Some results obtained in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory are discussed. Along with observables studied at energies lower than those at RHIC (particl...
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Some results obtained in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory are discussed. Along with observables studied at energies lower than those at RHIC (particle production ratios, soft spectra, flows, and so on), observables specific for RHIC energies related to the Jet Quenching effect and Color Glass Condensate (CGC) manifestation are discussed. Preliminary results and specifics of particular experiments at RHIC are not considered.
Inclusive cross section for pion production in proton - proton collisions are calculated based on unintegrated parton distribution functions (uPDFs). In addition to purely gluonic terms the present approach includes a...
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Inclusive cross section for pion production in proton - proton collisions are calculated based on unintegrated parton distribution functions (uPDFs). In addition to purely gluonic terms the present approach includes also quark degrees of freedom. Phenomenological fragmentation functions from the literature are used. The new mechanisms are responsible for pi+ - pi- asymmetry. In contrast to standard collinear approach, application of 2 -> 1 k(t) - factorization approach can be extended towards much lower transverse momenta, both at mid and forward rapidity region. The results of the calculation are compared with SPS and RHIC data.
The modification of the single inclusive spectrum of high transverse momentum (pT) pions emanating from an ultrarelativistic heavy-ion collision is investigated. The deconfined sector is modeled using a full three-dim...
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The modification of the single inclusive spectrum of high transverse momentum (pT) pions emanating from an ultrarelativistic heavy-ion collision is investigated. The deconfined sector is modeled using a full three-dimensional (3-D) ideal fluid dynamics simulation. Energy loss of high pT partons and the ensuing modification of their fragmentation is calculated within perturbative QCD at next-to-leading twist, where the magnitude of the higher twist contribution is modulated by the entropy density extracted from the 3-D fluid dynamics simulation. The nuclear modification factor (RAA) for pions with a pT⩾8 GeV as a function of centrality as well as with respect to the reaction plane is calculated. The magnitude of contributions to the differential RAA within small angular ranges, from various depths in the dense matter, is extracted from the calculation and demonstrates the correlation of the length integrated density and the RAA from a given depth. The significance of the mixed and hadronic phase to the overall magnitude of energy loss is explored.
Spin-dependent quark densities, matrix elements of specific density operators in proton states of definite spin-polarization, indicate that the nucleon may harbor an infinite variety of nonspherical shapes. We show th...
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Spin-dependent quark densities, matrix elements of specific density operators in proton states of definite spin-polarization, indicate that the nucleon may harbor an infinite variety of nonspherical shapes. We show that these matrix elements are closely related to specific transverse-momentum-dependent (TMD) parton distributions accessible in the angular dependence of the semi-inclusive processes ep→eπX and the Drell-Yan reaction pp→ll¯X. New measurements or analyses would allow the direct exhibition of the nonspherical nature of the proton.
Quark and gluon jets traversing through a quark-gluon plasma not only lose their energies but also can undergo flavor conversions. The conversion rates via the elastic q(q¯)g→gq(q¯) and the inelastic qq...
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Quark and gluon jets traversing through a quark-gluon plasma not only lose their energies but also can undergo flavor conversions. The conversion rates via the elastic q(q¯)g→gq(q¯) and the inelastic qq¯↔gg scatterings are evaluated in the lowest order in QCD. Including both jet energy loss and conversions in the expanding quark-gluon plasma produced in relativistic heavy ion collisions, we have found a net of quark jets to gluon jets. This reduces the difference between the nuclear modification factors for quark and gluon jets in central heavy ion collisions and thus enhances the p/π+ and p¯/π− ratios at high transverse momentum. However, a much larger net quark-to-gluon jet conversion rate than the one given by the lowest order QCD is needed to account for the observed similar ratios in central Au+Au and p+p collisions at the same energy. Implications of our results are discussed.
