Studies reveal how certain high-energy particle "rays" lose energy

Studies reveal how certain high-energy particle “rays” lose energy

STAR detector

image: Researchers used the STAR detector on the Relativistic Heavy Ion Collider (RHIC), shown here, to track how certain particle beams lose energy in the quark-gluon plasma (QGP) created when the nuclei of gold atoms collide in the center of the detector.
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Credit: Brookhaven National Laboratory

UPTON, NY — Researchers studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have revealed how certain particle beams lose energy as they cross the unique form of nuclear material created in these collisions. The results, published in Physical examination Cshould help them learn about the important “transport properties” of this hot particle soup, known as quark gluon plasma (QGP).

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

But there are several ways in which a jet can lose energy – or “go out”. So it can be difficult to say which of these causes creates the extinguishing effect.

With the new findings, for the first time, STAR has identified a specific population of jets for which physicists say they can identify the mechanism distinctly: individual quarks that emit gluons when interacting with QGP.

Theorists can now use data to refine their calculations that describe the basic properties of the hot quark soup.

“Jets are very useful because they tell how these quarks interact with themselves,” said Kolja Kauder, another lead author on the analysis, who is a physicist at the Brookhaven National Laboratory, where the RHIC is located. “This is the core of ‘quantum chromodynamics’ – the theory that describes the interaction of nuclear power between quarks and gluons. We learn more about the power of basic nature by studying how these jets go out. “

In the beginning

The strong force plays a big role in building the structure of everything we see in the universe today. This is because all visible matter is made up of atoms with protons and neutrons in the nucleus. These particles in turn consist of quarks, which are held together by the exchange of strong force-carrying particles – the glue-like gluons.

But quarks were not 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 surface of matter’s basic building blocks was cooled sufficiently for protons and neutrons to form. RHIC, a US Department of Energy office of the nuclear physics research facility, was built to recreate and study this quark gluon plasma.

RHIC recreates the quark soup of the early universe by directing the nuclei of heavy atoms like gold to frontal collisions at almost the speed of light. The energy released creates thousands of new subatomic particles, including quarks (remember that energy can create mass and vice versa through the famous equation E = mc2). It also “melts” the boundaries of the individual protons and neutrons to release the inner quarks and gluons.

Researchers have been tracking how different types of particles flow through the resulting quagmire-gluon plasma for more than two decades. These include collimated sprays, or jets, of particles that result from the fragmentation of a quark or gluon. Researchers have generally found that high-motion particles and jets lose energy when they cross the lump of hot QGP. Through this new study, they have identified details of a specific mechanism for jet extinguishing in a subgroup of jets.

Track “diets” at different angles

This study focused specifically on jet jets of particles produced back-to-back (called diets), where a jet near the surface of the QGP blob easily escapes with a lot of energy, while the recoil jet traveling a longer way in the opposite direction may be extinguished by the plasma . STAR physicists tracked the energy of particles that make up the “cone” in the recoil beam. Comparing it to the energy from the escaped (or “trigger”) jet tells them how much energy was lost.

They also divided all events into those that gave relatively narrow rays and those that gave a wider spray of particles.

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

Think of a large swimmer moving through the water in a non-streamlined way, he suggested. You can expect to see a wider wake that moves further from the person than after a narrow, streamlined swimmer. As for the particles, 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 does not matter what aperture the jet plane has; they all lose energy in the same way.”

For both narrow and wide jets, add together the energy from all the high momentum and low torque particles inside the “cone” can account for all the energy that “disappears” when extinguished. That is, while these beams experienced energy loss, in both the wide and narrow beams, the lost energy was converted into lower momentum particles that remained within the jet cone.

“When the jets lose energy, the lost energy is converted into particles with lower momentum. You can not just lose energy; it must be preserved,” says Brookhavens Kauder. The surprise was that all the energy stayed within the cone.

The consequences

The results have important implications for understanding when the extinguishing takes place for these jets.

“Not seeing any difference between the wide and narrow beams means that the mechanism of energy loss is independent of the substructure of the beam. The energy loss must have taken place before the rays were split – before there was an opening angle, narrow or wide “, said Kunnawalkam Elayavalli.

The most probable course of events: “Probably a single quark crossing plasma-emitting gluons (emitted energy) when interacting with other quarks in QGP, then it was split to produce the jet substructure. The gluons are transformed into other particles with lower momentum that stay within the cone, and it is the particles we measure, he said.

If the energy loss occurred after If the beam were split, each particle that forms the jet substructure would have lost energy, with more probability that particles would scatter outside the jet cone – in other words, form a “wake” beyond the area where 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 that are somewhat analogous to the viscosity and density of the water. It will also give physicists a way to understand more about the basic strong force interactions between quarks.

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

“This measurement essentially starts the next era of jet physics at RHIC, which will allow us to differentially study the space-time evolution of QGP.”

Raghav Kunnawalkam Elayavalli began this analysis as a postdoctoral fellow at Wayne State University and worked with Kauder (who later left Wayne State to join Brookhaven) and Wayne State physicist Joern Putschke, another lead author on the analysis. He completed the analysis under his current position at Yale / Brookhaven Lab with Yale physicist Helen Caines and Brookhaven Lab physicist Lijuan Ruan – the two spokespersons for STAR Collaboration – and will begin a faculty appointment at Vanderbilt University this summer.

This research was supported by the DOE Office of Science (NP), which also supports RHIC activities, and by the US National Science Foundation and a number of international agencies described in the scientific article. The STAR collaboration used computer resources in the RHIC & ATLAS Computing Facility at Brookhaven Lab; 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 at the US Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information visit

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