Hadron Multiplicities in Pb＋Pb Collisions at the Large Hadron Collider and Pomeron Loop Effects

We study the pseudo-rapidity distribution of hadron multiplicities of high energy Pb＋Pb collisions by using color glass condensate dynamics at LHC/ALICE in the fixed coupling case. It is found that after including the pomeron loop effects the charged hadron multiplicities at central rapidity are about 1500 for central Pb＋Pb collisions, which are significantly smaller than the saturation based calculations, ～ 1700 ÷ 2500 and compatible with that based on a study of multiplicities in the fragmentation region.     

Fragmentation of S nuclei in photoemulsion at 3.7 AGeV

Data on fragmentation of ³²S nuclei in emulsion at 3.7 AGeV are presented. The fragmentations are studied in terms of the impact parameter of collisions of the ³²S projectiles with the different target nuclei. The fragmentation cross sections and the multiplicities of the different fragments are nearly independent of the sulfur incident energy. The fragmentation of ³²S nuclei into alpha isotopes are extensively studied. The transverse momentum spectra of the emitted fragments are fitted by two Maxwell-Boltzmann distributions of different average transverse momenta. The characteristics (multiplicities and P_t-distributions) of the He-fragments produced in the central collisions are different from those emitted in the peripheral collisions.     

Multifragmentation of gold nuclei by light relativistic ions: Thermal breakup versus dynamical disintegration

The multiple emission of intermediate-mass fragments (IMF) is studied for collisions of p, ⁴He, and ¹²C on Au with the 4π FASA setup. The mean multiplicities of IMF saturate at a value of around 2 for incident energies above 6 GeV. An attempt at describing the observed IMF multiplicities in the two-stage scenario, a fast cascade followed by a statistical multifragmentation, fails. Agreement with the measured IMF multiplicities is obtained by introducing an intermediate expansion phase and modifying empirically the excitation energies and masses of remnants. The angular distributions and energy spectra from p-induced collisions are in agreement with the scenario of “thermal” multifragmentation of a hot and expanded target spectator. In the case of ¹²C + Au(22.4 GeV) and ⁴He (14.6 GeV) + Au collisions, deviations from a pure thermal breakup are seen in the fragment energy spectra, which are harder than those both from model calculations and from the measured ones for p-induced collisions. This difference is attributed to a collective flow with the expansion velocity at the surface of about 0.1c (for ¹²C + Au collisions).     

Thermal parameters in heavy ion collisions at SPS and RHIC: Centrality dependence

The centrality dependence of thermal parameters describing hadron multiplicities and intermediate-mass dilepton spectra in heavy ion collisions at SPS and RHIC is analyzed. From experimental hadron multiplicities we deduce evidence for strangeness saturation at high energy and maximum centrality. The observed dilepton spectra can be parameterized by a centrality independent temperature.     

Impact-parameter dependences of the number of interacting nucleons and mean multiplicities in heavy-ion collisions

The impact-parameter dependence of the mean number of interacting nucleons in high-energy heavy-ion collisions is considered in the case of identical atomic weights of colliding nuclei and in the case where one nucleus is much heavier than the other. It is shown that the use of a rare event—for example, an event of J/ψ or ? production—as a trigger may change significantly quantities representing the averages of the multiplicities for accompanying secondaries over impact-parameter values. The multiplicities of accompanying particles in central collisions can have but a slight dependence on the trigger. The observed multiplicity ratios for various secondaries in central and minimum-bias events can be used as a test in searches for quark-gluon plasma.     

Comment on entropy production in nuclear collisions

A recent analysis of the data for particle production in central nucleus-nucleus (AA) collisions finds an “enhancement” in particle multiplicities or entropy at the highest energies near 200A GeV (as compared to those at lower energies). This is interpreted within a relativistic photon gas model as an increase in the number of degrees of freedom, and the formation of the quark-gluon plasma between AGS and SPS energies is hypothesized. We find that particle multiplicities in A collisions also show an enhancement at large F, a nonlinear increase with the Fermi energy variable, . This suggests the possibility that the enhancement seen in AA collisions is also due to such a non-linearity in F. Received: 11 November 1997 / Published online: 10 March 1998     

The experimental and computer studies of the double-scattering effect for polycrystals

The effect of double scattering of 10–35 keV Ar ions by a copper polycrystal has been studied in the case of asymmetric reflection. Contributions from collisions of various multiplicities to the obtained energy spectra have been analysed theoretically. The spectra are computer-calculated using the statistical trial method with due account for a discrete set of possible distances between the surface atoms which scatter the ions. The charge-exchange processes are also included in the calculations.     

