We are now at the beginning of the second decade after the discovery of quark matter at the RHIC in Brookhaven, and a little after the beginning of the LHC heavy-ion programme. During this decade questions about heavy-ion collisions have become more pointed. The earlier assumptions of early thermalization, long hydrodynamic evolution, and abrupt freeze-out are being tested, probed, and improved.

Is matter thermally equilibrated, or does the initial condition influence even the late evolution of the fireball? A theoretical model for describing strongly coupled QCD-like theories (but not exactly QCD) has emerged and is being widely used. Can experiments be made precise enough that they test more than the generic features of strongly coupled systems? Lattice QCD gives a few quantitative predictions about the properties of thermally equilibrated matter. Can these be tested? These are some of the questions that become interesting in the modern context of heavy-ion collisions.

Lattice QCD also has predictions about the phase structure of QCD. Are they precise enough? Can the existence of a critical point be tested at colliders? Are there improved computational methods which may give quantitative answers to a wider variety of questions? Is local parity violation visible in experiments? These are new questions being asked, and the partial answers available are already extremely intriguing.

The classic questions now probe deeper: these are questions about photons and dileptons, about jets and heavy-quarkonia, and about the possible existence of quark matter in neutron stars.

This is a collection of reviews on the state of the field, which gives context and longer answers to these questions, and poses others.

Bedangadas Mohanty

National Institute of Science Education and Research,

Bhubaneswar 751 005, India

Sourendu Gupta

Tata Institute of Fundamental Research, Mumbai 400 005, India

(Editors)