The Tevatron experiments CDF and D0 reported 14 new hard QCD results in the period June 2008-June 2009. All of these use data taken with proton-antiproton collisions of center of mass energy 1.96 TeV. These results are reviewed here. The detectors themselves have been described previously

Seidel, Sally

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QCD AT THE TEVATRONSALLY SEIDELDepartment of Physics and AstronomyUniversity of New Mexico, MSC07 4220Albuquerque, 87131 NM, USAfor the CDF and D0 CollaborationsThe Tevatron experiments CDF and D0 reported 14 new hard QCD results in the periodJune 2008–June 2009. All of these use data taken with proton-antiproton collisions of centerof mass energy 1.96 TeV. These results are reviewed here. The detectors themselves have beendescribed previously. 1,2The measurement of the inclusive jet cross section probes the highest momentum transferscurrently available and is sensitive to new physics including quark substructure. The CDFmeasurement 3 uses the midpoint cone algorithm and characterizes data in five rapidity binsfrom less than 0.1 to 2.1. It is generally slightly below the prediction by a next-to-leadingorder (NLO) perturbative QCD (pQCD) calculation but consistent with it within systematicuncertainties up to jet transverse momenta (pT ) of 700 MeV/c. The systematics on data in theforward region are smaller than parton distribution function (PDF) uncertainties, providing aconstraint on the high-x gluon. The measurement shows improved agreement with the inclusivejet cross section measurement taken with the kT algorithm. The D0 measurement4 of theinclusive jet cross section agrees well with NLO pQCD over a similar range of jet pT in sixrapidity bins from 0.4 to 2.4. The D0 result favors the low edge of the CTEQ6.5M uncertaintyband and the shape of the MRST2004 functions.Both collaborations have searched for evidence of quark substructure in dijet angular distri-butions. Substructure can enhance the QCD cross section near 90◦ in the diquark (i.e., dijet)center of mass with associated amplitude that goes as ŝ/Λ2, where Λ is the contact interactionscale. CDF computes the ratio of the number of observed events to number of events predictedby PYTHIA with substructure turned off, as a function of dijet invariant mass, for angular re-gions defined through χ = exp|η1 − η2|, where η1 and η2 are the pseudo-rapidities of the highestand second highest pT jets, respectively. CDF finds no evidence for substructure and sets a 95%confidence level limit of Λ > 2.4 TeV. D0 compares data 5 to NLO pQCD as a function of χ aswell as yboost = 1/2|y1+y2|, which, in the approximation of massless 2 → 2 scattering, is equal to1/2 ln(xmax/xmin) where the xi are the parton momentum fractions. Finding no deviation fromtheory, D0 sets limits independent of Higgs mass using three consistent statistical approaches,two Bayesian and one frequentist, on Λ as well as Mc, the characteristic parameter for TeV−1extra dimensions, and MS , the characteristic parameter for Large Extra Dimensions.FERMILAB-CONF-09-409-EMany extensions of the Standard Model, motivated by the generational structure and ob-served mass hierarchy, predict resonances in the dijet mass spectrum. Like the inclusive jet crosssection, they can provide sensitive constraints on PDF’s. Using the midpoint jet algorithm, CDFcompares the dijet mass differential cross section for dijet masses up to 1400 GeV/c2 to predic-tions by NLOJET++ with PDF CTEQ6.1M, corrected to hadron level. Comparison to signalshapes predicted for excited quarks of various masses from 300 to 1100 GeV/c2 produces the moststringent lower mass limits presently available 6 on the excited quark, axigluon, flavor-universalcoloron, E6 diquark, and color-octet techni-ρ, as well as excluded mass limits for the W′ andZ ′. D0 extends their measurement range on the dijet mass cross section to |y| = 2.4 and, in acomparative study of the MSTW versus CTEQ6.6 PDF’s, finds MSTW favored and up to 40-60% variation in the cross section at the highest dijet masses. The D0 measurement systematicis comparable to the PDF uncertainty, implying constraints on future PDF predictions. 