Proton Decay Experiment On The Brink Of Extinction

The Proton Refuses To Decay, But Physicists’Funds Are Fading Fast Two thousand feet under the shores of Lake Erie, in a six-story salt cavern, one of the most sophisticated light detectors ever constructed is waiting. Every second, several particles speed through the instrument’s enormous pool of water and collide with atoms in it, setting off flashes of light to be detected and recorded. But these events are merely physics’ flotsam and jetsam—things to be identified, c

By | May 16, 1988

The Proton Refuses To Decay, But Physicists’Funds Are Fading Fast
Two thousand feet under the shores of Lake Erie, in a six-story salt cavern, one of the most sophisticated light detectors ever constructed is waiting. Every second, several particles speed through the instrument’s enormous pool of water and collide with atoms in it, setting off flashes of light to be detected and recorded. But these events are merely physics’ flotsam and jetsam—things to be identified, cataloged, and forgotten.

The detector and the scientists who tend its humming pumps are waiting for bigger game. They look for special flashes of light that would answer one of the most fundamental questions in physics and bring them fame, glory, and probably the Nobel Prize. They are waiting for a proton to fall apart.

Today, however, the proton is likely to outlast their expensive project. Seven years after it was switched on in a blaze of publicity and excitement, the detector has yet to see the demise of a single proton: Recently, the Department of Energy’s Division of High Energy Research has indicated its desire to shut off funding. Not even the detector’s dramatic and unprecedented observation of a supernova last year may save it.

The rise and possible fall of the Cleveland proton decay, experiment is a sobering story of the risks of Big Science. It is a tale of competing personnel, of expensive and rapidly aging equipment, and of rapid theoretical advances that can turn state-of-the-art instruments into expensive dinosaurs.

The tale begins a decade ago, when theoretical physicists appeared on the verge of a break-through that would unify all the particles and nearly all the forces in the universe. These so-called Grand Unified Theories or "GUT" have an extraordinarily small number of testable predictions; one of the few is that protons, the basic building block of the atom, are unstable and will eventually decay. In 1979, two architects of GUTS shared the Nobel Prize, and both pounded the pulpit during their acceptance speeches to encourage the search for proton decay.

Experimenters were naturally excited. The path to grand unification was already strewn with Nobel Prize winners, and the verification of proton decay would be the next great advance. Several scientists in the United States submitted proposals for detectors, but the Department of Energy refused to fund competing projects. The result was a collaboration between Frederick Reines of the University of California at Irvine, Dan Sinclair of the University of Michigan, Larry Sulak, then of Michigan and now of Boston University, and Maurice Goldhaber of Brookhaven National Laboratory Their enterprise became known as the IMB experiment, for Irvine, Michigan, and Brookhaven. Digging began at the end of 1979, and the first data was taken in 1982.

Stiff competition gave the project a special urgency. Four other teams around the world were also working feverishly to construct proton decay experiments; one in the Kolar Gold Field in India, one underneath Mt. Blanc on the Franco-Italian border, one further south along the same border in the Frejus tunnel and one in the Kamioka mines in Japan.

Each team soon produced a handful of so-called candidate events, things that looked like proton decay and had to be analyzed to make sure they weren’t cosmic rays or neutrinos. But as Goldhaber says dryly, “Not all candidates are elected.” Only the Indian scientists, apparently trying to secure a share of the Nobel, stuck their necks out and claimed to have seen the elusive event.

Time passed. More candidates entered the ring; none were elected. It became obvious that the proton is longer-lived than the first batch of GUTs had predicted. The experiments continued to run, however, in the hope that proton decay could still be detected. Still hotly competing, the experimenters raced to. incorporate the latest technology The IMB detector was upgraded in 1986 at a cost of one and a half million dollars, increasing its sensitivity fourfold.

Still no candidates were elected. Disappointment set in. The Frejus experiment dosed last year. Mt. Blanc is still running but is more interested in astrophysical particles than proton decay, partly just to keep its physicists busy. The Indians are keeping mum. The Japanese, as well as an Italian team, are still searching for approval and funding for larger detectors.

