Photon production in relativistic heavy ion collisions with equilibrium and non-equilibrium QGP

We consider the production sources of prompt and thermal photons which include the contribution of gluons in relativistic heavy ion collisions. Considering in our calculation the shadowing and iso-spin effects of the nucleus we can properly estimate the prompt photon production. We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region. The effect of the non-equilibrium quark-gluon plasma enhances the contribution of thermal photons.

Critical phenomena in a disc-percolation model and their application to relativistic heavy ion collisions

By studying the critical phenomena in continuum-percolation of discs, we find a new approach to locate the critical point, i.e. using the inflection point of P∞ as an evaluation of the percolation threshold. The susceptibility, defined as the derivative of P∞, possesses a finite-size scaling property, where the scaling exponent is the reciprocal of v, the critical exponent of the correlation length. A possible application of this approach to the study of the critical phenomena in relativistic heavy ion collisions is discussed. The critical point for deconfinement can be extracted by the inflection point of PQGP -- the probability for the event with QGP formation. The finite-size scaling of its derivative can give the critical exponent v, which is a rare case that can provide an experimental measure of a critical exponent in heavy ion collisions.

System size in relativistic heavy ion collisions

System size is more than a geometrical quantity in relativistic heavy ion collisions; it is closely related to evolution process, i.e. a different system size corresponds to a different evolution process, and whether QGP is produced depends on the system size. We propose that the system size should be under the same level when comparing the measurements from different colliding nuclei. The equivalence of the peripheral collisions of Au-Au and the central collisions of smaller nuclei is studied using the Monte Carlo method. Comparing the transverse overlapping area of the colliding nuclei, the number of participant nucleons and the number of nucleon-nucleon binary collisions in various colliding nuclei, we give an estimate of the correspondence in system size. This is helpful in the experimental comparison of the measurements from different colliding nuclei.

Energy dependence of resonance production in relativistic heavy ion collisions

The production of the hadronic resonances K^＊0（892）,φ（1020）,∑^＊（1385）,and Ξ^＊（1530） in central AA collisions at √SNN=17.3,200,and 2760 GeV is systematically studied.The direct production of these resonances at system hadronization is described by the quark combination model and the effects of hadron multiple-scattering stage are dealt with by a ultra-relativistic quantum molecular dynamics model（UrQMD）.We study the contribution of these two production sources to final observation and compare the final spectra with the available experimental data.The pT spectra of K^＊0（892） calculated directly by quark combination model are explicitly higher than the data at low pT≤1.5 GeV,and taking into account the modification of rescattering effects,the resulting final spectra well agree with the data at all three collision energies.The rescattering effect on φ（1020） production is weak and including it can slightly improve our description at low p_T on the basis of overall agreement with the data.We also predict the pT spectra of ∑^＊（1385） and Ξ^＊（1530）,to be tested by the future experimental data.

Neighboring azimuthal bin-bin multiplicity correlation as a direct measure for the shear viscosity in relativistic heavy ion collisions

Neighboring azimuthal bin-bin multiplicity correlation is suggested to be a good measure for internal layer-to-layer interactions of the formed matter in relativistic heavy ion collisions. It is shown to be directly related to the shear viscosity of the formed matter. As an application of this method, the shear viscosity in the samples generated by a multi-phase transport model （AMPT） is estimated. The results are in qualitative agreement with the theoretical calculation from microscopic interactions, i.e., the larger the scattering cross section, the smaller the shear viscosity.

Correlations of baryon and charge stopping in heavy ion collisions

Baryon numbers are theorized to be carried by valence quarks in the standard QCD picture of the baryon structure.Another theory proposed an alternative baryon number carrier,a non-perturbative Y-shaped configuration of the gluon field,called the baryon junction in the 1970s.However,neither of these theories has been verified experimentally.Recently,searching for the baryon junction by investigating the correlation of net-charge and net-baryon yields at midrapidity in heavy-ion collisions has been suggested.This paper presents studies of such correlations in collisions of various heavy ions from oxygen to uranium with the UrQMD Monte Carlo model.The UrQMD model implements valence quark transport as the primary means of charge and baryon stopping at midrapidity.Detailed studies are also conducted for isobaric ^(96)_(40)Zr+^(96)_(40)Zr and ^(96)_(44)Ru+^(96)_(44)Ru collisions.We found a universal trend of charge stopping with respect to baryon stopping and discovered that the charge stopping is always greater than the baryon stopping.This study provides a model baseline in valence quark transport for what is expected in net-charge and net-baryon yields at the midrapidity of relativistic heavy-ion collisions.

