The phenomenology of b-production and some of the previous Run I measurements
of CDF and D0 are briefly reviewed. A new analysis by CDF of Run I b-bbar
angular correlations and a new measurement by D0 of Run II b-jet cross-section,
are presented

Kajfasz, Eric

English

arXiv

The phenomenology of b-production and some of the previous Run I measurements of CDF and D0 are briefly reviewed. A new analysis by CDF of Run I b-bbar angular correlations and a new measurement by D0 of Run II b-jet cross-section, are presented

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b-production at the TevatronEric KajfaszTo cite this version:Eric Kajfasz. b-production at the Tevatron. 2003. <hal-00000584>HAL Id: hal-00000584https://hal.archives-ouvertes.fr/hal-00000584Submitted on 9 Sep 2003HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.ccsd-00000584 (version 1) : 9 Sep 2003b-production at the TevatronEric Kajfasz for the CDF and DØ collaborationsFermilab, P.O. Box 500, Batavia, IL 60510, USACPPM, Case 907, Faculte´ des Sciences de Luminy, 13288 Marseille Cedex 9, FranceThe phenomenology of b-production and some of the previous Run I measurements of CDFand DØ are briefly reviewed. A new analysis by CDF of Run I bb¯ angular correlations and anew measurement by DØ of Run II b-jet cross-section, are presented.1 IntroductionThe measurement of b quark production in high energy hadronic collisions provides an essentialtest bench for how well we understand QCD, especially in its perturbative regime. Indeed, theb quark is heavy enough (mb ≫ ΛQCD) to justify perturbative expansions, however, it is stilllight enough to be produced copiously at the Tevatron.After summarizing some of CDF and DØ Run I results, we present the new preliminary resultson B-hadron correlations that CDF recently extracted from its Run I data, and a preliminaryDØ b-jet cross-section coming from an analysis of Run II data collected with the upgradedDØ detector.2 Some of previous CDF and DØ Run I resultsTheoretical spectra at Next to Leading Order (NLO) have been available for some time 1, butthey show a discrepancy by up to a factor 2 to 4 with respect to experimental spectra measuredby CDF 2 and DØ 3 in Run I at the Tevatron.Fig. 1 shows a compilation of CDF and DØ measurements of the integrated b-quark cross-sectionas a function of the minimum transverse momentum of the b, PminT , in the central rapidity region|yb| < 1. The measurements are compared to the NLO QCD prediction shown as a dashed line.As a general trend, this discrepancy seems to be less accute at higher PminT . DØ also showed3(see fig. 2) that this discrepancy worsens at higher rapidities.Figure 1: CDF and D0 Run I data compared to NLOQCD predictions and to the model discussed in 5.Figure 2: D0 Run I measurement of b production in theforward region compared to NLO QCD predictions.PT distributions for open b-quark production are sensitive to large log terms, which need to beresummed to all orders, and to non-perturbative corrections required to account for the factthat hadrons, not quarks, are the measured final states. To prevent such a sensitivity, it hasbeen suggested 4 that one could look at ET distributions of b-jets, instead. Fig. 3 shows themeasurement performed by D0 in run I compared to the NLO QCD calculations 4. The betteragreement between theory and measurement seen here may hint at some possible improvementto be made e.g. in the heavy quark fragmentation functions.Figure 3: D0 Run I B-hadron production cross-sectioncompared to NLO QCD predictions.Figure 4: Predicted PYTHIA angular correlations forthe 3 sources of beauty (from 6). ∆φ is the azimuthalangle between b and b¯.The work done to try to better understand the situation mainly goes into three directions:Sources of b-quarks: at leading order, b-quarks are produced through qq¯ annihilations and ggfusion. But, at higher orders, two other mechanisms come into play, namely, flavor excitationand gluon splitting 6. The amount with which these different production mechanisms contributeat the Tevatron depends on a number of theoretical uncertainties. From an experimental pointof view, as shown in Fig. 4, the bb¯ azimuthal opening angle ∆φ for the three sources are quitedifferent and can be used to attempt to isolate their individual contributions 6. Flavor creationtends to produce b and b¯ mostly back to back. Its ∆φ distribution does not show any contribu-tion in the ”Towards” region (∆φ < 90◦). This will be elaborated on in section 3.Resummations and Fragmentation Functions:It has been shown 8 that NLO QCD with MS renormalization scheme, together with non-perturbative fragmentation functions extracted from LEP and SLC data, can give a good agree-ment with CDF Run I B meson cross-section measurement.Using a Fixed Order plus Next to Leading Log (FONLL) b quark spectrum and fitting the mo-ments rather than the shape of the non-perturbative fragmentation function from e+ e− data,has also proven 7 to provide a better agreement with CDF and D0 Run I measurements, as canbe seen in Fig. 5.New physics: an interesting possibility has been explored in 5. The dotted line on Fig. 1 showshow the production (pp¯ջ g˜g˜) of relatively light gluinos followed by their decay into b and anda light b˜ could help reduce the discrepancy.Figure 5: Run I data using tweaked resummation and fragmentation functions as discussed in 7.3 