This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: New results from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) provide strong evidence that smashing oxygen nuclei together at nearly the speed of light generates a quark-gluon plasma (QGP), a type of matter that existed just after the birth of the universe. The data, just published in Physical Review Letters, show that energetic jets of particles lose energy as they traverse the hot, dense matter left in the collision's wake.
This energy loss phenomenon, known as jet quenching, has been one of the "smoking gun" signatures of QGP formation in collisions of larger atomic nuclei at RHIC, a U.S. Department of Energy (DOE) Office of Science user facility that operated at DOE's Brookhaven National Laboratory from 2000 until earlier this year. This signature has also been observed in oxygen–oxygen collisions at the Large Hadron Collider (LHC), located at CERN, the European Particle Physics Laboratory.
The analysis from RHIC at significantly lower collision energy sets a new lower boundary for the conditions capable of producing the primordial QGP in this small collision system. "The QGP is believed to be the form of matter that filled the early universe—before the protons and neutrons that make up today's atoms formed. Understanding its properties is essential for uncovering how the early universe evolved in its first microseconds," said Li Yi, a professor of physics at Shandong University who oversaw the STAR analysis.
"One of the most powerful ways scientists study the QGP is by identifying where its signatures appear—and just as importantly, where these signatures turn off," she said. Rongrong Ma, a STAR physicist from Brookhaven Lab, said, "This analysis is pushing the boundaries to an extreme case to help us learn something about this jet quenching phenomenon and the conditions needed to generate a QGP." RHIC was built to create and study the QGP. Its collisions of gold nuclei, each packed with 79 protons and 118 neutrons, effectively melt the boundaries of these atoms' protons and neutrons, thus setting free their constituent quarks and gluons.
The collisions also deposit enough energy to be transformed via Einstein's famous equation, E=mc², into thousands of new quarks and gluons. But because the resulting QGP is a speck smaller than a single atomic nucleus and lasts less than a billionth of a billionth of a second before the quarks and gluons join up to form more ordinary particles, the only way to know if a QGP has formed is to study particles flying out of the collisions using giant detectors like STAR. Seeing signs that some particles emerge with less energy than expected implies that they've gotten "stuck," or quenched, by interacting with the soup of quarks and gluons on the way out.
Energy loss by collimated sprays of very energetic particles known as jets has been a particular interest. These jets are typically produced when a single quark or gluon in one nucleus interacts with a quark or gluon in the other, shooting out two sprays of energetic particles back-to-back from the point of that interaction. When these "di-jets" are produced near the edge of the collision zone, they give physicists a built-in way to study the plasma by tracking how each half of the jet gets affected as it moves different distances through the blob of hot nuclear matter.
Extract — continue reading at the source.
