“Oh, I got plenty o’ nuthin’, an’ nuthin’s plenty for me…”.
—”Porgy and Bess” George Gershwin, 1935
Bright opposing beams of gold ions finally were playing chicken within the tunnels of a vast new particle collider on Long Island. The scientists in the control room were delighted to have their machine up and running after 9 years of construction, but they also felt let down. They had expected to observe brilliant splats from head-on collisions between ions. During the 2 hours allotted for their run, they had seen zilch.
It was 8:30 p.m. on Monday, June 12, and even the die-hards were starting to grow weary. Should they pull the plug on this run? Not yet, argued one contingent, which somehow sensed that the hulking particle accelerator—the Relativistic Heavy Ion Collider, or RHIC—was quietly doing better than it seemed.
If that were the case, however, the detector was somehow missing the show, perhaps because it had been set to see less than a thousand charged particles spew from collisions rather than more. The scientists reset the detector.
In minutes, a fabulous starburst of particle tracks splashed across the large screen on the wall. The room filled with applause. “We were exuberant when we saw the first collision,” recalls John W. Harris of Yale University, head of the team that created the huge detector called STAR. Other bursts immediately followed. “All of our frustrations just melted away,” says Harris.
With the new accelerator, which is located at Brookhaven National Laboratory in Upton, N.Y., physicists are launching what they hope will be the decisive stage of an audacious quest that began in the 1980s (SN: 3/28/87, p. 202). The aim is nothing short of creating a primordial state of matter called the quark-gluon plasma.
In that extraordinary plasma, elementary particles, called quarks and gluons, roam freely like particles in a gas instead of being shackled together into protons and neutrons, as they usually are. Theorists say that quark-gluon plasma filled the infant universe during its first 10 or so microseconds and then cooled into matter.
To make the plasma, scientists say, they intend to melt a tiny chunk of empty space. That idea may seem bizarre, but to physicists, empty space is not really empty. In fact, they see it as the most complex and inscrutable entity in the universe. So, from this vacuum, they say, they should be able to conjure up something else remarkable, such as the quark-gluon plasma.
As that plasma recondenses into ordinary matter, researchers expect it to provide a new and telling window on the microcosms of quarks and gluons that make up each proton and neutron in the universe. The structure of those microcosms is almost as difficult to understand as the vacuum itself, physicists say.
“If we find out when we heat up the vacuum that the properties change dramatically in some way—like water changing into steam when we boil it—then that would be very interesting,” says theorist Mark G. Alford of the Massachusetts Institute of Technology.
Some answers to cosmological secrets may emerge from RHIC, too. After all, the cooling off of each collision roughly replays, in miniature, the instant when the plasma in the early universe rapidly coalesced into the constituents of ordinary matter. If those collisions reveal any irregularities in this plasma-to-matter transition, they may shed new light on the formation of the first magnetic fields, the first elements, and, possibly, a type of undetected dark matter known as strange matter.
Continual exchange
Each proton or neutron incorporates three quarks. A continual exchange of gluons, which are the carriers of what physicists call the strong force, binds those quarks together as they bob on a sea of so-called virtual quarks that wink in and out of existence. Like a rubber band connecting two balls, the strong force holds quarks together, becoming stronger as the quarks stray apart and weaker as they come together.
Scientists have been frustrated in their efforts to explore the insides of nucleons—a term for protons and neutrons—in their native states. The huge energies required for these experiments excite the quarks and gluons too much.
One of the attractions of RHIC, says Brookhaven theorist Larry McLerran, is that although the total energy of a typical collision is high, the energy is distributed among many thousands of quarks. That means the energy per quark in the resulting plasma is close to that inside the intact proton or neutron. So, the collider may provide an exceptionally revealing way to peer inside the stuff of which everything is made.
To create the plasma, the new collider first strips gold atoms of all electrons, splits them into two beams circling in opposite directions, and accelerates both beams to more than 99.99 percent the speed of light. At six points around a 3.8-kilometer tunnel, the beams intersect. There, the ions, which are flattened into pancakes by relativistic effects, slam together with gargantuan force and then pass through one another.
This collision of two ions, each of which contains 197 protons and neutrons, is like “two swarms of angry bees hitting each other,” says Brookhaven’s Thomas W. Ludlam. “They hit each other, have a fight, and then move on.”
That buzz of colliding nucleons is as dense in energy as 100 nuclei packed into the space of one. Its temperature is expected to soar to 100,000 times that of the sun’s core. Under those extreme conditions, the strong binding that holds a nucleon together should relax enough for a moment for quarks and gluons to roam freely, theorists predict.
Not that this will be easy to detect. Scientists have come up with many possible signatures of the quark-gluon plasma, but none of these alone can be considered unambiguous proof of the plasma’s presence. One clue would be a sudden outpouring of thousands of hadrons, which are particles made up of quarks and anti-quarks. Others would be specific patterns in the abundances of the different hadron types.
