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This article is by Natalie Crnosija, a Stony Brook University journalism graduate who spent four months in early 2012 as a science writing intern with the Media & Communications Office.
In the dead of night, while many experimental facilities at BNL are quiet, the Relativistic Heavy Ion Collider (RHIC) runs. When RHIC is operating, it runs every day, all day and all night. Until mid-April, the physicists used RHIC to collide polarized proton beams, which give a baseline for comparing results from heavy ion collisions at RHIC, and also are used to study the origin of what’s known as proton spin.
To conduct this basic research, shift crews of scientists operate and collect data from the collisions at two detectors, STAR and PHENIX. Each shift crew consists of a shift leader and three others who take responsibility for operating and monitoring various aspects of the detectors’ operation, even as the machine continues to run throughout each night.
Alex Mwai (left) and Damien Reynolds, both of SBU, and Ed O’Brien, BNL, in the PHENIX Counting House.
“It’s the only way we could do it,” said Edward O’Brien, senior physicist and director of operations at the PHENIX experiment. “It’s so complex. You have to essentially get thousands of pieces of electronics and equipment working simultaneously. You spend weeks getting everything running just so and you do your best to not disturb it, so you have to leave it running.”
Evening and overnight hours, explained O’Brien, actually permit the best data to be taken because there is little interference with the beam.
From 12:30 a.m. to 8:30 a.m. on April 21, the night “owl” crew for that week ran the PHENIX detector from their counting house and I kept watch with them. A large screen, showing the beams destined to collide and the level of the system’s voltage, commanded the room. Under it, a bank of computers were busy, each devoted to an aspect of the PHENIX detector’s function. Early in the shift, the team awaited the beam. Alex Mwai, a Stony Brook University (SBU) graduate student, said that the nightshift could sometimes be difficult, especially after 3 a.m.
“Other than that, the rest of it is actually a lot of fun, especially when you have beam and have stuff to do,” explained Mwai. In charge of voltage control, Mwai monitors the state of the high and low voltage for different systems, and ramps up high voltage when the team has a beam and is doing a run.
“The 2012 RHIC Run has been very successful, with both RHIC and PHENIX performing well,” said O’Brien. Success can be defined as recording the greatest number of proton collisions or, as the scientists call them, “events.”
“Every time there’s a collision, we make a determination as to whether we want to record the event or not,” said O’Brien. “We have the ability to take 6,000 to 7,000 events per second out of a total of a few million proton collisions every second.”
The PHENIX detector.
PHENIX’s 18 detector subsystems are used to measure the extent of various nuclear phenomena such as the production of the W-boson, which decays into high-energy muons, among the collision products. The W-boson could play a role in determining how a proton gets its overall spin. A proton is composed of quarks and gluons, subatomic particles that have spins of their own that contribute to the cumulative spin of the proton. These spins, however, do not add up to the proton’s one-half spin, leading scientists to look at the W-boson, among other clues to the origin of the missing spin.
“There’s a way to look for Ws by colliding protons and examining the various particles that are produced. It’s hard to produce the W-boson, so we are looking for something that is rare,” said O’Brien. “ When protons collide at RHIC, the quarks inside the protons also collide and sometimes form a W. We can get some insight into the quark spin by looking into the W spin.”
Another aspect of the collisions that interests Mwai is the nuclear phase diagram as it relates to quark-gluon plasma, the “perfect liquid” that existed after the Big Bang and is produced for a fraction of a second when heavy ions collide at RHIC. RHIC made a switch from colliding protons to colliding heavy ions around mid-April.
“The region of the nuclear phase that we are especially interested in is quark-gluon plasma,” said Mwai. “The questions are very interesting and most of these questions are really fundamental like, how do you form quark-gluon plasma, what are its characteristics and, of course, the transition to quark-gluon plasma,” which offers clues to how the universe evolved.
“It’s a new state of matter, that’s what drew me in,” said Damian Reynolds, an SBU chemistry graduate student and fellow PHENIX crewmember.
Based on the number of graduate students on shift, the experiment is like a “factory” for students, said Vlad Pantuev, a PHENIX collaborator from the Institute of Nuclear Research in Russia.
“It is experience first and, secondly, the wide range of tasks we have to do. You get to know the experiment,” said Pantuev. “You have to know programming, math, and physics.”
Bill Christie, BNL, in the STAR Control Room.
“What differentiates PHENIX from STAR, apart from PHENIX’s focus on muons, is how the detector examines collisions,” said Bill Christie, senior physicist and operations coordinator for the STAR detector.
“What happens in the collisions at STAR and PHENIX is the same, the difference is what we measure of what comes out of the collisions,” explained Christie. “There are many ways to look at the same collision and, depending on what you want to measure, you can have different technologies. You can try to measure a small solid angle or try to look at particles that come out in a small range of space but do it with very high precision, which is more what PHENIX is tailored to. At STAR, we try to measure everything that comes out, but we do it with slightly less precision, so our forte is large acceptance, good particle identification, good tracking, and good triggering.”
At another part of the RHIC ring, a night earlier, I watched as the STAR owl shift monitored the beam. Screens commanded the room. The computers, like those at PHENIX, were each focused on various aspects of the detector’s function. At STAR, however, there is a different suite of detectors, including the Time Projection Chamber, which identifies and traces the output of the collided particles.
A Ph.D. student from the University of São Paulo in Brazil, Renato Negrao had the job of watching and “recovering” the system.
“We don’t have many troubles here,” said Negrao. “The detectors work fine when we are taking data. Healthy plots mean good data. If we don’t have these guys [working], we don’t have data.”
The STAR detector.
For Negrao’s fellow detector operator, Geraldo Vasconcelos, a Ph.D. student from the University of Campinas, also in Brazil, another important aspect of taking shifts at STAR is the interactions with the shift crew, with whom he can discuss physics.
“It’s not just detectors,” said Vasconcelos.
According to shift leader Leonid Efimov, who has worked at STAR since 2000, the human element is crucial for the appropriate function of various detector components under different conditions of the accelerator.
“The number of such situational combinations is infinite,” Efimov said. Although it’s nice to think about building some programmable robots to support successful running with feedback from RHIC to the detectors and in the opposite direction, that seems an almost unsolvable task. “Even drawing up a robust algorithm of the completely automated operating mode looks like a utopia,” he said. “Thus, at present, a person is the most reliable operator.”
Still, as the years go by, Christie said, the automation of various facets of the detector keeps getting better. And the automation is continuous, with the detector’s being consistently upgraded when RHIC isn’t running.
“If you have a stable detector, it should take less and less human effort to run it. But we are always updating it,” said Christie, who is also in charge of upgrading the system off-season.
During RHIC’s run, however, Christie said he is never not on shift.
“Essentially, if I am awake, I know what’s going on at STAR,” he said.
Christie often leads tours and explains the function of the STAR detector and said this type of basic research is an important human endeavor.
“Basic research is how we’ve always figured out the new understanding of nature and the new technologies to allow us to study those things,” he explained. “These technologies can lead to useful developments in the future that you just can’t even conceive of when you start a program like this.”
And that is something worth staying up for.
Operated by Brookhaven National Laboratory, the Relativistic Heavy Ion Collider is sponsored by the U.S. Department of Energy Office of Science, Office of Nuclear Physics.
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