Sunday, October 16, 2011

The Tevatron’s Epitaph: Solid Science, No Surprises

    Next Friday, physicists will shut down the United States’s great atom smasher, the Tevatron collider at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. For a quarter of a century, the Tevatron reigned as the world’s highest-energy collider and scientists’ best probe of the structure of matter, until it was eclipsed by Europe’s Large Hadron Collider (LHC) 18 months ago. But even as physicists plan to gather at Fermilab next week to toast the Tevatron, they say its legacy is a mixed bag: The Tevatron produced a lot of excellent science but yielded no surprises to make physicists ethink their standard model of fundamental particles and forces.
   “It’s been a solid player,” says Paul Langacker, a theorist at the Institute for Advanced Study in Princeton, New Jersey. “It has not made big, unexpected discoveries, but it’s played its role.” Burton Richter, a Nobel Prize–winning experimenter at SLAC National Accelerator Laboratory in Menlo Park, California, says Tevatron researchers “have a proudhistory, but they didn’t have that great flash [of discovery] that suddenly
illuminates a whole new part of our field.”
   Built in what was open prairie—and is now suburbia—the Tevatron blasts protons into antiprotons. The 6.28-kilometer-long circular accelerator consists of more than 1000 tubelike superconducting magnets that guide the protons and antiprotons in opposite directions through an evacuated beam pipe. The particles collide within two 5000-ton detectors named CDF and D0 spaced around the ring. Those high-energy collisions can blast into existence new massive subatomic particles that quickly decay into telltale combinations
of familiar ones.
    The Tevatron’s biggest achievement was to blast out a particle called the top quark. It’s a cousin of the up quark and the down quark, which join in trios to form protons and neutrons that themselves clump together to make atomic nuclei. (Two ups and a down make a proton; two downs and an up make a neutron.) Announced on 3 March 1995, that discovery garnered headlines around the world and marked a triumph for the teams of physicists working with CDF and D0, each of which numbered about 450 members at the time (Science, 10 March 1995, p. 1423).
     But some physicists question whether the discovery merits the highest accolade—aNobel Prize—as the top  quark had to be there. “Everybody already knew everything about it,” says Gordon Kane, a theorist at the University of Michigan, Ann Arbor. 
     Scientists knew that the first family of the up quark and the down quark was mirrored in a second family comprising the heavier charm quark and strange quark. And they’d already found an even-more-massive analog of the down quark, dubbed the bottom quark, that needed a partner. By the 1990s, physicists had even inferred the top quark’s mass by measuring the masses of particles called the W and Z bosons. Thanks to quantum uncertainty, a Z can change into a top quark and an anti–top quark and then back into a Z. Not observable directly, such processes alter the masses of the W and the Z in ways that depend on the mass of the top quark.
    Even those who think the discovery of the top quark deserves a Nobel Prize say awarding one would be difficult. “The top quark is certainly worthy of a Nobel Prize,” says Sally Dawson, a theorist at Brookhaven National Laboratory in Upton, New York. But “the experimental collaborations are very big. So who are you going to give it to?”
    Beyond the discovery of the top quark, the Tevatron experiments confi rmed and fleshed out the standard model (see table). For example,CDF and D0 themselves precisely remeasured the mass of the W boson. That parameter put limits on the mass of the last missing piece of the standard model, the long-sought Higgs boson, which is key to physicists’ explanation of how all fundamental particles get their mass.
   The Tevatron itself was a seminal achievement, accelerator physicists say. It was the first accelerator to use magnets wound with superconducting wire, which carries electricity without resistance when cooled to near absolute zero. “The Tevatron is where I learned about building superconducting machines,” says Lyn Evans of the European particle physics laboratory, CERN, near Geneva, Switzerland, who worked on the Tevatron in its early days and later directed construction of CERN’s LHC. “If you like, it was a prototype for the LHC.”
    Some say it was a technical tour de force to make the machine work at all. The Tevatron was originally designed to acceleratea single beam of protons to be shot into fixed targets and not to accelerate protons and antiprotons in opposite ways within its one beam pipe, says Nicholas Samios, former director of Brookhaven. “They had to be very clever in separating the protons and antiprotons and controlling them as they bring them around,” he says.
    The Tevatron and Fermilab have their detractors. Kane says Fermilab could have bagged a prize much bigger than the top quark: the discovery of the W and Z bosons. Predicted in the 1960s, those particles convey the weak nuclear force just as photons carry the electromagnetic force. They were discoveredin 1983 at CERN in an accelerator called the Super Proton Synchrotron that was rejiggeredto collide protons and antiprotons. A year later, physicists Carlo Rubbia and Simon van der Meer shared the Nobel Prize for the discovery, which clinched a conceptual mechanism through which the electromagnetic and weak forces intertwine.
     However, Rubbia and two colleagues had proposed in 1976 that Fermilab hunt the W and Z by fashioning a collider out of the Tevatron’s predecessor, known as the Main Ring. Fermilab’s first director, Robert Wilson, and lab leaders declined because the Tevatron was already in the works, says Peter McIntyre, an accelerator physicist at Texas A&M University in College Station and one of the proposers. “Fermilab had a working accelerator and was building a new one, and this collidingbeams scenario seemed a distraction from
their main business,” he says. “It was one of the biggest disappointments in my life.”
    But Evans says the Main Ring wasn’t good enough to realize the plan. “McIntyre is dreaming,” he says. Lab officials were right to push on with the Tevatron, he says: “That decision has been totally vindicated. I don’t think that the top quark would have been discovered if they hadn’t taken that road.” 
    Now, some physicists argue that the Tevatron may miss out on the discovery of the Higgs boson. CDF and D0 researchers have been racing to collect enough data to spot it and last year pushed, unsuccessfully, for a 3-year extension of the Tevatron’s run (Science, 14 January, p. 131). But Kane argues that the Tevatron underperformed all along because of weak management at the lab and the Department of Energy, which funds Fermilab. “It could have performed much better and done much more,” he says. Others say Kane overstates the case. “I would not trust a theorist to talk about management,” Samios says.
    The Tevatron could have done far worse. In the 1980s, the TRISTAN collider at Japan’s KEK laboratory in Tsukuba sought the top quark and came up empty. The Tevatron will go down in history as a very good machine that, alas, found only what it was looking for.



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