Stage Of Heavy-ion Collisions Research Articles

A study of anisotropic flows at LHC by using purturbative simulation

We study the harmonic flows, for example, the directed, elliptic, third and fourth flow of the system of partons formed just after relativistic heavy ion collisions. We calculate the minijets produced during the primary collisions using standard parton distributions for the incomming projectile and target nucleus. We solve the Boltzmann equations of motion for the system of minijets by Monte Carlo method within only the perturbative sector. Based on the flow data calculated, we conclude the simulation can not explain the experimental results at RHIC and LHC so that the nonperturbative sector plays much more important roles even from the earliest stage of heavy ion collisions.

Magneto-sonoluminescence and its signatures in photon and dilepton production in relativistic heavy ion collisions

The matter produced in the early stages of heavy ion collisions consists mostly of gluons, and is penetrated by the coherent magnetic field produced by spectator nucleons. The fluctuations of gluonic matter in an external magnetic field couple to real and virtual photons through virtual quark loops. We study the resulting contributions to photon and dilepton production that stem from the fluctuations of the stress tensor ${T}_{\ensuremath{\mu}\ensuremath{\nu}}$ in the background of a coherent magnetic field $\stackrel{P\vec}{B}$. Our study extends significantly the earlier work [G. Basar, D. E. Kharzeev, and V. Skokov, Phys. Rev. Lett. 109, 202303 (2012)], in which only the fluctuations of the $trace$ of the stress tensor ${T}_{\ensuremath{\mu}\ensuremath{\mu}}$ were considered (the coupling of ${T}_{\ensuremath{\mu}\ensuremath{\mu}}$ to electromagnetic fields is governed by the scale anomaly). In the present paper we derive more general relations using the operator product expansion (OPE). We also extend the previous study to the case of dileptons, which offers the possibility to discriminate between various production mechanisms. Among the phenomena that we study are magneto-sonoluminescence [MSL, the interaction of magnetic field $\stackrel{P\vec}{B}(x,t)$ with the sound perturbations of the stress tensor $\ensuremath{\delta}{T}_{\ensuremath{\mu}\ensuremath{\nu}}(x,t)$] and magneto-thermoluminescence [MTL, the interaction of $\stackrel{P\vec}{B}(x,t)$ with smooth average $\ensuremath{\langle}{T}_{\ensuremath{\mu}\ensuremath{\nu}}\ensuremath{\rangle}$]. We calculate the rates of these process and find that they can dominate the photon and dilepton production at early stages of heavy ion collisions. We also point out the characteristic signatures of MSL and MTL that can be used to establish their presence and to diagnose the produced matter.

The Glasma, photons and the implications of anisotropy

We introduce distribution functions for quarks and gluons in the Glasma and discuss how they satisfy various relationships of statistical physics. We use these distributions to compute photon production in the early stages of heavy ion collisions. Photon rates satisfy geometric scaling, that is, the emission rate per unit area scales as a function of the saturation momenta divided by the transverse momentum of the photon. Photon distributions from the Glasma are steeper than those computed in the Thermalized Quark Gluon Plasma (TQGP). Both the delayed equilibration of the Glasma and a possible anisotropy in the pressure lead to slower expansion and mean times of photon emission of fixed energy are increased. This delayed emission might allow for larger photon elliptic flow.

The extraordinary glow

In this contribution I discuss some recent progress in understanding the evolution of the pre-thermal quark–gluon matter, known as the glasma, during the early stage in heavy ion collisions, and the implication for early time photon and dilepton emissions.

Effect of quark gluon plasma on charm quark produced in relativistic heavy ion collision

Charm quarks are produced mainly in the pre-equilibrium stage of heavy ion collision and serve as excellent probes entering the thermalized medium. They come out with altogether different momenta and energies and fragments into D-mesons and decay into non-photonic electrons which are observed experimentally. Here we present the effect of QGP on charm quark production using two different models: first one based on Wang-Huang-Sarcevic model of multiple scattering of partons and the second one is based on Parton Cascade Model with Boltzmann transport equation used for charm quark evolution in QGP.

