Narrow Results

Direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au collisions at center-of-mass energies and 200 GeV at RHIC is systematically investigated. We study the jet quenching effect, the medium-induced photon bremsstrahlung and jet-photon conversion in the hot QGP. We account for known cold nuclear matter effects, such as the isospin effect,the Cronin effect, shadowing and cold nuclear matter energy loss. It is shown that at high p_T the nuclear modification factor for direct photons is dominated by cold nuclear matter effects and there is no evidence for large cross-section amplification due to medium-induced photon bremsstrahlung and jet-photon conversion in the medium. Comparison of numerical simulations to experimental data also rules out large Cronin enhancement and incoherent photon emission in the QGP but the error bars in the current experimental data cannot provide further constraints on the magnitudes of other nuclear matter effects.

It is known that the presence of background magnetic field in cosmic plasma distorts the acoustic peaks in CMBR. This primarily results from different types of waves in the plasma with velocities depending on the angle between the magnetic field and the wave vector. We consider the consequences of these effects in relativistic heavy-ion collisions where very strong magnetic fields arise during early stages of the plasma evolution. We show that flow coefficients can be significantly affected by these effects when the magnetic field remains strong during early stages due to strong induced fields in the conducting plasma. In particular, the presence of magnetic field can lead to enhancement in the elliptic flow coefficient v₂.

We carry out hydrodynamical simulation of the evolution of fluid in relativistic heavy-ion collisions with random initial fluctuations. The time evolution of power spectrum of momentum anisotropies shows very strong correspondence with the physics of cosmic microwave anisotropies as was earlier predicted by us. In particular, our results demonstrate suppression of superhorizon fluctuations and the correspondence between the location of the first peak in the power spectrum of momentum anisotropies and the length scale of fluctuations and expected freeze-out time-scale (more precisely, the sound horizon size at freeze-out).

In this work, we have extracted the initial temperature from the transverse momentum spectra of charged particles in Au + Au collisions using STAR data at RHIC energies from $\sqrt{s_{NN}}$ = 7.7–200 GeV. The initial energy density $(𝜀)$, shear viscosity to entropy density ratio $(\eta /s)$, trace anomaly $(\mathrm{\Delta })$, the squared speed of sound $(C_s^2)$, entropy density, and bulk viscosity to entropy density ratio $(\zeta /s)$ are obtained and compared with the lattice QCD calculations for (2 + 1) flavor. The initial temperatures obtained are compared with various hadronization and chemical freeze-out temperatures. The analysis of the data shows that the deconfinement-to-confinement transition possibly takes place between $\sqrt{s_{NN}}$ = 11.5 and 19.6 GeV.

Based on the Pomeranchuk theorem, one constructs the $\delta (s)$ parameter to measure the difference between experimental data for the particle–particle and particle–antiparticle total cross-section at same energy. The experimental data for the proton–proton and proton–antiproton total cross-section were used to show that, at the same energy, this parameter tends to zero as the collision energy grows. Furthermore, one assumes a classical description for the total cross-section, dividing it into a finite number of non-interacting disjoint cells, each one containing a quark–antiquark pair subject to the confinement potential. Near the minimum of the total cross-section, one associates $\delta (s)$ with the entropy generated by these cells, analogously to the $XY$-model. Using both the Quigg–Rosner and Cornell confinement potentials and neglecting other energy contributions, one can calculate the internal energy of the hadron. One obtains that both the entropy and internal energy possess the same logarithmic dependence on the spatial separation between the pairs in the cell. The Helmholtz free energy is used to estimate the transition temperature, which is far from the temperature widely related to the Quark–Gluon Plasma.

We study the dynamics of first-order confinement-deconfinement phase transition through nucleation of hadronic bubbles in an expanding quark–gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the effects of curvature energy. We find that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pressure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.

Effective Lagrangian models of charmonium have recently been used to estimate dissociation cross sections with light hadrons. Detailed study of the symmetry properties reveals possible shortcomings relative to chiral symmetry. We therefore propose a new Lagrangian and point out distinguishing features amongst the different approaches. Using the newly proposed Lagrangian, which exhibits SU_L(4) × SU_R(4) symmetry and complies with Adler's theorem, we find dissociation cross sections with pions that are reduced in an energy dependent way, with respect to cases where the theorem is not fulfilled.

We review the results on the bottomonium system from the CMS experiment at the Large Hadron Collider. Measurements have been carried out at different center-of-mass energies in proton collisions and in collisions involving heavy ions. These include precision measurements of cross sections and polarizations, shedding light on hadroproduction mechanisms, and the observation of quarkonium sequential suppression, a notable indication of quark–gluon plasma formation. The observation of the production of bottomonium pairs is also reported along with searches for new states. We close with a brief outlook of the future physics program.

