Nestled into an atom smasher, a state-of-the-art particle tracker reveals unusual details about the building blocks of our world. The tracker offers the first direct measurement, at the Relativistic Heavy Ion Collider (RHIC), of how one of those building blocks gets caught up in the flow of a primordial soup. These blocks are heavy particles containing charm quarks. The soup, called a quark-gluon plasma, is a liquid-like matter that mimics the conditions of the early universe moments after the Big Bang. These findings suggest that the particles interact directly with other types of particles that make up the plasma. The results should help scientists make precise measurements of the plasma's properties. The measurements create a window into the early universe.
Physicists create quark-gluon plasma by steering the nuclei of heavy atoms, such as gold, into head-on collisions at RHIC. They didn't think heavy-quark particles would have time to interact with the plasma. Why? Because the plasma exists for only an infinitesimally small fraction of a second. The ability to track heavy particle interactions with the plasma will help scientists understand the properties and different phases of this unique form of nuclear matter.
Particle collisions at RHIC free quarks and gluons from their confinement within ordinary particles (e.g., protons and neutrons) so nuclear physicists can study particle interactions and the force that holds them together in the matter that makes up the visible parts of the universe today. By studying how different types of particles (e.g., light and heavy quarks) interact with the plasma, scientists can characterize its properties, learn how those properties relate to one another, and perhaps how they arise from those fundamental interactions.
The Heavy Flavor Tracker component of RHIC's STAR (Solenoidal Tracker at RHIC) experiment makes it possible to detect these heavy-quark-containing particles, which emerge from RHIC's collisions only rarely—like needles in a haystack—amid thousands of other particles made of lighter varieties of quarks. The tracker's ultrathin high position resolution silicon sensors sit very close to the central beam pipe in which particle collisions take place. The location makes it possible to track the decay products of particles containing heavy charm quarks.
By analyzing tens of thousands of these particles in 1.1 billion collisions, STAR physicists have concluded that heavy quarks exhibit the same characteristics of a flowing fluid as lighter particles do. This is evidence that the heavy particles are, surprisingly, moving along with and interacting in equilibrium with the free quarks and gluons in the plasma. Nuclear physicists will now track the flow and diffusion of these charm quarks to study detailed properties of the quark-gluon plasma—similar to the way scientists from the last century tracked the vibrations of pollen grains in water to learn about that liquid's unique properties.
STAR co-spokesperson, Brookhaven National Laboratory
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This research was supported by the Department of Energy (DOE), Office of Science, Nuclear Physics and by all the agencies and organizations that support research at STAR. The researchers used resources at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility.
L. Adamczyk, et al. (STAR Collaboration), "Measurement of D0 azimuthal anisotropy at midrapidity in Au + Au collisions at ?sNN=200 GeV." Physical Review Letters 118, 212301 (2017). [DOI: 10.1103/PhysRevLett.118.212301]
Brookhaven National Laboratory press release: Heavy Particles Get Caught Up in the Flow
Lawrence Berkeley National Laboratory article: Heavy Flavor Tracker for STAR