Recent jet results in $p\bar{p}$ collisions at $\sqrt{s}$=1.96 TeV from the
CDF experiment at the Tevatron are presented. The jet inclusive cross section
is compared to next-to-leading order QCD prediction in different rapidity
regions. The $b$-jet inclusive cross section is measured exploiting the long
lifetime and large mass of $B$-hadrons. Jet shapes, W+jets and W/Z+photon cross
sections are also measured and compared to expectations from QCD production.Comment: 5 pages, 6 figures Presented at the workshop on Low-x Physics,
  Lisbon, Portugal, June 28-July 1, 200

Gallinaro, Michele

English

arXiv

Recent jet results in p{bar p} collisions at {radical}s = 1.96 TeV from the CDF experiment at the Tevatron are presented. The jet inclusive cross section is compared to next-to-leading order QCD prediction in different rapidity regions. The b-jet inclusive cross section is measured exploiting the long lifetime and large mass of B-hadrons. Jet shapes, W+jets and W/Z+photon cross sections are also measured and compared to expectations from QCD production

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JETS AT CDF∗MICHELE GALLINARO†Laboratory of Experimental High Energy PhysicsThe Rockefeller University,1230 York Avenue, New York, NY 10021, USARecent jet results in pp¯ collisions at√s=1.96 TeV from the CDF experiment atthe Tevatron are presented. The jet inclusive cross section is compared to next-to-leading order QCD prediction in different rapidity regions. The b-jet inclusive crosssection is measured exploiting the long lifetime and large mass of B-hadrons. Jetshapes, W+jets and W/Z+photon cross sections are also measured and comparedto expectations from QCD production.1. IntroductionThe current physics program at the Tevatron hadron collider includes stud-ies of jets, with the goal of performing precision measurements to test andfurther constrain the validity of the Standard Model. Jets are collimatedsprays of particles originating from the fragmentation of the initial inter-acting partons. The full jet reconstruction allows to measure the primaryparton’s energy. Jet energies are measured experimentally by adding theenergy of the calorimeter cells associated to a cluster, using predeterminedalgorithms. During Run II, both the CDF and the DØ collaborations arestudying alternative methods to the cone-based JetClu algorithm1 used dur-ing Run I (1992-1995), in order to avoid problems of infrared and collineardivergences due to soft partons and below/above threshold particle emis-sion. Both MidPoint2 and kT3 jet reconstruction algorithms have beenused in Run II. The former is an improved version of the JetClu algorithmwhich reduces the sensitivity to infrared and collinear problems. The latterstarts by finding pairs of nearby particles and merges them together to formnew pseudo-particles, continuing until a set of stable well-separated jets arefound, and is infrared and collinear safe to all orders in perturbative QCD∗Presented at the workshop on “Low-x Physics”, Lisbon, Portugal, June 28-July 1, 2006†Representing the CDF collaboration.1FERMILAB-CONF-06-271-E2(pQCD). In addition to the energy from the primary parton, jets accruesoft contributions from the underlying event (UE) of beam remnants. Thesecontributions become more important at smaller jet pT . During Run I andRun II, the contribution from UE energy has been studied and a modifi-cation to Pythia4 MC has been determined (Tune A5) using CDF data.Pythia MC with the new set of parameters describes well the jet shapesmeasured in Run II6.2. Jet cross sectionMeasurement of the jet inclusive cross section provides a powerful test ofpQCD and a sensitive probe of quark sub-structure down to a scale of≈ 10−17cm. Run I data exhibited an excess in jet cross section for eventswith high ET jets7, when compared to the then current Parton DistributionFunctions (PDFs). The increase in center-of-mass energy from 1.80 to1.96 TeV from Run I to Run II results in a larger kinematic range formeasuring jet production. With a data sample ten times larger than inRun I, jet production can be measured at transverse energies far beyondthose of Run I and spanning more than eight orders of magnitude. Themeasurement of the inclusive jet production cross section is performed usingjets clustered by the MidPoint cone algorithm for jets with pT ≥ 54 GeV/cin five different jet rapidity regions: |y| < 0.1, 0.1 < |y| < 0.7, 0.7 < |y| <1.1, 