Quark-gluon plasma formation time and direct photons from heavy ion collisions

We investigated the information carried by the data of direct photons, ie, the transverse momentum spectrum and the elliptic flow $v_{2}$ from Pb+Pb collisions at $\sqrt{s_{NN}}$=2.76TeV measured at Large Hadron Collider (LHC) and from Au+Au collisions at $\sqrt{s_{NN}}$= 200 GeV measured at Relativistic Heavy Ion Collider (RHIC), in the frame work of (3+1)-dimensional ideal hydrodynamical models constrained with hadronic data. We found those direct photon data may serve as a useful clock at the early stage of heavy ion collisions. The time scales of reaching thermal and chemical equilibrium, extracted from those data, are about 1/3 and 1.5 fm/c, respectively. Thus the large elliptic flow of direct photons is explainable. High order harmonics, ie, $v_3$, $v_4$ and $v_5$, of direct photons from Pb+Pb collisions at 2.76TeV are also predicted, as a further test to compete with those who claim new sources of photons to account for the large elliptic flow of direct photons.

Isotropization of the quark gluon plasma

We report here recent analytical and numerical work on the theoretical treatment of the early stages of heavy ion collisions, that amounts to solving the classical Yang-Mills equations with fluctuating initial conditions. Our numerical simulations suggest a fast isotropization of the pressure tensor of the system. This trend appears already for small values of the coupling constant $\alpha_s$ . In addition, the system exhibits an anomalously small shear viscosity.

The Initial Stages of Heavy Ion Collisions

In this paper, I present the description of the early stages of heavy ion collisions at high energy in the Color Glass Condensate framework, from the pre-collision high energy nuclear wavefunction to the point where hydrodynamics may start becoming applicable.

LONG-RANGE RAPIDITY CORRELATIONS IN HEAVY-LIGHT ION COLLISIONS

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δϕ = π with the other one centered around Δϕ = 0 (here Δϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus-nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

Study the early stage of heavy ion collisions with direct photons

Based on the modelling of the collective motion in AuAu collisions at [Formula: see text] at centrality 0-20% and 20-40% and PbPb collisions at [Formula: see text] at centrality 0-40% with a 3+1D event-averaged ideal hydrodynamics constrained with hadronic data, we study the transverse momentum spectrum and elliptic flow of direct photons and find that the recent direct photon data from both PHENIX collaboration at RHIC and ALICE collaboration at LHC favour an early beginning of collective expansion (τ0 < 0.6 fm/c) and a late formation of quark gluon plasma (τ ~ 2 fm/c).

Hadronic effects on theX(3872)meson abundance in heavy ion collisions

We study the hadronic effects on the X(3872) meson abundance in heavy ion collisions. We evaluate the absorption cross sections of the X(3872) meson by pions and rho mesons in the hadronic stage of heavy ion collisions, and further investigate the variation in the X(3872) meson abundance during the expansion of the hadronic matter for its two possible quantum number states; $J^P=1^+$ and $2^-$. We show that the absorption cross sections and the time evolution of the X(3872) meson abundance are strongly dependent on the structure and quantum number of the X(3872) meson. We thus suggest that studying the abundance of the X(3872) meson in relativistic heavy ion collisions provides a chance to infer its quantum number as well as its structure.

Delineating the saturation boundary: linear vs non-linear QCD evolution

Abstract The forthcoming p + Pb run at the LHC will provide crucial information on the initial state effects of heavy ion collisions and on the gluon saturation phenomena. In turn, most of the saturation inspired phenomenology in heavy ion collisions borrows substantial empiric information from the analysis of e + p data, where abundant high quality data on the small-x kinematic region is available. Indeed, the very precise combined HERA data provides a testing ground in which the relevance of novel QCD regimes, other than the successful linear DGLAP evolution, in small-x inclusive DIS data can be ascertained. We present a study of the dependence of the AAMQS fits, based on the running coupling BK non-linear evolution equations (rcBK), on the fitted dataset. This allows for the identification of the kinematical region where rcBK accurately describes the data, and thus for the determination of its applicability boundary. It also sets important constraints to the saturation models used to model the early stages of heavy ion collisions. Finally we compare the rcBK results with NNLO DGLAP fits, obtained with the NNPDF methodology with analogous kinematical cuts. Further, we explore the impact on LHC phenomenology of applying stringent kinematical cuts to the low-x HERA data in a DGLAP fit.

