Quark Soup

Physicists create conditions not seen since the big bang.

While the Large Hadron Collider gets all the attention (it never hurts a physics experiment's street cred when rumors spread that it might create a mini black hole and swallow up the Earth), a lesser-known particle collider has been quietly making soup—quark soup. For the field of experimental particle physics, in which progress has been at a near-standstill since the glory days of the 1970s (yes, the top quark was discovered in an experiment at Fermilab in 1995, but really, everyone knew this last of the six quarks existed), this counts as the most notable achievement in years: a discovery that doesn't merely confirm what theory has long held, but points the way to new revelations about the creation and evolution of the universe.

The reason for that accolade is that quark soup was last seen when the universe was 1 microsecond old, physicists reported at the annual meeting of the American Physical Society. It was created at the 2.4-mile-around Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab on New York's Long Island, which smashes together gold ions traveling at nearly the speed of light. The result of the collisions is a tiny region of space so hot—4 trillion degrees Celsius—that protons and neutrons melt into a plasma of their constituent quarks and gluons, as Brookhaven describes here. The soup is 250,000 times hotter than the center of the sun, 40 times hotter than a typical supernova, and the hottest temperature in the universe today. Right there on Long Island. (For anyone wondering what kind of thermometer is used to measure a 4-trillion-degree soup, it is color: by analyzing the energy distribution (color) of light emitted from the soup, scientists can infer its temperature much as they infer the temperatures of stars or even of a glowing andiron.) In one of the truly helpful advances since the golden age of particle physics, several cool simulations of the RHIC collisions and resulting quark soup are on YouTube.

The last time such a quark-gluon plasma existed was 13.7 billion years ago, when the universe burst into existence in the big bang. By creating it in a lab for the first time, the RHIC teams have given scientists a chance to study how the cosmos came to evolve into the riot of galaxies and nebulae that we see today. And although the quark soup created at RHIC lasts not even 1 billionth of a trillionth of a second, there are already surprises. The quarks and gluons in the soup were expected to behave independently, for instance, but instead they behave cooperatively, almost like synchronized swimmers—or, in the spirit of the moment, like Olympic pairs skaters.

The behavior that has most intrigued the scientists so far is something called broken symmetry (of which there is a nice video here. Within the quark soup appear "bubbles" that violate a principle of physics called mirror symmetry, or parity. This form of symmetry means that events—in this case, the collisions of particles and the spray of subatomic debris that results—look the same if viewed in a mirror as they do when viewed directly. But one of the detectors monitoring the collisions inside RHIC observed an asymmetry in the electric charges of particles emerging from most of the collisions. Specifically, positively charged quarks seem to prefer to fly out of the collision parallel to the magnetic field, while negatively charged quarks prefer to emerge in the opposite direction. This behavior would appear reversed if reflected in a mirror, with negative quarks traveling parallel to the magnetic field and positive quarks traveling in the opposite direction. Hence the violation of mirror symmetry.

The quark soup also seems to contain bubbles that violate another form of symmetry, called charge-parity invariance. According to this bedrock principle of physics, when energy is converted to mass or vice versa as per Einstein's E=mc2, equal numbers of particles and antiparticles—matter and antimatter—are created or annihilated, respectively. That may seem like an abstruse point, but it may hold the key to how structure and form emerged from the otherwise homogeneous quark soup. Such symmetry-violating bubbles in the nascent universe, cosmologists suspect, tipped balance in the sea of otherwise equal amounts of matter and antimatter toward a preference for matter over antimatter. If the amounts of matter and antimatter had remained identical, no one would be here to notice: when a particle of matter encounters a particle of antimatter, they go poof in an annihilating burst of energy. By now, almost 14 billion years after creation, every particle of matter would have been destroyed through this process, leaving a universe awash in radiation and nothing else, an ethereally glowing world of light without substance. By re-creating conditions that last existed at the birth of the universe, says Steven Vigdor, Brookhaven's associate laboratory director for nuclear and particle physics, who oversees research at RHIC, "RHIC may have a unique opportunity to test in the laboratory some crucial features of symmetry-altering bubbles speculated to have played important roles in the evolution of the infant universe."

Previous experiments have found violations of charge-parity symmetry (a 1964 experiment discovering such violations brought the scientists who conducted it a Nobel Prize), but in each case the effect was too small to account for the amount of matter in the universe today. What RHIC found is "consistent with predictions of symmetry-breaking domains in hot quark matter," said Vigdor. "Confirmation of this effect and understanding how these domains of broken symmetry form at RHIC may help scientists understand some of the most fundamental puzzles of the universe." For a field that has been in the doldrums (especially in the United States) since the cancellation of the Superconducting Super Collider, and that seems plagued by gremlins (as when the Large Hadron Collider sprang a helium leak, particle physics really needed this one.