Production In Nucleus-nucleus Collisions Research Articles

Coherent final state interaction in jet production in nucleus-nucleus collisions

The coherent final state interaction of an energetic parton produced in AA collisions is studied. This interaction is due to the change in the cutoff scale and in the running coupling constant when the parton passes from a vacuum to a quark-gluon plasma. It is demonstrated that the contribution of this new mechanism to the energy loss may be of the same order of magnitude as the induced gluon radiation. However, an accurate evaluation of this medium effect is a difficult task, because there is a strong cancellation between the cutoff and running coupling constant effects. The uncertainties in the contribution of the coherent final state interaction restrict strongly the accuracy of jet tomographic analyses of the matter density produced in AA reactions.

Kaon production at subthreshold and threshold energies

We summarize what we have learnt about kaon production innucleus-nucleus collisions over the previous decade. We will address threequestions.(a) Is the K+ production sensitive to the nuclear equation of state? (b)How can it happen that at the same excess energy the same number of K+ and K- are produced in heavy-ion collisions although the elementarycross section in pp collisions differs by orders ofmagnitudes? and (c) Whydokaons not flow?

Dimuon and charm production in nucleus-nucleus collisions at the CERN-SPS

Muon pair production in p-A, S-U and Pb-Pb collisions has been studied by the NA38 and NA50 collaborations at the CERN SPS. In this paper we present an analysis of the dimuon invariant mass region between the $\phi$ and the J/ $\psi$ . In p-A collisions we find that, after the subtraction of the combinatorial background due to $\pi$ and K decays, the superposition of dimuons from semi-leptonic decays of D mesons and from the Drell-Yan process reproduces well the measured mass and transverse momentum spectra. The yield of open charm dimuons required to fit our data leads to a charm production cross section in good agreement with previous measurements. A linear extrapolation of the p-A sources with the product of the mass numbers of the projectile and target nuclei, $A\times B$ , underestimates the dimuon yield measured in S-U and Pb-Pb collisions. The excess increases with the number of participant nucleons, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distributions of the measured dimuon excess are compatible with those expected from the open charm contribution.

Kaon production in nucleus-nucleus collisions at 92 MeV per nucleon

K+ production far below the free nucleon-nucleon threshold has been investigated in collisions of 36Ar on 12C, natTi, and 181Ta targets at an incident energy of 92 MeV per nucleon. The cross sections for K+ production have been inferred from the observed muon decays of positive kaons. The results are discussed in the framework of a participant-spectator model and are compared to proton induced K+ production and to subthreshold pion production experiments.

Medium effect on photon production in ultrarelativistic nuclear collisions

The effect of in-medium vector and axial-vector meson masses on photon production is studied. We assume that the effective mass of a vector meson in hot nuclear matter decreases according to a universal scaling law, while that of an axial-vector meson is given by Weinberg's mass formula. We find that the thermal production rate of photons increases with reduced masses, and is enhanced by an order of magnitude at T=160 MeV with $m_\rho=300$ MeV. Assuming a hydrodynamic evolution, we estimate the effect of the reduced masses on photon production in nucleus-nucleus collisions. The result is compared to experimental data from the WA80/WA98 collaboration.

Particle production from disoriented chiral condensates described by a coherent state

The production of pions and sigma mesons from disoriented chiral condensates (DCC's) of finite size is studied using the linear sigma model and a continuous mode coherent state. Two models are used to describe the decay of DCC's. One assumes instantaneous decay and the other assumes radiation from the surface. It is found that, for a spherical DCC of radius of several fm, possible signals related to the momentum spectrum, multiplicity, and the neutral-to-charged pion ratio will be completely masked by the background which dominates multiparticle production in nucleus-nucleus collisions at high energies unless an appropriate cut or binning in the phase space is made on an event-by-event basis. We propose that a signal of DCC's may manifest itself as an abnormal peak in the rapidity distribution if pions with medium and large transverse momenta are cut off. The smearing effect due to sigma meson decay on the characteristic distribution of neutral-to-charged pion ratio is also studied.

A pixel detector system for ALICE

Two layers of Si pixel detectors are foressen in the proposed layout of the ALICE experiment at LHC. Their main task is to provide secondary vertex detection capability for the study of flavour production in nucleus-nucleus collisions at LHC. We illustrate the requirements for the ALICE pixel system, present a concept for its implementation and discuss the expected performance for the detection of charm and strange decay vertices.

Fusion of strings and its consequences at the highest cosmic ray energies

It is shown that the fusion of string is a source of particle production in nucleus-nucleus collisions outside the kinematical limits of nucleon-nucleon collisions. The spectrum of different particles is compared with the high energy data on p-A collisions obtaining a reasonable agreement. Results for A-B collisions at s =19.4 AGeV and s =200 AGeV are given and possible implications for cosmic rays are examined. Both the enhancement of the cumulative effect and the reduction of multiplicities implied by string fusion should strongly modify the first interactions and the profile of extensive air showers and should be taken into account in their simulation.

Quarkonium production and colour deconfinement in nuclear collisions

The suppression of quarkonium production in nucleus-nucleus collisions was originally proposed as a signal of colour deconfinement. Strong “anomalous” J ψ suppression in PbPb collisions has been reported at this Conference by the NA50 Collaboration. Is this suppression really anomalous? Can we conclude that the quark-gluon plasma is already discovered? What has to be done next? I address these questions basing on the current theoretical understanding of quarkonium production and new precise experimental information.

Fusion of strings and cosmic rays at ultrahigh energies

It is shown that the fusion of strings is a source of particle production in nucleus-nucleus collisions outside the kinematical limits of nucleon-nucleon collisions. Together with another effect of string fusion, reduction of multiplicities, this sheds some light on two of the main problems of ultrahigh energy cosmic rays, the chemical composition and the energy of the most energetic detected cosmic rays.

The production of residual nuclei in peripheral high energy nucleus-nucleus interactions

A formation zone intranuclear cascade model is applied to peripheral nucleus-nucleus collisions. We calculate the excitation energies of prefragments, treat their further nuclear disintegration and introduce a model for nuclear deexcitation by photon emission. Results are compared to data on target associated particle production in nucleus-nucleus collisions. We discuss implications of these models to the description of particle production in the fragmentation regions. Special emphasis is put on applications for air showers induced by cosmic ray nuclei and for residual nucleus production at heavy ion colliders.

Scaling between K + and proton production in nucleus-nucleus collisions

An empirical scaling involving the production of strange particle K + and protons has been observed at each rapidity over a large range covered by the E-802 spectrometer in reactions from minimum biased p+Be to central Si+Au at 14.6 A·GeV/c. This scaling has implications for the temporal evolution in nucleus-nucleus collisions.

On the photon production in nucleus-nucleus collisions at high energy

A modified Landau hydrodynamical model is applied to study hard thermal photon production in central heavy-ion collisions at LHC, RHIC and SPS energies. It is shown that the phase transition from quark-gluon plasma into hadrons in consequence of the thermodynamical expansion is close to the second order phase transition if a resonance production is taken into account. Hard direct photon emission is also investigated with consideration of nuclear shadowing effect on structure function of quarks and gluons. Also ππ photon background is investigated. It is demonstrated that at the LHC energy photon yield from the quark-gluon plasma in the photon transversal momentumk⊥ range from 5 to 25 GeV/c exceeds both the background and the direct photon yield. This conclusion may be important for the quark-gluon plasma diagnostic aims. It is also shown that for the LHC energy the thermal photon yield in the present model essentially exceeds this yield obtained in the Bjorken scaling model.

Mass dependence of antiproton production in nucleus-nucleus collisions at 3.65 GeV/nucleon

Antiproton production cross-sections have been measured for p+C, C+C, C+Cu and C+Pb collisions at 3.65 GeV/nucleon.\(\bar p\) laboratory momentum and angle are 0.8 GeV/c and 24°. The target mass dependence parameter is found to be 0.43±0.1. A strong increase in antiproton yield is observed from p+C, d+C to C+C collisions. Projectile mass parameter is 1.2±0.2 for d+C to C+C. The construction and calibration of APAKI, an annihilation detector for\(\bar p\) identification, are also described.

Delta excitations in compressed finite nuclei.

We treat $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, and $^{56}\mathrm{Ni}$ as systems of baryons which can exist in either the ground (nucleon) state or first excited (delta) state and follow their behavior under static comrpession using constrained spherical Hartree-Fock approximation (SHF). We use realistic effective nucleon-nucleon interactions with meson-exchange-based nucleon-delta transition potentials and delta-delta interactions and we make phenomenological adjustments to obtain SHF equilibrium properties in agreement with experiment. We then show how physical properties are affected by delta excitation under compression. We find that a significant fraction of the increase in energy of these nuclei under compression is stored in the form of \ensuremath{\Delta}-mass creation. This, in turn, may have implications for an enhanced role for nuclear compression in subthreshold pion production in nucleus-nucleus collisions. In addition, including the deltas leads to a lower compressibility of each of these nuclei.

Subthreshold antikaon production in nucleus-nucleus collisions

Antikaon production in nucleus-nucleus collisions at energies below the threshold for its production from the nucleon-nucleon interaction in free space is studied in the relativistic transport model. We include the in-medium self-energies of kaon and antikaon determined from either the mean-field approximation to the chiral Lagrangian or the empirical effective scattering lengths for the kaon-nucleon and the antikaon-nucleon interaction. Also, the final-state interactions of antikaons with nucleons through both elastic scattering and absorption are explicitly treated in the study. Our results indicate that an attractive scalar potential for kaon and antikaon is needed to explain quantitatively the experimental data from the Ni+Ni collision at 1.85 GeV/nucleon. To see more clearly the effect of the attractive scalar potential, we suggest that experiments at lower incident energies be carried out.

Rapidity dependence of soft mesons as a signature of resonance matter production in nucleus-nucleus collisions.

Enhanced production of soft---i.e., low transverse momentum---pions has been experimentally observed in nucleus-nucleus collisions at beam energies of 14.5{ital A} GeV [the Brookhaven National Laboratory Alternating Gradient Synchrotron (AGS)] and 200{ital A} GeV (CERN Super Proton Synchrotron). Here the rapidity dependence of soft pion and kaon production for the reaction Si(14.5{ital A} GeV)+Au is studied in the framework of the relativistic quantum molecular dynamics (RQMD) approach. The softest mesons from decaying resonance matter, which turns out to be the main production mechanism in RQMD, show a peculiar rapidity dependence, a weakening of the signal in the central rapidity region of the participant nucleons. This is in striking contrast to expectations from all other suggested mechanisms like mean fields, collective flow, and quantum statistical effects. The prediction can be tested by forthcoming experiments at the AGS in the near future. It is also predicted that a similar, though quantitatively much weaker, signal can be observed in the kaon spectra.

Subthreshold antiproton production in nucleus-nucleus collisions.

Antiproton production in nucleus-nucleus collisions at energies below the threshold for its production from the nucleon-nucleon interaction in free space is studied in the relativistic Vlasov-Uehling-Uhlenbeck model. The antiproton self-energies in the medium are obtained in the mean-field approximation from the G-parity transformation of the nucleon self-energies. Due to the decrease of antiproton mass in the medium as a result of the attractive scalar mean field, there is an enhanced production of primordinal antiprotons which are, however, mostly annihilated by nucelons as they propagate through the matter. The calculated final antiproton momentum spectrum is seen to agree reasonably with the experimental data.

Extrapolation of hadron production in nucleus-nucleus collisions to energies reached at the BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider with the two-component dual parton model.

We present a Monte Carlo version of the dual parton model for the description of particle production in hadron-nucleus and nucleus-nucleus collisions. Each nucleon-nucleon collision is described by the two-component dual parton model. Soft hadronic collisions and hard perturbative collisions (minijets) are the two components included in the unitarization scheme. Depending on the transverse momentum cutoff we find up to several thousand minijets in collisions of heavy ions at the TeV energies of the future CERN Large Hadron Collider (LHC). We get predictions up to energies reached at the LHC for the rapidity and transverse momentum distributions and the minijet component of the hadronic energy density.

Central12C-emulsion collisions at 4.5A GeV/c

Central collisions of12C with emulsion nuclei at 4.5A GeV/c have been investigated. Central collisions are defined as events withNh≥28 and having no observable projectile fragments emitted within 3° of the beam direction. The probability of central collisions is found to be 11.68% and it increases with the mass of the projectile. The multiplicity distribution of shower particles is a Poisson one which indicates the absence of correlations between the produced pions. F/B for black and grey particles are 1.40±0.04 and 2.95±0.10, respectively, and no peaks are observed in the angular distribution of target fragments which could be attributed to the shock-wave phenomenon. Evidence of strong correlations between the produced particles has been obtained. This indicates that the particle production in nucleus-nucleus collisions takes place via cluster production. However, the absence of higher-order correlations rules out the possibility of the production of quark-gluon plasma, in these collisions. Furthermore, no evidence is found the production of fireball-type heavy clusters predicted by Adamovich et al.