Anomalous J/ψ suppression in Pb---Pb interactions at 158 GeV/c per nucleon

The Drell-Yan and J/ψ cross-sections measured in Pb---Pb collisions are compared with the values extrapolated from the results obtained in proton and light ion induced reactions. While the Drell-Yan production exhibits the normal expected behaviour, the yield of J/ψ in Pb---Pb interactions is abnormally low, as it lies 9 standard deviations below the expected value. Moreover, the departure from the expected behaviour increases significantly from peripheral to central collisions.     

Some general characteristics of Si-emulsion interactions at 4.5 GeV/c per nucleon

We present our observations on the salient features of the interactions produced by ²⁸Si nuclei of 4.5 GeV/c per nucleon in photographic emulsions. The results on the inelastic mean-free path and interaction cross section are presented and compared with the predictions of a geometrical model. Measurements on the multiplicity distribution, mean multiplicities, multiplicity correlations and multiplicity scaling of charged secondary particles are studied. A comparison with other available data at same/nearby incident momentum per nucleon from proton-nucleus and carbon-nucleus collisions is presented in order to examine the dependence on the projectile mass.     

Suppression of charmonium production in PbPb collisions at 158 GeV/c per nucleon and quark-gluon deconfinement

An anomalous suppression of the charmonium yield in central collisions was observed in studying charmonium production in collisions of Pb nuclei accelerated to a momentum of 158 GeV/c per nucleon with Pb target nuclei. It is shown that, in peripheral collisions, the ratio of the cross section for J/ψ production to the cross section for the Drell-Yan process decreases exponentially (as in the case of collisions of lighter nuclei) owing to the ordinary absorption of J/gy in nuclear matter. The observed threshold effect of the anomalous suppression of charmonium production agrees well with the predictions based on the assumption of Debye color screening in the formation of quark-gluon plasma.     

J/psi production in Indium-Indium collisions at 158 GeV/nucleon

The NA60 experiment studies muon pair production at the CERN Super Proton Synchrotron. In this Letter we report on a precision measurement of J/psi in In-In collisions. We have studied the J/psi centrality distribution, and we have compared it with the one expected if absorption in cold nuclear matter were the only active suppression mechanism. For collisions involving more than approximately 80 participant nucleons, we find that an extra suppression is present. This result is in qualitative agreement with previous Pb-Pb measurements by the NA50 experiment, but no theoretical explanation is presently able to coherently describe both results.     

Strange baryon signals of deconfinement in Pb−Pb collisions at 158 GeV/c per nucleon

The main results of the CERN WA97 heavy ion experiment are summarized. The evidence for the formation of a deconfined state of matter in Pb?Pb collisions at CERN SPS energies, provided by these results, is discussed. The ongoing experimental efforts, by the CERN NA57 experiment, to investigate the onset of deconfinement are briefly reviewed.     

Collective sea-gull effect in πp interactions at 250 GeV/c

A collective “sea-gull” effect is observed for a system of fast hadrons produced in the fragmentation regions of nondiffractive π⁺p-interactions at 250 GeV/c. The effect is not reproduced by the FRITIOF fragmentation model in its details. It is demonstrated that hard-like processes, both in the collision phase and in the fragmentation phase, are not properly treated in the model.     

Single and double dissociation in πp collisions at 11 and 16 GeV/c

The reactions π⁻p → 2π⁻π⁺p, π⁻p → 2π⁻π⁺π^op and π⁻p → 2π⁻2π⁺n are analysed at 11 and 16 GeV/c using longitudinal phase space (LPS) plots. The weighted LPS distributions for π⁻p → 2π⁻π⁺p is dominated by two well separated structures corresponding to single diffraction dissociation of the pion, π⁻p → (2π⁻π⁺)p, and of the proton, π⁻p → π⁻(π⁻π⁺p). The former is more abundant than the latter, and both are approximately constant with energy. In contrast, processes of type π⁻p → (2π)(πp) decrease with increasing energy.

In the five-body reactions the weighted LPS distribution reveals especially at 16 GeV/c a maximum for single dissociation of the proton into 3πp, namely π⁻p → π⁻(π⁻π⁺π^op); this process is likely to be diffractive. The neutron channel has a corresponding maximum displaced toward a multiperipheral configuration π⁻p → π⁻(π⁻2π⁺)n. Another strong maximum corresponds to the pion dissociation π⁻p → (2π⁻π⁺π^o)p. This is interpreted to be an ω-exchange process because no analogous structure occurs in π⁻p → (2π⁻2π⁺)n. Finally, a broad structure reveals double dissociation of both incident particles; it occurs in the two channels π⁻p → (2π⁻π⁺)(π^op) and π⁻p → (2π⁻π⁺)(π⁺n), being stronger in the latter. Further analysis of this process in terms of isospin exchange suggests that it is partially diffractive. Factorization is also discussed.

An appendix gives general aspects of the LPS analysis for the asymptotic study of n-body collisions at very high energy.     

Multi-Regge analysis of the pp→ppππ reaction at 8.1 GeV/c

We present here an analysis of the reaction pp→ppπ⁺π⁻ at 8.1 GeV/c in terms of the two multi-Regge models proposed by Chan, ;oskiewicz and Allison and by Berger et al. in which we introduce some modifications to improve the agreement between experimental data and theoretical results. Comparison between various experimental distributions and corresponding theoretical curves is satisfactory with a very simple parametrization in both models.     

