We argue that the emission of hadrons with transverse momentum up to about 5 GeV/c in central relativistic heavy ion collisions is dominated by recombination, rather than fragmentation of partons. This mechanism provides a natural explanation for the observed constant baryon-to-meson ratio of about one and the apparent lack of a nuclear suppression of the baryon yield in this momentum range. Fragmentation becomes dominant at higher transverse momentum, but the transition point is delayed by the energy loss of fast partons in dense matter.

We discuss hadron production in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). We argue that hadrons at transverse momenta P T Ͻ5 GeV are formed by recombination of partons from the dense parton phase created in central collisions at RHIC. We provide a theoretical description of the recombination process for P T Ͼ2 GeV. Below P T =2 GeV our results smoothly match a purely statistical description. At high transverse momentum hadron production is well described in the language of perturbative QCD by the fragmentation of partons. We give numerical results for a variety of hadron spectra, ratios, and nuclear suppression factors. We also discuss the anisotropic flow v 2 and give results based on a flow in the parton phase. Our results are consistent with the existence of a parton phase at RHIC hadronizing at a temperature of 175 MeV and a radial flow velocity of 0.55c.

We calculate the production of high energy photons from Compton scattering and annihilation of a quark jet passing through a quark gluon plasma produced in a relativistic heavy ion collision. The contributions are large and reflect the momentum distribution of the jets and the initial conditions of the plasma.Relativistic heavy ion collisions are studied with the aim of producing a plasma of quarks and gluons (QGP). Photons are considered to be an important probe for the investigation of the formation and evolution of such a plasma due to their weak final-state interactions [1]. Once produced they carry the information about the conditions of the environment in which they were created, encoded in their momentum distribution, thus providing a glimpse deep into the bulk of strongly interacting matter. Though most of the measured photons have their origin in the decay of hadrons after the QGP phase, it has become possible to isolate the direct photons produced in such collisions [2].The sources of direct photons considered so far include quark annihilation, Compton scattering, and bremsstrahlung following the initial hard scattering of partons of the nuclei [3], as well as thermal photons from the QGP [4,5,6,7,8] and from hadronic interactions in the hot hadronic gas after the hadronization of the plasma [4,9]. The pre-equilibrium production of photons has also been investigated by several authors [10]. Results are available for production from the entire history of the system [11].In this letter we study a new source of direct photons originating from the passage of the produced high energy quark jets through the QGP (jet-photon conversion). A fast quark passing through the plasma will produce photons by Compton scattering with the thermal gluons and annihilation with the thermal antiquarks. This process is higher order in α s compared with photons from initial hard scatterings, but it is not a subleading contribution, since it corresponds to double scattering, which is enhanced by the size of the system. For cold nuclear matter this effect is encoded in multi-parton matrix elements which are enhanced by powers of A 1/3 [12]. Below, we find that this source is at least comparable in strength to the other direct photon sources and even dominates in the range p ⊥ ≤ 6 GeV for Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC).We also demonstrate that the p ⊥ -distribution of these photons is directly proportional to the momentum distribution of jets at an early stage after their production, before they have lost energy on their travel through the plasma. Since the measured high-p ⊥ hadron spectrum is proportional to the spectrum of partons after they have left the plasma, a comparision of both spectra could provide a quantitative determination of the energy loss and help confirm the mechanism of jet quenching [13,14].Furthermore, the photon yield depends on the integrated density of the matter traversed by the jets and thus can provide a measurement of this quantity. We emphasize that our mechanism is dis...

We present the first systematic study of higher-twist light-cone distribution amplitudes of the nucleon in QCD. We find that the valence three-quark state is described at small transverse separations by eight independent distribution amplitudes. One of them is leading twist-3, three distributions are twist-4 and twist-5, respectively, and one is twist-6. A complete set of distribution amplitudes is constructed, which satisfies equations of motion and constraints that follow from conformal expansion. Nonperturbative input parameters are estimated from QCD sum rules.

Employing nonperturbative transport coefficients for heavy-flavor (HF) diffusion through quark-gluon plasma (QGP), hadronization and hadronic matter, we compute D-and B-meson observables in Pb+Pb ( √ s=2.76 TeV) collisions at the LHC. Elastic heavy-quark scattering in the QGP is evaluated within a thermodynamic T -matrix approach, generating resonances close to the critical temperature which are utilized for recombination into D and B mesons, followed by hadronic diffusion using effective hadronic scattering amplitudes. The transport coefficients are implemented via Fokker-Planck Langevin dynamics within hydrodynamic simulations of the bulk medium in nuclear collisions. The hydro expansion is quantitatively constrained by transverse-momentum spectra and elliptic flow of light hadrons. Our approach thus incorporates the paradigm of a strongly coupled medium in both bulk and HF dynamics throughout the thermal evolution of the system.