Centrality dependence of multiplicities in ultrarelativistic nuclear collisions

We compute the centrality dependence of multiplicities of particles produced in ultrarelativistic nuclear collisions at various energies and atomic numbers. The computation is carried out in perturbative QCD with saturated densities of produced gluons and by including effects of nuclear geometry. Numbers are given for Au+Au collisions at RHIC energies.     

A consistent parametrisation for the production rates of negatively charged hadrons and neutral strange particles in nucleon-nucleon,nucleon-nucleus and nucleus-nucleus collisions

A parametrisation of negative hadron and neutral strange particle production was developed which consistently describes presently available data from nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions at a beam energy of 200 GeV per nucleon. Average multiplicities of negative hadrons are found to be proportional to the number of wounded nucleons, averageK _s ⁰ multiplicities proportional to the number of wounded quarks, and average and multiplicities proportional to the number of wounded quarks plus an additional contribution proportional to the number of interactions of secondary produced particles. Predictions are given for lead-lead collisions.On leave from Rudjer Boskovic Institute, Zagreb, Croatia     

Statistical hadronization model and transverse momentum spectra of hadrons in high energy collisions

A detailed analysis of transverse momentum spectra of several identified hadrons in high energy collisions within the canonical framework of the statistical model of hadronization is performed. The study of particle momentum spectra requires an extension of the statistical model formalism used to handle particle multiplicities, which is described in detail starting from a microcanonical treatment of single hadronizing clusters. Also, a new treatment of extra strangeness suppression is presented which is based on the enforcement of fixed numbers of pairs in the primary hadrons. The considered center-of-mass energies range from to 30 GeV in hadronic collisions ( and Kp) and from 15 to 35 GeV in collisions. The effect of the decay chain following hadron generation is accurately and exhaustively taken into account by a newly proposed numerical method. The exact conservation at low energy and the increasing hard parton emission at high energy bound the validity of the presently taken approach within a limited center-of-mass energy range. However, within this region, a clear consistency is found between the temperature parameter extracted from the present analysis and that obtained from fits to average hadron multiplicities in the same collision systems. This finding indicates that in the hadronization process the production of different particle species and their momentum spectra are two closely related phenomena governed by one parameter. Received: 31 October 2001 / Published online: 15 February 2002     

Hadron multiplicities

We review results on hadron multiplicities in high-energy particle collisions. Both theory and experiment are discussed. The general procedures used to describe particle multiplicity in quantum chromodynamics (QCD) are summarized. The QCD equations for the generating functions of the multiplicity distributions are presented both for fixed and running coupling strengths. The mean multiplicities of gluon and quark jets, their ratio, higher moments, and the slopes of multiplicities as a function of energy scale, are among the main global features of multiplicity for which QCD results exist. Recent data from high energy e⁺e⁻ experiments, including results for separated quark and gluon jets, allow rather direct tests of these results. The theoretical predictions are generally quite successful when confronted with data. Jet and subjet multiplicities are described. Multiplicity in limited regions of phase space is discussed in the context of intermittency and fractality. The problem of singularities in the generating functions is formulated. Some special features of average multiplicities in heavy quark jets are described.     

Neutrino propagation in a medium with a magnetic field

Data on the mean multiplicity of strange hadrons produced in minimum bias proton-proton and central nucleus-nucleus collisions at momenta between 2.8 and 400 GeV/c per nucleon have been compiled. The multiplicities for nucleon-nucleon interactions were constructed. The ratios of strange particle multiplicity to participant nucleon as well as to pion multiplicity are larger for central nucleus-nucleus collisions than for nucleon-nucleon interactions at all studied energies. The data at AGS energies suggest that the latter ratio saturates with increasing masses of the colliding nuclei. The strangeness to pion multiplicity ratio observed in nucleon-nucleon interactions increases with collision energy in the whole energy range studied. A qualitatively different behaviour is observed for central nucleus-nucleus collisions: the ratio rapidly increases when going from Dubna to AGS energies and changes little between AGS and SPS energies. This change in the behaviour can be related to the increase in the entropy production observed in central nucleus-nucleus collisions at the same energy range. The results are interpreted within a statistical approach. They are consistent with the hypothesis that the Quark Gluon Plasma is created at SPS energies, the critical collision energy being between AGS and SPS energies.     

