Recent RHIC data show evidence of multiple hadron production mechanisms in heavy ion collisions compared to simple fragmentation in vacuum. I will review the measurements of collective flow, high momentum quenching, and two particle angular correlations to show that neither thermal production nor string fragmentation can describe the abundances, the angular distributions or the kinematic properties of all hadrons produced at RHIC. The proposed new hadronization mechanisms not only serve as evidence for a deconfined partonic phase of matter, but also for strong coupling of the degrees of freedom in the deconfined phase. I will point out a surprising lack of flavor dependence in these properties at RHIC, though, which might have to lead to further revisions of our understanding of the relevant degrees of freedom in the partonic phase and during the hadronization process.
New physics and detector concepts for a future pp and heavy ion program at the RHIC-II accelerator facility will be discussed. I will focus on hadronic observables which enable us to gain a better understanding on the hadronization from a sQGP and the chiral symmetry restoration in a sQGP. The ultimate question of how matter acquires mass can be addressed by this program in a complementary way to the Higgs search in high energy physics. The contributions of the RHIC program to the study of QCD will be discussed in detail.
At the Relativistic Heavy Ion Collider (RHIC) collisions of heavy ions at nucleon-nucleon energies of 200 GeV appear to have created a new form of matter thought to be a deconfined state of the partons that ordinarily are bound in nucleons.We discuss the evidence that a thermalized partonic medium, usually called a Quark Gluon Plasma (QGP), has been produced. Then we discuss the effect of this high-density medium on the production of jets and their pair correlations. Next we look at direct photons as a clean electro-magnetic probe to constrain the initial hard scatterings. Finally we review th…
Measurements of identified particle momentum spectra at center of mass energy 200 GeV are reviewed. Emphasis is placed on the azimuthal dependence and the centrality dependence of hadron yields at intermediate transverse momentum (1.5 < p_T < 5 GeV/c). The first measurements of the fourth harmonic term (v_4) in the azimuthal variation of identified particle yields are shown. The recombination mechanism of hadron formation provides a consistent description of the dependence of these measurements on particle-type.
Nucleus-nucleus collisions at RHIC produce high temperature and high energy density matter which exhibits partonic degrees of freedom. We will discuss measurements of nuclear modification factors for light hadrons and non-photonic electrons from heavy quark decays, which reflect the flavor dependence of energy loss of high momentum partons traversing the dense QCD medium. The hadronization of bulk partonic matter exhibits collectivity in effective partonic degrees of freedom. Nuclear collisions at RHIC provide an intriguing environment, where many constituent quark ingredients are readily avai…
I review some important results from RHIC experiments. They were obtained in a unique environment for studying QCD bulk matter at temperatures and densities that surpass the limits where hadrons can exist as individual entities, raising the quark-gluon degrees of freedom to prominence. These findings support the major experimental observations from measuring the bulk properties of particle production, particle ratios and chemical freeze-out conditions, elliptic flow followed by hard probes measurements: di-jet fragment azimuthal correlations, high-pt hadron suppression, and heavy-flavors probe…
The study of the multi-strange production in the final state of ultra-relativistic Au+Au collisions at top RHIC energy gives information on the freeze-out conditions and probably on earlier stages of the collision. The chemical freeze-out temperature as well as the strangeness phase space occupancy factor gamma_s extracted from statistical models are studied as a function of the collision centrality. Within the framework of a hydrodynamically-inspired model the transverse collectivity of the collision is investigated. In this view, multi-strange particles seem to develop significant radial tra…
Extensive experimental data from high-energy nucleus-nucleus collisions were recorded using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The comprehensive set of measurements from the first three years of RHIC operation includes charged particle multiplicities, transverse energy, yield ratios and spectra of identified hadrons in a wide range of transverse momenta (p_T), elliptic flow, two-particle correlations, non-statistical fluctuations, and suppression of particle production at high p_T. The results are examined with an emphasis on implications for the formation of a …
We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate.