Multiplicity fluctuation and correlation of mesons and baryons in ultra-relativistic heavy-ion collisions at the LHC

We study the multiplicity fluctuation and correlation of identified mesons and baryons formed at hadronization by the quark combination mechanism in the context of ultra-relativistic heavy-ion collisions. Based on the statistical method of free quark combination, we derive the two-hadron multiplicity correlations, including meson-meson and meson-baryon correlations, and take the effects of quark number fluctuation at hadronization into account by a Taylor expansion method. After including the decay contributions, we calculate the dynamical fluctuation observable ν_（dyn） for Kπ, pπ and Kp pairs and discuss what underlying physics can be obtained by comparing with data from Pb-Pb collisions at sNN^（1/2）=2.76 Te V and simulations from the HIJING and AMPT event generators.

Large transverse momentum dilepton production in heavy ion collisions with two-photon processes

The cold component of large transverse momentum dilepton production via semi-coherent two- photon interaction is calculated. The cold contribution is essential to the dilepton spectra in the soft region for different mass bins. The results are compared with the PHENIX experimental data at RHIC, and we find that the modification of semi-coherent two-photon processes is more evident with the rising dilepton mass bins.

Flavor-dependent photoproduction in heavyion collisions

Strong electromagnetic fields produced in the non-central heavy-ion collisions can induce vector meson photoproduction.In this paper,we study the photoproduction J/ψandφmesons in the relativistic heavy-ion collision from ultra-peripheral nuclear collisions to peripheral hadronic heavy ion collisions.And then include both initial hadronic production and thermal production in quark-gluon plasma(QGP).We find,for the charm anti-charm bound state J/ψ,the photoproduced J/ψs are mainly in the very low momentum region and clearly exceed the hadronic production.However,considering the thermal production of strange quark anti-quark pairs in QGP produced in relativistic heavy-ion collisions,the photoproducedφis usually smaller than the thermal production and only evident at very peripheral collisions as even their photoproduction is much larger than J/ψ.

Jet-photon conversion with energy loss in heavy ion collisions

The rate of high energy photons produced from energetic jets during their propagation through the QGP at RHIC and LHC is studied by taking into account the contribution of jet quenching in the medium. It is shown that the jet quenching effect reduces the rate of jet-photon conversion at large transverse momemtum by about 40% at RHIC with √8= 200 AGeV, and by about 80% at LHC with √8 = 5500 AGeV.

Hadronic rapidity spectra in heavy ion collisions at SPS and AGS energies in a quark combination model

The quark combination mechanism of hadron production is applied to nucleus-nucleus collisions at the CERN Super Proton Synchrotron （SPS） and the BNL Alternating Gradient Synchrotron （AGS）.The rapidity spectra of identified hadrons and their spectrum widths are studied.The data of π -,K±,φ,Λ,Λ,Ξ - and Ξ＋ at 80 and 40 A GeV,and in particular at 30 and 20 A GeV where the onset of deconfinement is suggested to happen,are consistently described by the quark combination model.However,at AGS 11.6 A GeV below the onset,the π±,K±and Λ spectra cannot be simultaneously explained,indicating the disappearance of the intrinsic correlation of their production in the constituent quark level.The collision-energy dependence of the rapidity spectrum widths of the constituent quarks,and the strangeness of the hot and dense quark matter produced in heavy ion collisions,are obtained and discussed.

Relativistic correction to the dissociation temperature of B_(c) mesons in a hot medium

By solving two body Dirac equations with potentials at finite temperature,we calculate the dissociation temperature T_(d) of B_(c) mesons in quark-gluon plasma.It is found that T_(d) becomes higher with the relativistic correction than the T_(d) from the Schrodinger equation.Both the short range interaction and the constant term of the potential at the long-range scale have a contribution to the shift of T_(d),while the spin interaction is negligible.

