The Compact Muon Solenoid Heavy Ion program

Abstract

The Pb-Pb center of mass energy at the LHC will exceed that of Au-Au collisions at RHIC (Relativistic Heavy Ion Collider) by nearly a factor of 30, providing exciting opportunities for addressing unique physics issues in a completely new energy domain. The interest of the Heavy Ion (HI) Physics at LHC is discussed in more detail in the LHC-USA white paper and the Compact Muon Solenoid (CMS) Heavy Ion proposal. A few highlights are presented in this document. Heavy ion collisions at LHC energies will explore regions of energy and particle density significantly beyond those reachable at RHIC. The energy density of the thermalized matter created at the LHC is estimated to be 20 times higher than at RHIC, implying an initial temperature, which is greater than at RHIC by more than a factor of two. The higher density of produced partons also allows a faster thermalization. As a consequence, the ratio of the quark-gluon plasma lifetime to the thermalization time increases by a factor of 10 over RHIC. Thus the hot, dense systems created in HI collisions at the LHC spend most of the time in a purely partonic state. The longer lifetime of the quark-gluon plasma state widens significantly the time window available to probe it experimentally. RHIC experiments have reported evidence for jet production in HI collisions and for suppression of high p{sub T} particle production. Those results open a new field of exploration of hot and dense nuclear matter. Even though RHIC has already broken ground, the production rates for jets with p{sub T} > 30 GeV are several orders of magnitude larger at the LHC than at RHIC, allowing for systematic studies with high statistics in a clean kinematic region. High p{sub T} quark and gluon jets can be used to study the hot hadronic medium produced in HI interactions. The larger Q{sup 2} causes jets to materialize very soon after the collision. They are thus embedded in and propagate through the dense environment as it forms and evolves. Through their interactions with the environment, they measure its properties and are sensitive to the formation of quark-gluon plasma. Moreover large transverse momentum probes are easily isolated experimentally from the background of soft particles produced in the collision. From a theoretical point of view, the high p{sub T} ensures that the medium effects are perturbatively calculable, strengthening their usefulness as quantitative diagnostic tools. Another interesting aspect of heavy ion collisions at the energy of the LHC is the time evolution of the system. At the moment of the collision, this may be described in terms of classical color fields and the gross properties of the system are calculable in perturbative QCD. Lastly let us mention the fact that at central rapidities LHC is expected to reach a truly baryon-free regime. Results from RHIC have shown that not all the valence quarks are removed from central rapidities at RHIC.