Quark-gluon Plasma Formation Research Articles

Photon and Dilepton Production at LHC

We review various production mechanisms of photons and small mass dileptons at large transverse momentum at the LHC. Their relevance as a signal for quark-gluon plasma formation is discussed.

Jet Characterization at RHIC Energy

Jet quenching is considered to be one of the signatures of the formation of quark gluon plasma. In order to investigate the jet quenching, it is necessary to know the properties of jets produced in relativistic heavy ion collisions. In this calculation, we propose that determination of flow parameters may be used to identify and characterize the jets.

Design, construction and front end electronics of second station of Forward Muon Spectrometer in ALICE

Forward muon spectrometer in ALICE in the LHC experiments of CERN will look for signals of quark gluon plasma (QGP) formation in Pb-Pb collisions at 5.6 TeV/nucleon energy. This spectrometer is designed specifically to look for decays of heavy quarkoniums through the correlated muon pairs. The design and construction details with emphasis on the R & D contributions from the nodal agency. Saha Institute of Nuclear Physics (SINP) will be presented.

Hints of incomplete thermalization in RHIC data

The large elliptic flow observed in Au-Au collisions at RHIC is often put forward as a compelling evidence for the formation of a strongly-interacting quark-gluon plasma. The main argument is that the measured elliptic flow is as large as the value given by fluid-dynamics models that assume complete thermalization. It is argued that this claim may not be justified, since a detailed examination of experimental data rather suggests that the system created is not fully equilibrated at the time when anisotropic flow develops.

EXCITED HADRONS AS A SIGNAL FOR QUARK–GLUON PLASMA FORMATION

At the quark–hadron transition, when quarks get confined to hadrons, certain orbitally excited states, namely those which have excitation energies above the respective L = 0 states of the same order as the transition temperature Tc, may form easily because of thermal velocities of quarks at the transition temperature. We propose that the ratio of multiplicities of such excited states to the respective L = 0 states can serve as an almost model independent signal for the quark–gluon plasma (QGP) formation in relativistic heavy-ion collisions. For example, the ratio R* of multiplicities of [Formula: see text] and [Formula: see text] when plotted with respect to the center-of-mass energy of the collision [Formula: see text] (or vs. centrality/number of participants), should show a jump at the value of [Formula: see text] beyond which the QGP formation occurs. This should happen irrespective of the shape of the overall plot of R* vs. [Formula: see text]. Recent data from RHIC on Λ*/Λ vs. N part for large values of N part may be indicative of such a behavior, though there are large error bars. We give a list of several other such candidate hadronic states.

Re-hardening of hadron transverse mass spectra in relativistic heavy-ion collisions

We analyze the spectra of pions and protons in heavy-ion collisions at relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented hadron-string cascade model. In this energy region, hadron transverse mass spectra first show softening until SPS energies, and re-hardening may emerge at RHIC energies. Since hadronic matter is expected to show only softening at higher energy densities, this re-hardening of spectra can be interpreted as a good signature of the quark-gluon plasma formation

Compact strange stars with a medium dependence in gluons at finite temperature

The possible existence of strange stars in the universe will help in the understanding of various properties of quantum chromodynamics, like asymptotic freedom and chiral symmetry restoration, which is otherwise very difficult to prove in laboratory experiments. Strange star properties were calculated using large $N_c$ approximation with built-in chiral symmetry restoration. A relativistic Hartree Fock calculation was performed using the Richardson potential as an interquark interaction. This potential has the asymptotic freedom and a confinement-deconfinement mechanism built into it and the present calculation employs an application of this potential with modified two scale parameters $\Lambda$ and $\Lambda^{\prime}$, to find a new set of equations of state for strange quark matter. The linear confinement string tension from lattice calculations is 350 $MeV$ and the Coulomb -like part has the parameter 100 $MeV$ from deep inelastic scattering experiments. We also consider the effect of temperature, $T$, on gluon mass in a simple way, in addition to the usual density dependence, and find that the transition $T$ from hadronic matter to strange matter is at 80 MeV, close to the 100 MeV estimated in litarature. Therefore formation of strange stars may be the only signal for formation of quark-gluon plasma with asymptotic freedom and chiral symmetry restoration and this may be observable through many processes -such as for example through delayed $\gamma$ ray afterglow.