1.1 < |y| < 1.6, and 1.6 < |y| < 2.1 (Fig. 1, left). Results are based onover 1 fb−1 of Run II data. The measured cross section is in agreement withnext-to-leading order (NLO) pQCD after the necessary non-perturbativeparton-to-hadron corrections are taken into account. Figure 1 (right) showsthe ratio of Data/Theory as a function of jet pT , and good agreement isobserved in the whole range. It also shows the ratio using MRST2004 andCTEQ6.1M PDFs, indicating that the current measurement in the forwardregion has the power of reducing the PDF uncertainties. Comparison ofresults in the central and forward rapidity regions can be used as a powerfuldiscriminant to select new physics processes, free of common systematicuncertainties. The jet cross section has also been measured using the kTalgorithm yielding similar results8.3. Heavy flavorThe inclusive b−jet production cross section has been measured using300 pb−1. The measurement is performed for jets reconstructed using theMidPoint algorithm with transverse momentum in the range 38 < pjetT <3 (GeV/c)JETTP0 100 200 300 400 500 600 700(GeV/c)nb TdYdPσ2 d-1410-1110-810-510-21010410710101013101510)6|Y|<0.1 (x10)30.1<|Y|<0.7 (x100.7<|Y|<1.1)-31.1<|Y|<1.6 (x10)-61.6<|Y|<2.1 (x10=0.75merge=0.7, fconeMidpoint  R -1 L=1.04 fb∫    CDF Run II PreliminaryData corrected to the hadron levelSystematic uncertainty=1.3sep/2, RJETT=PµNLO: EKS CTEQ 6.1M  Data / Theory 0.511.522.533.5|Y|<0.1 0.1<|Y|<0.7Data / Theory0.511.522.533.50.7<|Y|<1.1 (GeV/c)JETTP100 200 300 400 500 600 7001.1<|Y|<1.6 (GeV/c)JETTP0 100 200 300 400 500 600 700Data / Theory 0.511.522.533.51.6<|Y|<2.1PDF Uncertainty on pQCDData (parton level) / NLO pQCDSystematic uncertaintySystematic uncertainty includinghadronization and UE=0.75merge=0.7,  fconeMidpoint  R=1.3sep/2, RJETT=PµNLO pQCD: EKS CTEQ 6.1M  CDF Run II Preliminary  -1 L=1.04 fb∫  Figure 1. Left: Measured inclusive jet cross section (circles) as a function of jet pTcompared to NLO pQCD predictions (histogram). The shaded band shows the totalsystematic uncertainty of the measurement. Right: Ratio Data/Theory as a functionof jet pT . The error bar (shaded area) shows the statistical (systematical) uncertainty.The solid lines indicate the PDF theoretical uncertainty.400 GeV/c and rapidity |y| < 0.7. It exploits the good tracking capabilitiesof the CDF detector to tag b−jets by reconstructing secondary vertices.The b−tagging algorithm uses displaced tracks associated with a jet andwithin a cone of ∆R =0.4 in the η − φ space with respect to the jet axis.Secondary vertices are searched for using the significance of the impact pa-rameter and the displacement of the secondary vertex from the primaryinteraction, L2D. The sign of L2D is used to distinguish heavy flavor can-didates from mis-tagged jets, and only positively tagged jets (L2D>0) areconsidered. The b−tagging algorithm is optimized for a jet cone of R = 0.4in order to avoid an increase in the mis-tagging rate. Both B and D hadronshave a displaced measured secondary vertex. Using Pythia MC, the shapeof the invariant mass distribution of all charged tracks attached to the sec-ondary vertex is used to extract the b−jet fraction. The measured crosssection is in agreement with NLO pQCD predictions (Fig. 2, left). Figure2 (right) shows the ratio (R) between measured and theoretical predic-tions, and the grey band represents the total systematic error from data.Systematic theoretical uncertainties are shown around R = 1. For b−jetswith transverse momentum below 90 GeV/c, good consistency between themeasured cross section and NLO pQCD prediction is found; for b−jets withpT > 90 GeV/c, agreement is observed within the systematic uncertainties.4 jet [GeV/c]TP50 100 150 200 250 300 350 400 (nb/[GeV/c])T/dYdPσ2 d10-710-610-510-410-310-210-1110DataSystematic errorsNLO prediction CTEQ6Mcorrected at hadron level / 2b2+mT2P=Fµ=RµNLO uncertainties )0µ < µ / 4 < 0µ(scale included =0.75 merge=0.7, fconeMidPoint jets, R-1 L ~ 300 pb∫ = 1.96 TeV, s|Y|<0.7CDF RunII Preliminary jet [GeV/c]TP50 100 150 200 250 300 350Data / NLO prediction0.511.522.533.544.55Data/NLO prediction (CTEQ6M)corrected at hadron level / 2b2+mT2P=Fµ=RµSystematic errorsNLO uncertainties / 4 (upper))0µ (lower) and 0µ(scale =0.75 merge=0.7, fconeMidPoint jets, R-1 L ~ 300 pb∫ = 1.96 TeV, s|Y|<0.7CDF RunII PreliminaryFigure 2. Left: Measured inclusive b−jet cross section (circles) as a function of