When protons and neutrons “melt,” new data reveals that they go through a “first-order” phase transition—a kind of stop-and-go change in temperature. This is comparable to how ice melts: energy raises the temperature first, then the temperature remains constant during the transition as the energy changes a solid into a liquid. The temperature can only rise again once all the molecules are liquid. The melted state of protons and neutrons is a soup of quarks and gluons. At the Relativistic Heavy Ion Collider (RHIC), scientists studying quark-gluon plasma (QGP) are seeing signs of stop-and-go transition.
A quark-gluon plasma, also known as a quark soup, is a quantum chromodynamics phase that exists at high temperatures and/or densities. This phase is made up of asymptotically unbound quarks and gluons, which are some of matter’s most fundamental building components. Because of the strength of the color force, unlike gas-like plasma, quark-gluon plasma acts like a near-ideal Fermi liquid, however, flow properties are still being studied.
Theorists have anticipated signals that researchers can search for, as proof, of a first-order phase change in QGP for more than 35 years. Finding those signals, however, necessitates examining QGP at a variety of energies and figuring out crucial properties in tiny specks that vanish a billionth of a trillionth of a second after forming. Scientists now have the measurements they require thanks to RHIC’s versatility and the complexity of the STAR (Solenoidal Tracker at RHIC) detector.
RHIC, a user facility run by the Department of Energy’s Office of Science, was created in part to examine how nuclear matter degrades into a soup of free quarks and gluons. RHIC studies how gold atom nuclei melt to form this QGP by accelerating and colliding them at various energies. A decrease in pressure and high durability of the QGP would be similar to the water temperature which stays stable during freezing or melting – a sign of a first-order phase transition.
STAR scientists examined these indicators by measuring the fractional fluctuation (a pressure drop reduces the flow) and the system size (longer-lived systems would occur larger in one dimension). For these modest variations in size, particles with less than a femtometer of the wavelength must be more than a billion times smaller than a human hair’s width. Operating RHIC with one particle beam interacting with a stationary gold foil inside the STAR detector was needed to generate collisions at the minimum energy for this research.
The data from these low-energy, “fixed-target” collisions extends the energy range and aligns with the anticipated characteristics of a first-order phase transition, which have been speculated for a long time. Scientists are still collecting and analyzing data from a more complete scan in order to learn more about the phase transition’s features at various collision energy.