Relativistic Heavy Ion Collider - Current Results

Current Results

For a complementary discussion, see also quark-gluon plasma.

For the experimental objective of creating and studying the quark-gluon plasma, RHIC has the unique ability to provide baseline measurements for itself. This consists of the both lower energy and also lower mass number projectile combinations that do not result in the density of 200 GeV Au + Au collisions, like the p + p and d + Au collisions of the earlier runs, and also Cu + Cu collisions in Run-5.

Using this approach, important results of the measurement of the hot QCD matter created at RHIC are:

• Collective anisotropy, or elliptic flow. The multiplicity of the particles' bulk with lower momenta exhibits a dependency as (p_T is the transverse momentum, angle with the reaction plane). This is a direct result of the elliptic shape of the nucleus overlap region during the collision and hydrodynamical property of the matter created.

• Jet quenching. In the heavy ion collision event, scattering with a high transverse p_T can serve as a probe for the hot QCD matter, as it loses its energy while traveling through the medium. Experimentally, the quantity R_AA (A is the mass number) being the quotient of observed jet yield in A + A collisions and N_bin × yield in p + p collisions shows a strong damping with increasing A, which is an indication of the new properties of the hot QCD matter created.

• Color glass condensate saturation. The Balitsky–Fadin–Kuraev–Lipatov (BFKL) dynamics which are the result of a resummation of large logarithmic terms in Q² for deep inelastic scattering with small Bjorken-x, saturate at a unitarity limit, with N_part/2 being the number of participant nucleons in a collision (as opposed to the number of binary collisions). The observed charged multiplicity follows the expected dependency of, supporting the predictions of the color glass condensate model. For a detailed discussion, see e.g. Kharzeev et al.; for an overview of color glass condensates, see e.g. Iancu & Venugopalan.

• Particle ratios. The particle ratios predicted by statistical models allow the calculation of parameters such as the temperature at chemical freeze-out T_ch and hadron chemical potential . The experimental value T_ch varies a bit with the model used, with most authors giving a value of 160 MeV < T_ch < 180 MeV, which is very close to the expected QCD phase transition value of approximately 170 MeV obtained by lattice QCD calculations (see e.g. Karsch).

While in the first years, theorists were eager to claim that RHIC has discovered the quark-gluon plasma (e.g. Gyulassy & McLarren), though the experimental groups were more careful not to jump to conclusions, citing various variables still in need of further measurement. The present results shows that the matter created is a fluid with a viscosity near the quantum limit, but is unlike a weakly interacting plasma (a widespread yet not quantitatively unfounded belief on how quark gluon plasma looks).

A recent overview of the physics result is provided by the RHIC Experimental Evaluations 2004, a community-wide effort of RHIC experiments to evaluate the current data in the context of implication for formation of a new state of matter. These results are from the first three years of data collection at RHIC.

New results were published in Physical Review Letters on February 16, 2010, stating the discovery of the first hints of symmetry transformations, and that the observations may suggest that bubbles formed in the aftermath of the collisions created in the RHIC may break parity symmetry, which normally characterizes interactions between quarks and gluons.

The RHIC physicists announced new temperature measurements for these experiments of up to 4 trillion kelvins, the highest temperature ever achieved in a laboratory. It is described as a recreation of the conditions that existed during the birth of the Universe.