Nav: Home

Heavy particles get caught up in the flow

May 30, 2017

Upton, NY - By teasing out signatures of particles that decay just tenths of a millimeter from the center of a trillion-degree fireball that mimics the early universe, nuclear physicists smashing atoms at the Relativistic Heavy Ion Collider (RHIC) are revealing new details about the fundamental particles that make up our world.

Particle collisions at RHIC--a U.S. Department of Energy (DOE) Office of Science User Facility located at DOE's Brookhaven National Laboratory--regularly recreate tiny specs of quark-gluon plasma (QGP), a mixture of quarks and gluons, the fundamental building blocks of visible matter, which last existed as free particles some 14 billion years ago. The collisions free the quarks and gluons from their confinement within ordinary particles (e.g., protons and neutrons) so nuclear physicists can study their interactions and the force that holds them together in the universe today.

The new measurements, described in a paper just published in Physical Review Letters, are the first to come from a precision upgrade to RHIC's STAR detector known as the "Heavy Flavor Tracker" (HFT). Specifically, the paper gives details about the first direct measurement at RHIC of how a type of heavy particle containing a "charm" quark gets caught up in the flow of the expanding fireball. This measurement--a testament to the capabilities of the HFT--gives scientists a new window into understanding the interactions of the particles that make up the subatomic soup.

"By comparing our measurements with theoretical predictions that include the various parameters that play a role in these interactions--things like the diffusion coefficient (how quickly the charm quarks spread throughout the plasma) and viscosity (how sticky the QGP is)--we can learn about how these different properties relate to one another, and ultimately why the QGP behaves the way it does," said Brookhaven physicist Flemming Videbaek, the project manager responsible for the overall fabrication of the STAR HFT.

Precision particle tracking

Particles containing heavy quarks are considered ideal probes for understanding quark-gluon plasma because they may interact differently with the plasma than light quarks do, offering up subtle clues about its properties. But the QGP spits out particles containing heavy quarks only rarely, amid thousands of other particles made of the lighter varieties of quarks. The few heavy particles that do emerge decay into other particles almost instantly--mere fractions of a millimeter from the QGP fireball in which they were created. This rarity and rapid decay make heavy particles difficult to detect.

STAR's HFT, a state-of-the-art tracking device now sitting at the center of the house-sized detector, was designed to track the elusive but important heavy particles. Developed by nuclear physicists at Lawrence Berkeley National Laboratory, the HFT is the first silicon detector at a collider that uses Monolithic Active Pixel Sensor technology--the same technology used in digital cameras. The ultrathin sensors--unlike many of the particle detection components of STAR--sit very close to the central beampipe in which the collisions take place. While not quite close enough to detect the heavy charm quark itself, this location and the detector's high resolution (360 million pixels measuring 20 x 20 microns each) allow it to pick up signs of the heavy particles' decay.

For this particular study, STAR physicists were tracking particles called kaons and pions that emerge when charm-quark-containing particles known as a D-zeros decay. A concerted effort from many groups of the collaboration--including researchers from Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, Kent State University, and the University of Illinois at Chicago--made this analysis successful in a short time.

"We use the HFT to look for kaons and pions that are very close to one another-- within fractions of a millimeter of one another--whose paths from the collision emerge from a single point that's away from the collision vertex, but not very far, about 100-500 microns," Videbaek said. That's the distance D0s travel before they decay, he explained. If the kaon and pion have just the right mass and trajectories emerging from such a point, the scientists can conclude that they originated from a D0 at that spot--and use these measurements to track the emergence of D0s from all around the QGP.

"The precision of our measurement is unprecedented," said Xin Dong, a physicist at Berkeley Lab who led the postdocs and students conducting the physics analysis on the heavy flavor results. "It was extremely challenging due to interference from thousands of other particles produced in the same heavy ion collisions--a bit like picking a needle out of a haystack."

Free flowing interactions

The findings--based on an analysis of tens of thousands of such "needles" in 1.1 billion collisions--were somewhat surprising.

Think of the shape created when two spherical gold ions collide off center forming an oblong overlap--something like a football standing on end. STAR physicists found more D0s emerging from the fat part of the "football" than from its pointy ends. This pattern of "elliptic flow" was familiar from measurements of lighter particles emerging from the QGP. But nuclear physicists didn't initially expect such heavy particles to get caught up in the flow.

"D0s are created in the very first part of the collision, when the quarks and gluons are free," Videbaek said. "Physicists didn't think these heavy-quark particles would have time to interact, or equilibrate, with the QGP, which exists for only an infinitesimally small fraction of a second."