At a seminar at Brookhaven on July 19, Wit Busza of MIT and his colleagues working at one of the collider’s detectors, PHOBOS, reported the first scientific results from RHIC. From their measurements, the group calculates that RHIC’s most energetic collisions so far have been ejecting between 6,000 and 7,000 particles apiece. Previously, the most energetic heavy ion collisions had taken place in Geneva at the European Laboratory for Particle Physics, or CERN. Those collisions generated about 2,500 particles, Busza says.
To give as complete a picture of the collisions as possible, each of RHIC’s four detectors measures the debris from gold-gold collisions in a different way. “Although we have pretty good models, no one knows exactly what to expect,” Ludlam says.
Even as scientists at RHIC strive toward sightings of the quark-gluon plasma, they may be retracing steps already taken by CERN physicists. Early this year, research teams at the European lab announced that they may already have caught a glimpse of the plasma. (SN: 2/19/00, p. 117: Melting nuclei re-create Big Bang broth). Still, many physicists are counting on the immensely powerful RHIC to bring the plasma into full view.
RHIC was at only about one-fourth of its full energy when its first collisions took place in June, but it already was outdoing its most powerful predecessor, a machine at CERN, by a factor of three. As of early August, the new collider was up to 130 billion electron volts (GeV) per nucleon pair, or some two-thirds of its intended top energy. To calculate the total energy of a full-energy collision, physicists multiply the top energy, 200 GeV, by 197, the number of nucleon pairs in each gold-gold collision. The product: a whopping 39.4 trillion electron volts.
Despite the hopes that scientists pin on RHIC’s brawn, “maybe they will not see the quark-gluon plasma where we expect to see it,” speculates cosmologist Angela V. Olinto of the University of Chicago. “That would be really interesting, too.”
Just a sideshow
Although collisions of gold nuclei create the fireworks in RHIC, that spectacle is just a sideshow to what is expected to happen in a surrounding speck of vacuum, researchers say. “What’s important is that we put a lot of energy into a small volume” of space, says theorist Edward Shuryak of the State University of New York at Stony Brook. Scientists will have their eyes on a patch of nothing the size of a nucleus. They hope the tiny vacuum will suck up enough energy from the ion collision to break down into another kind of nothingness.
Physicists suspect there are many possible types of nothingness. “We’re thinking of the vacuum as a substance that has different phases,” Alford explains. However, only one vacuum actually permeates the universe today, he says. Physicists refer to it as the zero-temperature vacuum because space hovers near absolute zero. Although theorists can’t yet calculate the properties of this vacuum, they suspect that it’s rich in odd characteristics.
For one thing, it’s full of ghostly activity. As a consequence of Heisenberg’s uncertainty principle and the equivalence of matter and energy (according to Einstein’s famous equation), pairs of so-called virtual particles of all types—always one of matter and the other of antimatter—continually pop into existence. Then the particles annihilate each other.
Sharing the vacuum with those short-lived visitors is an also-ghostly but enduring structure that permeates space, scientists propose. This structure, called a chiral condensate, consists of quark-antiquark pairs, but only certain types of quarks pair up with certain types of antiquarks. “There are so many of the pairs that they all lock together” to form an invisible “background,” Alford explains.
Within that tight-knit structure, these pairs also arrange themselves in a way analogous to the lining up of the minuscule magnetic fields of the atoms in certain magnetic materials. Because of this quark-antiquark framework, physicists say, the vacuum exerts a strong influence on matter and energy.
RHIC collisions are expected to pump enough energy into a tiny volume of that vacuum to destroy its structure. In effect, the collider is an “oven that can turn the temperature up incredibly high, but only over a very small region,” Alford says. “The basic idea is to see if something melts.”
When the zero-temperature vacuum is excited, physicists say, particles of ordinary matter—that is, configurations of confined quarks and gluons—appear. However, when a superheated vacuum is further excited, it produces quarks and gluons that roam freely through the plasma.
Although the strong force still operates in this plasma, “there are so many quarks around that they get in each other’s way,” Alford explains. The quarks “get distracted: ‘Am I supposed to be feeling attraction to this quark here or that one there?'”
Whereas the zero-temperature vacuum contains plenty of ghostly order, the quark-gluon plasma contains a vast amount of energy and minimal order, physicists say. By measuring properties of the relatively simple quark-gluon plasma, researchers expect to better understand not only it but also the complicated zero-temperature vacuum and that vacuum’s effects.
Theorists propose, for example, that a kind of pressure exerted by the zero-temperature vacuum keeps quarks and gluons cooped up as protons and neutrons. But the scientists have only a rough idea of how it works.
They predict that inside nucleons, gluons form tubelike structures that bind the quarks together. “Why is the glue living in tubes? The vacuum is forcing it to,” Alford says, “just like it’s forcing the quarks to be heavy at low temperatures.”
Physicists suspect that quarks are intrinsically lightweight particles, having no more than about 10 times the tiny mass of the electron. However, interactions between the quarks in nucleons and the zero-temperature vacuum add a large amount of mass to the nucleons’ quarks—making them about 60 times their intrinsic mass.
Because the zero-temperature vacuum brings about the largest share by far of the mass of every nucleus, scientists consider its effect the overriding factor in explaining why all matter in the universe has the mass that it does, Shuryak says.
Probing tiny space
Even as RHIC experiments probe the tiniest volumes of space, they also may help scientists make more sense of the biggest thing there is—the universe.
As a quark-gluon plasma cools, it condenses into hadrons. This occurs because the inwardly directed pressure of the zero-temperature vacuum has taken over, forcing the plasma’s constituents back into protons and neutrons. Physicists regard that condensation as a phase transition, like steam becoming water or water freezing into ice.
It’s common during phase transitions, like water’s familiar ones, for bubbles of unchanged matter to linger and then suddenly and violently burst in a belated transformation into the new phase. The creation of bubbles in plasma-to-hadron transitions would result in density fluctuations, which are “something RHIC can look at very carefully, ” says Berndt Mueller of Duke University in Durham, N.C.
In RHIC collisions, the primary evidence of a choppy transition would be an uneven distribution of matter and energy thrown off from the collisions as they cool down. If RHIC experimenters find signs of bubbles, some physicists say, that evidence may help crack a cosmological mystery: the origin of the first cosmic magnetic fields.
Magnetic fields often are associated with parity violation—a phenomenon in which the laws of physics yield a different answer if everything in an experiment is reversed, as in a mirror (SN: 2/20/99, p. 118). In the universe’s first instants, bubbles in which parity violation occurs may have formed in the cooling quark-gluon plasma, Dmitri E. Kharzeev of Brookhaven and his colleagues have proposed. They hope to test their ideas using data from RHIC.
The notion of cosmic phase transitions generating these original magnetic fields dates back to 1983, Olinto says. However, evidence from past heavy-ion experiments, as well as theoretical arguments, suggest that violent bubbling doesn’t occur as the quark-gluon plasma turns into hadrons, notes MIT’s Krishna Rajagopal. Nor is there cosmological evidence for such a bubbly transition, he adds.
Besides magnetic fields, RHIC scientists have set their sights on other cosmic targets. Craig J. Hogan of the University of Washington in Seattle points out that theories of how the first, light elements in the universe formed predict quite accurately the observed cosmic abundances of hydrogen, helium, and heavier elements. However, RHIC experiments can help determine if that process really started with a uniform cloud of protons and neutrons that froze out of the quark-gluon plasma, he notes.
Without dismissing the potential value of conclusions about the universe that may be drawn from RHIC, Hogan and other cosmologists sound a note of caution. “It’s important to realize that RHIC does not really reproduce the circumstances of the early universe,” Hogan warns.
For instance, besides being vast compared with an RHIC plasma, the primordial quark-gluon plasma is thought to have endured about a million trillion times as long.
Even amidst such caveats, theorists including Olinto have come up with another exotic possibility. The experiments might yield the first evidence of one proposed type of dark matter, the unseen substances that astronomers suspect make up almost all of the universe.
“This is a long shot,” Olinto admits. However, if bubbles in the cooling quark-gluon plasma are large enough, and if they chill in just the right way, some of the plasma might condense into hypothetical blobs dubbed strange matter because they contain many so-called strange quarks (SN: 3/4/89, p. 138).
Scientists have theorized that this strange matter is capable of transforming anything it touches into strange matter as well. That possibility, plus the chance that RHIC could precipitate other bizarre phenomena, such as certain unwelcome changes in the vacuum, prompted a backlash of fear against the collider last year (SN: 10/23/99, p. 271).
To investigate doomsday predictions circulating about the machine, Brookhaven lab officials convened a panel of scientists. The committee examined and refuted each proposed catastrophe scenario.
The fact that RHIC is now running “doesn’t prove it’s safe,” notes Robert Jaffe of MIT, who chaired the safety-review panel. “The best argument that nothing bad is going to happen is that good physics tells us nothing bad is going to happen.”
The panel’s analysis noted, for example, that cosmic rays regularly cause heavy-ion collisions of vastly higher energies than RHIC can muster. Yet those events have not wrecked the universe.
Although RHIC may already be producing the quark-gluon plasma, it’s too soon to tell for sure, scientists say. To make that judgment, teams are now studying the data. In the meanwhile, they say, the most obvious sign is encouraging—that’s the thousands of particle tracks that, time after time, splash across the control-room screens.