New CDF bb¯ angular correlations from Run I dataIn this analysis, CDF uses 90 pb−1 of a data sample taken during the run IB of the Tevatron(1994-1995) with collisions at a center of mass energy of 1.8 TeV. A muon or an electron triggeris required in order to enrich the b content of the data analyzed. Tracking information is used toreconstruct the decay vertices of both B hadrons. Decay vertices angular correlations are thencompared to PYTHIA predictions.3.1 secondary vertex correlationsFigure 6 shows, for the data with an electron trigger, the opening angle ∆φ between the mo-mentum vectors of the secondary vertices in the transverse plane. Detector effects are simulatedin Monte Carlo. The relative contribution from flavor creation, flavor excitation, and gluonsplitting are varied to give a best match to the data. Similar distributions are also produced fordata with a muon trigger.Figure 6: Measured secondary vertex correlationscompared to PYTHIA with relative contributions ofthe different sources adjusted to give best match.Figure 7: Measured B-hadron correlations.3.2 measured B-hadron correlationsFigure 7 shows, for a total of 17, 000 e+µ events, the opening angle ∆φ between the measured Bdirections in the transverse plane. The detector effects are unfolded from data using PYTHIA.Also taken into account are corrections for mistags, tags from prompt charm, and sequentialdouble tags. The fraction of events in the ”Towards” region (∆φ < 90◦), where flavor excitationand gluon splitting contribute, is measured to be 28.8  1.0(stat)  3.1(syst)%. The shadedregion shows the correlated systematic errors. The error bars show statistical errors only.4 DØ Run II b-jet cross-sectionThis analysis utilizes 3.4 pb−1 of data, collected with the upgraded DØ detector between end ofFebruary and mid-May 2002 as part of the Run II of the Tevatron, with collisions at a center ofmass energy of 1.96 TeV.4.1 µ+jet cross-sectionFirst, the cross-section for the muons associated with jets is measured. Jets are defined with aR =√∆η2 +∆φ2 = 0.5 cone algorithm. The muon track is measured by the muon system onlyand the kinematic cuts used are:|ηjet| < 0.6, |EjetT | > 20 GeV, |ηµ| < 0.8, PµT > 6 GeV, |∆R(jet, µ)| < 0.7The jet reconstruction efficiency is 100% for Ejet > 20 GeV, the muon reconstruction efficiencyis 43.7 0.8(stat) 2.2(syst)%.4.2 b-tagging and b-jet fractionThen, the b content of the sample is estimated by identifying the jets emanating from b quarks.In this analysis, the tagging of b-jets is done by calculating the P relT for the muons associatedwith jets (see P relT definition in Fig. 8). The method is based on the fact that, because of thehigh mass of the b quark, the muons produced in the decay of the b-quark have a higher P relTthan the muons produced in other processes.Figure 8: Definition of P relTSince the shape of the P relT distribution depends on the energy of the jet, the EjetT range is dividedinto several bins. In each of these bins, the P relT distribution for the data is adjusted to the sumof a signal template (extracted from a b ջ µ Monte Carlo simulation) and of a backgroundtemplate (extracted from 1.5 million QCD events). Because of the satistical limitations of thebackground templates, four EjetT bins are used. The resulting b-jet fraction as a function of EjetTis given in Fig. 9, including a functional form a+ b/EjetT fitted to the measurements.4.3 b-jet cross-sectionThe b-jet cross-section is obtained by folding the µ+jet cross-section with the b-jet fractionand by unfolding it from the calorimeter jet energy resolution using an ansatz function. Thepreliminary Run II result is shown in Fig. 10 compared to theoretical predictions (solid line).The band within the dashed lines covers the theoretical uncertainties. This measurement isconsistent with the corresponding Run I result shown in section 2.Figure 9: b-jet fraction as a function of EjetT .Figure 10: Measured Run II b-jet cross section com-pared to theoretical predictions.5 ConclusionsAlthough the agreement between calculated and measured b-production spectra is still notperfect, our understanding is steadily progressing, both on the theoretical and experimentalsides. There is still room left for improvement and maybe new physics. New Run II data at theTevatron from both CDF and DØ will hopefully help to shed light on the remaining obscurecorners of this long standing issue, soon.References1. P. Nason, S. Dawson, and R. K. Ellis Nucl. Phys. B303, 607 (1988); Nucl. Phys. B327,49 (1989);Nucl. Phys. B335, 260 (1990)W. Beenaker et al. Phys. Rev. D40, 54 (1993); Nucl. Phys. B351, 507 (1991)2. F. Abe et al. Phys. Rev. D50, 4252 (1994); Phys. Rev. Lett. 75, 1451 (1995); Phys. Rev.D53, 1051 (1996)D. Acosta et al. Phys. Rev. D65, 052005 (2002)3. S. Abachi et al. Phys. Rev. Lett. 74, 3548 (1995)B. Abbott et al. Phys. Lett. B487, 264 (2000); Phys. Rev. Lett. 84, 5478 (2000); Phys.Rev. Lett. 85, 5068 (2000)4. S. Frixione et M. L. Mangano Nucl. Phys. B483, 321 (1997)5. E. L. Berger et al. Phys. Rev. Lett. 86, 4231 (2001)6. R. D. Field Phys. Rev. D65, 094006 (2002) and hep-ph/02011127. M. Cacciari, P. Nason Phys. Rev. Lett. 89, 122003-1 (2002)P. Nason Heavy Flavor Production hep-ph/03010038. J. Binnewies et al. Phys. Rev. D58, 030416 (1998)B. A. Kniehl hep-ph/0211008

b-production at the Tevatron

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