ALICE/97{06 Internal Note/PHY 6 March 1997 SIMULATION OF CENTAURO EVENTS AT CASTOR 1 2 3 E. Gladysz-Dziadu s ,Yu. V. Kharlov ,A.D.Panagiotou , 2 S. A. Sadovsky 1 Institute of Nuclear Physics, Krakow, Poland 2 Institute for High Energy Physics, Protvino, Russia 3 University of Athens, Athens, Greece Abstract We present the rst Monte Carlo mo del of Centauro events based on the phe- nomenological mo del of Panagiotou et al. and show the quantitative consequences for kinematics, baryon numb er, mass and decay prop erties of the Centauro reball. The simulation of Centauro events for the CASTOR detector is p erformed. The signatures of these events are discussed in details. Intro duction In this pap er we present the rst Monte Carlo mo del of Centauro events based on the phenomenological mo del [1, 2 , 3] and discuss results on MC simulations of Centauro events for the CASTOR detector of the ALICE exp eriment at LHC [4]. Originally the mo del of Centauro event pro duction was based on exp erimental facts from cosmic ray studies and assumptions of some geometrical characteristics of the events; exp erimentally observed transverse momenta, energies of di erent spices of secondary particles and scenario of the Centauro reball evolution allow to calculate thermo dynamical parameters and the lifetime of the Centauro reball. The extrap olation of this mo del to the higher energy allowed to estimate some observables of Centauro events, when taking into account the collider kinematics [5]. In this approachwe attempt to predict more precisely the characteristics of such kind of events. In the current pap er we present this mo del of Centauro events in heavy ion collisions on assumption of some fundamental characteristics of the Centauro reball which lead to the predictions of observables in such kind of events. The mo del is for- mulated in terms of impact parameter of ion collisions, two thermo dynamical parameters 1 baryochemical p otential and temp erature which are assigned to the Centauro reball pro duced in the scenario of Centauro eventevolution, and the nuclear stopping p ower. Since we construct the fully quantitative mo del wehave to formalize all assumptions of the original mo del and intro duce some additional assumptions. The event generator cal- culates the Centauro reball parameters and pro duces the full event con guration. Thus the mo del predicts all kinematical parameters of the Centauro events whichwere observed in cosmic ray exp eriments. In section 1 we give the gradual thermo dynamical and kinematical description of pro duction and evolution of Centauro-typ e events in relativistic heavy ion collisions and showcharacteristic mass, energy and multiplicity distributions of these events. In section 2 results of the detection simulation of these events with the CASTOR apparatus are given. Centauro events are compared with minimum bias events pro duced by the HIJING generator. Signatures of Centauro events are discussed. 1 Physics of Centauro events Centauro reball evolution. The description of Centauro events [6] was intro duced in pap ers [1, 2 , 3 ]. According to this mo del Centauro events o ccur in nuclear collision in the pro jectile fragmentation region when the pro jectile nucleus p enetrating through the target nucleus transforms its kinetic energy into heating and formats of a slightly hot quark matter with high baryochemical p otential [2, 3]. We refer to this quark matter as a primary Centauro reball. On the rst stage of its evolution it contains u and d quarks and gluons. The high baryochemical p otential results in imp ossibil ity for gluons to fragmentinto uu and dd pairs due to Pauli blo cking [2]. Therefore gluons quickly fragmentinto ss pairs. The partial chemical equilibrium is achieved by couplings -quarks + 0 with u and d-quarks and emission of K and K from the primary reball which decreases the temp erature and entropy. After this stage the Centauro reball b ecomes a slightly 13 strange quark matter SQM with relatively long lifetime 10 sec [3]. Finally the SQM reball decays explosively into baryons and some light metastable strange matter ob jects with A> 6 called as strangelets. Baryon numb er of Centauro reball. We consider collisions of nuclei with atomic numb ers A and A and charges Z and Z resp ectively with impact parameter b. The 1 2 1 2 impact parameter is roughly restricted by 0 <b<R + R ; 1 2 1=3 fm i =1; 2 