Bound quarks loosen up

In the first 10 millionths of a second after the Big Bang, scientists believe that matter existed in a hot and dense state known as the quark–gluon plasma. Thirteen billion years later, physicists at Brookhaven National Laboratory’s (BNL) Relativistic Heavy Ion Collider (RHIC) are recreating the conditions necessary to observe this unusual state of matter by colliding high-energy gold nuclei.

Writing in Physical Review Letters¹, Ágnes Mócsy, a theoretical physicist at the RIKEN-BNL Research Center, and Péter Petreczky of the BNL nuclear theory group, make predictions with important implications for these experiments. They show that particles found in the quark–gluon plasma, known as ‘quarkonium states’, break apart at much lower temperatures than many scientists have recently believed. Since this suppression in the quarkonium dissociation temperature is a direct result of ‘free’ quarks in the quark–gluon plasma, scientists view the effect as further evidence that the plasma has formed in a high-energy collision.

Quarks, and the gluons that mediate the interactions between them, are fundamental particles so strongly attracted to one another that they are never found in isolation under normal conditions. Rather, quarks bind in pairs or triplets to form protons, neutrons and other more exotic particles and only extremely high temperatures and pressures can pull them apart.

To create such temperatures (>10¹² °C), gold nuclei at the RHIC are accelerated toward each other at very high energies (Fig. 1). The energy density generated at the collision of two nuclei is sufficiently great to free the quarks that are bound in the protons and neutrons.

Quarkonium states, however, contain such tightly bound quarks that they can survive above the theoretical temperature, Tc, at which the quark–gluon plasma forms. Unbound quarks in the plasma can shield—or ‘screen’—the interaction between these tightly bound quarks and lower the temperature at which quarkonium states dissociate.

Underlining the importance of screening, Mócsy explains: “The effect is an unambiguous signature of the quark–gluon plasma.”

Mócsy and Petreczky calculated the dissociation temperatures of several quarkonium states, including the so-called J/ψ particle. Members of the high-energy nuclear physics community have held that this particle survives to around 2Tc, but Mócsy and Petreczky have shown that screening suppresses its dissociation temperature to around 1.2Tc. “These results are therefore against the trend,” says Mócsy.

Detectors can measure the products of the J/ψ particle decay and determine its presence in the evolution of the quark–gluon plasma.