A combined analysis of both $e^{+}{e}^{-}$ (LEP, SLD) and $pp$ (RHIC-PHENIX and LHC-ALICE) hadroproduction processes are done for the first time for the vector meson nonet at the next-to-leading order (NLO) using a model with broken SU(3) symmetry. The transverse momentum ($p_T$) and rapidity ($y$) dependence of the differential cross-section for $\omega$ and $\varphi$ mesons of the $pp$ data are also discussed. The input universal quark (valence and singlet) fragmentation functions at a starting scale of $Q_0^2=1.5{\mathrm{\text{GeV}}}^2$, after evolution, have values that are consistent with the earlier analysis for $e^{+}{e}^{-}$ at NLO. However, the universal gluon fragmentation function is now well determined from this study with significantly smaller error bars, as the $pp$ hadroproduction cross-section is particularly sensitive to the gluon fragmentation since it occurs at the same order as the quark fragmentation, in contrast to the $e^{+}{e}^{-}$ hadroproduction process. Additional parameters involved in describing the strangeness and sea suppression and octet–singlet mixing are found to be close to the earlier analysis; in addition, a new relation between the gluon and sea suppression in $K^{\ast }$ and $\varphi$ hadroproduction has been observed.

Relativistic heavy-ion collisions provide an ideal environment to study the emergent phenomena in quantum chromodynamics (QCD). The chiral magnetic effect (CME) is one of the most interesting, arising from the topological charge fluctuations of QCD vacua, immersed in a strong magnetic field. Since the first measurement nearly a decade ago of the possibly CME-induced charge correlation, extensive studies have been devoted to background contributions to those measurements. Many new ideas and techniques have been developed to reduce or eliminate the backgrounds. This paper reviews these developments and the overall progress in the search for the CME.

Evolution of spatially anisotropic perturbation created in the system formed after Relativistic Heavy Ion Collisions has been studied. The microscopic evolution of the fluctuations has been examined within the ambit of Boltzmann Transport Equation (BTE) in a hydrodynamically expanding background. The expansion of the background composed of quark gluon plasma (QGP) is treated within the framework of relativistic hydrodynamics. Spatial anisotropic fluctuations with different geometries have been evolved through Boltzmann equation. It is observed that the trace of such fluctuation survives the evolution. Within the relaxation time approximation, analytical results have been obtained for the evolution of these anisotropies. Explicit relations between fluctuations and transport coefficients have been derived. The mixing of various Fourier (or k) modes of the perturbations during the evolution of the system has been explicitly demonstrated. This study is very useful in understanding the presumption that the measured anisotropies in the data from heavy ion collisions at relativistic energies imitate the initial state effects. The evolution of correlation function for the perturbation in pressure has been studied and shows that the initial correlation between two neighbouring points in real space evolves to a constant value at later time which gives rise to Dirac delta function for the correlation function in Fourier space. The power spectrum of the fluctuation in thermodynamic quantities (like temperature estimated in this work) can be connected to the fluctuation in transverse momentum of the thermal hadrons measured experimentally. The bulk viscous coefficient of the QGP has been estimated by using correlations of pressure fluctuation with the help of Green–Kubo relation. Angular power spectrum of the anisotropies has been estimated in the appendix.

The power spectrum of the momentum distributions of particles have been estimated with Optical Glauber and Monte Carlo Glauber initial conditions for relativistic heavy ion collisions. The evaluation procedure adopted in this work is analogous to the one used in the calculation of power spectrum in Cosmic Microwave Background Radiation (CMBR). The power spectrum due to perturbations in the phase space distribution of the particles has also been evaluated. The perturbation in phase space has been evolved through the Boltzmann transport equation in an expanding quark–gluon plasma (QGP) background. The expansion of the QGP has been treated within the purview of (3+1)-dimensional relativistic hydrodynamics. We observe that the nonequilibrium effects introduced as perturbations in the phase space distributions can be traced through its variation with temperature which is distinctly different from the case of vanishing perturbation. A relation has been derived between the power spectrum and the flow harmonics.

We study the thermodynamics of AdS–Schwarzschild black hole in the presence of an external string cloud. We observe that, at any temperature, the black hole configuration is stable with nonzero entropy. We further notice that when the value of the curvature constant equals to one, if the string cloud density has less than a critical value, within a certain range of temperature three black holes configurations exist. One of these black holes is unstable and other two are stable. At a critical temperature, a transition between these two stable black holes takes place which leads us to conclude that the bound state of quark and antiquark pairs may not exist. By studying the corresponding dual gauge theory, we confirm the instability of the bound state of quark and antiquark pair in the dual gauge theory.