7Several studies of vector particle plus jet have been released. A measurement by D0 of thedifferential cross section for production of an isolated photon with an associated jet 8 probes thegluon distribution and, generally, the dynamics of hard QCD interactions, over ranges 0.007 ≤x ≤ 0.8 and 900 ≤ Q2, i.e. (pγT )2 ≤ 1.6 × 104 (GeV/c)2. This study investigates qg → qγ andqq → gγ for four classes of events: those with leading jet central versus forward as well as thosewith photon and jet rapidities in the same direction and opposite. D0 finds that the predictionsof NLO QCD do not describe the shape over the full range in pγT and that scale variationscannot describe the normalization simultaneously in all four rapidity classes. A measurement ofthe differential cross section reduces some uncertainties through cancellations, but disagreementbetween data and theory persists. By contrast, a CDF measurement of the inclusive isolatedprompt photon production cross section, in which the data are corrected to the hadron level,finds agreement with predictions by JETPHOX with CTEQ6 and non-perturbative corrections.A recent measurement by D0 of the inclusive cross section for Z/γ∗(→ e+e−) + jets 9 testsNLO pQCD and provides an important control on background to new physics. Events arebinned in the pT of the Nth jet, for N = 1, 2, and 3. Data agree well with NLO-MCFM butdiverge from predictions by PYTHIA and HERWIG increasingly with jet pT and N . PYTHIAwith pT ordering is found to describe the leading jet well. SHERPA and ALPGEN are seento improve upon the particle shower-based generators. Some discrepancies persist nonethelessbetween data and predictions of production rates and jet pT spectra.The D0 measurement of the inclusive cross section for Z plus jet 10 tests pQCD at the MZscale while evaluating a major background to many mechanisms with smaller cross sectionsfor Higgs, top, and SUSY production. This first such measurement differential in Z pT andy is within 5% of predictions by MCFM but significantly above predictions by PYTHIA withALPGEN. The cross section shape is well described by pQCD.Motivated by the fact that gb → Zb and qq → Zbb are the largest background to the searchfor Standard Model Higgs through ZH → Zbb and to some searches for sbottom, CDF hasmeasured the cross section for b-jet production in events with a Z. The result is a measurementof σ(Z + jet)/σ(Z), per jet and per event, and differentially versus jet and Z kinematicalvariables η, ET , pT , number of jets, and number of b-jets.11 The data agree generally withpredictions by PYTHIA, ALPGEN, and MCFM (with and without hadronic correction) butshow scale-dependent differences of up to 2 standard deviations, suggesting that higher ordersare particularly important. The best agreement is found for low scale factors. The uncertaintyis 20% lower than in an earlier study by the same collaboration.CDF has also measured the cross section for b-jet production in events with a W , a searchchannel for Higgs through pp → W (→ eν)H(→ bb) and for new physics. The technique involvestagging the jet as originating from a b through a displaced secondary vertex. Light quarkcontaminants are removed by a maximum likelihood fit to the invariant mass of charged tracksassociated with the vertex. The measured product σ × BR is 2.5 to 3 times higher than fixedorder predictions by ALPGEN and PYTHIA.A measurement by D0 12 of the cross section for production of W + c-jet relative to that forproduction of W + jet is consistent with leading order pQCD and with an s-quark PDF evolvedfrom Q2 scales two orders of magnitude lower. This provides direct evidence for the processqg → Wq′ while controlling background to Higgs, stop, and top studies.D0 has released a first measurement13 of the triply differential cross section d3σ/dpγt dyγdyjet.This probes the b, c, and g PDFs through the process gQ → γQ. The study divides the datainto classes yγyjet > 0 and yγyjet < 0 separately for b and c jets, finding good agreement overthe full range in pγT up to 108.3 GeV/c for the b-jet case, but substantial disagreement abovepγT = 70 GeV/c for the c-jet case.References1. D.E. Acosta, et al., Phys. Rev. D 71, 032001 (2005); D.E. Acosta et al., Phys. Rev. D71, 052003 (2005); T. Aaltonen et al., Phys. Rev. D 77, 112001 (2008).2. V.M. Abazov et al., Nucl. Instr. and Meth. A 565 (2006) 463.3. T. Aaltonen et al., Phys. Rev. D 78, 052006 (2008).4. V.M. Abazov et al., Phys. Rev. Lett. 101: 062001 (2008).5. V.M. Abazov et al., FERMILAB-PUB-09-326-E, arXiv:0906.4819[hep-ex], June 2009.6. T. Aaltonen et al., Phys. Rev. D 79: 112002 (2009).7. V.M. Abazov et al., D0 Note 5919-CONF, April 2009.8. V. M. Abazov et al., Phys. Lett. B 666 (2008) 435.9. V.M. Abazov et al., Phys. Lett. B 678, 45 (2009).10. V.M. Abazov et al., Phys. Lett. B 669, 278 (2008).11. T. Aaltonen et al., arXiv:0812.4458[hep-ex], Dec. 2008.12. V.M. Abazov et al., Phys. Lett. B 666 (2008) 23.13. V.M. Abazov et al., Phys. Rev. Lett. 102: 192002 (2009).

QCD at the Tevatron

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QCD at the Tevatron

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