Scientists at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in the US have successfully generated quark-gluon plasma (QGP) by smashing oxygen nuclei at nearly the speed of light.
QGP is the type of matter that existed in the first few microseconds just after the universe was born and helps physicists determine how protons, neutrons, and atoms were first formed.
Studying the QGP can also help scientists understand other phenomena in physics, such as the limits of nuclear stability and the search for exotic particles like X-particles.
However, since QGP only lasted for a few microseconds even after the Big Bang, it is difficult for scientists to observe it directly, even inside particle colliders like RHIC or the Large Hadron Collider (LHC) at CERN. So,
The only way for scientists to know if QGP is formed is to study particles flying out of collisions of nuclei using detectors like the Solenoidal Tracker at RHIC or the STAR detector.
“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,” said Li Yi, a professor of physics at Shandong University involved in the work.
Researchers have previously found that energetic jets of particles lose their energy when they travel through the hot, dense matter left after nuclei collide inside a particle smasher. This is called jet quenching and is considered a signature of QGP formation.
What is jet quenching?
When heavier nuclei like gold are smashed into each other by a particle collider, their boundaries collapse, and their protons and neutrons are set free as constituent elements of quarks and gluons.
These collisions follow Einstein’s famous equation of E=mc2 and release energy accordingly. Since this QGP formed is smaller than the size of the nuclei themselves, researchers use detectors to spot other particles that fly out as a result of these nuclei collisions.
However, in their observations, researchers have found that some particles emerge from these collisions with less energy than expected, as though they were stuck behind quarks and gluons of the plasma.
To understand this further, researchers study jets of energy that are released when a nucleus containing a single quark or gluon in its nucleus collides with another containing a single quark or gluon in its nucleus.
Such a collision produces two jets of energetic particles near the edge of the collision zone and helps researchers study the QGP by observing the loss of energy of these particles. This quenching of the jets is critical in understanding the QGP, while also being a telltale sign of its existence.
Studying oxygen collisions
Researchers at RHIC had observed jet quenching in gold-gold collisions. With 78 protons and 118 neutrons, these are relatively large nuclei, and lower-energy particles emerging from these collisions were found to move collectively, like a perfect liquid with no resistance.
The researchers wondered if similar observations would be made if they worked with a system that contained nuclei of lighter elements like oxygen. With just eight protons and eight neutrons in each colliding nucleus, oxygen offers a simpler system for observing jet quenching than heavy nuclei like gold or lead.
Using data from oxygen-oxygen collisions conducted at 200 billion electron volts (GeV) at RHIC in 2021, the team found 20 percent suppression of high-energy particles due to jet quenching. While lower than the gold and lead counterparts, the suppression is still proof of QGP formation.
Because researchers at the LHC have also studied oxygen-oxygen collisions, the RHIC team could compare its findings with those of CERN researchers.
“We are pushing the boundaries in two ways to explore the minimal conditions for creating a QGP, both in terms of the nucleus size by looking at these small oxygen-oxygen collisions, but also by looking at data from lower collision energies — and we still see this crucial QGP signature. That’s a unique feature of our measurement,” explained Rongrong Ma, a STAR physicist from Brookhaven National Laboratory in a press release.
With a new sPHENIX detector now part of the RHIC’s particle collider setup, there is much more that will be discovered regarding oxygen-oxygen collisions in the time to come.