Study reveals how some ‘jets’ of high-energy particles lose energy

image: Scientists used the STAR detector at the Relativistic Heavy Ion Collider (RHIC), shown here, to track how certain jets of particles lose energy in the quark-gluon plasma (QGP) that forms when the nuclei of gold atoms collide in the center of the detector.
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Credit: Brookhaven National Laboratory

UPTON, NY — Scientists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have revealed how certain particle beams lose energy as they traverse the unique shape of nuclear matter created in these collisions. The results, published in Physical assessment Cshould help them learn about the key “transport properties” of this hot particle soup known as a quark-gluon plasma (QGP).

“By watching particle rays slow down as they move through the QGP, we can learn about its properties in the same way that studying how objects move through water can tell you something about its density and viscosity,” said Raghav Kunnawalkam Elayavalli, a researcher. postdoctoral fellow at Yale University and member of RHIC’s STAR experiment collaboration.

But there are multiple ways a fighter jet can lose energy – or be “extinguished”. So it can be difficult to say which of those causes is causing the quenching effect.

With the new findings, STAR has identified for the first time a specific population of jets that the physicists say can distinctively identify the mechanism: individual quarks that eject gluons as they interact with the QGP.

Theorists can now use the data to refine their calculations describing the fundamental properties of the hot cottage cheese soup.

“Jets are very useful because they tell you how these quarks interact with themselves,” said Kolja Kauder, another lead author of the analysis, who is a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, where RHIC is located. “This is the essence of ‘quantum chromodynamics’ – the theory describing the strong nuclear force interactions of quarks and gluons† We learn more about that fundamental force of nature by studying how these jets are extinguished.”

At the beginning

The strong force plays an important role in building the fabric of everything we see in the universe today. That’s because all visible matter is made of atoms with protons and neutrons in their nucleus. Those particles in turn are made up of quarks, which are held together by the exchange of strong force carrier particles – the glue-like gluons.

But quarks weren’t always bound together. Scientists believe that quarks and gluons roamed freely very early in the universe, just a microsecond after the Big Bang, before the primordial soup of matter’s fundamental building blocks had cooled enough to form protons and neutrons. RHIC, a US Department of Energy Office of Science user facility for nuclear physics research, was built to recreate and study this quark-gluon plasma.

RHIC mimics the quark soup of the early universe by sending the nuclei of heavy atoms like gold into head-on collisions at nearly the speed of light. The released energy creates thousands of new subatomic particles, including quarks (remember that energy can create mass and vice versa via the famous equation E=mc2† It also “melts” the boundaries of the individual protons and neutrons to release the internal quarks and gluons.

Scientists have been tracking how different types of particles flow through the resulting quark-gluon plasma for more than two decades. These include collimated sprays, or jets, of particles resulting from the fragmentation of a quark or gluon. The scientists have generally found that high momentum particles and jets lose energy as they pass through the blob of hot QGP. Through this new study, they have identified details of a specific mechanism for jet quenching in a subset of jets.

Tracking ‘Dijets’ from different angles

This study specifically focused on jets of particles that were produced back-to-back (called dikets), where one jet close to the surface of the QGP blob easily escapes with a lot of energy, while the recoil jet that travels a longer route in the opposite direction. direction is extinguished by the plasma. STAR physicists tracked the energy of particles that make up the “cone” of the recoil beam. If you compare that to the energy of the escaped (or “trigger”) jet, it tells them how much energy was lost.

They also divided all events into those that produced relatively narrow beams and those that produced a wider nebula of particles.

“Our intuition tells us that something wider moving through the medium should lose more energy,” Kunnawalkam Elayavalli said. “If the jet is narrow, it can punch through and you would expect less energy loss than with a wider jet, which sees more of the plasma. That was the expectation.”

Think of a large swimmer moving through the water in a non-streamlined manner, he suggested. You would expect a wider wake to move further away from the person than a slender, streamlined swimmer’s wake. In the case of the particles, the physicists expected that the wider “wake” produced by wider jets would push particles beyond the limits of their detection.

“But what we found is that with this particular subset of jets that we studied at RHIC, it doesn’t matter what the jet’s opening angle is; they all lose energy in the same way.”

For both the narrow and wide jets, where the energy of all high thrust and low momentum particles within the “cone” could account for all the energy “lost” extinguishing. That is, while these jets did experience energy loss, in both the wide and narrow jets, the lost energy was converted into lower momentum particles that remained within the jet cone.

“When the jets lose energy, that lost energy is converted into particles with lower momentum. You can’t just lose energy; it has to be conserved,” Brookhaven’s Kauder said. The surprise was that all the energy stayed in the cone.

The implications

The results have important implications for understanding when the extinguishing is done for these jets.

“Not seeing any difference between the wide and narrow jets means that the mechanism of energy loss is independent of the substructure of the jet. The energy loss must have happened before the jets split — before there was an opening angle, narrow or wide,” Kunnawalkam Elayavalli said.

The most likely sequence of events: “Probably a single quark passing through the plasma radiated gluons (released energy) while interacting with other quarks in the QGP, then it split to produce the jet substructure. The gluons turn into other lower momentum particles that stay in the cone, and those are the particles we’re measuring,” he said.

When the energy loss has happened after at the beam split, any particle making up the beam substructure would have lost energy, with a greater chance of particles dispersing outside the beam cone — in other words, forming a “wake” outside the area where the physicists could measure them.

Knowing the specific mechanism of energy loss for these jets will help theorists refine their calculations of how the energy loss relates to the QGP transport properties – properties somewhat analogous to the viscosity and density of water. It will also give physicists a way to understand more about the fundamental strong-force interactions between quarks.

“Obtaining a quantitative understanding of the properties of this plasma is paramount to studying the evolution of the early universe,” Kunnawalkam Elayavalli said, “including how that primordial soup of particles became the protons and neutrons of the nuclei of atoms that make up our world today.

“This measurement essentially kicks off the next era of jet physics at RHIC, allowing us to differentially study the space-time evolution of the QGP.”

Raghav Kunnawalkam Elayavalli began this analysis as a postdoctoral researcher at Wayne State University in collaboration with Kauder (who later left Wayne State to join Brookhaven) and Wayne State physicist Joern Putschke, another lead author of the analysis. He completed the analysis during his current position at Yale/Brookhaven Lab with Yale physicist Helen Caines and Brookhaven Lab physicist Lijuan Ruan — the two co-spokespersons for the STAR collaboration — and will begin a faculty position at Vanderbilt University this summer.

This research was supported by the DOE Office of Science (NP), which also supports RHIC operations, as well as the US National Science Foundation and a range of international agencies detailed in the scientific paper. The STAR collaboration leveraged computing resources at the RHIC & ATLAS Computing Facility at Brookhaven Lab; the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility at Lawrence Berkeley National Laboratory; and the Open Science Grid consortium.

Brookhaven National Laboratory is supported by the Office of Science of the United States Department of Energy. The Office of Science is the largest proponent of basic science research in the United States, working to address some of the most pressing challenges of our time. For more information visit

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