New results on $ϕ$-meson production and elliptic flow $v_{2}$ measurements from RHIC 2004 run (Run-IV) have been reviewed. In addition, the di-hadron correlation function between the trigged $ϕ$ and $Ω$ and the associated soft particles was simulated. Knowledge about these results are discussed.
When heavy ions with high energy collide, a hot and dense matter is produced. As the matter expands, the matter undergoes cross-over phase transition from partonic matter to hadronic matter. Jets are created by high pT partons in the early phase of the collisions. Leptons and photons can penetrate the matter without strong interaction. For that reason, jets, leptons and photons are good probes to study the partonic matter in the early phase of the collisions. We report about two probes associated with photon and their results for PHENIX.
We will review recent results on quantitative description of global properties of bulk partonic matter at RHIC. These results include strangeness phase space factor of the partonic matter, azimuthal angular anisotropy $v_2$, and transverse momentum $p_T$ distributions of effective partons at the hadronization of bulk partonic matter. We present empirical constraints on parton energy loss in the high $p_T$ region ($>$ 5 GeV/c). A flat $R_{AA}$ as a function of $p_T$ at mid-rapidity implies a constant fraction of the parton energy loss ($Δp_T/p_T$) and the fraction reaches 25% for neutral $π$, c…
We report recent results from a multiphase transport (AMPT) model on the azimuthal anisotropies of particle momentum distributions in heavy ion collisions at the Relativistic Heavy Ion Collider. These include higher-order anisotropic flows and their scaling, the rapidity dependence of anisotropic flows, and the elliptic flow of charm quarks.
We present experimental features of identified particle production from nucleus-nucleus collisions at RHIC. These features reflect hadronization from a deconfined partonic matter whose particle formation scheme is distinctly different from fragmentation phenomenology in elementary collisions. Multi-parton dynamics, such as quark coalescences or recombinations, appear to be essential to explain the experimental measurements at the intermediate transverse momentum of 2-5 GeV/c. Constituent quarks seem to be the dominant degrees of freedom at hadronization and gluon degrees of freedom are not exp…
We present first measurements of the $ϕ$-meson elliptic flow ($v_{2}(p_{T})$) and high statistics $p_{T}$ distributions for different centralities from $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions at RHIC. In minimum bias collisions the $v_{2}$ of the $ϕ$ meson is consistent with the trend observed for mesons. The ratio of the yields of the $Ω$ to those of the $ϕ$ as a function of transverse momentum is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}\sim 4$ GeV/$c$, but disagrees at higher momenta. The nuclear modification factor ($R_{CP}$) of $ϕ$ follows th…
Measurements of identified particles over a broad $p_T$ range may provide strong evidence for the existence of a thermalized partonic state in heavy-ion collisions (\textit{i.e.} a quark-gluon plasma). Of particular interest are the centrality dependence and the azimuthal anisotropy in the yield of baryons and mesons at intermediate $p_T$. The first measurements of $v_2$ and the number-of-binary-collisions scaled centrality ratio $R_{CP}$ for $K_S^0$ and $Λ+\barΛ$ production in Au+Au collisions at RHIC are presented. These measurements from the STAR detector establish the particle-type depende…
We investigate the role of the initial condition used for the hydrodynamic evolution of the system formed in ultra-relativistic heavy-ion collisions and find that an appropriate choice motivated by the models of early-stage dynamics, specifically a simple two-dimensional Gaussian profile, leads to a uniform description of soft observables measured in the Relativistic Heavy-Ion Collider (RHIC). In particular, the transverse-momentum spectra, the elliptic-flow, and the Hanbury-Brown--Twiss correlation radii, including the ratio R_out/R_side as well as the dependence of the radii on the azimuthal…
Investigating partonic interactions is one of the primary goals of the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). RHIC is specially tailored for studying intrinsic partonic spin-momentum correlations due to its unique ability to collide polarized proton beams. Transverse single-spin asymmetries of order 10% have been measured in PHENIX at center of mass energies from 62.4 GeV to 200 GeV, similar to previous measurements. These results indicate that there exist partonic transverse momentum effects within the proton and/or within the process of hadronization. The MPC-EX, a …
High-energy nuclear collision provide a unique tool to study the strongly interacting medium. Recent results from the Relativistic Heavy Ion Collider (RHIC) on ϕ-meson production has revealed the formation of a dense partonic medium. The medium constituents are found to exhibit collective behaviour initiated due to partonic interactions in the medium. We present a brief review of the recent results on ϕproduction in heavy-ion collisions at RHIC. One crucial question is where, in the phase diagram, does the transition happen for the matter changing from hadronic to partonic degrees of freedom. …
We present STAR measurements of K^{0}_{S}, ϕ, Λ, Ξ, and Ωat mid-rapidity from Au+Au collisions at \sqrt{s_{NN}} = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). Nuclear modification factors and baryon-to-meson ratios are measured to understand recombination and parton energy loss mechanisms. Implications on partonic versus hadronic dynamics at low beam energies are discussed.
