More accurately than ever, new experiments can recreate the early universe
when it was a jumble of elementary particles.
Imagine having a microscope that would enable you to closely examine a
single atom. Let's assume that it is an ultra-small hydrogen atom. You can
identify the nucleus—in this example, a single proton—if you zoom in past
the lone electron circulating at the periphery. High school physics would
have you think that this proton is made up of a straightforward quark triad
consisting of two up quarks and one down quark. Physicists are still
striving to understand the interior structure of a proton and how its
constituent parts interact to generate its mass, spin, and other attributes
since the reality within a proton is so much more complicated.
Only the "valence quarks"—buoys floating on top of a turbulent sea of
quarks and antiquarks (their antimatter counterparts), as well as the gooey
"gluon" particles that hold them together—represent the three quarks in the
fundamental model of the inside of a proton. A proton has a variable amount
of quarks and gluons overall. Quark-antiquark couples appear and disappear
often, and gluons frequently divide and proliferate, especially as a proton
speeds up. It's essentially total anarchy. This chaos is contained to the
interiors of protons and neutrons thanks to the strong force, the most
potent of the four basic forces of nature. Unless it doesn't, that is.
The cosmos was too hot and dense in the first minuscule fractions of a
second after the big bang for the strong force to bind quarks and gluons
together. Instead, they transformed into a quark-gluon plasma, a perfect
liquid of particles that flows with practically no resistance. The history
of the universe at this point was brief. Quarks and gluons were trapped into
protons and neutrons in less than 10-6 seconds. The quark-gluon plasma might
potentially be recreated inside particle accelerators 13.7 billion years
later. The temperatures and pressures required for droplets of quark-gluon
plasma to develop, momentarily, before dissolving are created by the
collision of two massive atomic nuclei, such as gold atoms, moving at close
to the speed of light.
Towering structures made of stacked detectors and other sensors placed in
concentric rings and linked by hundreds of cables are used to record these
encounters. Last year, when I went to see two of them at the Brookhaven
National Laboratory's Long Island location, I was astounded by the laborious
process of vast teams of technicians reaching the devices by mounting
numerous levels of scaffolding. These are some of the biggest and most
complex machines ever created, all for the study of a drop of primordial
slime even smaller than an atom. Standing beneath such a colossus is like
witnessing the apex of what humanity can do. Scientists have the opportunity
to discover how matter first came into being by examining quark-gluon plasma
droplets. Approximately 10 microseconds after the great bang, according to
theoretical physicist Bjoern Schenke of Brookhaven, this is what filled the
whole cosmos. We can travel back in time as far as we can by studying
it.
The study also provides insight into the strong force, which is the least
known of all the natural forces. Quantum chromodynamics (QCD), a theory that
describes this force, is so challenging that researchers seldom ever use it
to perform direct calculations. They can only acquire approximative answers
by using supercomputers running simulations. The strong force and quantum
chromodynamics are two concepts that are essential to comprehending nature,
according to scientist Haiyan Gao, assistant laboratory director for nuclear
and particle physics at Brookhaven. To fully comprehend how this theory
operates, quark-gluon plasma experiments are required.
Scientists from Brookhaven will start the most recent experiment to examine
quark-gluon plasma in April 2023. At the lab's Relativistic Heavy Ion
Collider (RHIC), one of the biggest particle accelerators in the world, the
gadget, known as sPHENIX, is one of two detectors. The Solenoidal Tracker at
RHIC (STAR), the other detector there, is also reopening following
significant modifications. The world's largest accelerator, the Large Hadron
Collider (LHC), just started a new run at the European CERN physics facility
in Geneva. It now has updated detectors and the capacity to smash many more
atoms at once. Together, these instruments should provide the most
comprehensive image yet of this primordial fluid, advancing our
understanding of the mysteries behind the smallest building blocks of
matter.
A SURPRISE FINDING
Long before it was ever found, scientists anticipated quark-gluon plasma,
albeit they thought it would look very different. Following the
identification of quarks in the late 1960s and gluons in 1979, the
predictions were realized in the 1970s and 1980s. When liberated from their
nuclear bonds, quarks and gluons were predicted by physicists to manifest as
a uniformly expanding gas. According to Berndt Mueller, a physicist at Duke
University who began developing theoretical models for quark-gluon plasma in
the 1980s, fluids often change to gas as they warm up. It seemed sense that
quarks and gluons wouldn't leave nuclei until temperatures in the billions
of degrees were reached.
Because of the broad range of theoretical possibilities and the impending
arrival of experimental evidence, Mueller was drawn to the field. I was
approximately 30 years old at the time, and you search about for new
projects to work on when there is so much fascinating information to learn.
