ALICE (A Large Ion Collider Experiment) at the Large Hadron Collider (LHC) is designed to study the physics of strongly interacting matter at extreme energy densities. The ALICE experiment, like other High Energy Physics experiments, comprises many electronic detectors each of which measure some properties of the various elementary particles produced in collisions of heavy ions. These measurements are used to study the properties of a phase of matter called Quark-Gluon Plasma (QGP) formed at extreme densities.
All ordinary matter in today's universe is made up of atoms. Each atom contains a nucleus composed of protons and neutrons (except hydrogen, which has no neutrons), surrounded by a cloud of electrons. Protons and neutrons are in turn made of quarks bound together by other particles called gluons. No quark has ever been observed in isolation: the quarks, as well as the gluons, seem to be bound permanently together and confined inside composite particles, such as protons and neutrons. This is known as confinement.
Collisions in the LHC generate temperatures more than 100,000 times hotter than the centre of the Sun. For part of each year the LHC provides collisions between lead ions, recreating in the laboratory conditions similar to those just after the big bang. Under these extreme conditions, protons and neutrons "melt", freeing the quarks from their bonds with the gluons. This is quark-gluon plasma. The existence of such a phase and its properties are key issues in the theory of quantum chromodynamics (QCD), for understanding the phenomenon of confinement, and for a physics problem called chiral-symmetry restoration. The ALICE collaboration studies the quark-gluon plasma as it expands and cools, observing how it progressively gives rise to the particles that constitute the matter of our universe today.
The ALICE collaboration uses the 10,000-tonne ALICE detector - 26 m long, 16 m high, and 16 m wide - to study quark-gluon plasma. The detector sits in a vast cavern 56 m below ground close to the village of St Genis-Pouilly in France, receiving beams from the LHC.
The collaboration counts more than 1000 scientists from over 100 physics institutes in 30 countries.
STAR (Solenoidal Tracker at RHIC) is an experiment at the RHIC (Relativistic Heavy-Ion Collider). The STAR detector specializes in tracking the thousands of particles produced by each ion collision at RHIC. Weighing 1,200 tons and as large as a house, STAR is a massive detector. It is used to search for signatures of the form of matter that RHIC was designed to create: the quark-gluon plasma (QGP).
Detecting and understanding the QGP allows us to understand better the universe in the moments after the Big Bang, where the symmetries (and lack of symmetries) of our surroundings were put into motion. Unlike other physics experiments where a theoretical idea can be tested directly by a single measurement, STAR must make use of a variety of simultaneous studies in order to draw strong conclusions about the QGP. This is due both to the complexity of the system formed in high-energy nuclear collisions and the unexplored landscape of the physics being studied.