Inelastic J/Ψ photoproduction off nuclei: gluon enhancement or double color exchange?

The nuclear enhancement observed in inelastic photoproduction of J/gY should not be interpreted as evidence for an increased gluon density in nuclei. The nuclear suppression of the production rate due to initial and final state interactions is calculated and a novel two-step color exchange process is proposed, which is able to explain the data.     

The study of gluon energy loss in cold nuclear matter from J/ψ production

The energy loss effects of the incident quark, gluon, and the color octet ccˉ on J/ψ suppression in p-A collisions are studied by means of the experimental data at E866, RHIC, and LHC energy. We extracted the transport coefficient for gluon energy loss from the E866 experimental data in the middle x F region(0.20 x F 0.65) based on the Salgado-Wiedemann(SW) quenching weights and the recent EPPS16 nuclear parton distribution functions together with nCTEQ15. It was determined that the difference between the values of the transport coefficient for light quark, gluon, and heavy quark in cold nuclear matter is very small. The theoretical results modified by the parton energy loss effects are consistent with the experimental data for E866 and RHIC energy, and the gluon energy loss plays a remarkable role on J/ψ suppression in a broad variable range. Because the corrections of the nuclear parton distribution functions in the J/ψ channel are significant at LHC energy level, the nuclear modification due to the parton energy loss is minimal. It is worth noting that we use the color evaporation model(CEM) at leading order to compute the p-p baseline, and the conclusion in this paper is CEM model dependent.     

Effective dilaton lagrangian and gluon condensate in a nucleon medium

The gluon condensate as a function of temperature and baryon density in a nucleon medium is obtained from an effective dilaton Lagrangian. It is shown that, at a normal nuclear density of nucleons, n ₀ = 0.17 fm^?3, the gluon condensate decreases by about 10%.     

Kaon and pion ratio probes of jet quenching in nuclear collisions

Non-Abelian energy loss in quark gluon plasmas is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. Here we investigate pion and kaon production in pp and AA collisions in a perturbative QCD frame, suppression pattern and hadron ratios. The K^?/K⁺ and K⁺/π⁺ ratios are found to be most sensitive to the opacity (density) of the plasma. Experimental data indicate that the fragmentation dominated pQCD region will be reached only at higher p _T; in an intermediate p _Tregion other particle production mechanisms dominate the K/π ratios.     

相对论重离子碰撞中的重味产生

重味粒子是新的物质形态——夸克胶子等离子体的敏感探针。 利用相对论流体力学描述夸克胶子等离子体的时空演化, 采用输运方程模拟重味粒子在夸克胶子等离子体中的运动, 既考虑重味粒子的热胶子离解, 也通过细致平衡原理包含重味粒子在热密媒质中的重产生。 正是由于离解与重产生之间的竞争以及竞争对于碰撞能量、 横动量和快度的依赖性, 自然解释了在RHIC能区的J/ψ疑难, 预言了在LHC能区由于重产生取得主导地位, J/ψ的核修正因子在中心和半中心碰撞中将随着参与反应核子数的增大而升高, 同时其平均横动量会受到强烈的压低。 Heavy quarkonium is a sensitive signature of the new state of matter-quark gluon plasma produced in high energy nuclear collisions. We describe the space time evolution of the quark gluon plasma by relativistic hydrodynamic equations and the quarkonium motion by transport equation. We found that the competition between the gluon dissociation and regeneration can explain naturally the J/ψ puzzles at RHIC energy. We predict the increase of the nuclear modification factor in semi central and central collisions and the related transverse momentum suppression at LHC energy.     

Gluon recombination and shadowing at small values of x

We determine the recombination probabilities for gluons to go into gluons or into quarks in a low-density limit. A modified Altarelli-Parisi equation expressing this recombination is given. We find recombination is very small compared to normal evolution in all interesting circumstances except that of nuclear shadowing. Thus, for evolution above Q₀ ≈ 2 GeV, the gluon density remains far below saturation except at unusually small values of x.     

Spin dependent gluon densities from largeP T photon and dimuon production

We investigate spin asymmetries in highp _T photon and dimuon production and show that the spin dependent gluon density can be extracted reliably frompp collisions with these final states. The statement remains true even when the balancing jet is not observed. Hence fixed target experiments can be used to measure the spin dependent gluon density. We also compute the asymmetries in \(p\bar p\) collisions, and show that the EMC measurement of the spin structure function can be verified and the spin dependent gluon density can be extracted from these experiments.     

Charged hadron suppression in Au+Au collisions at high p T from STAR

We report high statistics measurements of inclusive charged hadron production in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and 130 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5 ‖p _T ‖ 12 GeV/c. The collision energy dependence of the yields and the centrality and p _T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. The p _T -dependent suppression expected from models incorporating jet attenuation in cold nuclear matter or absorption of fragmentation hadrons is not observed.     

J/<Emphasis Type="Italic">ψ</Emphasis> production measurements by the PHENIX experiment

Quarkonia suppression is considered to be one of the key probes of the Quark Gluon Plasma (QGP) created in heavy ion collisions. The PHENIX experiment has measured J/ψ production in a variety of colliding systems. Measurements made in p+p collisions show good agreement with pQCD predictions and serve as baseline for other systems at the same collision energy. The cold nuclear matter contribution to the suppression is constrained through measurements in d+Au collisions. In Au+Au, the suppression observed at mid rapidity is smaller than that at forward rapidity, a tendency opposite to what is expected from the higher gluon density at mid rapidity. The results will be presented and discussed in this article.     

Nuclear effects on J/ψ production in proton–nucleus collisions

The study of nuclear effects for J/ψ production in proton–nucleus collisions is crucial for a correct interpretation of the J/ψ suppression patterns experimentally observed in heavy-ion collisions. By means of three representative sets of nuclear parton distribution, the energy loss effect in the initial state and the nuclear absorption effect in the final state are taken into account in the uniform framework of the Glauber model. A leading order phenomenological analysis is performed on J/ψ production cross-section ratios R _W/Be (x _F) for the E866 experimental data. The J/ψ suppression is investigated quantitatively due to the different nuclear effects. It is shown that the energy loss effect with resulting in the suppression on R _W/Be (x _F) is more important than the nuclear effects on parton distributions in high x _F region. The E866 data in the small x _F keep out the nuclear gluon distribution with a large anti-shadowing effect. However, the new HERA-B measurement is not in support of the anti-shadowing effect in the nuclear gluon distribution. It is found that the J/ψ–nucleon inelastic cross section $\sigma^{J/\psi}_{\mathrm{abs}}$ depends on the kinematical variable x _F, and increases as x _F in the region x _F>0.2.     

Baryon-to-entropy ratio in very high energy nuclear collisions

We compute as a function of rapidity y the baryon number carried by quarks and antiquarks with p _T > p ₀ ≈ 2 GeV produced in Pb+Pb collisions at TeV energies. The computation is carried out in lowest order QCD perturbation theory using structure functions compatible with HERA results. At p ₀ = 2 GeV the initial gluon density is both transversally saturated and thermalised in the sense that the energy/gluon equals to that of an ideal gas with the same energy density. Even at these high energies the initial net baryon number density at y = 0 at τ = 0.1 fin will be more than the normal nuclear matter density but the baryon-toentropy ratio is only (B-B?)/S ～ 1/5000. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.     

Simple models of the contribution of intermediate stage gluons to J/ψ suppression at the CERN SPS

We construct three simple scenarios of the time dependence of density of intermediate stage gluons in nuclear collisions in the CERN SPS energy range. Gluons with energy of about 0.6–1.0 GeV are assumed to be produced in nucleon-nucleon collisions in a Glauber type model. The rate of gluon production is given by the parameter n _g/nn equal to the average number of gluons produced per nucleon-nucleon collision. The value of this parameter determines the behaviour of the gas of gluons. The number of gluons increases due to gluon branching and processes like g+g → g+g+g and decreases due to the hadronization. Gluons are assumed to be able to dissociate J/ψ in g+J/ψ collisions, the dissociation cross-section σ _gγ is taken as a free parameter. In the first scenario, the energy density of the gas of gluons never reaches the critical energy density ε _c ≈ 0.7 GeV/fm³ and gluons rapidly hadronize. In the second scenario, the critical energy density is reached but the system of gluons is unable to reach thermalization. In the third scenario gluons reach thermalization and the thermalized system suppresses J/ψ by the Matsui-Satz mechanism. The third scenario under the assumption of a small value of σ _gψ is able to describe qualitatively the data on J/ψ suppression in Pb?Pb interactions obtained by the NA50 Collaboration. Other scenarios have problems with getting the rather abrupt onset of J/ψ suppression.     

