Overview of experiments and detectors at RHIC

Abstract

The term ‘‘mini-bang’’ has been used to describe the interactions explored at RHIC, as each collision unfolds a sequence of events that calls to mind the formation of matter in the evolution of the cosmos. The sequence of events is thought to be as follows: After a formation time of about 1 fm/ c, the colliding system becomes thermalized in a plasma of quarks and gluons at extremely high temperature and density. This quark gluon plasma then expands and cools, and when it reaches the critical temperature of the phase transition, hadrons begin to form. After that it becomes a gas of interacting hadrons, possibly in thermal equilibrium. As expansion continues, a freeze-out density is reached at which the hadrons no longer interact with each other. The detected particles are those that emerge from this freeze-out volume. Thus, experiments at RHIC are in many ways analogous to certain astronomical observations. The object of interest is an extended source emitting copious radiation whose spectra may reflect thermal, and possible chemical equilibrium of the radiating source. As with stellar observations, different types of radiation may carry signals characteristic of distinct stages as the system evolves in space and time. In the case of nuclear collisions at RHIC, most of the many thousands of particles emitted are hadrons produced at the surface of the interacting volume. They provide information about the size, the thermal properties, and the relative abundance of particle species at freeze-out. Information from the hot interior of the expanding volume can only be carried to the outside by radiation that is not coupled to the strong interaction. Such signals can be carried by photons, which may be real (gamma rays) or virtual (electron–positron or muon– antimuon pairs). These direct, electromagnetic signals are much less abundant than the hadrons, comprising about 0.01% of all the particles emerging from the collision, and require very specialized instrumentation to resolve them from the hadronic background. The RHIC detectors are specially designed for this environment. They are capable of measuring very high densities of relatively soft (low-momentum) particles with precise determination of the kinematic properties and quantum numbers of each particle within selected solid angles. Because of the near-thermal nature of the interactions, RHIC detectors are not designed to reconstruct the entire event over all angles (i.e., to be ‘‘hermetic’’), as is generally the case at other high-energy colliders. In 1993–1994, after a 3-year period of R&D [1], conceptual designs were approved for two large detectors of quite different and complementary designs. STAR, a solenoidal detector based on particle tracking in a large time-projection chamber, focuses on large solid angle detection of hadrons, while PHENIX is designed to exploit a number of tracking and particle identification technologies for detection of leptons and photons, as well as hadrons, over a more limited range of solid angles. Two smaller, more specialized detectors, PHOBOS and BRAHMS, were subsequently