jet pTcompared to NLO pQCD predictions (histogram). The shaded band shows the total sys-tematic uncertainty of the measurement. Right: Ratio of measured to NLO cross sectionas a function of jet pT . The error bar (shaded area) shows the statistical (systematical)uncertainty.4. W+jet productionThe study of inclusive W boson production in association with jets providesanother useful test of QCD in a large Q2 environment. The measurementof W+jet production cross section may also improve the understanding ofthe background to top pair production and possibly increase the sensitivityto Higgs boson searches. Recent work has been focused on improving theMonte Carlo simulation with multi-particle production in the final state.A new measurement of W + n jet (n ≥ 1, ...4) cross section as a functionof jet pT has been performed (Fig. 3, left). Cross sections have been cor-rected to particle level jets, in the effort of minimizing the detector effectsand reducing the dependence from a particular model. Data show goodagreement with the Alpgen9+Pythia MC over the entire jet pT spectrum.Other kinematic variables studied indicate good agreement between dataand MC predictions. Figure 3 (right) shows the measured differential crosssection as a function of the dijet invariant mass.5. W/Z+photon productionThe study of dijet mass resolution is very important as many new physicssignatures are expected to appear as dijet mass bumps10. One example isthe neutral Higgs boson which is expected to decay predominantly to bb¯,and the ability to precisely reconstruct the dijet mass peak can thus enhancethe Higgs discovery potential. A study of the process pp¯ → W (Z)γ → qq¯γhas been performed using ≈ 184 pb−1 of CDF data11. A dedicated triggerwas studied and implemented to enhance the sensitivity of this analysis.5Jet Transverse Energy [GeV]0 50 100 150 200 250 300 350[pb/GeV]T/dEσd-510-410-310-210-110110CDF Run II Preliminary n jets≥) + νe→(WCDF Data  -1dL =  320 pb∫W kin:  1.1≤| eη 20[GeV]; |≥ eT E 30[GeV]≥ νT]; E2 20[GeV/c≥ WT MJets: |<2.0ηJetClu R=0.4; |hadron level; no UE correctionLO Alpgen + PYTHIA normalized to DataσTotal jetst1jetnd2jetrd3jetth4]2) [GeV/c2-jet1Di-jet Invariant Mass M(jet0 100 200 300 400 500)]2[pb/(5GeV/cjj/dMσd-310-210-1101CDF Run II Preliminary 2 jets≥) + νe→(WCDF Data  -1dL =  320 pb∫W kin:  1.1≤| eη 20[GeV]; |≥ eT E 30[GeV]≥ νT]; E2 20[GeV/c≥ WT MJets:  15[GeV]≥ jetT|<2.0; EηJetClu R=0.4; |hadron level; no UE correctionLO Alpgen + PYTHIA normalized to DataσTotal Figure 3. Left: Differential cross section dσ(W → eν+ ≥ n jets)/dEjetTas a function ofthe jet transverse energy. Data are compared to Alpgen+Pythia MC and normalized tothe measured inclusive cross section in each jet multiplicity sample. Right: differentialcross section as a function of the dijet invariant mass.The event sample is selected by requiring a central photon with ET >12 GeV/c and two jets with ET > 15 GeV/c. Thanks to the low thresholdof the photon transverse energy, the W/Z mass peak is not biased and thebackground is estimated from data. An advanced Neural Network eventselection improves the signal-to-noise ratio from 1/333 to 1/71. Furtheroptimization around the W/Z dijet mass region increases the ratio to 1/41.No evidence of a W/Z → qq¯ peak is found, and the 95% upper limit of theproduction cross section σ(pp¯ → Wγ) is calculated to be 54 pb, consistentwith the Standard Model expectations of 20.5 pb.References1. F. Abe et al. (CDF Collaboration), Phys. Rev. D 45, 1448 (1992).2. G. U. Flanagan, Ph.D. thesis, Michigan State University, 2005.3. S. Catani, Y. L. Dokshitzer, M. H. Seymour and B. R. Webber, Nucl. Phys.B 406, 187 (1993).4. T. Sjostrand, S. Mrenna and P. Skands, JHEP 0605, 026 (2006).5. T. Affolder et al. (CDF Collaboration), Phys. Rev. D 65, 092002 (2002).6. D. Acosta et al. (CDF Collaboration), Phys. Rev. D 71, 112002 (2005).7. T. Affolder et al. (CDF Collaboration), Phys. Rev. D 64, 032001 (2001)[Erratum-ibid. D 65, 039903 (2002)].8. A. Abulencia et al. (CDF Collaboration), Phys. Rev. Lett. 96, 122001 (2006).9. M. Mangano, M. Moretti, F. Piccinini, R. Pittau, and A. Polosa, JHEP 0307,001 (2003).10. A. Bocci, S. Lami, S. Kuhlmann and G. Latino, Int. J. Mod. Phys. A 16S1A,255 (2001).11. A. Bocci, Ph.D. thesis, The Rockefeller University, 2005.

Jets at CDF

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Jets at CDF

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