert. Uncertainty = 7 TeV    |y| < 1.1s    -1L = 2.9 pb/2π /3π2 /6π5 π[ rad ] dijetϕΔ dijetϕΔddijetσd dijetσ1  THEORYDATA123 CMS  > 300 GeVTmax p123  < 300 GeVTmax 200 < p123  < 200 GeVTmax 140 < p123  < 140 GeVTmax 110 < p123  < 110 GeVTmax 80 < p=1.0ISRk=2.0ISRk=2.5ISRk=3.0ISRk=4.0ISRk  Systematic Uncert. = 7 TeV    |y| < 1.1s    -1L = 2.9 pb/2π /3π2 /6π5 π[ rad ] dijetϕΔ dijetϕΔddijetσd dijetσ1  PYTHIA6DATAFig. 4. – Angular decorrelation effects in data compared to the fixed order calculations at NLO(left) and compared to the PYTHIA MC generator with different values of kISR parameter(right).An alternative way to open the phase-space to the soft radiation is provided by the socalled Mueller-Navelet (MN) di-jet events [22]. The MN jets are defined as the pair of themost forward and backward jets in the event with the rapidity gap ΔMNy = |yjet1−yjet2|.The understanding of the inter-jet radiation in the rapidity gap is fundamental to be ableto separate a vector-boson fusion mechanism in the Higgs production (VBF) from theQCD background.The study of the additional jet production as function of ΔMNy was performed bythe CMS Collaboration using the 35 pb−1 collected at√s = 7TeV in 2010 [23] and5 pb−1 collected in 2011 [24]. Those data sample was characterised by the lower PUconditions with respect to the data collected in 2011. This point is important for thephysics implying the forward jets lying outside of the tracker reach (|η| > 2.5).On fig. 5 (left) the ratio of the number of the inclusive MN dijet events events withpT > 35GeV to the exclusive one is shown. The study as function of ΔMNy shows aslow increase of the inter-jet activity together with the opening of the phase-space. TheMC inspired by the DGLAP evolution, as implemented in PYTHIA, limits the rise ofthe ratio due to the strong kT ordering of the emission. In contrast the CCFM inspiredevolution consider a random walk in kT and predicts more activity in the inter-jet region.Unexpectingly the results clearly tend to prefer the DGLAP like evolution. This resultis confirmed by fig. 5 (right) where the azimuthal decorrelation between the MN jetsis shown for 3 < ΔMNy < 6. The CCFM inspired evolution predicts a much largerdecorrelation than the DGLAP like models.JET PHYSICS AT CMS 103y|Δ|0 1 2 3 4 5 6 7 8 9MNR11.522.533.544.552010 dataPYTHIA6 Z2PYTHIA8 4CHERWIG++ UE-7000-EE-3HEJ + ARIADNECASCADE = 7 TeVsCMS, pp, dijets > 35 GeVTp|y| < 4.7Fig. 5. – The inter-jet activity in the Mueller-Navlet events.5. – ConclusionThe measurements of the hard jets production by the CMS Collaboration was pre-sented. The inclusive multi-jet production appears to be reasonably well described bythe fixed order calculations under the condition that a sufficiently larger radius param-eter is used. In general the larger is the jet radius the less the observable appears to besensitive to the details of the soft radiation. The maximal size is mainly limited by theexperimental ability to mitigate the PU effects. Under those conditions the jet observ-ables provides a very powerful tool to measure directly the QCD parameters such as thestrong coupling running or the proton PDFs.A detailed study of the different components of the parton showering models andtheir ability to describe the data in the sensitive phase-space regions (azimuthal decorre-lation, jet shapes, angular coherence etc. . . ) was performed by the CMS Collaboration:ISR, FSR, colour coherence. In general the main features are rather well captured bythe LO+PS MC generators even if more tuning might be necessary in some cases. Incontrary to describe the same observables the fixed order calculations needs soft gluonresummation.The soft gluon production was considered in the events with a large rapidity gapbetween the hard jets. It appears that the kT ordered PS models inspired by the DGLAPevolution predict coherent results with respect to the measurements. In contrary the 1/xemission-ordered models inspired by the CCFM evolution predict too much inter-jetactivity. Those results are encouraging the use of the DGLAP inspired models to predictthe background contamination of the electroweak induced interactions like VBF whichare tagged by requesting a rapidity gap between two most outward jets.104 M. G. GOUZEVITCHREFERENCES[1] CMS Collaboration, JINST, 3 (2008) S08004.[2] CMS Collaboration, CMS-PAS-PFT-10-001.[3] Cacciari M. and Salam G. P., Phys. Lett. B, 641 (2006) 57 hep-ph/0512210.[4] Cacciari M., Salam G. P. and Soyez G., JHEP, 04 (2008) 063 arXiv:0802.1189 [hep-ph].[5] Chatrchyan S. et al. 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Jet physics at CMS

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