We review some important results from the PHENIX experiment at RHIC. They were obtained in a unique environment for studying QCD bulk matter at temperatures and densities that sur- pass the limits where hadrons exist as individual entities, so raising to prominence the quark- gluon degrees of freedom. We present measurements of nuclear modification factors for neutral pions, light favors (strange hadrons), direct-photons and non-photonic electrons from decays of particles carrying charm or beauty quarks. We interpret the large suppression of hadron produc- tion at high transverse momenta as re…
Production of W bosons in longitudinally polarized pp collisions provides an excellent tool to probe the flavor-dependence of sea quark polarizations in the polarized proton. Current status and future plans for the W physics program with the STAR detector at RHIC are presented along with remarks concerning our knowledge of the nature and origin of the partonic sea.
We will present a review of selected results on strange baryon production from nucleus-nucleus collisions at the AGS, SPS and RHIC. From the AGS and SPS heavy ion physics program several intriguing aspects: centrality dependence of strangeness production, anti-hyperon yield, hyperon production enhancement and baryon number transport will be highlighted. Our discussion on RHIC results will focus on experimental probes of partonic degree of freedom. Measurements of the elliptic flow v2 and the nuclear modification factor and their distinctive particle dependence are presented as a function of tr…
I will review RHIC data with respect to the intriguing possibility that the hadron production mechanism in the produced partonic medium might be different than in vacuum. I will use the measurements of collective features, such as flow and quenching of identified particles, to show that different regions of the particle momentum spectrum are likely populated through different mechanisms, and that the medium seems to play an important role in hadronization. Finally I will address the question whether the different initial conditions achievable in heavy ion collisions at LHC energies, compared t…
I will present the latest results on particle identified spectra and two particle correlations at high transverse momentum measured with the STAR detector at RHIC. I will compare those measurements with the projected capabilities for the LHC using the ALICE detector. The effects of string fragmentation and quark recombination in the intermediate momentum range will be discussed in the context of hadron formation in proton-proton and Au-Au collisions, as well as energy loss mechanisms in the dense partonic phase. The necessity for gamma-jet and heavy quark jet measurements will be detailed base…
From the wealth of data obtained from the first three years of RHIC operation, the four RHIC experiments, BRAHMS, PHENIX, PHOBOS and STAR, have concluded that a high density partonic matter is formed at central Au+Au collisions at 200 GeV. The research focus now shifts from initial discovery to a detailed exploration of partonic matter. Particles carrying heavy flavor, i.e. charm or beauty quarks, are powerful tool for study the properties of the hot and dense medium created in high-energy nuclear collisions at RHIC. At the relatively low transverse momentum region, the collective motion of th…
Prompt photon production in ultra-relativistic heavy-ion collisions provides a calibrated probe for the study of the properties of high energy density QCD matter. Especially interesting are the measurements of $γ$-tagged jets where the hard scattering scale is known and can be used to determine the partonic energy loss in the dense matter. We discuss the potential of $γ$-jet measurements at the Relativistic Heavy Ion Collider (RHIC) and argue that the observed supression of the away-side correlations for di-jet production in central Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV should significan…
Recent results on high transverse momentum (pT) hadron production in p+p, d+Au and Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) are reviewed. Comparison of the nuclear modification factors, $R_{dAu}(pT)$ and $R_{AA}(pT)$, demonstrates that the large suppression in central Au+Au collisions is due to strong final-state effects. Theoretical models which incorporate jet quenching via gluon Bremsstrahlung in the dense partonic medium that is expected in central Au+Au collisions at ultra-relativistic energies are shown to reproduce the shape and magnitude of the observed suppressio…
We review hadron production in heavy ion collisions with emphasis on pion and kaon production at energies below 2 AGeV and on partonic collectivity at RHIC energies.
