Nuclear modification factor for light and heavy flavors within pQCD and recent data from the LHC

The flavor dependence of the nuclear modification factor R _AA in the pQCD calculations at LHC energies has been examined. The computations are performed accounting for radiative and collisional parton energy loss with running coupling constant. Our results show that the recent LHC data on the R _AA for charged hadrons, D-mesons, and non-photonic electrons agree reasonably with the pQCD picture of the parton energy loss with the dominating contribution from the radiative mechanism.     

Heavy-flavour spectra in high-energy nucleus–nucleus collisions

The propagation of the heavy quarks produced in relativistic nucleus–nucleus collisions at RHIC and LHC is studied within the framework of Langevin dynamics in the background of an expanding deconfined medium described by ideal and viscous hydrodynamics. The transport coefficients entering into the relativistic Langevin equation are evaluated by matching the hard-thermal-loop result for soft collisions with a perturbative QCD calculation for hard scatterings. The heavy-quark spectra thus obtained are employed to compute the differential cross sections, the nuclear modification factors R _AA and the elliptic-flow coefficients v ₂ of electrons from heavy-flavor decay.     

Surface Emission of Quark Gluon Plasma at RHIC and LHC

Within the framework of a factorization model, we study the behaviour of nuclear modification factor in Au Au collisions at RHIC and Pb-Pb collisions at LHC. We find that the nuclear modification factor is inversely proportional to the radius of the quark-gluon plasma and is dominated by the surface emission of hard jets. We predict the nuclear modification factor R^LHC AA - 0.15 in central Pb-Pb collisions at LHC. The study shows that the factorization model can be used to describe the centrality dependence of nuclear modification factor of the high transverse momentum particles produced in heavy ion collisions at both RHIC and LHC.     

Corona effect in <Emphasis Type="Italic">AA</Emphasis> collisions at the LHC

Following our earlier finding based on RHIC data on the dominant jet production from nucleus corona region, we reconsider this effect in nucleus–nucleus collisions at the LHC energies. Our hypothesis was based on experimental data, which raised the idea of a finite formation time for the produced medium. At the RHIC energy and in low-density corona region, this time reaches about 2 fm/c. Following this hypothesis, the nuclear modification factor R _AA at high p _t should be independent on particle momentum, and the azimuthal anisotropy of high p _t particles, v ₂, should be finite. A separate prediction held that, at the LHC energy, the formation time in the corona region should be about 1 fm/c. New LHC data show that R _AA is not flat and is rising with p _t . We add to our original hypothesis an assumption that a fast parton traversing the produced medium loses the fixed portion of its energy. A shift of about 7 GeV from the original power law p ^?6 production cross section in pp explains well all the observed R _AA dependencies. The shift of about 7 GeV is also valid at the RHIC energy. We also show that the observed at the LHC dependence of v ₂ at high p _t and our previous predictions agree.     

Hydrokinetic predictions for femtoscopy scales in A + A collisions in the light of recent ALICE LHC results

A study of energy behavior of the pion spectra and interferometry scales is carried out for the top SPS, RHIC and for LHC energies within the hydrokinetic approach. The main mechanisms that lead to the paradoxical, at first sight, dependence of the interferometry scales with an energy growth, in particular, a decrease R _out/R _side ratio, are exposed. The hydrokinetic predictions for the HBT radii at LHC energies are compared with the recent results of the ALICE experiment.     

Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions

We study the induced photon bremsstrahlung from a fast quark produced in AA collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions, the induced photon spectrum is sharply peaked at a photon energy close to the initial quark energy. In this region, the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses [4–7], our results show that, at RHIC and LHC energies, the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production and may even enhance it at p _T ～5–15 GeV.     

Jet color chemistry and anomalous baryon production in <Emphasis Type="Italic">AA</Emphasis>-collisions

We study anomalous high-p _T baryon production in AA-collisions due to formation of the two parton collinear gq system in the anti-sextet color state for quark jets and gg system in the decuplet/anti-decuplet color states for gluon jets. Fragmentation of these states, which are absent for NN-collisions, after escaping from the quark–gluon plasma leads to baryon production. Our qualitative estimates show that this mechanism can be potentially important at RHIC and LHC energies.     

