Whirling particles are fastest ever detected

Rice physicists help find speediest vortex in ‘perfect liquid’ plasma

The spin of the strongest tornado is a pale shadow of the vortex seen at the heart of matter colliding into a nearly perfect fluid, according to researchers at Rice University and their colleagues at Brookhaven National Laboratory.

Tracking particle spins reveals that the quark-gluon plasma created at the Relativistic Heavy Ion Collider spins faster than the cores of supercell tornadoes, Jupiter’s Great Red Spot or any other fluid, according to researchers at Brookhaven National Laboratory. Courtesy of Brookhaven National Laboratory

The collaboration that operates the STAR detector, part of the Relativistic Heavy Ion Collider (RHIC) at the New York facility, reported in a cover story in Nature this week that collisions between gold ions produce a quark-gluon plasma (QGP) that swirls faster than any other fluid ever observed.

The phenomenon lasts only a fraction of a millionth of a second, but while it does, particles in the frictionless fluid described by physicists as “nearly perfect” spin 10 billion trillion times faster than the most powerful tornado, according to Brookhaven researchers.

The low viscosity in the QGP allowed the vorticity to persist. “Viscosity destroys whirls,” said Michael Lisa, a member of the STAR collaboration from Ohio State University. “With QGP, if you set it spinning, it tends to keep on spinning.”

The plasma of the combined quarks and gluons is hundreds of thousands of times hotter than the center of the sun, the researchers reported.

“We’re usually interested in having these heavy ions collide as head-on as possible, but we’ve come to realize that if we look at the off-center incursions, there are a lot of interesting things happening,” said Frank Geurts, an associate professor of physics and astronomy at Rice and deputy spokesperson of the STAR collaboration.

This shows telltale signs of a lambda hyperon decaying into a proton and a pion as tracked by the Time Projection Chamber of the STAR detector. Because the proton comes out nearly aligned with the hyperon’s spin direction, tracking where these “daughter” protons strike the detector can be a stand-in for tracking how the hyperons’ spins are aligned. Courtesy of Brookhaven National Laboratory

Because collisions between the heavy ions that travel at nearly the speed of light are off-center, the nucleons on the extremities continue on almost as if nothing had happened. But those closer to the impact may create a QGP that will take on a substantial angular momentum, Geurts said. Particles that form as the plasma cools are expected to spin in a manner that is, on average, aligned with this angular momentum.

Rice scientists and their students designed, led construction of and continue to operate and calibrate the time-of-flight detector that is part of STAR (short for Solenoidal Tracker at RHIC). This detector plays an important role in this discovery, as it helps identify particles flying away from the collision.

“The STAR experiment is built around a big gas detector, called the Time Projection Chamber, which helps track the particles that emerge from the whirling plasma,” Geurts said. “Together with the time-of-flight detector, particles can be accurately identified.

Lambda particles are of particular interest. These particles align their spin with the QGP’s global spin. As lambdas decay, they pass that information along to their daughter particles, including protons.

Frank Geurts

“Measuring the protons’ emission angles tells us that, indeed, with respect to the event plane in which heavy ions collide, on average all lambda spins seem to be aligned,” Geurts said. “That agrees with a picture in which the original lambda particles have been emitted from a whirling system.”

The lab’s work is far from done, as it hopes future experiments will better measure the plasma’s vorticity, as well as measure the massive magnetic field created by these collisions, which they estimate may be the largest ever seen.

“There’s a lot that needs to be done, because we think this is an opening to looking at fundamental symmetries in nature,” Geurts said. “Our group feels its responsibility to make sure the detector is able to deliver exciting physics. And we see here again a very exciting result.”

Along with Geurts, Rice co-authors of the Nature paper are graduate students Daniel Brandenburg and Joseph Butterworth, postdoctoral researcher David Tlusty, research scientist Geary Eppley and Jabus Roberts, a professor of physics and astronomy.