Quark-gluon Plasma Formation Research Articles

Hard π0 and η production in S+ Au nuclear collisions at SPS energies and possible signature of quark-gluon plasma

Hard π 0 and η production is investigated in Landau and Bjorken hydrodynamical models taking into account great number of hadronic resonances (16 and 42) in hadronic phase. We consider two different scenario: with quark-gluon plasma (QGP) formation and with only hadronic gas one without QGP. The Cronin effect and hard direct pions emission are taken into account. It is shown that these two scenario give similar p ⊥ spectra which agrees with experimental data obtained by WA80 collaboration. Therefore we conclude that from hadronic spectra it is difficult to extract the proof of QGP formation. However we calculate also the η/ π 0 ratio. We show that this value agrees with experiment only for scenario with QGP and phase transition to hadrons. The hadronic gas scenario without QGP disagrees with experimental data.

Multipion correlations as a signal for quark-gluon plasma formation in high energy heavy ion collisions

The multiparticle Bose-Einstein correlations for quark-gluon plasma (QGP) formation in high energy heavy ion collisions are analyzed. The average correlation intensities of multipions as a function of pion multiplicity are investigated. It is shown that the average correlation intensities of multipions have obvious enhancements in the high multiplicity region. This signature can be used to detect the existence of QGP in rare high pion multiplicity events in high energy heavy ion collisions.

Novel scaling behavior for the multiplicity distribution under second-order quark-hadron phase transition

Deviation of the multiplicity distribution ${P}_{q}$ in a small bin from its Poisson counterpart ${p}_{q}$ is studied within the Ginzburg-Landau description for a second-order quark-hadron phase transition. The dynamical factor ${d}_{q}\ensuremath{\equiv}{P}_{q}{/p}_{q}$ for the distribution and the ratio ${D}_{q}\ensuremath{\equiv}{d}_{q}{/d}_{1}$ are defined, and novel scaling behaviors between ${D}_{q}$ are found which can be used to detect the formation of quark-gluon plasma. The study of ${d}_{q}$ and ${D}_{q}$ is also very interesting for other multiparticle production processes without a phase transition.

What causes ψ suppression in Pb+Pb collisions?

A reexamination of hadronic comover scattering indicates that this mechanism cannot explain the observed ψ suppression in Pb+Pb interactions. The possibility of quark-gluon plasma formation is therefore considered. Implications for RHIC and LHC are also discussed.

The quartz-fiber Zero-Degree Calorimeter for the NA50 experiment at CERN SPS

Quartz-fiber calorimeters have been the object of an intense and fruitful work of research and development in the last few years. In this paper we report about the first application of this technique in an experiment. Namely, we describe the design and performance of the Zero-Degree Calorimeter (ZDC) used in NA50, a heavy-ion experiment at CERN SPS aiming to detect the formation of quark-gluon plasma in Pb–Pb collisions at 158 AGeV by studying the production of the charmonia states J/ψ and ψ′. The unique properties of the quartz-fiber calorimeters turn out to be well suited to match the specific requirements of this experiment that operates at beam intensity of about 10 7 Pb-ion/s: radiation hardness (up to a few Grads), short duration of the signal (∼10 ns), spatial resolution of a few hundreds of μm, small size of the detector (5×5×65 cm 3) and energy resolution adequate for providing a precise measurement of the collision centrality.

ψ suppression in Pb+Pb collisions: a new look at hadrons vs. plasma

A reexamination of hadronic comover scattering indicates that this mechanism cannot explain the observed $\psi$ suppression in Pb+Pb interactions. The possibility of quark-gluon plasma formation is therefore considered.

CHARMONIUMSUPPRESSION INHEAVY-IONCOLLISIONS

▪ Abstract Data and their interpretations are reviewed for J/ψ and ψ′ production in heavy-ion collisions with particular emphasis on the recently reported anomaly in Pb-Pb reactions. The observed suppressions in pA collisions are discussed in terms of final-state absorption of a pre-meson and the pTdistributions due to parton scattering in the initial state. The experimental setup is described and the various results for J/ψ and ψ′ suppressions, including their pT-dependence, are presented. The data, in particular the observed anomalous suppression in Pb-Pb collisions, are discussed in the light of the proposed ideas, among them quark-gluon plasma formation and suppression via co-movers.

