Volume XXVII Number 1
Steven Vigdor, left, Will Jacobs, and Scott Wissink
Photo © Tyagan Miller
Many of the scientists and others participating in the STAR experiment surround the fully assembled STAR detector.
Photo courtesy Steven Vigdor and STAR Collaboration
A real-time image shows many hundreds of outgoing particle tracks from a single collision of two gold nuclei, as reconstructed by the STAR detector pole tip.
Photo courtesy Steven Vigdor and STAR Collaboration
Making a Mini-Bang
It's been two hours since I sat down to talk with Indiana University physics professor Scott Wissink and Will Jacobs, a senior research scientist, about their work, and my headis spinning. Sitting at a table in the pleasant, if vaguely airport-like, lounge of the IU Cyclotron Facility (IUCF), we've just concluded a quick crash-course in quantum physics.
"It's kind of like how a top might spin around on a table," says Wissink, a tall, friendly man whose years of teaching experience are evident in the helpful way he handles my question about what, exactly, "spin" means when it comes to quarks, gluons, and other subatomic particles. "Except not precisely," he continues, "since gluons have no size, mass, or diameter. They're really just virtual particles."
My understanding of quantum mechanics is admittedly murky, but after talking to Wissink and Jacobs, a few things are perfectly clear. First, the IUCF nuclear physics group is on the verge of important discoveries about quarks and gluons--the smallest, most fundamental forms of matter in the universe. Second, they're working on one of science's largest and most prominent stages: the recently completed Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory in Long Island, New York. Finally, as key participants in the STAR (Solenoidal Tracker at RHIC) experiment at Brookhaven, the IUCF nuclear physics group is helping to solve one of the most intriguing scientific mysteries of our time: what was the nature of the universe immediately after the Big Bang?
Quarks, gluons, and a 'mini bang'
Microseconds after the Big Bang, the infant cosmos consisted of a particle soup. Many physicists speculate that just before the formation of the first atomic nuclei, and well before individual atoms came together to form stars and galaxies, the universe existed in its most basic form: as a writhing, unimaginably hot sea of quarks and gluons. Quarks are tiny particles that form protons; gluons are the flashes of energy that bind quarks and keep them confined within the proton. A large international team of physicists currently working at Brookhaven is determined to simulate the condition of the universe immediately after its birth--known by the physicists as the "quark-gluon plasma" (QGP)--inside a particle collider.
Subatomic particles have an inverse relationship to the particle colliders and detectors used to study them: the smaller the particle, the larger and more powerful the machine. Colliders (which used to be known as "atom smashers") and detectors are massive structures, including miles of "track" through which atomic nuclei are accelerated almost to the speed of light, and hulking devices that are, in effect, exquisitely sensitive digital cameras used to record what happens when two nuclei collide head on. Surround the STAR detector with four walls, and it would resemble a very large house.
The collider to which the STAR detector is attached is an equally impressive marvel of engineering. (The "solenoidal" in STAR refers to the geometry of giant magnets that bend particles to facilitate their detection.) While not the largest or most powerful collider in existence, the RHIC at Brookhaven is currently the world's highest energy heavy ion accelerator.
A heavy ion, such as a gold nucleus, is one that contains an abundance of subatomic matter. In other words, heavy ions are stuffed with protons and neutrons and, consequently, harbor at their core a potentially dense mix of quarks and gluons. Due to their miniscule size, the actual impact of colliding heavy ions is small--about the same as a mosquito flying into a screen door. But for an incomprehensibly short amount of time--approximately 0.00000000000000000000001 seconds--the temperature caused by a collision will exceed one trillion degrees, or 10,000 times hotter than the Sun's core.
Physicists are hoping that RHIC's ability to generate such extraordinarily energetic collisions of heavy ions will turn back time, so to speak, inducing a "mini-bang" that will unchain quarks from their web of gluons and give rise, for a brief instant, to the quark-gluon plasma condition of the universe immediately after the Big Bang.
Alongside the goal of better comprehending the nature of the universe by revisiting its nascent period, the STAR collaboration also seeks to better understand the intrinsic qualities of the universe's most basic forms of matter. It's in this area of nuclear physics--the detailed examination of quarks and gluons--that Wissink, Jacobs, and their IU colleagues are making their mark. The IU team's primary contribution to STAR has been the design and construction of an Endcap Electromagnetic Calorimeter--a 30-ton disk-shaped detector that fits onto one end of the STAR detector like a plug. The massive calorimeter is used to detect and record something infinitesimal: the contribution of quarks and gluons to the proton's "spin."
Similar to how the earth spins on its axis, subatomic particles--protons and their quarks and gluons--have a specific spin that helps determine their magnetic properties. Physicists know that spin exists, but unraveling the contribution of quarks and gluons to the proton's spin remains a mystery.
