Scientists have reported new clues to solving a cosmic conundrum: How the
quark-gluon plasma—nature's perfect fluid—evolved into matter.
A few millionths of a second after the Big Bang, the early universe took on
a strange new state: a subatomic soup called the quark-gluon plasma.
And just 15 years ago, an international team including researchers from the
Relativistic Nuclear Collisions (RNC) group at Lawrence Berkeley National
Laboratory (Berkeley Lab) discovered that this quark-gluon plasma is a
perfect fluid—in which quarks and gluons, the building blocks of protons and
neutrons, are so strongly coupled that they flow almost friction-free.
Scientists postulated that highly energetic jets of particles fly through
the quark-gluon plasma—a droplet the size of an atom's nucleus—at speeds
faster than the velocity of sound, and that like a fast-flying jet, emit a
supersonic boom called a Mach wave. To study the properties of these jet
particles, in 2014 a team led by Berkeley Lab scientists pioneered an atomic
X-ray imaging technique called jet tomography. Results from those seminal
studies revealed that these jets scatter and lose energy as they propagate
through the quark-gluon plasma.
But where did the jet particles' journey begin within the quark-gluon
plasma? A smaller Mach wave signal called the diffusion wake, scientists
predicted, would tell you where to look. But while the energy loss was easy
to observe, the Mach wave and accompanying diffusion wake remained elusive.
Now, in a study published recently in the journal Physical Review Letters,
the Berkeley Lab scientists report new results from model simulations
showing that another technique they invented called 2D jet tomography can
help researchers locate the diffusion wake's ghostly signal.
"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," said study leader Xin-Nian Wang, a senior scientist in
Berkeley Lab's Nuclear Science Division who was part of the international
team that invented the 2D jet tomography technique.
To find that supersonic needle in the quark-gluon haystack, the Berkeley Lab
team culled through hundreds of thousands of 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. Some of the computer simulations for the current study
were performed at Berkeley Lab's NERSC supercomputer user facility.
Wang says that 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 there is a depletion of particles. 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 understand
what signals to look for in their quest to understand how the quark-gluon
plasma—nature's perfect fluid—evolved into 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," he
said.
Reference:
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