Heavy Ion Physics at the LHC: What's new ? What's next ?

Towards the end of 2010, some 25 years after the very first collisions of ultra-relativistic heavy ions at fixed target energies, and some 10 years after the start of operation of the Relativistic Heavy Ion Collider (RHIC), the LHC opened a new era in heavy ion physics with lead on lead collisions at $\sqrt{s_{NN}} = 2.76$ TeV. After a short reminder of the main results from lower energies, this review highlights a few selected areas where significant progress has been made during the first three years of ion operation at the LHC.Comment: Talk given at the 'Nobel Symposium on LHC results', Krusenberg, Sweden, 13 - 17 May 2013, to be published in Physica Script

Heavy Ion physics with the ALICE experiment at the CERN LHC

After close to 20 years of preparation, the dedicated heavy ion experiment ALICE took first data at the CERN LHC accelerator with proton collisions at the end of 2009 and with lead nuclei at the end of 2010. After a short introduction into the physics of ultra-relativistic heavy ion collisions, this article recalls the main design choices made for the detector and summarizes the initial operation and performance of ALICE. Physics results from this first year of operation concentrate on characterizing the global properties of typical, average collisions, both in pp and nucleus-nucleus reactions, in the new energy regime of LHC. The pp results differ, to a varying degree, from most QCD inspired phenomenological models and provide the input needed to fine-tune their parameters. First results from Pb-Pb are broadly consistent with expectations based on lower energy data, indicating that high density matter created at LHC, while much hotter and larger, still behaves like a very strongly interacting, almost perfect liquid.Comment: Talk given at Royal Society meeting on "Physics at the high energy frontier - the Large Hadron Collider project", London, 16 - 17 May 2011, to be published in "Philosophical Transactions of the Royal Society A

Hard Probes 2012: Experimental Summary

The 5th international Conference on Hard and Electromagnetic Probes in High-Energy Nuclear Collisions was held in May 2012 in Cagliari, Italy. This contribution summarises some of the experimental highlights presented at the meeting, concentrating on new results from LHC and RHIC on parton energy loss ('jet-quenching') and heavy quark meson production ('quarkonia suppression').Comment: Writeup of experimental summary talk of the 5th international Conference on Hard and Electromagnetic Probes in High-Energy Nuclear Collisions ('Hard Probes 2012'). Version 2: some minor typos corrected, references added, version as publishe

CERN, a working example of global scientific collaboration

The topic of this conference is 'South-South and North-South Collaboration in Science and Technology', which is addressed in this contribution in the context of basic research in high energy physics (HEP). The question whether developing countries can or should invest scarce resources in big science is not covered. HEP may be less expensive than one might fear, but cheap it is not, so priorities have to be set and these may indeed differ from country to country. The scope of this article is not to argue one way or another, but rather to give an indication and practical examples of both the requirements and the opportunities for scientific collaboration with CERN.Comment: Invited talk at the international meeting 'South-South and North-South Collaboration in Science and Technology', Islamabad, Pakistan, 12-13 March 2004; 3 pages, no figure

The Future of High Energy Nuclear Physics in Europe

In less than two years from now, the LHC at CERN will start operating with protons and later with heavy ions in the multi TeV energy range. With its unique physics potential and a strong, state-of-the complement of detectors, the LHC will provide the European, and in fact worldwide Nuclear Physics community, with a forefront facility to study nuclear matter under extreme conditions well into the next decade.Comment: Invited talk at the 'D. A. Bromley Memorial Symposium', Yale University, USA, 8-9 December 2005; to be published in the proceedings; 6 pages, 4 figure

ALICE results from the first Pb-Pb run at the CERN LHC

After 20 years of preparation, the dedicated heavy ion experiment ALICE took first data at the CERN LHC accelerator with proton collisions at the end of 2009 and with lead beams at the end of 2010. This article will give a brief overview of the main results presented at the Quark Matter 2011 conference.Comment: Inited talk at the 22nd International Conference on Ultra-relativistic Nucleus-Nucleus Collision (Quark Matter 2011), 23 - 28 May 2011, Annecy, Franc

Results from the first heavy ion run at the LHC

Early November 2010, the LHC collided for the first time heavy ions, Pb on Pb, at a centre-of-mass energy of 2.76 TeV/nucleon. This date marked both the end of almost 20 years of preparing for nuclear collisions at the LHC, as well as the start of a new era in ultra-relativistic heavy ion physics at energies exceeding previous machines by more than an order of magnitude. This contribution summarizes some of the early results from all three experiments participating in the LHC heavy ion program (ALICE, ATLAS, and CMS), which show that the high density matter created at the LHC, while much hotter and larger, still behaves like the very strongly interacting, almost perfect liquid discovered at RHIC. Some surprising and even puzzling results are seen in particle ratios, jet-quenching, and Quarkonia suppression observables. The overall experimental conditions at the LHC, together with its set of powerful and state-of-the-art detectors, should allow for precision measurements of quark-gluon-plasma parameters like viscosity and opacity.Comment: Invited talk at the Rutherford Centennial Conference on Nuclear Physics, July 25 - 29, 2011, Manchester, U

NA49/NA61: results and plans on beam energy and system size scan at the CERN SPS

This paper presents results and plans of the NA49 and NA61/SHINE experiments at the CERN Super Proton Synchrotron concerning the study of relativistic nucleus-nucleus interactions. First, the NA49 evidence for the energy threshold of creating quark-gluon plasma, the onset of deconfinement, in central lead-lead collisions around 30A GeV is reviewed. Then the status of the NA61/SHINE systematic study of properties of the onset of deconfinement is presented. Second, the search for the critical point of strongly interacting matter undertaken by both experiments is discussed. NA49 measured large fluctuations at the top SPS energy, 158A GeV, in collisions of light and medium size nuclei. They seem to indicate that the critical point exists and is located close to baryonic chemical potential of about 250 MeV. The NA61/SHINE beam energy and system size scan started in 2009 will provide evidence for the existence of the critical point or refute the interpretation of the NA49 fluctuation data in terms of the critical point.Comment: 11 pages, invited talk at Quark Matter 201

Medium information from anisotropic flow and jet quenching in relativistic heavy ion collisions

Within a multiphase transport (AMPT) model, where the initial conditions are obtained from the recently updated HIJING 2.0 model, the recent anisotropic flow and suppression data for charged hadrons in Pb+Pb collisions at the LHC center of mass energy of 2.76 TeV are explored to constrain the properties of the partonic medium formed. In contrast to RHIC, the measured centrality dependence of charged hadron multiplicity dN_ch/deta at LHC provides severe constraint to the largely uncertain gluon shadowing parameter s_g. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller s_g to be consistent with dN_ch/deta data at LHC. With the parton shadowing so constrained, hadron production and flow over a wide transverse momenta range are investigated in AMPT. The model calculations for the elliptic and triangular flow are found to be in excellent agreement with the RHIC data, and predictions for the flow coefficients v_n(p_T, cent) at LHC are given. The magnitude and pattern of suppression of the hadrons in AMPT are found consistent with the measurements at RHIC. However, the suppression is distinctly overpredicted in Pb+Pb collisions at the LHC energy. Reduction of the QCD coupling constant alpha_s by ~30% in the higher temperature plasma formed at LHC reproduces the measured hadron suppression.Comment: Talk given by Subrata Pal at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS