by Anthony Badea
figures by Anastasia Ershova
Long before our world took shape, The Big Bang sent a shockwave of energy irradiating through a violently expanding Universe. In one millionth of a millionth of a second, the primordial fabric of existence transitioned through three distinct phases as the four fundamental forces, electromagnetism, gravity, and the weak and strong forces, took shape. At this point temperatures were still too high for everyday matter such as protons and neutrons to form. Instead, a soup made out of their insides, known as quarks and gluons, permeated throughout space. In the blink of an eye, however, the quark soup cooled, giving rise to the first signs of ordinary matter.
Quark soup, known as quark-gluon plasma, is exactly as its name suggests: a liquid-like material formed out of quarks and gluons, the most basic known building blocks of matter. Currently, physicists don’t understand how larger particles such as protons form from quarks and gluons. Without knowing the details of this process, physicists will never have a complete understanding of how matter that makes up our Universe was constructed in the moments after the Big Bang. Scientists can only attain this knowledge by studying in detail the transition from free quarks and gluons to confined ones inside of larger particles such as protons and neutrons. The only place that quarks and gluons exist as free particles, however, is inside of quark-gluon plasma. This means that the only way to gain insight into the formation process is to study the plasma as it cools to form composite particles.
Though physicists want to study the quark-gluon plasma, obtaining it is another matter entirely. Most scientists theorize that quark soup can only naturally exist in the cores of neutron stars, city-sized stellar objects that are roughly 1.5 times as massive as the sun. The closest neutron star is around 400 lightyears away, however, and it will be technically challenging, if not impossible, to penetrate its billion-degree environment. Therefore, the quark-gluon plasma needs to be produced here on earth to be studied experimentally.
Mini-Big Bangs give you mini-bowls of quark soup
Fast forward a few billion years to the year 2000, when the quark-gluon plasma had long dissipated from our current reaches of the Universe. Soon, though, it would appear in another place–the Relativistic Heavy-Ion Collider (RHIC) in Brookhaven, New York. This nearly billion-dollar 3.8-kilometer accelerator and associated detectors became the first facility capable of producing and detecting quark-gluon plasma. By colliding two gold particles at near the speed of light, physicists were able to briefly observe a droplet of quark-gluon plasma only a billionth of a hair strand wide. This discovery ushered in a new era of humankind’s ability to study the formation of the Universe because, for the first-time, researchers could experimentally investigate an early-universe phase of matter in the lab rather than just on a chalkboard. Since then, physicists have produced hotter, larger, and longer-living droplets of quark soup in order to understand how droplets convert into larger particles.
Today, the droplets produced at the upgraded RHIC and the more advanced Large Hadron Collider (LHC) in Geneva, Switzerland are extraordinary. They reach temperatures a million times that of the core of the sun, have lifetimes of only a trillionth of the time it takes a computer to perform a single operation, and are so dense that if one were to fill a coffee cup with them it would weigh 1 trillion pounds. Essentially, each violent collision that produces quark-gluon plasma is a mini-Big Bang that physicists can photograph using the detectors housed underground at the LHC and RHIC. Unlike the singular actual Big Bang, however, researchers can produce millions of mini-Big Bangs, enabling experimentalists and theorists to test and refine hypotheses leveraging enormous amounts of data.
A few bowls of soup are not enough for hungry physicists
Though these experiments have produced a lot of data and experimental facilities are improving, understanding the physics of the quark-gluon plasma is still not easy because the droplets made in colliders cool too quickly to study directly. Instead, researchers use the particles that are produced when a droplet decays to reconstruct information about the plasma itself. This task is in many ways similar to looking at the aftermath of a car accident to infer what happened during the collision. Though physicists have made great strides in using this analysis over the past twenty years, such as modeling the plasma’s fluid-like behavior with increasing accuracy, there are still many steps necessary to understand how composite particles in the early universe formed.
Luckily, work is already underway to upgrade existing colliders with new experimental apparatuses that will allow physicists to more precisely measure properties of quark soup droplets. In Geneva, the LHC is undergoing upgrades to dramatically increase the amount of collision data produced, enabling researchers to study exotic physics that rarely occur. For example, an increased amount of data would allow detailed studies of the interactions that cause larger particles, such as the Upsilon (ϒ) and J-Psi (J/ѱ), to melt inside of the plasma. At the RHIC in Brookhaven, a state-of-the-art detector known as sPHENIX is under construction to study the microscopic structure of the quark-gluon plasma. And elsewhere physicists are planning the next iterations of experimental colliders, such as the Electron-Ion Collider, to further probe the mechanisms that govern matter. Together these projects, among others, will enable scientists to study quark soup, as well as many other exciting areas of physics such as the inner workings of stars, in a new light.
As physicists attempt to capitalize on these new technologies, they are faced with the question of which research avenues are the most exciting. For some, the answer may be relating the measurements made in labs on Earth to those made by telescopes of objects in space to better understand how the Big Bang gave rise to matter. Others may be drawn to ultra-precise measurements of known phenomena, such as quark-gluon plasma production, to seek out faults in theoretical models. Regardless, the ultimate goal is clear–understanding the basic building blocks of nature. Whether one is an academic who craves a detailed timeline of the universe’s formation, an engineer who wants to utilize this knowledge to manipulate matter, or a scientific aficionado who loves pondering nature’s many puzzles, this endeavor is worthwhile. After all, even after years of scientists cranking away in labs and on chalkboards, there are still many more unanswered questions than there are answered ones–and this is exciting.
Anthony Badea is a first-year Ph.D. student in the Physics department at Harvard University, where he is working in the Laboratory for Particle Physics and Cosmology in association with the ATLAS experiment at the Large Hadron Collider.
Anastasia Ershova is a second-year Ph.D. student in the Biological and Biomedical Sciences program at Harvard University, where she is working on DNA nanotechnology. You can find her on Twitter as @aersh0va.
For More Information:
- Check out this overview of the 30-year adventure of colliding heavy ion particles to produce quark-gluon plasma.
- See this discussion with experimental particle physicists at the LHC to learn more about the quark-gluon plasma and the machines that detect it.
- Watch this video by the national research facility Fermilab to understand better how quark-gluon plasma fits into our picture of matter.
- For a more advanced scientific review of heavy ion collisions, see this article.