Physical Society Colloquium

Penetrating Probes of the Quark-Gluon Plasma in Nuclear Collisions at RHIC and the LHC

Brian Cole

Columbia University

In 2001 the Relativistic Heavy Ion Collider facility at Brookhaven National Laboratory on Long Island, NY started colliding gold (Au) nuclei at an energy of 200 GeV per colliding nucleon pair with the goal of creating and studying the `quark-gluon plasma' (QGP) in the laboratory. The QGP is a unique state of strongly interacting matter that is predicted to exist at temperatures in excess of 1011 �K when the protons and neutrons (and other hadrons) in nuclear matter dissolve, releasing the normally confined quarks and gluons. Measurements using `penetrating' probes such as energetic photons and jets indicate that collisions at RHIC produce matter with energy densities of order 10 GeV/fm³ - an order of magnitude higher than the energy density needed to create the QGP. In 2008 (ca.), the experimental search for and/or studies of the quark-gluon plasma will take a giant leap forward when the Large Hadron Collider at CERN starts colliding lead (pb) nuclei at an energy >20 times higher than the top RHIC energy. The study of `penetrating probes' like jets and high-energy photons will be revolutionized at the LHC due to the increased production rate for energetic particles and because these particles will better stand out above the low-momentum `debris' from the collision. The ATLAS experiment has recently decided to participate in the LHC heavy ion program. While ATLAS was designed to studying proton-proton collisions it has significant advantages in the measurement of penetrating probes in heavy ion collisions due to its unique calorimeter design.

I will provide a general overview of the status of the RHIC program and highlight the role of penetrating probes in elucidating the properties of the matter created in nuclear collisions at RHIC and in the search for specific signatures of QGP formation. I will then discuss the unique opportunity provided by the LHC heavy ion physics program - in particular the study of heavy ion collisions within ATLAS.