During this time, scientists were inventing methods to collide heavy ions,
which are nuclei that contain several protons and neutrons. They predicted
that these collisions would produce temperatures and densities that would
shatter subatomic particles. The Super Proton Synchrotron (SPS) accelerator
at CERN started its own heavy ion collisions in 1986, and those produced the
first proof for the new state of matter. The earliest heavy ion collisions,
which took place in the 1970s at Lawrence Berkeley National Laboratory, were
not strong enough to create quark-gluon plasma.
It took some time. The CERN team ultimately made its findings public in
2000, but even at that time there was disagreement among scientists on
whether the data was reliable enough to declare a discovery. The RHIC at
Brookhaven debuted the same year and began smashing heavy ions at energy
greater than those at the SPS. This accelerator gathered enough evidence in
just five years for scientists to proclaim quark-gluon plasma to be
definitively discovered.
They had no idea what it would be like. The quark-gluon plasma appeared
more like a nearly flawless liquid with practically no viscosity than an
expanding gas. Particles move independently in a gas while cohesively in a
liquid. The "better" the liquid is at being a liquid, the greater the
interactions between the particles are—the more they can drag one another
along. The RHIC measurements revealed that quark-gluon plasma has the lowest
flow resistance of any material yet seen. This "was very much unexpected,"
according to Mueller.
The temperature of the quark-gluon plasma was for the first time measured
in 2010, according to RHIC researchers. It was a blistering four trillion
degrees Celsius, almost 250,000 times hotter than the center of the sun and
much hotter than any other material ever made by humanity. According to
Mueller, "things often become less of a perfect fluid the hotter they get."
However, when you get to the critical temperature, it does the reverse and
changes into a liquid. Scientists surmise that this peculiar behavior is
caused by a powerful force. The strong force works across the whole plasma
when the particles are hot enough to break free from protons and neutrons.
This causes the particle mass to interact strongly with one another.
THE STRONG FORCE'S MYSTERY
The precise moment at which the quarks and gluons escape their confinement
is one of the most important unanswered mysteries about quark-gluon plasma.
Where does quark-gluon plasma and ordinary matter meet? Gao queries. Where
is the alleged crucial region where the quark-gluon plasma and nuclear
matter coexist? One of the primary objectives of the new and improved tests
will be to determine when and where that transition occurs as well as the
minimum number of particles required to start the collective behavior.
Whether quark-gluon plasma is a fractal, or if its structure has a
complicated, repeating pattern that seems the same at any size, whether you
zoom in or out, is another matter of debate. These two characteristics of
quark-gluon plasma, according to some researchers, might be explained by
fractal theory, which may also provide light on the plasma's behavior.
Quark-gluon plasma appears to have fractal structure, claims Airton Deppman
of the University of So Paulo's Institute of Physics. We are also looking at
whether the fractal structure endures the plasma to proton phase
change.
Understanding the strong force, the most perplexing of nature's fundamental
forces, may be improved with the aid of the answers to these questions. The
interactions between quarks and gluons are described by quantum
chromodynamics by giving them a characteristic termed color charge. It also
explains why quantum chromodynamics spirals out of control so fast. In the
theory of electromagnetism, color charge is analogous to electrical charge.
Unlike electromagnetism, which only has positive or negative charges, QCD
has three different charges: red, green, or blue. Furthermore, antimatter
particles can be charged in antired, antigreen, or antiblue.
The electromagnetic force is carried by a particle called a photon in
electromagnetism, which makes things very straightforward. But in QCD, the
force carrier, the gluon, also has a color charge and may interact with both
itself and quarks through the strong force. QCD has become unreasonably
difficult due to these self-interactions and additional fees. The idea can
be expressed in simply two lines, although Schenke claims that the problem
still hasn't been fully solved. It has not yet been figured out how quarks
and gluons, for instance, get confined in the proton during the confinement
process.
Researchers believe that investigating quark-gluon plasma, the only
environment in which they have ever been able to investigate unbound quarks,
would help them understand confinement better. Freeing them and seeing how
they recombine into protons, neutrons, and other particles that we can see
from the detector is one method to discover that, says Schenke. As a result,
the mechanisms in QCD that cause confinement may be better understood using
experimental data from heavy-ion collisions.
NEW AND BETTER
The improved STAR detector and the brand-new RHIC experiment sPHENIX should
allow researchers to monitor the plasma with the highest level of accuracy
ever. For instance, the superconducting magnet on sPHENIX is nearly three
times more powerful than the one on STAR. According to Brookhaven physicist
David Morrison, "that's critical for many of the things we want to measure."