Production of secondaries in relativistic nuclei collisions at high energies

Using the multiple-scattering theory and quark-gluon string model, the averaged and central collisions between high-energy nuclei are studied. Production of light flavour hadrons as well as charmed and beauty mesons and baryons is considered. We present predictions for mean multiplicities of secondaries, inclusive spectra and distributions over multiplicity for the case of Au?Au collisions with energy 100 GeV per nucleon in every beam.     

Heavy quark energy loss in high multiplicity proton-proton collisions at the LHC

One of the most promising probes to study deconfined matter created in high energy nuclear collisions is the energy loss of (heavy) quarks. It has been shown in experiments at the Relativistic Heavy Ion Collider that even charm and bottom quarks, despite their high mass, experience a remarkable medium suppression in the quark gluon plasma. In this exploratory investigation we study the energy loss of heavy quarks in high multiplicity proton-proton collisions at LHC energies. Although the colliding systems are smaller than compared to those at the Relativistic Heavy Ion Collider (p+p vs Au+Au), the higher energy might lead to multiplicities comparable to Cu+Cu collisions at the Relativistic Heavy Ion Collider. The interaction of charm quarks with this environment gives rise to a non-negligible suppression of high momentum heavy quarks in elementary collisions.     

相对论性核—核碰撞中的弹核碎片多重数与角分布

讨论了相对论性核-核碰撞中弹核碎片的多重数分布与发射角分布.分别给出了多重数及发射角的几率分布公式.计算结果与15—200GeV/N能区核核碰撞的实验数据进行比较,二者在实验误差范围内符合.     

Exclusive measurements of mean pion multiplicities in 4He-nucleus reactions from 200 to 800 MeV/nucleon

Mean multiplicities of pi+ and pi- in 4He collisions with C, Cu, and Pb at 200, 600, and 800 MeV/u, and with C and Pb at 400 MeV/u have been measured using the large solid angle detector Diogene. The independence of pion multiplicity on projectile incident energy, target mass and proton multiplicity is studied in comparison with intra-nuclear cascade predictions. The discrepancy between experimental results and theory is pointed out and discussed.     

Jet production from nuclei at 400 GeV/c

Results are presented from a calorimeter study on the production of jets in large-E_T proton-nucleus collisions at 400 GeV/c. Jet-like large-p_T events from eight nuclear targets are seen with a special jet trigger. As previously observed, the cross section for such events increases slightly faster than the atomic number, much like in the production of large-p_T single hadrons. Data on energy flows and multiplicities suggest that a heavy nucleus does not significantly widen the angular distribution of the beam remmants. In the central region the particle flow in hard nuclear collisions is similar to that in soft nuclear collisions.     

On chemical equilibrium in nuclear collisions

The data on average hadron multiplicities in central A+A collisions measured at CERN SPS are analysed with the ideal hadron gas model. It is shown that the full chemical equilibrium version of the model fails to describe the experimental results. The agreement of the data with the off–equilibrium version allowing for partial strangeness saturation is significantly better. The chemical freeze–out temperature of about 180 MeV seems to be independent of the system size (from S+S to Pb+Pb) and in agreement with that extracted in , and collisions. The strangeness suppression is discussed at both hadron and valence quark level. It is found that the hadronic strangeness saturation factor increases from about 0.45 for interactions to about 0.7 for central A+A collisions with no significant change from S+S to Pb+Pb collisions indicating that the strangeness enhancement in heavy ion collisions cannot be fully attributed to the increased system size. The quark strangeness suppression factor is found to be about 0.2 for elementary collisions and about 0.4 for heavy ion collisions independently of collision energy and type of colliding system. Received: 31 October 1997 / Published online: 26 February 1998     

Multiplicity of charged secondaries emitted in association with neutral strange particles in antiproton-nucleus collisions at 40 GeV/c

In collisions of 40-GeV/c antiprotons with D, Li, C, S, Cu, and Pb nuclei, mean multiplicities of various secondary particles are investigated as functions of the mass number A. The mass-number dependence of the mean multiplicities of positively charged particles suggests that the effect of intranuclear cascades is strong for the emission of Λ hyperons, but that it is relatively weak for the emission of either K ⁰ or $\bar \Lambda $ . Also measured are the yields of various neutral strange particles with respect to those of charged secondaries.