One of the central goals of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is to probe the phase diagram of quarks and gluons, which are the building blocks of atomic nuclei. The familiar phase diagram for water indicates when we can expect to find steam, liquid water, or ice. The phase diagram for quarks and gluons is considerably more exotic, with a quark-gluon plasma phase, a color superconductor phase, and a phase that consists of a gas of quarks bound together by gluons.
Heavy-ion collisions at RHIC produce quarks and gluons in one of these three phases. By varying the collision energy, RHIC can map out the phase diagram, and potentially find an intriguing predicted critical point beyond which the quark-gluon plasma can smoothly transform into the gas phase. However, theoretical advances to support a comprehensive framework for modeling these heavy-ion collisions will be necessary to interpret the RHIC measurements.
This research will fill in critical theoretical inputs by performing the first state-of-the-art lattice QCD calculations to (i) extend the reach of the equation of state that describes nuclear matter to cover almost the entire beam energy range of the RHIC program; (ii) provide more stringent constraints on the location of the critical point in the phase diagram, vastly improving the present status; and (iii) establish the equilibrium baseline for certain experimentally-measured parameters for a large range of RHIC beam energies.Phase diagrams describe not only everyday matter, but also the more exotic “nuclear matter”, matter inside nuclei. Nuclei are made of strongly interacting protons and neutrons, which are themselves composed of quarks, bound together by gluons. Just as ice melts into water with sufficiently high temperatures, so the protons and neutrons in nuclei “melt” into a remarkable phase of matter, the “quark gluon plasma” (QGP). Establishing the boundary between atomic nuclei and the QGP in the nuclear matter phase diagram is a central goal of the heavy-ion research programs at major experimental facilities such as the Relativistic Heavy Ion Collider (RHIC) at BNL and ALICE at CERN.
As the theory that describes quarks and gluons, Quantum Chromodynamics (QCD), is precisely known mathematically and can be studied on supercomputers, one might hope to predict the phase diagram of QCD directly. This can already be done for gluons alone, but determining the phase diagram of the real-world mixture of quarks and gluons requires new computational techniques. This ALCC project supports the investigation of a promising method for predicting the QCD phase diagram in the presence of both quarks and gluons. A special goal of this research is to attempt to find the conjectured “critical point” in the QCD phase diagram, the end point of the line along which nuclei and the QGP phase coexist . The results of this project are expected to help in the interpretation of experimental observations at RHIC, and to contribute to our understanding of the physics of quarks and gluons more generally.
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