Study of QGP signatures with the ϕ → K + K – signal in Pb-Pb ALICE events

The phi->K+K- decay channel in Pb-Pb collisions at LHC is studied through a full simulation of the ALICE detector. The study focuses on possible signatures in this channel of quark-gluon plasma (QGP) formation. On a basis of 10^6 collisions at high centrality some proposed QGP signatures are clearly visible both in K+K- invariant mass and transverse mass distributions. The high significance of this observation appears to reside heavily on the use of the TOF (Time Of Flight) system of ALICE in addition to its central tracking detectors.

Study of QGP signatures with the ϕ → K + K – signal in Pb-Pb ALICE events

The ϕ → K+K– decay channel in Pb-Pb collisions at LHC is studied through a full simulation of the ALICE detector. The study focuses on possible signatures in this channel of quark-gluon plasma (QGP) formation. On a basis of 106 collisions at high centrality some proposed QGP signatures are clearly visible both in K + K– invariant mass and transverse mass distributions. The high significance of this observation appears to reside heavily on the use of the TOF (Time Of Flight) system of ALICE in addition to its central tracking detectors.

Multiplicity fluctuations in a hadron gas with exact conservation laws

The study of fluctuations of particle multiplicities in relativistic heavy-ion reactions has drawn much attention in recent years, because they have been proposed as a probe for underlying dynamics and possible formation of quark-gluon plasma. Thus, it is of uttermost importance to describe the baseline of statistical fluctuations in the hadron gas phase in a correct way. We have performed a comprehensive study of multiplicity distributions in the full ideal hadron-resonance gas in different ensembles, namely grand-canonical, canonical and microcanonical, using two different methods: asymptotic expansions and full Monte Carlo simulations. The method based on asymptotic expansion allows a quick numerical calculation of dispersions in the hadron gas with three conserved charges at primary hadron level, while the Monte-Carlo simulation is suitable to study the effect of resonance decays. Even though mean multiplicities converge to the same values, major differences in fluctuations for these ensembles persist in the thermodynamic limit, as pointed out in recent studies. We observe that this difference is ultimately related to the non-additivity of the variances in the ensembles with exact conservation of extensive quantities.

Statistical mechanics of Yang-Mills classical mechanics

Statistical mechanics (SM) of Yang-Mills classical mechanics is studied by using a toy model that resembles chaotic quartic oscillators. This nonlinear system attains the thermodynamic equilibrium not by collisions, which is generally assumed in SM, but by chaotic dynamics. This is a new mechanism of thermalization that may be relevent to the quark-gluon plasma (QGP) formation in relativistic heavy-ion collisions because the interactions governing QGP involve quantum chromodynamics (QCD), which is a Yang-Mills theory [SU(3)]. The thermalization time is estimated from the Lyapunov exponent. The Lyapunov exponent is evaluated using the recently developed monodromy matrix method. We also discuss the physical meaning of thermalization and SM in this system of few degrees in terms of chromo-electric and chromomagnetic fields. One of the consequence of thermalization, such as equipartition of energy and dynamical temperature, is also numerically verified.

The future of hard and electromagnetic probes at RHIC

Potential near- and long-term physics opportunities with jets, heavy flavors and electromagnetic probes at the Relativistic Heavy Ion Collider (RHIC) are presented. Much new physics remains to be unveiled using these probes, due to their sensitivity to the initial high density stage of RHIC collisions, when quark-gluon plasma (QGP) formation is expected. Additional physics will include addressing deconfinement, chiral symmetry restoration, properties of the strongly-coupled QGP and a possible weakly-interacting QGP, color glass condensate in the initial state, and hadronization. To fully realize the physics prospects of the RHIC energy regime, new detector components must be added to existing experiments, the RHIC machine luminosity upgraded, and a possible new detector with significantly extended coverage and capabilities added.

QGP SIGNALS AND THE DUAL PARTON MODEL: IS THERMAL EQUILIBRIUM REACHED IN HEAVY ION COLLISIONS?

Some of the so-called signals of Quark Gluon Plasma (QGP) formation are examined in the framework of a string model, the Dual Parton Model, supplemented with final state interaction. The aim is to determine the strength and duration time of the final state interaction required to describe the data, in order to gain some insight on whether or not the system reaches thermal equilibrium.