The virtual photon–gluon impact factor with massive quarks and exact gluon kinematics

We calculate the impact factor coupling a virtual photon to a gluon via a massive quark–antiquark pair at LL order, but with the imposition of the correct gluon kinematics. Exact analytical results are presented in triple Mellin space with respect to scaled Bjorken x, gluon transverse momentum and heavy quark mass. The application of these results to the calculation of approximate NLL coefficient functions needed to relate structure functions to the BFKL gluon is presented. The NLL effects with running coupling are seen to lead to a suppression of the small x divergence when compared with the fixed and running coupling LL results, but less than in the massless case.     

Shadowing correction to the gluon distribution behavior at small x

We determined the saturation exponent of the gluon distribution using the solution of the QCD nonlinear Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (NLDGLAP) evolution equation at small x . The very small-x behavior of the gluon distribution is obtained by solving the Gribov, Levin, Ryskin, Mueller and Qiu (GLR-MQ) evolution equation with the nonlinear shadowing term incorporated. The form of the initial condition for the equation is determined. We find, with decreasing x , the emergence of a singular behavior and the eventual taming (at R = 5 GeV^-1) and the essential taming (at R = 2 GeV^-1) of this singular behavior by the shadowing term. The nonlinear gluon density functions are calculated and compared with the results for the integrated gluon density from the Balitsky-Kovchegov (BK) equation for the different values of Q². It is shown that the results for the gluon density function are comparable with the results obtained from the BK equation solution. Also we show that for each x , the Q²-dependence of the data is well described by gluon shadowing corrections to the GLR-MQ equation. The resulting analytic expression allows us to predict the logarithmic derivative \( {\frac{{\partial F^{s}_{2} (x,Q^{2})}}{{\partial \ln Q^{2}}}}\) and to compare the results with H1 data and a QCD analysis fit.     

Charmonium suppression in the presence of dissipative forces in a strongly-coupled quark–gluon plasma

We study the survival of charmonium states in a strongly-coupled quark–gluon plasma in the presence of dissipative forces. We consider first-order dissipative corrections to the plasma equation of motion in the Bjorken boost-invariant expansion with a strongly-coupled equation of state for QGP and study the survival of these states with the dissociation temperatures obtained by correcting the full Cornell potential not its Coulomb part alone with a dielectric function encoding the effects of deconfined medium. We further explore the sensitivity of prompt and sequential suppression of these states to the shear viscosity-to-entropy density ratio, η/s from perturbative QCD and AdS/CFT predictions. Our results show nice agreement with the recent experimental results at RHIC.     

Stars of strange matter?

We investigate suggestions that quark matter with strangeness per baryon of order unity may be stable. We model this matter at nuclear matter densities as a gas of close packed Λ-particles. From the known mass of the Λ-particle we obtain an estimate of the energy and chemical potential of strange matter at nuclear densities. These are sufficiently high to preclude any phase transition from neutron matter to strange matter in the region near nucleon matter density. Including effects from gluon exchange phenomenologically, we investigate higher densities, consistently making approximations which underestimate the density of transition. In this way we find a transition density ρ_tr≳7ρ₀, where ρ₀ is nuclear matter density is not far from the maximum density in the center of the most massive neutron stars that can be constructed. Since we have underestimated ρ_tr and still find it to be ∼7ρ₀, we do not believe that the transition from neutron to quark matter is likely in neutron stars. Moreover, measured masses of observed neutron stars are ≅1.4 M_⊙, where M_⊙ is the solar mass. For such masses, the central (maximum) density is ρ_c<5ρ₀. Transition to quark matter is certainly excluded for these densities.     

The QCD generalized parton model and a next-to-leading approximation of the gluon density

Using recent neutrino data, we calculate in the framework of the generalized parton model the modification of the gluon density caused by next-to-leading logarithmic effects. We find that they are of the same order of magnitude as the experimental uncertainties, and that the gluon density tends to be more peaked towards small x.     

On nuclear dependence of direct photon production

We have calculated the impact on theA dependence of direct photon production at large transverse momenta (p _T ) of models that were used successfully to explain the EMC effect. We find that, for energies andp _T 's typical of present direct photon measurements, the change in the photon yield due to nuclear modification of parton distributions can be expected to be about 15–20% on Fe. The effect is quite sensitive to the gluon distribution. In particular, for the models used in our calculations, theA dependence is affected more by the gluon distribution than by the quark distributions.