QCD critical point, baryon number fluctuations, and final state interactions

Baryon number cumulants are important quantities to diagnose the primordial stage of heavy ion collisions. In experiments, however, proton number cumulants have been measured as substitutes. In fact, proton number fluctuations are further modified in the hadron phase and different from those of baryon number. We explore the relation between proton and nucleon number fluctuations in the final state in relativistic heavy ion collisions.

Long-range rapidity correlations in heavy–light ion collisions

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δ ϕ = π with the other one centered around Δ ϕ = 0 (here Δ ϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus–nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

Production of photons and dileptons in the Glasma

We study the production of photons and dileptons during the pre-equilibrium Glasma stage in heavy ion collisions and discuss the implications in light of the PHENIX data. We find that the measured distributions of such electromagnetic emissions, while having some features not well understood if hypothesized to entirely arise from a thermalized Quark–Gluon Plasma, have some qualitative features that might be described after including effects from a thermalizing Glasma. The shape and centrality dependence of the transverse momentum spectra of the so-called “thermal photons” are well described. The mass and transverse momentum dependence of intermediate mass dileptons also agree with our estimates. The low transverse momenta from which the excessive dileptons (in low to intermediate mass region) arise is suggestive of emissions from a Bose condensate. We also predict the centrality dependence of dilepton production. Uncertainties in the current approach and improvements in the future are discussed.

Effective dynamics of a non-Abelian plasma out of equilibrium

Starting from kinetic theory, we obtain a nonlinear dissipative formalism describing the nonequilibrium evolution of scalar colored particles coupled selfconsistently to nonabelian classical gauge fields. The link between the one-particle distribution function of the kinetic description and the variables of the effective theory is determined by extremizing the entropy production. This method does not rely on the usual gradient expansion in fluid dynamic variables, and therefore the resulting effective theory can handle situations where these gradients (and hence the momentum-space anisotropies) are expected to be large. The formalism presented here, being computationally less demanding than kinetic theory, may be useful as a simplified model of the dynamics of color fields during the early stages of heavy ion collisions and in phenomena related to parton energy loss.

Baryon number cumulants and proton number cumulants in relativistic heavy ion collisions

Abstract Baryon number cumulants are invaluable tools to diagnose the primordial stage of heavy ion collisions. In experiments, however, proton number cumulants have been measured as substitutes. In fact, proton number fluctuations are further modified in the hadron phase and different from those of baryon number. We give formulas that express the baryon number cumulants solely in terms of proton number fluctuations, which are experimentally observable.

Particlization in hybrid models

In hybrid models, which combine hydrodynamical and transport approaches to describe different stages of heavy-ion collisions, conversion of fluid to individual particles, particlization, is a non-trivial technical problem. We describe in detail how to find the particlization hypersurface in a 3+1 dimensional model, and how to sample the particle distributions evaluated using the Cooper-Frye procedure to create an ensemble of particles as an initial state for the transport stage. We also discuss the role and magnitude of the negative contributions in the Cooper-Frye procedure.

The early stages of a high energy heavy ion collision

At high energy, the gluon distribution in nuclei reaches large densities and eventually saturates due to recombinations, which plays an important role in heavy ion collisions at RHIC and the LHC. The Colour Glass Condensate (CGC) provides a framework for resumming these effects in the calculation of observables. In this talk, I present its application to the description of the early stages of heavy ion collisions.

Revisiting quark stars under the influence of strong magnetic fields

AbstractQuark matter at finite temperature and subject to strong magnetic fields is possibly present in the early stages of heavy ion collisions and in the interior of protoneutron stars. We use the mean field approximation to investigate this type of quark matter described by the Nambu–Jona-Lasinio model. The energy per baryon of magnetized quark matter becomes more bound than nuclear matter made of iron nuclei, for magnetic fields around 1019 G. When the su(3) NJL model is applied to stellar matter, the maximum mass configurations are always above 1.45 solar masses and may be as high as 1.9 solar masses for a central magnetic field of 1018 G. These numbers are within the masses of observed neutron stars but exclude the recently measured star with 1.97 solar mass.The effect of the magnetic field on the effective quark masses and chemical potentials is only felt for quite strong magnetic fields, above 5 × 1018 G, with larger effects for the lower densities. Spin polarizations are more sensitive to weaker magnetic fields and are larger for lower temperatures and lower densities.