J/ψ production in indium–indium collisions

Dilepton measurements are one of the most powerful tools for investigating the quark–gluon plasma formation in high-energy heavy-ion collisions. The NA60 experiment, taking place at the CERN SPS, studies the production of prompt dimuons and open charm in collisions induced by proton and heavy-ion beams on nuclear targets. In this paper, some preliminary results from the indium–indium dataset are presented.

Experimental and theoretical challenges in the search for the quark–gluon plasma: The STAR Collaboration's critical assessment of the evidence from RHIC collisions

We review the most important experimental results from the first three years of nucleus–nucleus collision studies at RHIC, with emphasis on results from the STAR experiment, and we assess their interpretation and comparison to theory. The theory-experiment comparison suggests that central Au + Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a quark–gluon plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages, and have not found ready explanation in a hadronic framework. However, the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter. The theoretical treatment of the collision evolution, despite impressive successes, invokes a suite of distinct models, degrees of freedom and assumptions of as yet unknown quantitative consequence. We pose a set of important open questions, and suggest additional measurements, at least some of which should be addressed in order to establish a compelling basis to conclude definitively that thermalized, deconfined quark–gluon matter has been produced at RHIC.

Study of the J/ψ production and suppression in In–In collisions at the CERN SPS

The NA60 experiment aims at measuring the kinematics of the muon pairs produced in ultra-relativistic nuclear collisions. The use of dileptonic probes in the search for quark–gluon plasma formation has been carried on by several experiments at the CERN SPS, which have provided some of the most interesting observations; nevertheless certain important questions remain open. One of them is the interpretation of the anomalous suppression of the J/ψ meson in Pb–Pb collisions with respect to the yields extrapolated from the proton–nucleus data taken by the NA50 experiment. The NA60 experiment has recently taken data on J/ψ production in indium–indium collisions: the comparison of the suppression patterns in the two systems (In–In with respect to Pb–Pb) should help in understanding which is the physical variable driving the J/ψ suppression. In this paper, after briefly summarizing the NA60 physics motivation and detector concept, we present the first results from In–In collisions at 158 GeV/nucleon, focusing on the preliminary analysis of charmonia production and suppression.

Hunt for the quark-gluon plasma: 20 years later

We review briefly the recent progress in the search for the quark-gluon plasma (QGP) at CERN SPS and BNL RHIC. Several model analyses and observables, such as hadronic thermal equilibrium, hydrodynamical flow, HBT, jet quenching, etc. which lead to the present conception of the formation of QGP are described.

Characterization of jets in relativistic heavy ion collisions

Jet quenching is considered to be one of the signatures of the formation of quark gluon plasma. In order to investigate the jet quenching, it is necessary to detect jets produced in relativistic heavy ion collisions, determine their properties and compare those with the jets one obtains in hadron-hadron or $e^+-e^-$ collisions. In this work, we propose that calculation of flow parameters may be used to detect and characterize jets in relativistic heavy ion collisions.

Vacuum creation of quarks at the time scale of QGP thermalization and strangeness enhancement in heavy-ion collisions

The vacuum parton creation in quickly varying external fields is studied at the time scale of order 1 fm/c typical for the quark–gluon plasma formation and thermalization. To describe the pre-equilibrium evolution of the system the transport kinetic equation is employed. It is shown that the dynamics of production process at times comparable with particle inverse masses can deviate considerably from that based on classical Schwinger-like estimates for homogeneous and constant fields. One of the effects caused by non-stationary chromoelectric fields is the enhancement of the yield of ss̄ quark pairs. Dependence of this effect on the shape and duration of the field pulse is studied together with the influence of string fusion and reduction of quark masses.

Strangeness and quark–gluon plasma

A brief summary of strangeness milestones is followed by a chemical nonequilibrium statistical hadronization analysis of strangeness results at SPS and RHIC. Strange particle production in AA interactions at GeV can be understood consistently as originating from the deconfined quark–gluon plasma (QGP) in a sudden hadronization process. Onset of QGP formation as function of energy is placed in the beam energy interval 10–30 A GeV/c. Strangeness anomalies at LHC are described.