The primary scientific objective of STAR is to study the formation and characteristics of the quark-gluon plasma (QGP), a state of matter believed to exist at sufficiently high energy densities. Detecting and understanding the QGP allows physicists to understand better the Universe in the seconds after the Big Bang, when the presently-observed symmetries (and asymmetries) of the Universe were established.
Unlike other physics experiments where a theoretical prediction 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 the high-energy nuclear collision and the unexplored landscape of the physics studied. STAR therefore consists of several types of detectors, each specializing in detecting certain types of particles or characterizing their motion. These detectors work together in an advanced data acquisition and subsequent physics analysis that allows definitive statements to be made about the collision.
The physics of STAR
In the immediate aftermath of the Big Bang, the expanding matter was so hot and dense that protons and neutrons could not exist. Instead, the early universe comprised a plasma of quarks and gluons, which in today's cool universe are confined and exist only within composite particles (bound states) – the hadrons, such as protons and neutrons. Collisions of heavy nuclei at sufficiently high energies allow physicists to study whether quarks and gluons become deconfined at high densities, and if so, what the properties of this matter (i.e. quark–gluon plasma) are.
In particular, STAR studies the collective expansion of the hot quark-gluon matter, such as the elliptic flow. This allows to extract the transport coefficients that characterize the quark-gluon matter, including the shear and bulk viscosity, and to investigate macroscopic quantum phenomena, such as the chiral magnetic effect.