QCD predicts a phase transition between hadronic matter and a Quark Gluon Plasma at high energy density. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is a new facility dedicated to the experimental study of matter under extreme conditions. I will discuss the first round of experimental results from colliding heavy nuclei at RHIC and our current understanding of the state of matter generated in such collisions, concentrating on partonic energy loss and jet quenching as a probe of the medium.
The observation of meson and baryon grouping in the $R_{CP}$ and $v_2$ measurements at intermediate $p_T$ has been interpreted as a manifestation of bulk partonic matter hadronization through multi-parton dynamics such as recombination of partons. $ϕ$ mesons provide unique sensitivity to test these theoretical scenarios, since the $ϕ$ has a mass heavier than the proton and close to the hyperons. The $R_{CP}$ and $v_2$ measurements of $ϕ$ mesons from Run IV Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV at STAR are presented. Energy and colliding system dependence of the $ϕ$ yields at mid-rapidity…
We report STAR preliminary results on multi-strange baryon production in Au+Au collisions at sqrt(s_NN)=200 GeV at RHIC. Its implication for the formation of a new state of matter is discussed. The system size dependence on the production of strange baryons is investigated to study the onset of strange quark equilibration in the medium. The nuclear modification factor of Lambda, Xi and Omega is also presented. Its suppression at p_T>3 GeV/c supports the formation of a dense interacting medium at RHIC. The spectra of multi-strange baryons reveal that within a hydro-inspired model, they may deco…
We present an analysis of $Ω$, $Ξ$, $Λ$ and $ϕ$ spectra from Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV in terms of distributions of effective constituent quarks at hadronization. Consistency in quark ratios derived from various hadron spectra provides clear evidence for hadron formation dynamics as suggested by quark coalescence or recombination models. We argue that the constituent quark distribution reflects properties of the effective partonic degrees of freedom at hadronization. Experimental data indicate that strange quarks have a transverse momentum distribution flatter than that of up…
We study the mechanism and probability of in-medium hadronization in the deconfined medium produced in heavy-ion collisions at RHIC and LHC. We show the likelihood of color-neutral objects to be formed inside the partonic fireball and the probability of these states to escape the medium with reduced interaction strength and energy loss. We will suggest specific measurements that are sensitive to the early degrees of freedom and show predictions for these measurements at RHIC and the LHC
We investigate the role of partonic degrees of freedom in high multiplicity p-Pb collisions at $\sqrt{s_{NN}}=$ 5.02 TeV carried out at the Large Hadron Collider (LHC) by studying the production and collective flow of identified hadrons at intermediate $p_T$ via the coalescence of soft and hard partons generated from viscous hydrodynamics (VISH2+1) and the energy loss model (LBT), respectively. We find that combining the intermediate $p_T$ hadrons from the coalescence with the low $p_T$ hadrons from hydrodynamics and high $p_T$ hadrons from the jet fragmentation, our Hydro-Coal-Frag model prov…
Recent experimental and theoretical developments have motivated interest in a more detailed exploration of heavy ion collisions in the range sqrt(sNN)=5-15 GeV. In contrast to interactions at the full RHIC energy of sqrt(sNN)=200 GeV, such collisions result in systems characterized by much higher baryon chemical potential, muB. Extensions of lattice QCD calculations to non-zero values of muB suggest that a critical point may exist in this region of the QCD phase diagram. Discovery of the critical point or, equivalently, determining the location where the phase transition from partonic to hadro…
