MCSTHAR++ is a new Monte Carlo code implementing the Statistical
Hadronization Model. This model assumes that hadronization proceeds through the
microcanonical decay of massive extended clusters. Unlike other hadronization
models, in this approach very few free parameters are needed, as has been
demonstrated in previous studies. The tuning of the model and the comparison
with the data is ongoing.Comment: 3 pages; To appear in the proceedings of the conference IFAE 2010,
  Roma, Italy, 7-9 April 201

Becattini, F.

Bignamini, C.

Piccinini, F.

English

arXiv

MCSTHAR++ is a new Monte Carlo code implementing the Statistical Hadronization Model. This model assumes that hadronization proceeds through the microcanonical decay of massive extended clusters. Unlike other hadronization

models, in this approach very few free parameters are needed, as has been demonstrated in previous studies. The tuning of the model and the comparison with the data is ongoing

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DOI 10.1393/ncc/i2011-10766-4Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010MCSTHAR++, a Monte Carlo code for the microcanonicalhadronizationC. Bignamini(1)(∗), F. Becattini(2) and F. Piccinini(3)(1) Dipartimento di Fisica Nucleare e Teorica, Universita` di Pavia and INFN, Sezione di PaviaPavia, Italy(2) Dipartimento di Fisica, Universita` di Firenze and INFN, Sezione di FirenzeFirenze, Italy(3) INFN, Sezione di Pavia - Pavia, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 2 Febbraio 2011)Summary. — MCSTHAR++ is a new Monte Carlo code implementing the StatisticalHadronization Model. This model assumes that hadronization proceeds throughthe microcanonical decay of massive extended clusters. Unlike other hadronizationmodels, in this approach very few free parameters are needed, as has been demon-strated in previous studies. The tuning of the model and the comparison with thedata is ongoing.PACS 12.40.Ee – Statistical models.PACS 13.66.Bc – Hadrons production in e−e+ interaction.1. – IntroductionThe transition from the partonic state to the confined state is described using variousphenomenological models like the Cluster Models [1, 2] and the String Model [3]. Acompletely independent model, based on a statistical formulation and known as StatisticalHadronization Model [4], has been studied in the literature but it is not available in anyofficial release of the Monte Carlo event generators for High Energy Physics. In this papera short overview of the microcanonical formulation of the Statistical Hadronization Modelwill be given and a new Monte Carlo code performing the hadronization process accordingto that model, MCSTHAR++, and its interface to the event generator HERWIG6.510 [5] willbe described, showing also some preliminary results obtained for the hadronization oflight quarks only.(∗) Speaker.c© Societa` Italiana di Fisica 185186 C. BIGNAMINI, F. BECATTINI and F. PICCININI2. – The Statistical Microcanonical Hadronization Model: a short overviewAs described in [4], the statistical model is based on the hypothesis that in a highenergy collision some extended objects made of pre-hadronic matter, called clusters orfireballs, are produced. These objects are supposed to be colorless and characterized bywell defined physical quantities like energy, momentum and abelian charges. Each one ofthese clusters hadronizes according to a pure statistical law, which for the microcanonicalformulation is the following: every multihadronic state confined inside the cluster andconserving all the physical quantities of the cluster itself is equally likely. In this pictureof the hadronization process the cluster is treated as a microcanonical ensemble, whosepartition function Ω is given by [6]Ω =∑(Nj)〈(Nj) |PQPV PQ| (Nj)〉, where PV =∑hV|hV 〉〈hV |(1)is the sum over all the multihadronic states |hV 〉 confined inside the cluster, while thesum on the left equation is over all possible channels, corresponding to the asymptoticstates, which are identified by the K-tuple of integer (Nj) = (N1, N2, . . . , NK), one foreach hadron species j, and where PQ is the projector on the conserved set of charges.The above partition function is then modified introducing the strangeness suppressionparameter γs and multiplying the weight of each channel by the factor γNss , where Nsis the total number of strange and antistrange quarks contained in the hadrons of thechannel itself. At this point this hadronization model requires only two parameters,which need to be tuned on the experimental data: the γs parameter and the energydensity of the clusters ρ, which is taken to be equal for each cluster and which is used toconvert the mass of a cluster into its volume, a quantity which appears in Ω, as can beseen in [6].3. – MCSTHAR++: code description and preliminary resultsMCSTHAR++ (Monte Carlo STatistical HAdronization in high energy Reactions) is aMonte Carlo code implementing the Statistical Hadronization Model in its microcanoni-cal formulation. It is written in Object Oriented C++ and it is built to take as input a setof clusters and to give in output a set of hadrons (stable and unstable), coming from themicrocanonical hadronization of each one of the incoming clusters. For the hadronizationof these objects the exact conservation of energy-momentum, electric charge, strangeness,baryonic number, charm and beauty charge is imposed.In the present case, MCSTHAR++ is interfaced to HERWIG6.510 [5], the clusters used asinput for MCSTHAR++ are the “standard” HERWIG’s clusters (allowed to have non-zero bary-onic number) and the primary hadrons produced during the hadronization are decayedusing the HERWIG’s routines for the strong and electroweak hadron decay.Even if a full tuning of the model is mandatory, it is already possible to see thegood behavior of the hadronization code in the preliminary results shown in this sec-tion. The theoretical predictions are obtained with no tuning at all of the HERWIG’sparameters and with a reasonable choice of the MCSTHAR++’s parameters: γs = 0.65 andρ = 0.35GeV/fm3. The charged-particle scaled momentum distributions of fig. 1 andthe multiplicity values of table I show a comparison among MCSTHAR++, HERWIG6.510and LEP (OPAL [7] and DELPHI [8]) data for the process e+e− → γ/Z0 → uu¯, dd¯, ss¯at 91.2GeV center-of-mass energy.MCSTHAR++, A MONTE CARLO CODE FOR THE MICROCANONICAL HADRONIZATION 187Fig. 1. – Charged-particle scaled momentum xp = 2p/√s and log(1/xp) distribution.Table I. – Mean values of the charged-particle multiplicity and of the production rate of chargedpions, charged kaons and (anti)protons, for the hadronization of light quarks only.Nch Nπ± NK± Np,p¯MCSTHAR++ 19.53± 0.14 16.64± 0.11 1.65± 0.04 0.98± 0.07HERWIG6.510 18.601± 0.006 15.022± 0.006 1.628± 0.002 1.736± 0.002DELPHI 19.94± 0.34 16.84± 0.87 2.02± 0.07 1.07± 0.054. – ConclusionsMCSTHAR++, a new Monte Carlo code for the Statistical Microcanonical Hadronization,has been presented. This code, written in C++, will be soon available to be used with theMonte Carlo event generators alternatively to the standard hadronization modules, as ithas been shown in this paper, where MCSTHAR++ has been interfaced to HERWIG6.510.The next steps of this work will be focused on the tuning of MCSTHAR++, completedwith the hadronization of c and b quarks, on LEP and Tevatron data, and on the com-parison with other available hadronization models.∗ ∗ ∗One of us (CB) would like to thank S. Gieseke for support and useful discussionsand the ITP of the Karslruhe University for warm hospitality. Work supported in partby the EU Marie Curie Research Training Network MCnet (MRTN-CT-2006-035606).REFERENCES[1] Webber B. R., Nucl. Phys. B, 238 (1984) 492; Marchesini G. and Webber B. R., Nucl.Phys. B, 310 (1988) 461.[2] Winter J. C., Krauss F. and Soff G., Eur. Phys. J. C, 36 (2004) 381.[3] Andersson B., Gustafson G., Ingelman G. and Sjo¨strand T., Phys. Rep., 97 (1983)31.[4] Becattini F., arXiv:0901.3643 [hep-ph] (2009) and references therein.188 C. BIGNAMINI, F. BECATTINI and F. PICCININI[5] Corcella G., Knowles I. G., Marchesini G., Moretti S., Odagiri K., RichardsonP., Seymour M. H. and Webber B. R., JHEP, 0101 (2001) 010.[6] Becattini F. and Ferroni L., Eur. Phys. J. C, 35 (2004) 243; 38 (2004) 225.[7] Ackerstaff K. et al., Eur. Phys. J. C, 7 (1999) 369.[8] Abreu P. et al., Eur. Phys. J. C, 5 (1998) 585.

MCSTHAR++, a Monte Carlo code for the microcanonical

hadronization

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MCSTHAR++, a Monte Carlo code for the microcanonical hadronization

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