March 17, 2010 | 10
In the high-energy physics community, all eyes have been on Europe for some time, as the Large Hadron Collider, or LHC, has proceeded in fits and starts to become, in 2009, the most powerful atom smasher the world has ever seen. But as the LHC has taken shape in an underground tunnel outside Geneva, colliders stateside have been fading into retirement. At the start of 2008 the U.S. had four colliders; if Fermilab’s Tevatron shuts down as planned in 2011, the U.S. will soon be down to one: the Relativistic Heavy-Ion Collider, or RHIC, at Brookhaven National Laboratory in Upton, N.Y.
That point was made by RHIC physicist Ed O’Brien on March 10, when lab personnel led a group of reporters on a tour of the Long Island facility last week. RHIC accelerates gold ions (atoms stripped of some of their electrons) to nearly light speed as they race around a 3.8-kilometer ring—roughly the size of the Indianapolis Motor Speedway, as physicist Todd Satogata noted. RHIC is one of the largest colliders in the world: for comparison, Cornell University’s recently retired CESR collider was 0.77 kilometers in circumference, the Tevatron’s ring is 6.3 kilometers, and the mammoth LHC ring is 27 kilometers. (Like the Large Hadron Collider, RHIC saw its share of apocalyptic hand-wringing before start-up, thanks in part to an offhand remark made by physicist Frank Wilczek in Scientific American in 1999.)
At detectors around the RHIC ring, clockwise gold-ion beams plow into counterclockwise beams, and the detectors track the subatomic debris to investigate the physics of the interaction. Because the collider was turned off for a maintenance day, the media group got a peek inside the STAR [pictured above] and PHENIX detectors—two massive instruments that fill their chambers so tightly, they are almost impossible to see in full.
RHIC had recently been in the news following the February announcement that the quark–gluon plasma created from ion collisions in the detectors reaches temperatures of roughly four trillion degrees Celsius. That plasma "flows more freely than any fluid known to mankind," but it lasts just yoctoseconds, said Brookhaven physicist Peter Steinberg. Wait—yoctoseconds? "I would advise you to Wikipedia it," Steinberg added. Consider it done: a yoctosecond is one septillionth of a second, or 10–24 seconds.
Even that ferocious fireball is relatively tame given how small a volume it fills, Steinberg noted. A high-speed ion collision in RHIC packs about the same energy as a midair crash between two mosquitoes—the much slower speed of the mosquito crash is offset by the vastly greater mass of an insect compared to an atom. Out of the quark–gluon plasma appears a shower of recognizable particles, which allows RHIC physicists to unwind that yoctosecond-scale process, much of which is not directly observable. "No one’s ever seen a quark; you can’t see a gluon," Steinberg said, noting that their existence is inferred from the behavior of larger composites such as protons and neutrons. "We live our lives," he added, "as if they exist as particles."
One of RHIC’s missions is to investigate the nature of the proton, specifically the origin of its quantum-mechanical spin, or angular momentum, which happens to be the same as the electron’s, a fact that Steinberg called "totally weird." The spin could be wrapped up in the individual spin of the proton’s constituent quarks or in their orbital motion within the proton’s structure. "There are lots of places for the spin to be," Steinberg said, adding that the harder physicists look, the murkier the picture becomes. "It’s one of the most amazing null results I’ve seen, I think," he said. RHIC’s ability to swap in spin-polarized protons for gold ions in its collisions makes the collider uniquely positioned to investigate this wrinkle of nuclear physics, even though the Tevatron and the LHC have more brute strength.
Brookhaven physicists have been deeply involved in building and operating the LHC and its support infrastructure, which the lab demonstrated by hosting a video chat (albeit a choppy one) with three Brookhaven physicists currently stationed at CERN, the European lab for particle physics that runs the LHC. It’s only natural that domestic physicists would want to hitch their wagon to the so-called big bang machine over in Europe, for which expectations are sky-high. "The questions that we’re addressing at the LHC are the most fundamental that we know of," said Howard Gordon, a Brookhaven physicist who leads the U.S. program office for ATLAS, one of the two all-purpose detectors at the LHC. Among the issues LHC researchers hope to resolve: finding the Higgs boson, a hypothetical particle that lends other particles mass; identifying the signature of dark matter particles, the mysterious stuff that fills the universe but has yet to be directly observed; and looking for the extra dimensions beyond the traditional four (three dimensions of space, one of time) that string theory would require. If there are indeed extra dimensions to be found, Gordon says, the signal from the LHC "would be unmistakable."
Between RHIC and the LHC, Brookhaven will soon find itself deluged with petabytes of experimental data. The computing facility is one of two tier 1 centers for LHC data in the U.S., meaning that it sits one rung below CERN (the tier 0 center) on the network hierarchy. According to Brookhaven’s Ofer Rind, the facility has about 10,000 computing cores, lined up in a library of rack systems, dedicated to RHIC and ATLAS alone. The computing facility, running at capacity, pulls about 4.5 megawatts, enough to power about 3,500 U.S. homes. Much of that electricity is tied up in cooling systems to pull waste heat away from the computers—always a challenge for large-scale computing centers. "We turn electricity into science and heating," Rind said, "and I won’t say which one we do more."
Photographs © John Matson/Scientific American