I examine the role of final state interactions in cold nuclear matter in modifying hadron production on nuclear targets with leptonic or hadronic beams. I demonstrate the extent to which available experimental data in...
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I examine the role of final state interactions in cold nuclear matter in modifying hadron production on nuclear targets with leptonic or hadronic beams. I demonstrate the extent to which available experimental data in electron-nucleus collisions can give direct information on final state effects in hadron-nucleus and nucleus-nucleus collisions. For hadron-nucleus collisions, a theoretical estimate based on a parton energy loss model tested in lepton-nucleus collisions shows a large effect on midrapidity hadrons at fixed target experiments. At energies currently available at the BNL Relativistic Heavy Ion Collider, the effect is large for negative rapidity hadrons but mild at midrapidity. This final state cold hadron quenching needs to be taken into account in jet tomographic analysis of the medium created in nucleus-nucleus collisions.
The ability of experimental signatures to resolve the spatiotemporal profile of an expanding quark-gluon plasma is studied. In particular, the single inclusive suppression of high-momentum hadrons versus the centralit...
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The ability of experimental signatures to resolve the spatiotemporal profile of an expanding quark-gluon plasma is studied. In particular, the single inclusive suppression of high-momentum hadrons versus the centrality of a heavy-ion collision and with respect to the reaction plane in noncentral collisions is critically examined. Calculations are performed in the higher twist formalism for the modification of the fragmentation functions. Radically different nuclear geometries are used. The influence of different initial gluon distributions as well as different temporal evolution scenarios on the single inclusive suppression of high-momentum pions are outlined. It is demonstrated that the modification versus the reaction plane is quite sensitive to the initial spatial density. Such sensitivity remains even in the presence of a strong elliptic flow.
The unexpectedly strong suppression of high pT heavy quarks in heavy-ion collisions has given rise to the idea that partons propagating through a medium in addition to energy loss by induced radiation also undergo sub...
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The unexpectedly strong suppression of high pT heavy quarks in heavy-ion collisions has given rise to the idea that partons propagating through a medium in addition to energy loss by induced radiation also undergo substantial energy loss due to elastic collisions. However, the precise magnitude of this elastic energy-loss component is highly controversial. Although for a parton inside a medium it is surprisingly difficult to define the difference between elastic and radiative processes rigorously, the main phenomenological difference is in the dependence of energy loss on in-medium path length: in a constant medium radiative energy loss is expected to grow quadratically with path length, elastic energy loss linearly. In this article, we investigate a class of energy-loss models with such a linear path-length dependence and demonstrate that they are incompatible with measured data on hard hadronic back-to-back correlations where a substantial variation of path length is probed. This indicates that any elastic energy-loss component has to be small.
We discuss the measured nuclear suppression of p+p¯ production in 200A GeV Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC) within radiative energy loss. For the Albino-Kniehl-Kramer (AKK) set of fr...
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We discuss the measured nuclear suppression of p+p¯ production in 200A GeV Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC) within radiative energy loss. For the Albino-Kniehl-Kramer (AKK) set of fragmentation functions, proton production is dominated by gluons, giving rise to the expectation that the nuclear suppression for p+p¯ should be stronger than for pions due to the stronger coupling of gluons to the quenching medium. Using a hydrodynamical description for the soft matter evolution, we show that this is indeed seen in the calculation. However, the expected suppression factors for pions and protons are sufficiently similar that a discrimination with present data is not possible. In the high pT region above 6 GeV where the contributions of hydrodynamics and recombination to hadron production are negligible, the model calculation is in good agreement with the data on p+p¯ suppression.
The evolution of shower parton distributions in a jet is investigated in the framework of a quark recombination model. The distributions are parameterized and the Q^2 dependence of the parameters is given by polynomia...
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The evolution of shower parton distributions in a jet is investigated in the framework of a quark recombination model. The distributions are parameterized and the Q^2 dependence of the parameters is given by polynomials of In Q^2 for a wide range of Q^2
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