The ALICE Experiment
The ALICE Experiment is one of 4 large detectors at the Large Hadron Collider (LHC). It is located at CERN, the European Organization for Nuclear Research in Geneva, Switzerland. ALICE is dedicated to the study of the Quark-Gluon Plasma, or QGP, which is created in high-energy collisions of large nuclei. These collisions create a medium of extreme temperature and density, in which the nucleons dissolve and their constituents - quarks and gluons - are freed.
ALICE utilizes a wide variety of different detector technologies to reconstruct charge particles (silicon and gaseous detectors), identify different particle species (using specific energy loss, Cherenkov and Transition Radiation as well muon detection techniques) and measure the energy of photons and electrons (electromagnetic calorimetry).
Photons in Heavy-Ion Collisions
Photons are an important probe to study the properties of the Quark-Gluon Plasma, because they are created throughout the entire space-time evolution of the fireball, and because they do not interact strongly and can therefore escape the hot medium, carrying information about the conditions at their creation. Photons of different energies provide access to different stages of the collisions: high-energy photons are only created in the earliest hard interactions and thus serve as a reference for other hard processes, like the production and modification of jets (“jet quenching”) in heavy-ion collisions. At low energies, thermal production of photons is expected to be dominant, and the first results of direct photon measurements by ALICE show an excess of photons for low transverse momenta. The interpretation of this excess, including the extraction of the medium temperature, is still being debated.