Measurements of dihadron correlations relative to the event plane in Au+Au collisions at√^(S)NN=200 GeV

Dihadron azimuthal correlations containing a high transverse momentum(pr)trigger particle are sensit-ive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium,ie.jet-quenching.Previous measurements revealed a strong modification to di-hadron azimuthal correlations in Au+Au collisions with respect to ptp and d+Au collisions.The modification in-creases with the collision centrality,suggesting a path-length or energy density dependence to the je-quenching ef-fect.This paper reports STAR measurements of dihadron azimuthal correlations in mid-central(20%-60%)Au+Au collisions at√^(S)NN=200 GeV as a function of the trigger particle's azimuthal angle relative to the event plane,Ф_(s)=|Ф_(t)-ψ_(Ep)|.The azimuthal correlation is studied as a function of both the trigger and associated particle pr.The subtractions of the combinatorial background and anisotropic flow,assuming Zero Yield At Minimum(ZYAM),are described.The correlation results are first discussed with subtraction of the even harmonic(elliptic and quadrangu-lar)flow backgrounds.The away-side correlation is strongly modifed,and the modification varies withФ_(s),with a double-peak structure for out-of-plane trigger particles.The near-side ridge(long range pseudo-rapidity△_(η)correla-tion)appears to drop with increasingФ_(s)while the jet-like component remains approximately constant.The correla-tion functions are further studied with the subtraction of odd harmonic triangular flow background arising from fluc-tuations.It is found that the triangular flow,while responsible for the majority of the amplitudes,is not sufficient to explain theφs-dependence of the ridge or the away-side double-peak structure.The dropping ridge withФ_(s)could be attributed to aФ_(s)-dependent lliptie anisotropy;however,the physics mechanism of the ridge remains an open ques-tion.Even with aФ_(s)-dependent elliptic flow,the away-side correlation structure is robust.These results,with extens-ive systematic studies of the dihadron correlations as a function ofФ_(s),trigger and associated particle pT,and the pseudo-rapidity range△_(η),should provide stringent inputs to help understand the underlying physics mechanisms of jet-medium interactions in high energy nuclear collisions.

Rapidity dependence of hadron p_T spectra in central Au+Au collisions at s_(NN)～(1/2)=200 GeV

The transverse momentum spectra for identified hadrons at different rapidities in central Au＋Au collisions at √^SNN= 200 GeV are studied in a quark combination model. The results for PT spectra of π^±, K^±, p（p^-） and for the p/π ratios in a broader PT range at midrapidity agree well with the data. The transverse momentum spectra of pions, protons and antiprotons at various rapidities y - 1, η= 2.2 and y ≈ 3.2 are calculated and compared with the data.

Rapidity distributions of net protons from AGS to LHC energy regions

Taking the conservation of baryon number into account in a non-uniform flow model, the rapidity distribution of the net protons at the LHC is predicted. The energy dependence of the rapidity distribution, baryon stopping and collective flow from BNL/AGS to CERN/LHC are systematically investigated.

Different coalescence sources of light nucleus production in Au-Au collisions at √sNN=3 GeV

We study the production of light nuclei in the coalescence mechanism of Au-Au collisions at midrapidity at √sNN=3GeV.We derive analytic formulas of the momentum distributions of two bodies,three bodies,and four nucleons coalescing into light nuclei and naturally explain the transverse momentum spectra of the deuteron(d),triton(t),helium-3(3He),and helium-4(4He).We reproduce data on the yield rapidity densities,yield ratios,and averaged transverse momenta of d,t,3He,and 4He and provide the proportions of contributions from different coalescence sources for t,3He,and 4He in their production.We find that besides nucleon coalescence,nucleon+nucleus coalescence and nucleus+nucleus coalescence may play requisite roles in light nucleus production in Au-Au collisions at √sNN=3 GeV.

Revisiting the spin effects induced by thermal vorticity

We revisit the spin effects induced by thermal vorticity by calculating them directly from the spin-dependent distribution functions.For spin-1/2 particles,we provide the polarization up to the first order of thermal vorticity and compare it with the usual results calculated from the spin vector.For spin-1 particles,we show that all the non-diagonal elements vanish and there is no spin alignment up to the first order of thermal vortcity.We present the spin alignment at second-order contribution from thermal vorticity.We also show that the spin effects for both Dirac and vector particles receive an extra contribution when the spin direction is associated with the momentum of the particle.