"Until recently we thought that each quark carried one-third of a proton's energy, meaning that the collective spin of a proton's quarks would determine the direction of its spin," explains Jacobs. "But it turns out that quarks have relatively little impact on the direction of a proton's spin."
In the larger picture, Jacobs notes, spin is only a handle, a way to streamline the gargantuan task of observing and measuring the complex quark-gluon interactions at work inside a proton. For example, the apparently minimal effect of quarks on the proton's spin suggests that gluons may play an unexpectedly large role. Although individual gluons exist only briefly, overall they are much more abundant than quarks inside the proton, and thus physicists suspect they contribute significantly to the proton's spin. The question is how and to what extent. The IU nuclear physics team hopes answering such questions with the aid of their calorimeter will greatly advance our knowledge about the nature of the particles at play in the quark-gluon plasma.
RHIC was designed with spin in mind. Unlike other colliders, it can fire "spin polarized" proton beams at each other, meaning that in one beam, the majority of protons spin in the same direction, while protons in the other beam spin in the opposite direction. The idea is that when the beams' individual protons collide, the collision will be powerful enough to knock loose individual quarks and gluons in mid-spin, so to speak.
"It's like smashing two alarm clocks together so hard that individual pieces fly out before they get caught in the general wreckage," Wissink explains. "Our hope is that quarks and gluons in colliding protons will fly away from the nuclear debris so quickly that we'll be able to detect traces of them as they were in the proton and measure as accurately as possible the nature of their spin and how it contributes to the spin of the proton."
And that's important, Wissink adds, because knowing more about how and why quarks and gluons work together to "build" a proton helps us understand more about the universe at the most basic levels.
It may come as a surprise to learn that IU nuclear physicists are currently engaged in such pathbreaking research on a national stage. From the mid-1970s to 2000, the IU Cyclotron Facility was internationally known as a hub of nuclear physics research--a "national user" facility made available to researchers around the world. But since the National Science Foundation ended the facility's national user status in 2000, the cyclotron has become more well known for medical-related projects related to proton therapy--the use of a proton beam to destroy cancer cells. Although they never stopped working, the IU nuclear physics groups largely faded from public view.
"When I tell people I work at the cyclotron, they always ask if I work on medical physics," says Wissink wistfully. "For most people outside the IU physics community, it's as though the nuclear physics group had drifted away or died off."
Yet the "demotion" of nuclear physics at the IUCF has ultimately resulted in a step forward for the nuclear physics group, according to Steven Vigdor.
"When we were operating a national facility (at the IUCF), all of our research tended to start from the question: What important research can we do using the locally available beams and experimental facilities?" says Vigdor, a professor of physics and senior member of the IUCF nuclear physics group. (Vigdor has been asked by the 500-person team at STAR to draft an evaluation of the first three years of experiments at RHIC.) "Now," he says, "we are freer to ask: What is the most important research going on in the field, and how can we make an impact on it?"
Forced to look elsewhere to further their research, members of IU's nuclear physics group aimed high. Their participation in the search for quark-gluon plasma places them at the center of advanced experimental physics.
Although physicists are not yet willing to claim victory in their quest to re-create quark-gluon plasma, so far, RHIC has produced a growing pile of tantalizing evidence of QGP's existence in the collider.
"Normally, when nuclei collide, elementary particles shoot away from the center in opposite directions at equal speeds," Jacobs explains, using a digital representation of exploding nuclear matter that looks like the work of an overzealous child with a Spirograph toy. "But when heavy nuclei such as gold collide, something strange happens. This particle here flies out as expected," he says, pointing to a spidery blue line representing the path of quark jetting away from the center of a collision, "but the particle it knocked into either doesn't show or comes out a lot slower than expected. Under normal conditions it should come flying out the other way. But it doesn't, which suggests that these are not normal conditions."
The STAR collaborators suspect, and hope, that the abnormal conditions described by Jacobs are in fact quark-gluon plasma. In theory, such a super hot, soupy mixture of free-floating quarks and gluons would impede the progress of a quark or gluon rocketing through it.
It will most likely be years before any such conclusion is reached. Meanwhile, STAR physicists continue to accumulate and analyze data and calibrate the machinery to produce ever more detailed results. For their part, IU's STAR team continues to fine-tune its calorimeter, which is scheduled to be fully operational in 2005. When it is, Jacobs, Vigdor, Wissink, and their colleagues will begin to probe what makes the universe tick.
"The nature of the universe has fascinated people for centuries," says Wissink. "When you try to fill that in with more detail, you don't just want to say there was a big bang and leave it at that. You need to fill in the steps along the way--why does the universe look like it does, why are there stars and galaxies?"
To figure that out, Wissink and others are grappling with the fundamentals of nature--the quarks and gluons that ruled the cosmos in its beginning.
Jeremy Shere, a freelance science writer, is completing his Ph.D. in English at IU Bloomington.