Can the RHIC <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> puzzle(s) be settled at LHC?

One observes strong suppression effects for hard probes, e.g. the production of J/ψ or high-p _T particles, in nucleus–nucleus (AA) collisions at RHIC. Surprisingly, the magnitude of the suppression is quite similar to that at SPS. In order to establish whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate shadowing for gluons in the Glauber–Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated by energy-momentum conservation effects. Furthermore, final-state suppression due to interaction with comoving matter (hadronic or pre-hadronic) has been shown to describe the data at SPS. We extend this model by including the backward reaction channel, i.e. recombination of open charm, which is estimated directly from pp data at RHIC. Strong suppression of charmonium both in pA and AA collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization of heavy quarks.     

Measurement of π0 and η mesons with PHENIX in ?[`(sNN )]\surd \overline {s_{NN} } 200 GeV Au+Au collisions at RHIC

The π ^o meson has been a crucial probe for observing jet quenching in ultrarelativistic heavy-ion collisions at RHIC. Measurements of the η meson in the same collisions have also shed light on a possible dependence of the observed suppression on the particle species. The preliminary π ⁰ nuclear modification factor R _AA from the 2004 RHIC run allowed a first systematic comparison between a precise measurement with high statistics and theoretical calculations, constraining model parameters such as the initial gluon density dN^g/dy, and the transport coefficient [^(q)]\hat q. The final π ⁰ spectra and R _AA are shown as well as the first η results obtained with both PHENIX electromagnetic calorimeters.     

Low- x QCD physics from RHIC and HERA to the LHC

We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of the parton momentum fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced.     

Resummed pQCD for <Emphasis Type="Italic">W</Emphasis><Superscript><Emphasis Type="Italic">±</Emphasis></Superscript> and <Emphasis Type="Italic">Z</Emphasis><Superscript>0</Superscript> transverse momentum spectra at RHIC and LHC

The transverse momentum distributions of W_± and Z₀ are predicted at RHIC and LHC. A resummation formalism with power corrections to the renormalization group equations is used. Shadowing effects are discussed and found to be unimportant at RHIC, but important for LHC. We study the enhancement of power corrections due to multiple scattering in nuclear collisions and numerically illustrate the weak effects of the dependence on the nuclear mass.     

Cold nuclear modifications at RHIC and LHC

We use recent nuclear parton distributions, among them the Hirai — Kumano — Nagai (HKN) and Eskola — Paukkunen — Salgado (EPS08) parameterizations, in our pQCD-improved parton model to calculate the nuclear modification factor, R _AA′ (p _T ), at RHIC and at the LHC. At RHIC, the deuteron-gold nuclear modification factor for pions, measured at p _T ≥ 10 GeV/c in central collisions, appears to deviate more from unity than the model results. The slopes of the calculated R _dAu (p _T ) are similar to the slopes of the PHENIX pion and photon data. At LHC, without final-state effects we see a small enhancement of R _dPb (p _T ) in the transverse momentum range 10 GeV/c ≥ p _T ≥ 100 GeV/c for most parameterizations. The inclusion of final-state energy loss will reduce the R _dPb (p _T ) values.     

v<Subscript>2</Subscript> and R<Subscript>AA</Subscript> of non-photonic single electrons: which restriction do the current measurements pose on the production of charm,beauty?

In relativistic heavy ion collisions, heavy flavor is expected to result predominately from initial hard parton–parton scatterings. Hence, in the absence of later stage effects, the production of heavy flavor in A+A collisions can be viewed as a superposition of N+N collisions. Measurements of v₂ or R_AA of heavy flavor (or their decay electrons) in A+A collisions violate the simple superposition picture and therefore present themselves as probes of the medium formed in such collisions. On the basis of the measured v₂ and R_AA of non-photonic single electrons in A+A collisions at RHIC, we will investigate the interplay between these observables as well as the restrictions they pose for charm and bottom production.     

