Introduction to Low x Physics and Saturation

The idea of saturation of parton densities in small x physics is briefly introduced. Some aspects of saturation are described, mainly focusing on the status of our knowledge on the non-linear equations describing the high parton density regime. Implications of saturation ideas on the description of nuclear collisions at the Relativistic Heavy Ion Collider are discussed.Comment: 8 pages in LaTeX using enclosed appolb.cls, 3 eps figures included using epsfig; minireview at the XXXIIIrd International Symposium on Multiparticle Dynamics (Kracow, Poland, September 5th-11th 2003

Predictions for the heavy-ion programme at the Large Hadron Collider

I review the main predictions for the heavy-ion programme at the Large Hadron Collider (LHC) at CERN, as available in early April 2009. I begin by remembering the standard claims made in view of the experimental data measured at the Super Proton Synchrotron (SPS) at CERN and at the Relativistic Heavy Ion Collider (RHIC) at the BNL. These claims will be used for later discussion of the new opportunities at the LHC. Next I review the generic, qualitative expectations for the LHC. Then I turn to quantitative predictions: First I analyze observables which characterize directly the medium produced in the collisions - bulk observables or soft probes -: multiplicities, collective flow, hadrochemistry at low transverse momentum, correlations and fluctuations. Second, I move to calibrated probes of the medium i.e. typically those whose expectation in the absence of any medium can be described in Quantum Chromodynamics (QCD) using perturbative techniques (pQCD), usually called hard probes. I discuss particle production at large transverse momentum and jets, heavy-quark and quarkonium production, and photons and dileptons. Finally, after a brief review of pA collisions, I end with a summary and a discussion about the potentiality of the measurements at the LHC - particularly those made during the first run - to further substantiate or, on the contrary, disproof the picture of the medium that has arisen from the confrontation between the SPS and RHIC data, and theoretical models.Comment: 64 pages, 40 figures, 7 tables; invited review for "Quark-Gluon Plasma 4"; v2: small changes, some predictions and references added, final versio

Nuclear Structure Functions at Small x in Multiple Scattering Approaches

A simple model for nuclear structure functions in the region of small $x$ and small and moderate $Q^2$, 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 ratios of nuclear structure functions is obtained. The unintegrated gluon distribution and the behavior of the saturation scale which result from this model are discussed.Comment: LaTeX2e, uses enclosed moriond.sty, 4 pages, 2 eps figures included using epsfig, contribution to the XXXVIIIth Rencontres de Moriond: QCD and Hadronic Interactions, Les Arcs, France, March 22nd-29th 200

A simple model for nuclear structure functions at small xx in the dipole picture

A simple model for nuclear structure functions in the region of small x and small and moderate $Q^2$, 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 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.Comment: LaTeX, 27 pages, 9 postscript figures included using epsfig; final version: discussion on geometrical scaling clarified, comparison of gluon densities with other approaches added, Fig. 8 redone, references added and updated, some misprints corrected, results and conclusions unchange

Jet quenching

Jet quenching studies play a prominent role in our current understanding of ultra-relativistic heavy ion collisions. In this review I first present the available formalism to compute medium-induced gluon radiation. Then I discuss its effect on single particle spectra, with dedicated attention to the case of the radiating parton being a massive quark. Next I examine more differential observables like jet shapes and multiplicities, and the consequences of flow on radiative energy loss. I conclude with some remarks.Comment: LaTeX, 8 pages, 7 eps figures, uses enclosed cjpsuppl.cls; invited talk at Physics at LHC, Vienna, Austria, July 13th-17th 200

Heavy-ion collisions at the Large Hadron Collider: a review of the results from Run 1

We present an overview of the results obtained in pPb and PbPb collisions at the Large Hadron Collider during Run 1. We first discuss the results for global characteristics: cross sections, hadron multiplicities, azimuthal asymmetries, correlations at low transverse momentum, hadrochemistry, and femtoscopy. We then review hard and electromagnetic probes: particles with high transverse momentum, jets, heavy quarks, quarkonium, electroweak bosons and high transverse momentum photons, low transverse momentum photons and dileptons, and ultraperipheral collisions. We mainly focus on the experimental results, and present very briefly the main current theoretical explanations.Comment: 33 pages, 29 figure

On the long-range correlations in hadron-nucleus collisions

Long-range correlations between multiplicities in different rapidity windows in hadron-nucleus collisions are analyzed. After recalling the standard results in the probabilistic model, we study them in the framework of perturbative QCD. Considering interacting BFKL pomerons in the form of fan diagrams coupled to a dilute projectile, analytic estimates are done for very large rapidities. The correlation strength results weakly depending on energy and centrality or nuclear size, and generically greater than unity. Finally, we turn to the Color Glass Condensate framework. For a saturated projectile and considering the most feasible experimental situation of forward and backward rapidity windows symmetric around the center-of-mass, the resulting correlation strength turns out to be larger than unity and shows a non-monotonic behavior with increasing energy, first increasing and then decreasing to a limiting value. Its behavior with increasing centrality or nuclear size depends on the considered rapidity windows.Comment: 17 pages, LaTeX, 6 eps figures included using graphicx; v2: error in the CGC formula corrected, conclusions of the corresponding section changed accordingl

On correlations and discreteness in non-linear QCD evolution

We consider modifications of the standard non-linear QCD evolution in an attempt to account for some of the missing ingredients discussed recently, such as correlations, discreteness in gluon emission and Pomeron loops. The evolution is numerically performed using the Balitsky-Kovchegov equation on individual configurations defined by a given initial value of the saturation scale, for reduced rapidities $y=(\alpha_s N_c/\pi) Y<10$. We consider the effects of averaging over configurations as a way to implement correlations, using three types of Gaussian averaging around a mean saturation scale. Further, we heuristically mimic discreteness in gluon emission by considering a modified evolution in which the tails of the gluon distributions are cut-off. The approach to scaling and the behavior of the saturation scale with rapidity in these modified evolutions are studied and compared with the standard mean-field results. For the large but finite values of rapidity explored, no strong quantitative difference in scaling for transverse momenta around the saturation scale is observed. At larger transverse momenta, the influence of the modifications in the evolution seems most noticeable in the first steps of the evolution. No influence on the rapidity behavior of the saturation scale due to the averaging procedure is found. In the cut-off evolution the rapidity evolution of the saturation scale is slowed down and strongly depends on the value of the cut-off. Our results stress the need to go beyond simple modifications of evolution by developing proper theoretical tools that implement such recently discussed ingredients.Comment: 20 pages, LaTeX, 18 eps figures included using epsfig; changes in v3: new kind of averaging added, 5 new plots, discussions and conclusions extended, final versio