A closer look inside nature’s perfect fluid

Berkeley Lab research brings us closer to understanding how our universe began.


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How the quark-gluon plasma evolved into the matter?

A new study offered clues to solve this puzzle.

After few seconds of the Big Bang, the universe underwent a strange state of subatomic soup called the quark-gluon plasma. The quark-gluon plasma is also known as nature’s perfect fluid.

A decade ago, it was discovered that the quarks and gluons in the quark-gluon plasma are so strongly coupled that they flow almost friction-free. There are highly energetic jets of particles that fly through the quark-gluon plasma faster than the velocity of sound. These jets also generate a supersonic boom called a Mach wave.

In 2014, scientists used an atomic X-ray imaging technique called jet tomography to study the properties of these jet particles. The results demonstrate that these jets scatter and lose energy as they propagate through the quark-gluon plasma.

But from where did the journey of jet particles begin?

According to scientists, a smaller Mach wave signal called the diffusion wake could give a clue. But, while the energy loss was easy to observe, the Mach wave and accompanying diffusion wake remained elusive.

This time-lapse video clip shows a supersonic Mach wave as it evolves in an expanding quark-gluon plasma. The computer simulation provides new insight into how matter formed during the birth of the early universe. (Credit: Berkeley Lab)

A new study by the scientists from Berkeley Lab suggests that another technique they invented called 2D jet tomography can help locate the diffusion wake’s ghostly signal.

Study leader Xin-Nian Wang, a senior scientist in Berkeley Lab’s Nuclear Science Division, said, “Its signal is so tiny, it’s like looking for a needle in a haystack of 10,000 particles. For the first time, our simulations show one can use 2D jet tomography to pick up the tiny signals of the diffusion wake in the quark-gluon plasma.”

To find that signal, scientists culled through several lead-nuclei collision events simulated at the Large Hadron Collider (LHC) at CERN and gold-nuclei collision events at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. They also performed some computer simulations at Berkeley Lab’s NERSC supercomputer user facility.

Wang said, “Their unique approach “will help you get rid of all this hay in your stack—help you focus on this needle.” The jet particles’ supersonic signal has a unique shape that looks like a cone—with a diffusion wake trailing behind, like ripples of water in the wake of a fast-moving boat. Scientists have searched for evidence of this supersonic “wakelet” because it tells you that particles are depleted. Once the diffusion wake is located in the quark-gluon plasma, you can distinguish its signal from the other particles in the background.”

“Their work will also help experimentalists at the LHC, and RHIC understands what signals to look for in their quest to understand how the quark-gluon plasma—nature’s perfect fluid—evolved into the matter.”

“What are we made of? What did the infant universe look like in the few microseconds after the Big Bang? This is still a work in progress, but our simulations of the long-sought diffusion wake get us closer to answering these questions.”

Journal Reference:
  1. Wei Chen et al., Search for the Elusive Jet-Induced Diffusion Wake in Z/γ -Jets with 2D Jet Tomography in High-Energy Heavy-Ion Collisions, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.127.082301


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