Nuclear modification at ?{sNN }\sqrt {s_{NN} } = 17.3 GeV, measured at NA49

Transverse momentum spectra up to 4.5GeV/c were measured around midrapidity in Pb+Pb reactions at ?{s_NN }\sqrt {s_{NN} } = 17.3GeV, for π ^±, p, [`(p)]\bar p and K ^±, by the NA49 experiment. The nuclear modification factors R _AA , R _AA/pA and R _CP were extracted and are compared to RHIC results at ?{S_NN }\sqrt {S_{NN} } = 200GeV. The modification factor R _AA shows a rapid increase with transverse momentum in the covered region. The modification factor R _CP shows saturation well below unity in the π ^± channel. The extracted R _CP values follow the 200GeV RHIC results closely in the available transverse momentum range for all particle species. For π ^± above 2.5GeV/c transverse momentum, the measured suppression is smaller than that observed at RHIC. The nuclear modification factor R _AA/pA for π ^± stays well below unity.     

Parton energy loss in glasma

We study the synchrotron-like gluon emission in AA-collisions from fast partons due to interaction with the coherent glasma color fields. Our results show that for RHIC and LHC conditions the contribution of this mechanism to parton energy loss is much smaller than the radiative energy loss in the plasma phase.     

Quantum analysis of fluctuations of electromagnetic fields in heavy-ion collisions

We perform quantum calculations of fluctuations of the electromagnetic fields in AA collisions at RHIC and LHC energies. The analysis is based on the fluctuation–dissipation theorem. We find that in the quantum picture the field fluctuations are very small. They turn out to be much smaller than the predictions of the classical Monte Carlo simulation with the Woods–Saxon nuclear density.     

A simplified model for calculating RAA and RCP of cold nuclear matter effects

An incoherent binary nucleon-nucleon collision model of AA collisions is presented for simulating particle production in cold nuclear matter. With a simple phenomenological parameter, the mean nucleon energy loss fraction, this model yields pseudorapidity density distributions that are comparable to those of experiment, as well as those of HIJING. Particle production data for a given binary collision is extracted from the PYTHIA event generator. The nuclear geometry is described by the Glauber model. The preliminary R _AA and R _CP results are also presented and discussed, with a proposal that R _AA be redefined.     

Distinguishing ‘Higgs’ spin hypotheses using γγ and WW ∗ decays

A multi-step setup for heavy-flavor studies in high-energy nucleus-nucleus (AA) collisions—addressing within a comprehensive framework the initial $Q\overline{Q}$ production, the propagation in the hot medium until decoupling and the final hadronization and decays—is presented. The initial hard production of $Q\overline{Q}$ pairs is simulated using the POWHEG pQCD event generator, interfaced with the PYTHIA parton shower. Outcomes of the calculations are compared to experimental data in pp collisions and are used as a validated benchmark for the study of medium effects. In the AA case, the propagation of the heavy quarks in the medium is described in a framework provided by the relativistic Langevin equation. For the latter, different choices of transport coefficients are explored (either provided by a perturbative calculation or extracted from lattice-QCD simulations) and the corresponding numerical results are compared to experimental data from RHIC and the LHC. In particular, outcomes for the nuclear modification factor R _AA and for the elliptic flow v ₂ of D/B mesons, heavy-flavor electrons and non-prompt J/ψ’s are displayed.     

Strange particle production at the intersection of soft and hard processes at RHIC

Nuclear suppression factor and u ₂ measurements as a function of transverse momentum for neutral strange particles are shown and compared to particle identified spectra from PHENIX and WA98 and to models that attempt to describe the suppression of particles at moderate pt in central RHIC Au?Au collisions. It is shown that identified spectra carry additional information to the published charged particle spectra. In particular, dependencies of the R _AA and u ₂ values on the measured particle species will be discussed in the context of several models.