The influence of the Pt_hat cutoff on the production of bB events in Pythia has been studied. Versions 5.75, 6.125 and 6.152 have been considered

Gennai, Simone

arXiv

The influence of the Pt_hat cutoff on the production of bB events in Pythia has been studied. Versions 5.75, 6.125 and 6.152 have been considered.The influence of the Pt_hat cutoff on the production of bB events in Pythia has been studied. Versions 5.75, 6.115 and 6.152 have been considered

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arXiv:hep-ex/0010069v3  12 Feb 2001A study of bb production mechanisms in PYTHIA ‡Simone Gennai†§† I.N.F.N. Pisa, ItalyAbstract. The influence of the Pˆt cutoff on the production of bb events in Pythiahas been studied. Two samples of events generated with Pˆt ≥ 1GeV (default) andwith Pˆt ≥ 10 GeV are investigated. In versions 5.75 and 6.115 of Pythia there aresome critical differences between the two samples for what concerns contribution in bbproduction mechanisms. Version 6.152, taken as the CMS official one, has been testedalso.1. IntroductionCMS (Compact Muon Solenoid) is a general purpose experiment under construction atCERN. The description of the apparatus is given elsewhere [1]. CMS will be built onLHC (Large Hadron Collider) a pp collider with√s=14 TeV and a maximum luminosityof 1034 cm−2 s−1 [2] . One of the main physics topics of the Collaboration will be thestudy of the B hadron physics. In fact apart from the interest “per se” of this topic,b-jets are also the main background for several other physics channels [3], [4]. In thisarticle, I present a study on bb production mechanisms at LHC energies using the MonteCarlo package Pythia [5] as event generator.At a hadron collider bb pairs are produced by the following mechanisms:(i) Pair production (gluon or quark fusion): gg → bb, qq → bb. For each event a bbpair is produced in the hard interaction.(ii) Flavour excitation: bq → bq, bg → bg. A heavy flavour from the partondistribution of one beam particle is put on mass shell by scattering against a partonof the other beam. As b quark is not a valence flavour it must come from a branchingg → bb.(iii) Gluon splitting: g → bb. It occurs in the initial- or final-state shower but noheavy flavours are produced in the hard scattering.A more detailed definition of the mechanisms can be found in ref [6]. As thesemechanisms give rise to different kinematic configurations of the final state is importantto understand their contribution to the total cross section.The only way to generate (i), (ii) and (iii) in Pythia is using the steering card MSEL=1,‡ Talk presented at Phenomenology Workshop on Heavy Flavours and CP Violation, Durham UK§ E-mail: simone.gennai@pi.infn.it2which generates QCD 2 → 2 events. In order to have higher statistics, and savecomputation time the user can set kinematic cuts at generation level which limits theavailable portion of phase space [7], increasing the efficiency of selection cuts. One ofthis cuts is the Pˆt cutoff. ‖In the following sections I will show differences between the use of the Pythia defaultcutoff (1 GeV) and a 10 GeV cutoff. Three versions of the generator have beenconsidered: 5.75, 6.115 and 6.152 which has been chosen as event generator for thenext CMS physics studies.2. bb production cross sectionsTwo samples have been prepared with the three versions of Pythia to investigate theinfluence of the Pˆt cutoff on the production of bb events. Both of them have beengenerated using MSEL=1 and contain events with only one bb pair. The first sample(SAMPLE A) has been generated with the default Pˆt cutoff (1 GeV). The second sample(SAMPLE B) has been generated with Pˆt ≥ 10 GeV. For both samples the bb productioncross section for Pˆt ≥ 10 GeV has been computed using the following formula:σ(Pˆt≥10 GeV )bb=N(Pˆt≥10 GeV )bbNtot· σtot (1)where N(Pˆt≥10 GeV )bbis the number of bb events with Pˆt ≥ 10 GeV, N tot is the totalnumber of events generated and σtot is the total (QCD 2 → 2) cross section given byPythia . The results of the computation for the three versions of the package are shownin Tab.1 (the values of the cross section are given in µb).cross section (Pˆt ≥ 10 GeV) (µb) P 5.75 P 6.115 P 6.152A B A B A Bσ pair production 20 20 47 51 48 52σflavour excitation - 110 - 221 211 236σgluon splitting 130 130 173 193 159 173σbb150 260 220 465 418 461Table 1. Contributions to the bb production cross section for Pˆt ≥ 10 GeV withPythia 5.75, 6.115 and 6.152 (in µb).All double countings have been avoided requiring the number of b-quarks comingfrom the hard interaction to be 2 (pair production), 1 (flavour excitation) or 0 (gluonsplitting). As it appears clear from the table for version 5.75 and 6.115 the cross sec-tion is Pˆt dependent. The problem is