Higher baryon resonances in carbon-carbon collisions at 4.2 GeV/c per nucleon

The production of the N(1440), the so-called Roper, and N(1520) resonances in high-energy collisions of carbon nuclei with the carbon nucleus, using a 2 m propane bubble chamber, was investigated. Attention was paid to the two-pion decay mode of the higher baryon resonances. From the invariant masses of three-particle states the mass and width of the resonances were obtained. The ratio of the resonances decay to and states was estimated.Received: 1 July 2003, Revised: 29 September 2003, Published online: 20 April 2004PACS: 25.70.Ef Resonances - 14.20.Gk Baryon resonances with S = 0     

Hadronic expansion dynamics in central Pb+Pb collisions at 158 GeV per nucleon

Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to ext ract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced. Received: 25 November 1997 / Published online: 24 March 1998     

Reconstruction of hadronization stage in Pb+Pb collisions at 158 A GeV/c

Recent data on hadron multiplicities in central Pb+Pb collisions at 158 A GeV/c at mid-rapidity are analyzed within the concept of chemical freeze-out. A non-uniformity of the baryon chemical potential along the beam axis is taken into account. An approximate analytical solution of the hydrodynamic equations for a chemically frozen Boltzmann-like gas is found. The Cauchy conditions for hydrodynamic evolution of the hadron resonance gas are fixed at the thermal freeze-out hypersurface from analysis of one-particle momentum spectra and HBT correlations. The proper time of chemical freeze-out and physical conditions at the hadronization stage, such as energy density and averaged transverse velocity, are found.     

Structures and angular distributions of slow and relativistic charged particles produced in 7Li-emulsion interactions at 3 GeV/c per projectile nucleon

Experimental results on the multiplicity distributions of various particles produced in the interactions of ⁷Li with emulsion nuclei at a momentum of 3 GeV/c per projectile nucleon are reported. A comparison with data on collisions induced by other nuclei at a nearly identical momentum per nucleon is presented in order to reveal the dependence on the projectile mass. The internal structure of ⁷Li is explored by studying the projectile fragment. The mean multiplicity of shower particles, 〈n _s〉, induced by ⁷Li is found to be less than that in the case of ⁶Li projectiles. The angular distributions of target fragments and relativistic charged secondaries are investigated. No shock-wave phenomena are observed. Forward-to-backward ratios are calculated for each case. The probability distributions for relativistic secondaries produced per unit rapidity are studied in detail, along with the rapidity densities and their dependence on the projectile and the target mass. A comparison of the angular spectra of shower particles produced in central and peripheral collisions supports the limiting-fragmentation hypothesis. The collisions in question seem to become more central with increasing shower-particle multiplicity.     

Small-angle correlations of identical particles in carbon-carbon interactions atP=4.2 GeV/c per nucleon

Two-particle small-angle correlations between negative pions and between protons in carbon-carbon collisions atP=4.2 GeV/c per nucleon have been studied, both for an unbiased sample and for “central” events. A comparison of experimental π^? π^? andpp correlation functions with theoretical predictions has been made. A possible evidence for the existence of two fireballs in C+C interactions atP=4.2 GeV/c per nucleon is presented.     

158GeV/c的Pb–Pb碰撞中的J/ψ抑制现象研究

在Blaizot等人建立的模型基础上,我们考虑了核–核碰撞中的参与者数与核子–核子碰撞数的起伏效应对J/?ψ产生的影响,并对NA50合作组在入射动量为158GeV/c的Pb-Pb碰撞中的实验数据进行了分析,理论结果与实验数据符合的很好     

Projectile stopping in nucleus-nucleus and hadron-nucleus collisions at 4.2 and 10 GeV/c per nucleon

The collisions ofp,²H,⁴He and C with carbon and tantalum nuclei at 4.2 GeV/c per nucleon as well as the collisionsp-C andp-Ta at 10 GeV/c from 2-m propane bubble chamber have been studied. New results on nuclear stopping have been obtained from the examination of proton rapidity distributions and average rapidity of leading protons for collisions of various degree of centrality: our study points out that a proton projectile is fully stopped in the centralp-Ta collisions at 4.2 GeV/c but only partly stopped at 10 Gev/c. The proton multiplicity in the centralp-Ta collisions at 10 GeV/c can be described by the binomial distribution,P(n), which expresses the probability that the projectile meetsn protons among the nucleons being along the diameter of a target nucleus.     

A new measurement of $J/\psi$ suppression in Pb-Pb collisions at 158 GeV per nucleon

We present a new measurement of production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions. Received: 25 August 2004, Revised: 15 December 2004, Published online: 21 January 2005     

Features of CC interactions at a momentum of 4.2 GeV/c per nucleon for various degrees of nuclear-collision centrality

The multiplicity, rapidity, and transverse-momentum distributions of secondaries (negative pions and protons) from CC interactions characterized by various numbers of participant protons are presented. It is shown that, in contrast to the mean transverse momenta of protons, the mean transverse momenta of pions depend only slightly on the degree of collision centrality. The shape of the rapidity distributions of negative pions is also weakly dependent on the degree of collision centrality. With decreasing impact parameter, the product protons are found to concentrate in the central rapidity region. The data in question are analyzed within the modified version of the FRITIOF model. It is shown that, as soon as elastic nucleon rescatterings are taken into account, the model describes satisfactorily the experimental features of negative pions and fast protons. The spectra of slow protons are described only qualitatively because of limitations of the evaporation model used here to treat the deexcitation of residual nuclei.