Instead, the fact that the heavy quarks exhibit the same elliptic flow as lighter particles do is evidence that they are in equilibrium, interacting with the free quarks and gluons in the QGP.

"The type of flow we observed for particles with heavy quarks suggests that their interactions inside the quark-gluon plasma are so strong that the heavy quarks themselves become part of the quark-gluon 'soup,'" said Dong.

Grazyna Odyniec, leader of Berkeley Lab's Relativistic Nuclear Collisions Program, added, "The discovery of the elliptic flow of a very massive charm quark is of fundamental importance for our understanding of quark-gluon plasma phase dynamics. It opens up a broad range of theoretical speculations about the nature of a possible mechanism (or mechanisms) behind this observation."

Brookhaven Lab physicist and STAR collaboration spokesperson Zhangbu Xu noted that the ability to track the flow and diffusion of the heavy particles gives nuclear physicists a new way to "see" and study the interactions of the freely moving quarks and gluons and other properties of the QGP--somewhat analogous to the way scientists from the last century tracked the vibrations of pollen grains in water to learn about its properties.

"Einstein proved in 1905 that atoms and molecules exist, and that we could use the so-called Brownian motion of pollen grains to measure the properties of the fluid and other fundamental physics constants," Xu said. "Now we can use the charm quarks like the pollen grains to measure the flow and other properties of the QGP."
-end-
This research was supported by the DOE Office of Science and by all the agencies and organizations that support research at STAR.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

DOE/Brookhaven National Laboratory

Related Plasma Articles:

How bacteria protect themselves from plasma treatment
Considering the ever-growing percentage of bacteria that are resistant to antibiotics, interest in medical use of plasma is increasing.
A breakthrough in the study of laser/plasma interactions
Researchers from Lawrence Berkeley National Laboratory and CEA Saclay have developed a particle-in-cell simulation tool that is enabling cutting-edge simulations of laser/plasma coupling mechanisms.
Researchers turn liquid metal into a plasma
For the first time, researchers at the University of Rochester's Laboratory for Laser Energetics (LLE) have found a way to turn a liquid metal into a plasma and to observe the temperature where a liquid under high-density conditions crosses over to a plasma state.
How black holes power plasma jets
Cosmic robbery powers the jets streaming from a black hole, new simulations reveal.
Give it the plasma treatment: strong adhesion without adhesives
A Japanese research team at Osaka University used plasma treatment to make fluoropolymers and silicone resin adhere without any adhesives.
Chemotherapeutic drugs and plasma proteins: Exploring new dimensions
This review provides a bird's eye view of interaction of a number of clinically important drugs currently in use that show covalent or non-covalent interaction with serum proteins.
The coming of age of plasma physics
The story of the generation of physicists involved in the development of a sustainable energy source, controlled fusion, using a method called magnetic confinement.
Intense microwave pulse ionizes its own channel through plasma
More than 30 years ago, researchers theoretically predicted the ionization-induced channeling of an intense microwave beam propagating through a neutral gas (>103 Pa) -- and now it's finally been observed experimentally.
Plasma thruster: New space debris removal technology
A Japanese and Australian research group has discovered new technology to remove space debris using a single propulsion system in a helicon plasma thruster.
Separating the sound from the noise in hot plasma fusion
For fusion power plants to be effective, scientists must find a way to trigger the low-to-high confinement transition, associated with zonal flows of plasma.
More Plasma News and Plasma Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

Risk
Why do we revere risk-takers, even when their actions terrify us? Why are some better at taking risks than others? This hour, TED speakers explore the alluring, dangerous, and calculated sides of risk. Guests include professional rock climber Alex Honnold, economist Mariana Mazzucato, psychology researcher Kashfia Rahman, structural engineer and bridge designer Ian Firth, and risk intelligence expert Dylan Evans.
Now Playing: Science for the People

#540 Specialize? Or Generalize?
Ever been called a "jack of all trades, master of none"? The world loves to elevate specialists, people who drill deep into a single topic. Those people are great. But there's a place for generalists too, argues David Epstein. Jacks of all trades are often more successful than specialists. And he's got science to back it up. We talk with Epstein about his latest book, "Range: Why Generalists Triumph in a Specialized World".
Now Playing: Radiolab

Dolly Parton's America: Neon Moss
Today on Radiolab, we're bringing you the fourth episode of Jad's special series, Dolly Parton's America. In this episode, Jad goes back up the mountain to visit Dolly's actual Tennessee mountain home, where she tells stories about her first trips out of the holler. Back on the mountaintop, standing under the rain by the Little Pigeon River, the trip triggers memories of Jad's first visit to his father's childhood home, and opens the gateway to dizzying stories of music and migration. Support Radiolab today at Radiolab.org/donate.