Charmonium — In and out of the nucleus

The bound states of charm and anticharm quarks, charmonium, offer a unique opportunity to explore the relationship between the traditional way of understanding hadronic interactions in nuclear physics, i.e., in terms of meson exchanges, and the QCD based description in terms of gluon exchanges. Inserting charmonium, for example J ψ , into the nucleus leads to many interesting possibilities. Several of these are mentioned. One, namely J ψ attenuation in nuclei, is discussed in some detail because of its importance as the possible signature of quark-gluon plasma(QGP) formation in relativistic heavy ion collisions. A technique for the direct measurement of J ψ -nucleon cross section is proposed.

Strangeness enhancement — a potential signature for QGP phase

Abstract Strangeness enhancement has always been considered as a potential signature for deconfining as well as chiral symmetry restoring quark-hadron phase transition. We obtain the ratios Λ Λ , Ξ Ξ and K + K − from a quark-gluon plasma (QGP) using a modified equation of state (EOS). Similarly these ratios are also obtained from a hadron gas (HG) by using a thermodynamically consistent equation of state (EOS) which incorporates the finite size, hard-core repulsive interactions among baryons as an excluded volume effect. We then suggest that the variations of these ratios either with the energy density or with baryon density can serve as a potential signature for detecting a QGP formation in the ultra-relativistic heavy-ion collisions.

New source term for QGP formation in the background-field model

We consider pair production in a space-time-dependent background field and derive a source term, i.e., production rate in the one-particle phase space. Such a source term is required in Boltzmann-equation-based models of quark-gluon plasma formation and evolution. We compare the source term derived here with the one that has been used in the literature so far. Significant differences are observed.

Fast hadronization of a quark gluon plasma and meson production

Abstract Production of mesons from an equilibrated but rapidly hadronizing quark gluon plasma (QGP) is studied. The proposed model assumes the formation of hadrons such that the constituent quarks from the hadronizing QGP combine to form hadrons of given masses. The QGP is assumed to hadronize without undergoing through an equilibrated hadron gas (HG) phase. The results obtained are compared with the case of a pure HG without QGP formation.

On the theory of instabilities arising at quark-gluon plasma formation

Collisions of two degenerate quark Fermi liquids are studied. It is shown that there arise instabilities, which manifest themselves in propagation of growing oscillations corresponding to the modes existing in a Fermi liquid at rest. Quark jets are likely to appear in the directions of the growing oscillations propagation. The instabilities studied in this work are similar to the beam instability in ordinary electron plasma.

Thermal particle and photon production in Pb+Pb collisions with transverse flow

Particle and photon production is analyzed in the presence of transverse flow using two approximations to describe the properties of the hadronic medium, one containing only {pi},{rho},{omega}, and {eta} mesons ({ital simplified equation of state}) and the other containing hadrons and resonances from the particle data table. Both are considered with and without initial quark gluon plasma formation. In each case the initial temperature is fixed by requiring dN{sub ch}/dy{approximately} 550 in the final state. It is shown that most observables are very sensitive to the equation of state. This is particularly evident when comparing the results of the simplified equation of state in the scenarios with and without phase transition. The hadronic gas scenario leads to a substantially higher rate for the p{sub T} distribution of all particles. In the complete equation of state with several hundreds of hadronic resonances, the difference between the scenarios with and without phase transition is rather modest. Both photon and particle spectra, in a wide p{sub T} range, show very similar behavior. It is therefore concluded that from the p{sub T} spectra it will be hard to disentangle quark gluon plasma formation in the initial state. It is to be stressed, however, thatmore » there are conceptual difficulties in applying a pure hadronic gas equation of state at SPS energies. The phase transition scenario with a quark gluon plasma present in the initial state seems to be the more natural one. {copyright} {ital 1997} {ital The American Physical Society}« less

Signatures of quark-gluon plasma formation in nuclear collisions: News from QM96

Three generic signatures of quark-gluon plasma production in nuclear collisions are reviewed. First, we discuss whether the recent J Ψ suppression data in Pb + Pb are the “smoking gun” for the deconfinement transition. Second, claims that the excess production of low mass dileptons provides evidence for chiral restoration are reviewed. Finally, the pion interferometry time delay signature of a rapid cross-over region in the QCD equation of state is analyzed.