"When particles collide, their courses are bent by magnetic fields because
they shoot out in all directions. We can take a look at that to begin
determining the type of particle it was and how much energy and momentum it
had. For instance, the team is looking for composite particles called
upsilons. Upsilon particles, which are made up of a bottom quark and an
anti-bottom quark, can arise in collisions and then fly through the
quark-gluon plasma as test probes to see how the plasma affects them. He
continues, "We can actually untangle the physics underpinning many of the
peculiar features of the quark-gluon plasma.
The experiment will also gain from the ability to capture far more data
than was previously feasible, which entails recording many more collisions
and the particles they produce. Around 10 petabytes of data are collected
yearly by STAR; 150 petabytes will be collected annually by sPHENIX. With
such growth, hitherto unanswerable questions will become possible.
STAR also possesses cutting-edge capabilities, including brand-new
calorimeters for calculating particle energies and tracking detectors for
recognizing particles with various electrical charges. The "forward"
detectors, which can capture particles flying out of collisions at greater
angles than before, including particles going in the same direction as the
beams that flowed into the crash, are among the most important innovations,
according to Lijuan Ruan of Brookhaven, one of STAR's spokesmen. Ruan, who
has been working on STAR for many years and helped to create some of its
first components about 20 years ago as a doctoral student, says, "Now that's
basically it—we're not going to update anymore." When you really construct a
detector that the entire cooperation can use, it's a different experience
from when you only use one, she claims. "I feel honored," Before it shuts
down, STAR, one of the initial RHIC experiments that contributed to the
identification of quark-gluon plasma, will continue to run for a further
three years.
The LHC's third phase, which started in July 2022 and will last until 2025,
just got going in Europe. Approximately 100 times as many lead-lead
collisions may now be studied at the LHC thanks to the most recent
enhancements. The additional collisions will also improve measurement
accuracy. Understanding the start of the quark-gluon plasma and how it
progressively develops is one of the key objectives for run three, according
to Luciano Musa, an ALICE experiment participant at the LHC.
The LHC collisions take place at greater energy than the RHIC experiments
and result in a hotter, denser, and more durable quark-gluon plasma. In
addition to producing a wider range of particles, these violent collisions
also produce information on the plasma's characteristics. According to Musa,
"the research at RHIC and LHC actually complement one other." Every time a
discovery is made at the LHC, the RHIC team investigates if the same
phenomena have been seen at lower energies.
Different facets of the plasma are shown by the various energy ranges.
Vanderbilt University physicist Raghav Kunnawalkam Elayavalli completed
their doctoral work at the LHC, but more recently joined the STAR and
sPHENIX teams to concentrate on the particles emerging from lower-energy
collisions. According to Kunnawalkam Elayavalli, "They are closer to the
size of the plasma; they talk to it a lot more." "Imagine it as a party
where there are plenty of people and you're running for the door. However,
if you take your time and don't want to go right away, you have the
opportunity to chat with individuals as you depart. They can get more
information from the quark-gluon plasma at RHIC because particles have to go
through it more slowly. The average distance a particle may travel without
interacting with another particle is what scientists call the "transport
characteristics," they continue. It provides information about the plasma's
basic scale.
GOING BACK TO THE START
Quark-gluon plasma research is entering a new age that ought to take it
beyond the fundamentals and toward real solutions to open problems. At RHIC,
there came a time when physics was essentially, "Wow, this is happening—this
is new physics," according to Kunnawalkam Elayavalli. "And right now, we're
in the era of accuracy. We might inquire, "Why is this taking place?
The attempt to comprehend this unique form of matter is being led by RHIC
and the LHC, but planned experiments elsewhere will also provide insights.
In addition to the LHC, the SPS accelerator is still operational at CERN.
The critical point at which protons and neutrons transform into quark-gluon
plasma will be determined by colliding moving ions onto a stationary target
in the NA61/SHINE experiment, which is scheduled to take place there. The
Facility for Antiproton and Ion Research (FAIR) at GSI Darmstadt in Germany
is slated to launch a second fixed-target experiment in 2028. Additionally,
a collider known as the Nuclotron-based Ion Collider Facility (NICA) at the
Joint Institute for Nuclear Research in Dubna, close to Moscow, will also
test the critical point.
It's a thrilling period, adds Mueller. We are aware that the quark-gluon
plasma was there in the early cosmos, but we lack the means to investigate
it. This is how we can investigate a physical state that would be impossible
to attain otherwise.
