Search

We present our recent studies of the medium-induced radiative energy loss of heavy quarks and its experimental signatures in heavy ion collisions. In particular, we discuss the ratio of nuclear modification factors of high-pTheavy-flavoured mesons to light-flavoured hadrons (heavy-to-light ratio). Depending on the transverse momentum of the heavy quark, this ratio turns out to be sensitive either to the mass-dependence or to the colour charge dependence expected for radiative parton energy loss. Based on this observation, we show how the simultaneous analysis of Dand Bmesons at the LHC in the region 10 < pT< 20 GeV allows one to disentangle the dependence of parton energy loss on parton identity. We then discuss to what extent high-pTelectron spectra, expected to be dominated by heavy quark decay contributions, can give access to the microscopic mechanism underlying parton energy loss.

Abstract.: A simple model for nuclear structure functions in the region of small x and small and moderate Q2 is presented. It is a parameter-free extension, in the Glauber-Gribov approach to nuclear collisions, of a saturation model for the nucleon. A reasonable agreement with experimental data on the ratios of nuclear structure functions is obtained. Nuclear effects in the longitudinal-to-transverse cross section ratios are found to be small. Predictions of the model for values of x smaller than those available to present experiments are given. The unintegrated gluon distribution and the behaviour of the saturation scale which result from this model are shown and discussed.

We propose a new criterium for saturation of the density of partons both in nucleons and nuclei. It is applicable to any multiple scattering model which would be used to compute the number of strings exchanged in epand eAcollisions. The criterium is based on percolation of strings, and the onset of percolation is estimated from expectations coming from the study of heavy-ion collisions at high energies. We interpret this onset as an indication of saturation of the density of partons in the wave function of the hadron. In order to produce quantitative results, a particular model fitted to describe present HERA data and generalized to the nuclear case is used. Nevertheless, with the number of scatterings controlled by the relation between inclusive and diffractive processes, conclusions are weakly model-dependent as long as different models are tuned to describe the experimental data. This constitutes a new approach, based on the eikonal description of soft hadronic collisions, and different from others which employ either perturbative QCD ideas or semiclassical methods. It offers an alternative picture for saturation in the small Q2region.

Abstract.: Nuclear structure functions and cross-sections for heavy flavour production in lepton–nucleus collisions are investigated in the low x region accessible now or in the near future. The scattering on a heavy nucleus is described by the sum of fan diagrams of BFKL pomerons, which is exact in the high-colour limit. The initial condition for the evolution at x=0.01 is taken from a saturation model, which reproduces the experimental data on the proton. The dependence of the structure functions is well described by a power factor , with reaching values as low as 1/2 at extremely low x. The total cross-sections for heavy flavour production reach values of the order of mb, and the corresponding transverse momentum distributions are sizable up to transverse momenta larger than the initial large scale .

Abstract.: A Monte Carlo model to simulate nuclear collisions in the energy range going from SPS to LHC is presented. The model includes in its initial stage both soft and semihard components, which lead to the formation of color strings. Collectivity is taken into account considering the possibility of strings in color representations higher than triplet or antitriplet, by means of string fusion. String breaking leads to the production of secondaries. At this point, the model can be used as an initial condition for the further evolution by a transport model. In order to tune the parameters and see the results in nucleus–nucleus collisions, a naive model for rescattering of secondaries is introduced. Results of the model are compared with experimental data, and predictions for RHIC and LHC are shown.

Abstract.: Unintegrated gluon densities in nuclei, dipole-nucleus cross-sections and quark densities are numerically investigated in the high-color limit, with the scattering on a heavy nucleus exactly described by the sum of fan diagrams of BFKL pomerons. The initial condition for the evolution in rapidity is quickly forgotten, and the gluon density presents a “supersaturation” pattern, as previous studies indicated. Both dipole-nucleus cross-sections and quark densities present the expected saturation features. Identifying the position in transverse momentum l of the maximum of the gluon distribution with the saturation momentum , at large rapidities all distributions depend on only one variable, or .

It is shown that the ratio D /〈 n 〉 and the forward-backward correlations present a characteristic behavior close to the critical point of percolation of strings. This behavior can be detected in the new incoming heavy ion experiments.

The KNO scaling, the scaling of multiplicities, the behaviour of fluctuations in the transverse momentum as a function of the multiplicity and the long range correlations in nucleus-nucleus collisions are studied by a Monte Carlo code based on the string fusion model. It is shown that the fusion of strings produces a strong reduction of the long range correlations at high multiplicities, which could be detected experimentally. On the contrary the KNO scaling, the scaling of multiplicities and the behaviour of fluctuations in the transverse momentum are not modified when string fusion is included.

The absorptive corrections resulting from the rescattering of a heavy flavour (the so-called nuclear absorption) are usually calculated with a probabilistic formula valid only in the low-energy limit. We extend this formula to all energies using a quantum field theoretical approach. For charmonium and bottonium we find that the absorptive corrections in the rigorous treatment are very similar to the ones in the probabilistic approach. On the contrary, at sufficiently high energy, open charm and bottom are absorbed as much as charmonium and bottonium — in spite of the fact that their absorptive cross sections are zero, and therefore they are not absorbed in the probabilistic model. At high enough energies there are also absorptive corrections due to the shadowing of the nucleus structure function, which are present for all systems including Drell-Yan pair production. These shadowing corrections cancel in the low-energy limit.