by Panos Charitos. Published: 08 July 2012

On Friday 29 June 2012, a town meeting was held at CERN to collect input from the European physics community for an update to the European Strategy for Particle Physics

In particular, the town meeting aimed to solicit input from a large number of stakeholders including informal groups of physicists, experimental collaborations, communities for future facilities but also from laboratories and national committees. The main topics under discussion at the meeting were soft probes, flow and hydrodynamic responses of the medium; hard probes and quarkonia and also future opportunities for colliders and fixed target experiments.

The meeting featured short presentations of existing and planned future heavy ion experiments at the CERN LHC, the Brookhaven RHIC, the CERN SPS, the FAIR facility in Darmstadt and the JINR in Dubna. In addition, the meeting provided a forum in which individual scientists and groups could contribute with short comments and statements. The meeting counted 237 registered participants that covered all experimental and theoretical activities in the field and concluded with an open two hour discussion about the priorities of the field, at the end of which the participants reached a consensus. These conclusions will be submitted to the Open Symposium of the European Strategy Group in Cracow by the convenors of the town meeting, namely J.P. Blaizon, K.Redlich, J.Wambach and U.A. Wiedemann.

The study of strong-interaction matter under extreme conditions in temperature and density is central to our understanding of how complex phenomena emerge from the Standard Model; it has implications for our understanding of the early Universe and the evolution of massive stars. A rich phase diagram is expected in which hadrons are ultimately dissolved into their constituents, quarks and gluons. This new state of matter, the quark gluon plasma, is studied by colliding heavy ions at ultra-relativistic energies. At the highest energies available at the Large Hadron Collider, the quark gluon plasma is created and diagnosed at nearly vanishing baryo-chemical potential, i.e. under conditions prevailing in the very early Universe. At lower beam energies, currently available at the CERN-SPS, RHIC in Brookhaven and at future facilities such as FAIR in Darmstadt and NICA in Subnam the phases of strong interaction matter are probed at high (net) baryon density creating conditions as they are encountered in proto-neutron stars after core collapse supernova explosions, the merger of binary neutron stars or in the final stages of neutron star black-hole systems. For a comprehensive understanding of the equilibrium properties of hot and dense matter a large range of beam energies is indispensable.

Considering the fundamental physics questions that are coming into experimental reach in the coming decade, the town meeting highlighted in particular the following opportunities for fundamental progress:

1) The top priority for future quark matter research in Europe is the full exploitation of the physics potential of colliding heavy ions in the LHC.

2) At lower centre of mass energies where the baryochemical potential is large, advances in accelerator and detector technologies provide opportunities for a new generation precision measurements that address central questions about the QCD phase diagram.

3) The complementarity of LHC and RHIC is an essential resource in efforts to quantify properties of the Quark Gluon Plasma.

4) Dedicated investments in theoretical research are needed to fully exploit the opportunities arising from the upcoming precision era of nuclear research at collider and fixed target energies.

It can be concluded that the Town Meeting on “Relativistic Heavy-Ion Collisions” was an important step towards shaping the future of particle physics in Europe by taking into account the different stakeholders of the physics community in accordance with the scope of the European Strategy Group formed by CERN.