The color confinement in Quantum Chromodynamics (QCD) remains an interesting and intriguing phenomenon. It is considered as a very important nonperturbative effect to be taken into account in all models intended to describe the QCD many-parton system. During the deconfinement phase transition, the non-Abelian character of the partonic plasma manifests itself in an important manner. A direct consequence of color confinement is that all states of any partonic system must be colorless and the requirement of the colorlessness condition is more than necessary. Indeed, the colorless state is a result of the multiparton interactions, from which collective phenomena can emerge, inducing strong correlations and giving rise to a long-range order of liquid-like phase, a behavior fundamentally different from that of a conformal ideal gas. Within our Colorless QCD MIT-Bag Model and using the ${\mathcal{ℒ}}_{m,n}$-method, three Thermal Response Functions, related to the Equation of State, like pressure $\mathcal{𝒫}(T,V)$, sound velocity ${\mathcal{𝒞}}_s^2(T,V)$ and energy density $𝜖(T,V)$ are calculated and studied as functions of temperature $(T)$ and volume $(V)$. Also and in the same context, two relevant correlation forms $\frac{\mathcal{𝒫}}{𝜖}(𝜖)$ and ${\mathcal{𝒞}}_s^2(𝜖)$ are calculated and studied intensively as functions of $(𝜖)$ at different volumes. A detailed comparative study between our results and those obtained from lattice QCD simulation, hot QCD and other phenomenological models is carried out. We find that the Liquid Partonic Plasma Model is the model which fits our Equation of State very well, in which the Bag constant term is revealed very important. Our Colorless Partonic Plasma, just beyond the finite volume transition point, is found in a state where the different partons interact strongly showing a liquid behavior in agreement with the estimate of the plasma parameter $7.92\le {\mathrm{\Gamma }}_{\mathrm{\text{CPP}}}\le 10.10$ and supporting the result obtained from the fitting work. This allows us to understand experimental observations in Ultra-Relativistic Heavy-Ion Collisions and to interpret lattice QCD results.

We discuss the statistical mechanics and thermodynamics of quark matter at zero temperature and finite chemical potential using a thermodynamically consistent framework of quasiparticle model for QGP without the need of any reformulation of statistical mechanics or thermodynamical consistency relation. Using that equation of state, we solve the Tolman–Oppenheimer–Volkoff equation to obtain the mass-radius relation of dense quark star.

In this paper, one uses a damped potential to present a description of the running coupling constant of QCD in the confinement phase. Based on a phenomenological perspective for the Debye screening length, one compares the running coupling obtained here with both the Brodsky–de Téramond–Deur and the Richardson approaches. The results seem to indicate the model introduced here corroborate the Richardson approach. Moreover, the Debye screening mass in the confinement phase depends on a small parameter, which tends to vanish in the nonconfinement phase of QCD.

The Quantum Chromo Dynamics (QCD), with its two main properties of confinement and asymptotic freedom, is considered as the fundamental theory of the strong interaction from which the phenomenology of string model and hadronic bag models can be derived as particular cases. In QCD, besides quark masses, there is only one parameter, the renormalization scale parameter $\mathrm{\Lambda }$, which comes into play. In nonperturbative models, like string model and bag models, the string constant $\sigma$, the hadronic bag surface tension $\mathrm{\Sigma }$ and the concept of the hadronic bag pressure ${ℬ}$ emerge naturally. Since these parameters characterize the same color confining force, this explains the existence of simple mathematical relationships that connect them. Although in most applications of the MIT bag model, considering ${ℬ}$ with its phenomenologically determined value as a constant does not generate any inconsistency, however, there are some indications that ${ℬ}$ is not a universal constant. The concept of hadronic bag pressure ${ℬ}$ as a parameter was introduced long before, and we plan in this work to extrapolate the concept of ${ℬ}$ to become a full-fledged thermodynamic variable and to explore the consequences of its variability in the context of our colorless MIT bag model. To achieve this, we carry out a detailed thermodynamic study based on the three variables $(T,V,{ℬ})$ using an improved ${\mathcal{ℒ}}_{m,n}$-method and considering the hadronic bag pressure ${ℬ}$ as a thermodynamic variable like the temperature $T$ and volume $V$. The findings in the small ${ℬ}$ region, in themselves, are very unexpected and exciting that essentially concern the abnormal behavior of the order parameter ${\mathscr{H}}(T,V,{ℬ})$. This shows that, from an experimental point of view, a low value of the hadronic bag pressure ${ℬ}$ would have dramatic consequences. Thus, it is argued that the abnormal behavior observed in the order parameter ${\mathscr{H}}(T,V,{ℬ})$ well reflects the appearance of the mixed state at low temperature before the deconfining process. We also indicate how this result might be avoided by imposing a lower bound on the hadronic bag pressure: ${ℬ}\ge {{ℬ}}_{\mathrm{\text{min}}}$. A detailed study of the global properties of the order parameter ${\mathscr{H}}(T,V,{ℬ})$ as function on the three variables is given, and as an illustration of our ideas we construct three different criteria from which we extract the lower bound of the hadronic bag pressure ${{ℬ}}_{\mathrm{\text{min}}}$. We succeeded to show that the physically genuine colorless deconfining phase transition is feasible only if the thermodynamic lower bound is taken as ${{ℬ}}_{\mathrm{\text{min}}}^{1∕4}\simeq 56$MeV. As a consequence, the corresponding lower bound of the hadronic bag surface tension is ${\mathrm{\Sigma }}_{\mathrm{\text{min}}}^{1∕3}\simeq 47.77$MeV. Our results are consistent with each other, and seem to corroborate the results obtained by other approaches, such as the thermodynamic model of chromo-magnetism. We focus not only on certain technical details, but also on physical understanding and interpretation. The results are relevant for the parton phenomenology of stars and for URHIC experiments. We review some of the underlying physics and discuss outstanding questions regarding this variability.

Identified charged hadron (${\pi }^{\pm }$, $K^{\pm }$, p and $\bar{p}$) nuclear modification factors and ratios of $p/{\pi }^{+}$, $\bar{p}/{\pi }^{-}$, $K^{+}/{\pi }^{+}$ and $K^{-}/{\pi }^{-}$ measured by the PHENIX experiment in p$+$Al, ³He$+$Au, Cu+Au collisions at $\sqrt{s_{{}_{\mathrm{\text{vNN}}}}}$ = 200GeV and in U+U collisions at $\sqrt{s_{{}_{\mathrm{\text{NN}}}}}$ = 193 GeV are presented. The interpretation of experimental results has been carried out by comparison to theoretical predictions involving recombination and fragmentation models, implemented in the string melting version of AMPT package and in PYTHIA/ANGANTYR package, respectively. It is shown that in low $p_T$ range ($p_T<3$GeV/c) fragmentation processes seem to dominate over recombination processes in collisions, characterized by small numbers of participant nucleons ($〈N_{\mathrm{\text{part}}}〉\lesssim 10$). The contribution of recombination processes increases with increase of $〈N_{\mathrm{\text{part}}}〉$ values and becomes dominant in collisions with $〈N_{\mathrm{\text{part}}}〉\gtrsim 100$ which correspond to central collisions of large systems (Cu+Au and U+U).

The QCD phase transition exhibits different periods during the evolution time of the universe. We explore the cosmological implications of QCD phase transition in the early universe, which was mainly full of QGP matter, through the Friedmann equations. This is performed by studying the effect of GUP on the MIT bag model. The obtained modified MIT bag model alongside with the Friedmann equations is utilized to study the thermodynamical quantities in terms of the evolution time. The effect of GUP is set by inserting the parameter $\alpha =10^{-2}{\mathrm{\text{GeV}}}^{-1}$. It is found that the GUP effect on the evolution time of the universe makes it shorter, i.e. the time span of each phase takes shorter time to transfer to another phase than their corresponding ones without GUP effect.

Using our recently developed one parameter quasiparticle model, we analyze more recent (refined) results of (2+1)-flavor quark–gluon plasma (QGP) in lattice simulation of quantum chromodynamics (QCD) by various groups [S. Borsanyi et al., arXiv:1007.2580v2; A. Bazavov et al., Phys. Rev. D80 (2009) 014504; T. Umeda et al., arXiv:1011.2548v1; C. DeTar et al., Phys. Rev. D81 (2010) 114504]. We got a remarkable good fit to lattice thermodynamics of [S. Borsanyi et al., arXiv:1007.2580v2] and reasonable good fit to [A. Bazavov et al., Phys. Rev. D80 (2009) 014504] by adjusting single parameter of the model which may be related QCD scale parameter. The same model also fits the lattice results of [S. Borsanyi et al., arXiv:1007.2580v2] on (2+1+1)-flavor QGP. Further, we extend our model for above system with zero chemical potential to nonzero chemical potential and predict quark density without any new parameters which may be compared with future lattice data.