The measurement of resonance production in ultrarelativistic heavy-ion collisions provides a glimpse of the hadronic medium properties and its evolution at different stages. Resonances decaying into hadrons are used to estimate the time span and hadronic interaction cross section in the hadronic phase between chemical and kinetic freeze-out. Specifically, the comparison of K*0(892) and phi(1020) resonances is interesting as the lifetimes of these particles differ by about a factor of 10. Moreover, the nuclear modification factor and azimuthal anisotropy measurements of mesonic resonances, whic…
At top RHIC and LHC energies the suppression of high transverse momentum ($p_{\mathrm{T}}$) hadrons provides evidence for partonic energy loss in QGP. We study partonic energy loss in the RHIC Beam Energy Scan (BES) by investigating the centrality dependence of the binary-collision-scaled high-$p_{\mathrm{T}}$ yields. Observing a decrease of the scaled yield in more central collisions is proposed as a possible signature for jet quenching. Even at energies and centralities where this signature is lost a QGP may still be formed since competing phenomena responsible for enhancements may overwhelm…
We use an event-by-event hydrodynamical description of the heavy-ion collision process with Glauber initial conditions to calculate the thermal emission of photons. The photon rates in the hadronic phase follow from a spectral function approach and a density expansion, while in the partonic phase they follow from the AMY perturbative rates. The calculated photon elliptic flows are lower than those reported recently by both the ALICE and PHENIX collaborations.
Systematic trend of the hadron freeze-out conditions from AGS/SPS to RHIC is discussed. The most interesting results from collisions at RHIC are that the system is indeed approaching net-baryon free and the transverse expansion is much stronger than that from collisions at AGS/SPS energies. In order to understand the trend of the collective velocity, an energy scan between $\sqrt{s_{NN}} = 20 - 200 GeV, is important. In addition, systematic studies on the anisotropy parameter $v_2$ and the transverse momentum distributions of $ϕ, Ω,$ and $J / ψ$ are necessary as they will help in determining w…
Recent experimental advances at the Relativistic Heavy Ion Collider (RHIC) and the large center-of-mass energies available to the heavy-ion program at the Large Hadron Collider (LHC) will enable strongly interacting matter at high temperatures and densities, that is, the quark-gluon plasma (QGP), to be probed in unprecedented ways. Among these exciting new probes are fully-reconstructed inclusive jets and the away-side hadron showers associated with a weakly or electromagnetically interacting boson, or, tagged jets. Full jet reconstruction provides an experimental window into the mechanisms of…
The experimental and theoretical status of the ``near perfect fluid'' at RHIC is discussed. While the hydrodynamic paradigm for understanding collisions at RHIC is well-established, there remain many important open questions to address in order to understand its relevance and scope. It is also a crucial issue to understand how the early equilibration is achieved, requiring insight into the active degrees of freedom at early times.
Recent results on jet production in heavy ion collisions at RHIC and the LHC are discussed, with emphasis on inclusive jet yields and semi-inclusive hadron-triggered and vector boson-triggered recoil jet yields as well as their azimuthal angular correlations. I will also discuss the constraints that these observables impose on the opacity of the medium, the flavour dependence of energy loss, the interplay of perturbative and non perturbative effects and the change of the degrees of freedom of the medium with the resolution of the probe.
