'This sign change is a robust indication, supported by first-principles calculations, that the formation of a quark-gluon plasma is turned off at RHIC's lowest collision energy,' Xu said. The order of the hierarchy among the analyzed characteristics flipped - and so did the sign of the key relationships, from negative to positive. 'For these energies, we need more data,' Xu said.īut at the lowest energy, 3 GeV, the scientists saw a dramatic shift. 'At all of these energies, we found the predicted hierarchy and negative values - meaning the data at these energies are all consistent with a thermalized QGP.'īelow 19.6 GeV, the data continued to match the predictions, though the error bars indicating the range of uncertainty about those measurements were large.

'We know at 200 GeV that RHIC collisions create a QGP, but what about the next energy, 62.4 GeV, 54.4 GeV, 39, 27, 19.6?' said Nu Xu, a physicist at LBNL and former spokesperson for STAR. The STAR data indicate that these thermodynamic patterns generally persist at all but the lowest collision energy. The QCD calculations predict a hierarchical ordering of the net-proton distribution characteristics - and that certain relationships among these characteristics should all have negative values. 'These numerical simulations of QCD have the formation of a thermalized quark-gluon plasma built in, so if the data match the predictions, it is evidence that QGP is present,' Mohanty said. Then they compared their observations with predictions calculated using the equations of quantum chromodynamics (QCD), the theory that describes the interactions of quarks and gluons, simulated on a discrete spacetime lattice. Mohanty and the STAR team analyzed data on a variety of characteristics of the distribution, including the mean value, the variance, how skewed the data were, and so on, up to what are known as 5th and 6th order characteristics. 'We measured, event by event, the number of protons minus the number of antiprotons produced, and the distribution of that net-proton production,' said Bedangadas Mohanty, a physics professor at NISER. To determine whether a QGP was created at each collision energy the scientists looked at the distribution of protons produced in each collision event. 'These data give us the widest coverage to date of the nuclear phase diagram - the map of how nuclear matter changes with temperature and density.' 'We analyzed 10 collision energies - from a center of mass energy of 200 GeV, which is RHIC's highest collision energy between two gold beams, down to 3 GeV, where one gold beam collides with a stationary gold target,' said Ashish Pandav, a student at India's National Institute of Science Education and Research (NISER), now stationed at DOE's Lawrence Berkeley National Laboratory (LBNL). The new analysis used data collected by RHIC's STAR detector during the first phase of the RHIC Beam Energy Scan to systematically search for the energy at which production of this thermalized state of quarks and gluons is turned off. Various measurements have shown that the QGP exists down to 19.6 GeV.

Since the collider began operating in 2000, a wide range of measurements have shown that the most energetic smashups of atomic nuclei - at 200 billion electron volts (GeV) - 'melt' the boundaries of protons and neutrons to set free, for a fleeting instant, the quarks and gluons that make up ordinary nuclear particles. Generating and studying QGP has been a central goal of research at RHIC. The findings, just published by RHIC's STAR Collaboration in Physical Review Letters, will help physicists map out the conditions of temperature and density under which the exotic matter, known as a quark-gluon plasma (QGP), can exist and identify key features of the phases of nuclear matter. The 'off' signal shows up as a sign change - from negative to positive - in data that describe 'higher order' characteristics of the distribution of protons produced in these collisions. Department of Energy's (DOE) Brookhaven National Laboratory - can be 'turned off' by lowering the collision energy. Physicists report new evidence that production of an exotic state of matter in collisions of gold nuclei at the Relativistic Heavy Ion Collider (RHIC) - an atom-smasher at the U.S.

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