In the Ohio salt mine, strains appeared in the marriage. Dan Sinclair had been keenly interested in the problem of identifying the hundreds of fragments created when particles crash into the detector. It was a difficult task, because the fragments leave behind only faint trails of light as sole clues to their identity. But by last year, all the puzzles had been solved and written into a computer program, and Sinclair was ready to take the "M" out of IMB. “We already did what we set out to do,” Sinclair says. He announced plans to look around for a more challenging project.

Sulak and Reines, in contrast, still wanted to push the detector to its limits. The longer the experiment rims without seeing a disintegrating proton, the more it raises the lower limit of the particle’s life-time. In addition, Sulak was particularly intrigued by the technical challenge of making the detector run automatically at low cost.

Then came what looked like a godsend, a rare event that promised to bring new life to the old detector. On February 23, 1987, a supernova burst into the heavens, the first in a nearby galaxy in almost 400 years. Scientists around the globe were overjoyed

Nowhere was the rejoicing greater than among the IMB scientists. Supernovas release enormous quantities of neutrinos—exactly the kind that the IMB detector, thanks to its upgrade, was now able to observe. Moreover, due to the peculiar physics of supernovas, the neutrinos arrived in the salt mine four hours before the light appeared in astronomers’ telescopes. As a result, the IMB collaborators were able to collect new information about both the neutrinos and the dynamics of supernovas.

Sulak was wildly enthusiastic. "This was an absolutely fabulous discovery," he says. In his vision, the beleaguered Ohio salt mine experiment could spring back from obsolescence to a new role in the forefront of physics as the world’s best supernova detector.

But not all his collaborators were impressed. "These things are very rare," says Sinclair. "The time between supernovas is decades or centuries. That’s not a good match to the human lifespan, and I’ve already sunk 10 years of my life into this thing." Sinclair turned his attention to a gamma ray detector under construction in Utah. Last fall he informed the DOE of his decision, adding that the IMB detector had reached the point of diminishing returns.

Reines then told the DOE that he wanted to continue running the experiment partly as a supernova detector. In January, the DOE fired back. Although the IMB’s supernova sighting is "historic," DOE’s letter said, "the usefulness of the IMB detector for proton decay will have passed the point of diminishing returns by 1989." If the remaining IMB members wanted to wait around for more supernovas, the letter suggested, they ought to solicit money from agencies that fund astrophysics projects, not from the DOE.

"The DOE should have been impressed that we spotted the supernova,"Sulak says. "Instead, they used it as a pretext to give us our walking papers." Sulak took his case public. At a session of the AAAS meeting this February, he spelled out the virtues of the project, and displayed a transparency of the DOE letter. The talk inspired a series of newspaper articles attacking the DOE decision, including one in Newsday quoting Sulak regarding "the total irresponsibility of the government."

Annoyed DOE officials agree that the experiment has done a superb job of pushing back the proton decay lifetime limit, but say that is little reason to keep going. "Our question is whether it is worth continuing to run the experiment just to refine the limits a little more," says Williams.

On April 22, a subcommittee within DOE’s High Energy Physics Division began to debate whether the IMB experiment is worth funding purely for its astrophysics results. Its decision, which will go before the full committee in July, could be favorable for the project. But if so, the decision will also probably strengthen the resolve of the DOE Division of High Energy Physics to force the collaboration to seek funds here by next year.

Even Sulak realizes that the remnants of the IMB collaboration will face an uphill battle to snare such funds, because astrophysics money is scarce. If they fail, the experiment will end and its equipment pillaged for other projects. Along the way, there will no doubt be more letters back and forth to the DOE, more public pressure, maybe the intervention of a congressman. In short, it’s business as usual in Big Science, as hot areas of research wax and wane and scientists hitch their careers to chancy projects in a high-stakes game of craps. In the meantime, one of the most sensitive light collectors ever built sits beneath Lake Erie, silently waiting.

New York writer Robert P. Crease is CO-author (with Charles C. Mann) of
The Second Creation (Macmillan, 1986;
Collier 1987), a history of particle physics.

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