Data taken over the last several years have demonstrated that RHIC has created a hot, dense medium with partonic degrees of freedom. Identified particle spectra at high transverse momentum ($p_T$) and heavy flavor are thought to be well calibrated probes thus serve as ideal tools to study the properties of the medium. We present $p_T$ distributions of particle ratios in p+p collisions from STAR experiment to understand the particle production mechanisms. These measurements will also constrain fragmentation functions in hadron-hardon collisions. In heavy ion collsions, we highlight: 1) recent m…
We construct the nature of the matter found in RHIC when its temperature has dropped down close to, and below, $T_c$. Just above $T_c$ it is composed of extremely strongly bound quark-antiquark pairs forming chirally restored mesons of the quantum numbers of the $π, S, ρ$ and $a_1$ with very small size and zero energy and just below $T_c$, it is composed of mesons of the same quantum numbers with zero mass. We invoke infrared slavery for the former and the vector manifestation (VM) of hidden local symmetry for the latter. As the temperature drops below $T_c$, the strongly bound quark-antiquark…
We discuss experimental features of identified particle production from nucleus-nucleus collisions. These features reflect hadronization from a deconfined partonic matter whose particle formation scheme is distinctly different from fragmentation phenomenology in elementary collisions. Multi-parton dynamics, such as quark coalescences or recombinations, appear to be essential to explain the experimental measurements at the intermediate transverse momentum of 2--5 GeV/c. Constituent quarks seem to be the dominant degrees of freedom at hadronization. Heavy quark production should help quantify de…
Direct photon spectra and elliptic flow v2 in heavy-ion collisions at RHIC and LHC energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases - the Parton-Hadron-String Dynamics (PHSD). The results suggest that a large v2 of the direct photons - as observed by the PHENIX Collaboration - signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late stages of the collision. In order to further differentiate the origin of the direct photon azimuthal asymmetry, we compare our predictions fo…
Strange particles have been a very important observable in the search for a deconfined state of strongly interacting matter, the quark-gluon plasma (QGP), which is expected to be formed in ultra-relativistic heavy ion collisions. We review the main experimental observations made at the Super Proton Synchrotron (SPS) at CERN, Geneva, and at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). The large amount of recently collected data allows for a comprehensive study of strangeness production as a function of energy and system size. We review results on yields, t…
We study the charged particle and transverse energy production mechanism from AGS, SPS, RHIC to LHC energies in the framework of nucleon and quark participants. At RHIC and LHC energies, the number of nucleons-normalized charged particle and transverse energy density in pseudorapidity, which shows a monotonic rise with centrality, turns out to be an almost centrality independent scaling behaviour when normalized to the number of participant quarks. A universal function which is a combination of logarithmic and power-law, describes well the charged particle and transverse energy production both…
We update the study of the total Psi and Upsilon production cross section in proton-proton collisions at RHIC energies using the QCD-based Color-Singlet (CS) Model, including next-to-leading order partonic matrix elements. We also include charm-quark initiated processes which appear at leading order in alpha_s, but which have so far been overlooked in such studies. Contrary to earlier claims, we show that the CS yield is consistent with measurements over a broad range of J/Psi rapidities. We also find that charm-quark initiated processes, including both intrinsic and sea-like charm components,…
We compute \frac{dσ}{dp_T} of the J/ψproduction in pp collisions at RHIC at \sqrt s = 500 and 200 GeV by using both the color octet and singlet models in the framework of non-relativistic QCD. The J/ψwe compute here includes the direct J/ψfrom the partonic fusion processes and the J/ψcoming from the radiative decays of χ_J's both in the color octet and singlet channel. The high p_T J/ψproduction cross section is computed within the PHENIX detector acceptance ranges: -0.35 < η< 0.35 and 1.2 < \|η| < 2.4, the central electron and forward muon arms. It is found that the color octet contribution t…
In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $ψ$ of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au+Au collisions is based on the recent STAR data within the RHIC Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data for $\sqrt{s_{NN}}=$7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at $\sqrt{s_{NN}}=$39 GeV. For reference, th…
We consider production of prompt photons in high energy gold-gold and deuteron-gold collisions in the forward rapidity region of RHIC ($y \sim 3.8$). In this kinematics, the projectile partons typically have large $x_{bj}$ while the target partons are mostly at very small $x_{bj}$ so that the primary partonic collisions involve valence quarks from the projectile and gluons from the target. We take the target nucleus to be a Color Glass Condensate while the projectile deuteron or nucleus is treated as a dilute system of partons. We show that the photon production cross section can be written as…