Estimates of relative yields of strange baryons and antibaryons from pp and pA interactions

Relative production rates of hyperons and antihyperons from heavy ion collisions are expected to be sensitive signals of quark-gluon plasma formation. They have to be confronted to corresponding quantities from pp and pA collisions which are evaluated here.

Radiative energy loss and p⊥-broadening of high energy partons in nuclei

The medium-induced p ⊥-broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that − dE/ dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton p 2 ⊥ w . We find −dE/dz = 1 8 α sN cp 2 ⊥w holds independent of the details of the parton-nucleon scatterings so long as L is large and so long as perturbative QCD applies at short distances. Numerical estimates suggest that p ⊥-broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.

The time-delay signature of quark-gluon plasma formation in relativistic nuclear collisions

The hydrodynamic expansion of quark-gluon plasmas with spherical and longitudinally boost-invariant geometries is studied as a function of the initial energy density. The sensitivity of the collective flow pattern to uncertainties in the nuclear matter equation of state is explored. We concentrate on the effect of a possible finite width, ΔT ∼ 0.1 T c, of the transition region between quark-gluon plasma and hadronic phase. Although slow deflagration solutions that act to stall the expansion do not exist for ΔT > 0.08 T c, we find, nevertheless, that the equation of state remains sufficiently soft in the transition region to delay the propagation of ordinary rarefaction waves for a considerable time. We compute the dependence of the pion-interferometry correlation function on ΔT, since this is the most promising observable for time-delayed expansion. The signature of time delay, proposed by Pratt and Bertsch, is an enhancement of the ratio of the inverse width of the pion correlation function in out-direction to that in side-direction. One of our main results is that this generic signature of quark-gluon plasma formation is rather robust to the uncertainties in the width of the transition region. Furthermore, for longitudinal boost-invariant geometries, the signal is likely to be maximized around RHIC energies

J/ psi Suppression in Pb-Pb Collisions: A Hint of Quark-Gluon Plasma Production?

The NA50 Collaboration has recently observed a strong suppression of J/Psi production in Pb-Pb collisions at 158 Gev/n. We show that this recent observation finds a quantitative explanation in a model which relates the suppression mechanism to the local energy density, whose value is higher in Pb-Pb collisions than in any other system studied previously. The sensitivity of the phenomenon to small changes in the energy density could be suggestive of quark-gluon plasma formation.

A scheme to identify collective transverse flow in relativistic heavy ion collisions at CERN SPS

A judicious selection of projectile-target combinations in relativistic heavy ion collisions at CERN SPS may provide us with a number of systems having a similar particle rapidity density per unit transverse area. There may be a formation of quark gluon plasma, followed by hadronization, if the energy density attained is large enough. Electromagnetic radiations from such systems normalized to the initial transverse area would be identical if they do not undergo a transverse expansion. A deviation from this scaling behaviour is shown to provide a novel signature for the transverse flow effects in such experiments. Such a deviation is expected to emerge clearly in a comparison of results for Pb+Pb system, e.g., with those for S+Au.

PHYSICS OF STRANGE MATTER FOR RELATIVISTIC HEAVY-ION COLLISIONS

Relativistic heavy-ion collisions offer the possibility to produce exotic metastable or even absolutely stable states of nuclear matter containing (roughly) equal number of strangeness compared to the baryon number: Strangelets, small pieces of strange quark matter, were proposed as a signal of quark-gluon plasma formation. As their hadronic counterpart, also small pieces of strange hadronic matter may also show up with rather similar properties. The reasoning of both their stability and existence, the possible separation of strangeness necessary for their formation, and the chances for their detection, are reviewed.