related to flavour excitation which in this two‖ Pˆt is defined as the transverse momentum of the outgoing partons in the center of mass system ofthe ingoing partons.3versions cannot be generated with default Pˆt cutoff (sample A). The difference in theσbb(for sample A) between 5.75 and 6.115 is due to the different choice of the defaultparton distribution function. The interpretation of the observed phenomena, given byT. Sjo¨strand and E. Norrbin, is reported in [8]. In version 6.152 the problem related toflavour excitation has been set but there is still a difference of about 10% between thecross sections of the two samples (for all versions statistical errors can be neglected).The difference in the value of cross sections is not very important, because the resultsare usually normalized to the total bb cross section of 500 µb. On the other hand, thedifferent contributions for the production mechanisms could be more dangerous, as theycan lead to different kinematic distributions, and therefore affect the efficiencies for dif-ferent selections.Fig. 1 illustrates the difference in the bb production cross sections due to the additionalcontribution of the flavour excitation mechanism in sample B.Figure 1. Cross section with the two different cutoff on Pˆt. The solid line is with thedefault cutoff (1 GeV). The picture is obtained with Pythia 5.75.3. Kinematic distributionsThe main kinematic parameters which define the signature of bb event are the transversemomenta and pseudorapidities of the b quarks, and the angular distance ∆φ betweentheir directions in the transverse plane. The first two parameters have similardistributions in both samples. The ∆φ distribution is shown in fig. 2 for the threedifferent mechanisms. For what concerns gluon splitting, the distribution is slightlypeaked at small ∆φ. The angle between the two b-quarks produced by the gluon-fusion4delta_phi (rad)0204060801001200 0.5 1 1.5 2 2.5 3delta_phi (rad)0510152025300 0.5 1 1.5 2 2.5 3delta_phi (rad)0123450 0.5 1 1.5 2 2.5 3Figure 2. Distribution of the angle between the two b quarks in the transverse plane:the upper one is gluon splitting, the middle is gluon fusion, the last is related to flavourexcitation. The plots are in arbitrary units.mechanism has a peak at ∆φ ∼ pi, as expected, since in the process gg → bb the b-quarksare produced back-to-back in the transverse plane. The last distribution correspondsto the flavour excitation production mechanism, for which the back-to-back topologyis preferred. We can conclude that the total ∆φ distributions of sample A and sampleB are slightly different. Some care should be taken about this, as it could affect theestimated efficiency of selection cuts.54. ConclusionsVersions 5.75 and 6.115 show a big discrepancy in the total bb production cross sectionwhen using two different cutoffs on Pˆt. The problem is due to flavour excitation whichcan be generated only in sample B but gluon splitting and pair production are stableand show the same cross section in the two samples. In both versions the use of adifferent Pˆt. cutoff from the default one leads to slightly different ∆φbb distributionscaused by the presence of flavour excitation.In version 6.152 everything is set, a part a little inconsistency of about 10% on which Iam working in order to understand the cause.5. AcknowledgementsI really thank Andrei Starodumov, Fabrizio Palla and Roberto Dell’Orso who helpedme to make this study.I also would like to thank M. Mangano, P. Nason and G. Ridolfi for their collaborationand fruitful discussions. Special thanks to T. Sjo¨strand and E. Norrbin for detailedexplanations of observed Pythia behaviour and recomendations for the generation of bbevents.references[1] CMS Collaboration 1994 The Compact Muon Solenoid Technical Proposal (CERN/LHCC 94-38)[2] The LHC Study Group 1991 Design Study Of The Large Hadron Collider (LHC) (CERN 91-03)[3] A. Acuto 1993 CP asymmetries in neutral B-meson decays (Phys. Rev. D 47 3945-3954)[4] S. Abdullin, IReS, Strasbourg, France, D. Denegri, Saclay, France 1999 B- And Tau-MultiplicitiesPer Event In SUSY (mSUGRA) And Instrumental Implications (CMS-NOTE-1999-035)[5] T. Sjostrand 1993 Pythia 5.7 And Jetset 7.4 Phisics And Manual (CERN-TH.7112/93)[6] E. Norrbin, T. Sjostrand 2000 Production and Hadronizatrion of Heavy Quarks (Eur. Phys. J. C17(2000) 137), (hep-ph 0005110)[7] G. Wrochna, CERN, Geneva 1996 CMS Level-1 Trigger for b-physics studies (CMS-CR-1996-002)[8] S. Gennai, F. Palla, A. Starodumov, R. Dell’Orso 2000 A Study Of bb Production Mechanisms InPythia published in Proceedings Of The Workshop On Standard Model Physics (And More)At LHC (CERN 2000-004)

A study of bB production mechanisms in Pythia

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