are radii of colliding nuclei. Centauro reball is where R =1:15A i i pro duced in a region of the twonuclei overlapping. The baryon number N of the reball b can b e estimated from a simple geometrical consideration. We assume that all nucleons of the pro jectile nucleus which o ccur in the overlapping region with the target nucleus can interact. Really only the most central part of the overlapping region of the pro jectile 2 900 800 700 ), arbitrary units b 600 f(N 500 Figure 1: Baryon numb er of the reball 400 pro duced in Pb-Pb collisions with impact 300 parameters 0 <b<5 fm. 200 100 140 150 160 170 180 190 Nb forms the reball. Assuming the uniform distribution of nucleons in a nucleus one can nd N through the volume ratio of overlapping region V and the whole pro jectile b ovrlp nucleus V : 1 V ovrlp N =0:9 A : 1 b 1 V 1 Here the factor 0:9 gives the most central part of the overlapping region. In other words the primary reball baryon number N is de ned bya number of nucleons of the pro jectile b nucleus which impacts in the interaction region. It is naturally to assume that pro jectile and target nuclei are distributed uniformly in 2 the transverse plane, i.e. the squared impact parameter b is distributed uniformly. All cosmic Centauro events were observed with rather high hadron multiplicityN > 70, h hence in our mo del we restrict the Centauro reball pro duction by N > 50. In our b quantitative mo del we use a simple assumption that eachnucleus collision with di erent impact parameters results in Centauro reball pro duction with one and the same ther- mo dynamical characteristics. In the real nature it is not so but it seems to b e reasonable when the impact parameter varies a little. Central collisions are more likely to pro duce the Centauro quark matter reball than p eripheral collisions are. Therefore we calculate the distribution of the baryon numb er of the primary Centauro reball for Pb-Pb colli- sions for impact parameters 0 <b<5 fm which corresp ond to the central collisions, the plot is shown in Fig. 1. The form of the distribution f N is rough enough b ecause of b the motivation discussed ab ove. From the other hand this distribution gives the right representation on the baryon number N range b ecause N is de ned strictly for the xed b b impact parameter. This remark also concerns shap es of other distributions given in the pap er. 3 Mass of Centauro reball. The pro duced reball is a glob of decon ned quark matter whichcharacterized by a temp erature T and baryochemical p otential of a nucleon .As b the basic phenomenological mo del [2, 3] predicts, the Centauro reball has a very high baryochemical p otential which do es not p ermitu and d to b e pro duced. This phase of 23 the Centauro reball is unstable yet and after t 10 sec [3] gluons fragmentinto ss pairs. After that a chemical equilibrium in the reball is achieved. In the rst-order p erturbative QCD the energy density of the quark-gluon plasma containing u, d, s quarks and gluons at the temp erature T around a critical one T is expressed by see, e.g. [7, 8 , 9] c and references therein " = " + " + " : g q s Here q = u; d. Gluon and quark contributions " , " and " are q q s 2 8 15 4 " = T ; 1 g s 15 4 2 2 50 3 7 4 4 2 2 1 1 T + 3 T + ; " = s s q q q 2 10 21 2 " ! 2 4 2 18T m m T m m m s s s s s " = +6 K K : s s 1 2 2 T T T T Here K are i-order mo di ed Bessel functions. The strong coupling constant should b e i s taken at a scale Q 2T and equals =0:3 at a critical temp erature T = 170 MeV [8]. s c The is the strangeness equilibration factor 0:4. The net energy density for all s s degrees of freedom is given by 2 37 110 3 2 4 4 2 2 " = T 1 T + 1 + 3 + " : 2 s s s q q 2 30 37 2 Here baryochemical p otential of a quark can b e taken as = =3. q q b The other thermo dynamical quantities of interest, pressure P and quark numb er den- sity n = N =V are obtained from equation 2: q q fb ! 1 @P P = "; n = ; q 3 @ q T ! 3 2 q 2 1 : 3 n =2 T + s q q 2 Since the numb er of quarks N in the primary Centauro reball is de ned from the collision q geometry as N =3N one can obtain from 1 and 3 the volume of the reball V in q b fb the order O : s 3N 2 b ! V = : 4 1+ s fb 3 q 2 2 T + q 2 4 T = 130 MeV 18000 T = 190 MeV 16000 T = 250 MeV 14000 ), arbitrary units fb 12000 f(M tauro re- 10000 Figure 2: Mass of the Cen ball in Pb-Pb collisions at = 8000 b 1:8 GeV and T = 130, 190 and 6000 250 MeV.