Tantalizing signs of phase-change ‘turbulence’ in RHIC collisions

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The STAR detector on the U.S. Department of Energy’s Brookhaven National Laboratory Credit: Brookhaven National Laboratory

Physicists learning collisions of gold ions on the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science consumer facility for nuclear physics analysis at DOE’s Brookhaven National Laboratory, are embarking on a journey by way of the phases of nuclear matter—the stuff that makes up the nuclei of all of the seen matter in our universe. A brand new evaluation of collisions performed at completely different energies reveals tantalizing signs of a important level—a change in the way in which that quarks and gluons, the constructing blocks of protons and neutrons, remodel from one section to a different. The findings, simply printed by RHIC’s STAR Collaboration in the journal Physical Review Letters, will assist physicists map out particulars of these nuclear section modifications to higher perceive the evolution of the universe and the situations in the cores of neutron stars.

“If we are able to discover this critical point, then our map of nuclear phases—the nuclear phase diagram—may find a place in the textbooks, alongside that of water,” stated Bedanga Mohanty of India’s National Institute of Science and Research, one of a whole bunch of physicists collaborating on analysis at RHIC utilizing the delicate STAR detector.

As Mohanty famous, learning nuclear phases is considerably like studying in regards to the strong, liquid, and gaseous varieties of water, and mapping out how the transitions happen relying on situations like temperature and stress. But with nuclear matter, you may’t simply set a pot on the range and watch it boil. You want highly effective particle accelerators like RHIC to show up the warmth.

RHIC’s highest collision energies “melt” odd nuclear matter (atomic nuclei made of protons and neutrons) to create an unique section referred to as a quark-gluon plasma (QGP). Scientists consider the complete universe existed as QGP a fraction of a second after the Big Bang—earlier than it cooled and the quarks sure collectively (glued by gluons) to kind protons, neutrons, and finally, atomic nuclei. But the tiny drops of QGP created at RHIC measure a mere 10-13 centimeters throughout (that is 0.0000000000001 cm) they usually final for less than 10-23 seconds! That makes it extremely difficult to map out the melting and freezing of the matter that makes up our world.

“Strictly speaking if we don’t identify either the phase boundary or the critical point, we really can’t put this [QGP phase] into the textbooks and say that we have a new state of matter,” stated Nu Xu, a STAR physicist at DOE’s Lawrence Berkeley National Laboratory.

Tantalizing signs of phase-change 'turbulence' in RHIC collisions
Mapping nuclear section modifications is like learning how water modifications below completely different situations of temperature and stress (internet baryon density for nuclear matter). RHIC’s collisions “melt” protons and neutrons to create quark-gluon plasma (QGP). STAR physicists are exploring collisions at completely different energies, turning the “knobs” of temperature and baryon density, to search for signs of a “critical point.” Credit: Brookhaven National Laboratory

Tracking section transitions

To monitor the transitions, STAR physicists took benefit of the unbelievable versatility of RHIC to collide gold ions (the nuclei of gold atoms) throughout a variety of energies.

“RHIC is the only facility that can do this, providing beams from 200 billion electron volts (GeV) all the way down to 3 GeV. Nobody can dream of such an excellent machine,” Xu stated.

The modifications in power flip the collision temperature up and down and in addition fluctuate a amount generally known as internet baryon density that’s considerably analogous to stress. Looking at information collected through the first section of RHIC’s “beam energy scan” from 2010 to 2017, STAR physicists tracked particles streaming out at every collision power. They carried out an in depth statistical evaluation of the web quantity of protons produced. A quantity of theorists had predicted that this amount would present giant event-by-event fluctuations because the important level is approached.

The purpose for the anticipated fluctuations comes from a theoretical understanding of the pressure that governs quarks and gluons. That concept, generally known as quantum chromodynamics, means that the transition from regular nuclear matter (“hadronic” protons and neutrons) to QGP can happen in two alternative ways. At excessive temperatures, the place protons and anti-protons are produced in pairs and the web baryon density is near zero, physicists have proof of a clean crossover between the phases. It’s as if protons regularly soften to kind QGP, like butter regularly melting on a counter on a heat day. But at decrease energies, they count on what’s referred to as a first-order section transition—an abrupt change like water boiling at a set temperature as particular person molecules escape the pot to grow to be steam. Nuclear theorists predict that in the QGP-to-hadronic-matter section transition, internet proton manufacturing ought to fluctuate dramatically as collisions strategy this switchover level.

“At high energy, there is only one phase. The system is more or less invariant, normal,” Xu stated. “But after we change from excessive power to low power, you additionally improve the web baryon density, and the construction of matter could change as you’re going by way of the section transition space.

Tantalizing signs of phase-change 'turbulence' in RHIC collisions
As physicists turned the collision power down at RHIC, they anticipated to see giant event-by-event fluctuations in sure measurements comparable to internet proton production–an impact that is just like the turbulence an airplane experiences when getting into a financial institution of clouds–as proof of a “critical point” in the nuclear section transition. Higher degree statistical analyses of the information, together with the skew (kurtosis), revealed tantalizing hints of such fluctuations. Credit: Brookhaven National Laboratory

“It’s just like when you ride an airplane and you get into turbulence,” he added. “You see the fluctuation—boom, boom, boom. Then, when you pass the turbulence—the phase of structural changes—you are back to normal into the one-phase structure.”

In the RHIC collision information, the signs of this turbulence usually are not as obvious as meals and drinks bouncing off tray tables in an airplane. STAR physicists needed to carry out what’s generally known as “higher order correlation function” statistical evaluation of the distributions of particles—in search of extra than simply the imply and width of the curve representing the information to issues like how asymmetrical and skewed that distribution is.

The oscillations they see in these increased orders, notably the skew (or kurtosis), are reminiscent of one other well-known section change noticed when clear liquid carbon dioxide immediately turns into cloudy when heated, the scientists say. This “critical opalescence” comes from dramatic fluctuations in the density of the CO2—variations in how tightly packed the molecules are.

“In our data, the oscillations signify that something interesting is happening, like the opalescence,” Mohanty stated.

Yet regardless of the tantalizing hints, the STAR scientists acknowledge that the vary of uncertainty in their measurements remains to be giant. The crew hopes to slim that uncertainty to nail their important level discovery by analyzing a second set of measurements constructed from many extra collisions throughout section II of RHIC’s beam power scan, from 2019 by way of 2021.

The complete STAR collaboration was concerned in the evaluation, Xu notes, with a selected group of physicists—together with Xiaofeng Luo (and his pupil, Yu Zhang), Ashish Pandav, and Toshihiro Nonaka, from China, India, and Japan, respectively—assembly weekly with the U.S. scientists (over many time zones and digital networks) to debate and refine the outcomes. The work can be a real collaboration of the experimentalists with nuclear theorists around the globe and the accelerator physicists at RHIC. The latter group, in Brookhaven Lab’s Collider-Accelerator Department, devised methods to run RHIC far beneath its design power whereas additionally maximizing collision charges to allow the gathering of the mandatory information at low collision energies.

“We are exploring uncharted territory,” Xu stated. “This has never been done before. We made lots of efforts to control the environment and make corrections, and we are eagerly awaiting the next round of higher statistical data,” he stated.


Theory gives roadmap in quest for quark soup ‘important level’


More info:
J. Adam et al, Nonmonotonic Energy Dependence of Net-Proton Number Fluctuations, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.126.092301

Provided by
Brookhaven National Laboratory


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Tantalizing signs of phase-change ‘turbulence’ in RHIC collisions (2021, March 5)
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