Particle colliders, like the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory, help physicists investigate the nature of matter and its evolution. By heating up and smashing together particles, like neutrons and protons, at nearly the speed of light, researchers can recreate phases of matter that only existed during the earliest days of the universe. These are the states beyond solid, liquid and gas, like the quark-gluon plasma.
“We’re effectively turning back the clock and making tiny Big Bangs,” said Zachary Sweger, a Ph.D. student studying nuclear physics at the College of Letters and Science at UC Davis. “It tells us about our history and how nuclei change with temperature and pressure.”
Sweger works with the STAR detector, part of RHIC. Like many other particle collider experiments, the collaboration is large, with hundreds of scientists participating. For years, RHIC scientists have conducted collision experiments to recreate the quark-gluon plasma state beyond normal matter.
One challenge of particle collider experiments is background noise. When particles collide, numerous other particles are produced, manifesting as signals in the data. In new-particle and dark matter search experiments, background noise is accounted for in data analyses. But when trying to discern the supercritical phase of a substance, the point when it shifts from a gas to a plasma, background noise is still a problem.
In a study appearing in Physical Review C, Sweger and colleagues use computational models of RHIC data to show how scientists can refine their search for signals of the supercritical phase of nuclear matter, divorcing background noise from legitimate signals in the data.
“Sometimes we don’t really understand everything that’s going on with our detector, which is the size of a three-story house and is made up of thousands of components,” said Sweger. “We want to make sure that we’re getting it right, especially with a measurement that is as highly anticipated as this one.”
Fluctuations in the data
When smashing together large nuclei, researchers count the number of particles that result from the collision. They then create a bell curve distribution graph of those numbers.
“With our signal, we know pretty well the distribution that these should follow in the absence of some supercritical fluid,” Sweger said. “If there is a supercritical fluid, we expect fluctuations. We’re looking for these rare events in which the number of particles that come out of the collision is very different from the rest of the collisions.”
In a bell curve distribution graph, fluctuations manifest as upticks in the graph’s tail ends. Such signals were observed by STAR detector scientists in previous experiments.
Sweger and colleagues had been analyzing new data from the STAR detector for years, crosschecking them with other analyzers and even using a different detector. Using a different detector yielded slightly different results.
Puzzled, Sweger couldn’t stop thinking about the discrepancies. He continued analyzing the data as a side project.
“Eventually, I realized that the entire time that we’ve been thinking about this, we’ve been picturing this measurement as one-dimensional, like the number of protons that we’re measuring in any collision,” he said. “Really we should be thinking about this in two-dimensional space, where the number of protons that come out of our collisions are measured against other particles.”
“When you measure those two numbers with different detectors, you can very easily see a false signal,” Sweger added.
Pileups and convincing the STAR community
On top of false signals, Sweger and colleagues also illuminated the effects of pileup on the collision experiments. Pileup is when two collisions occur in the collider when only one was expected. It’s a fairly common occurrence.
Using a computational model, Sweger created simulations that showcased the results of pileup collisions.
“These toy models of our detectors showed that, depending on how sensitive our detectors are to this pileup, that you’re going to see a much bigger signal than you ever would have seen otherwise,” Sweger said.
According to Sweger, the research was motivated by a need to convince the rest of the STAR collaboration that these false signals are a problem.
“This is going to define our measurement if we’re not careful,” he said.
In April, Sweger will join other scientists of the STAR collaboration at a conference at the Goethe University of Frankfurt. There, he and colleagues will share their results.
“With many people in the collaboration, it’s likely that some will want to do things the traditional way — the way we’ve always kind of done this measurement — but that really makes this measurement very unstable,” Sweger said.
The research illuminates the iterative process of scientific experimentation and how that process is always being refined to unlock new discoveries.
The research was supported in part by the U.S. Department of Energy and the National Science Foundation.
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