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Most-Wanted Particle Appears, Perhaps

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Tantalizing new clues of a long-sought subatomic particle have set the particle-physics community abuzz. Vivid but sparse signs of the so-called Higgs boson at the Large Electron-Positron Collider (LEP) in Switzerland have also sparked a desperate race against time to catch further glimpses before researchers pull the plug on the 11-year-old machine.

"It's amazing. People are working around the clock," says Christopher Tully, a Princeton University physicist and a LEP researcher.

The European Laboratory for Particle Physics (CERN) near Geneva was ready to shut down LEP at the end of this month to make way for construction of the Large Hadron Collider (LHC). However, because of the recent findings, the laboratory's managers announced on Sept. 14 that they would extend LEP's life by a month.

Despite frenzied efforts, the researchers stand only a slim chance of proving the Higgs boson exists, say physicists at LEP and elsewhere. The collider will most likely have too little time to collect enough data to stake a claim on the elusive Higgs.

Theorists have proposed Higgs boson as the answer to one of physics' most basic questions: Why does matter have mass? Interactions with the boson, they say, would confer mass on otherwise massless particles.

Finding the Higgs boson would also put the finishing touch on the so-called standard model of particle physics, which explains all the known particles and forces except gravity (SN: 7/1/95, p. 10). Of the roughly two dozen fundamental particles predicted by the model, only the Higgs boson, named after British physicist Peter W. Higgs of the University of Edinburgh, remains undetected.

"The situation at this moment is extremely exciting," comments theorist Marcela Carena of Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. Theorists predicted the Higgs boson in the 1960s, and experimentalists have been searching for it since the 1970s.

The hints that it may be within reach have emerged from an extraordinarily high-strung LEP. Over the years, researchers had steadily pushed the accelerator to higher energies — over 200 billion electron volts — but now the machine is on the verge of breakdown, physicists say. "We're balancing on the edge of a precipice," says Roger Cashmore, CERN research director.

Running at those extreme energies since June, LEP has recorded four electron-positron collisions whose explosive patterns hint at the fleeting presence of a Higgs particle. The scientists monitored tens of thousands of electron-positron impacts.

During the 1-month postponement of LEP's shutdown, researchers there expect to double the number of collisions measured by their detectors at the record-breaking energy. LEP scientists hope that the count of Higgs-like events will also at least double. If so, the case for the Higgs boson will grow stronger. Alternatively, the recent rash of apparent sightings could prove a fluke.

Even if the cranked-up LEP fails to nail down the particle before the machine's demise, the recent close calls could help guide a Higgs search slated to begin at Fermilab in the next few years, scientists say.

However, should four or five more events show up by Nov. 2, "there will be pressure on the CERN management to extend" LEP's life again, predicts Sau Lan Wu of the University of Wisconsin-Madison, a member of the LEP team that detected three of this summer's events.

Acquiring enough events to declare discovery of the Higgs boson might require another full year of LEP operations, scientists speculate. But adding even 3 months to LEP's life would excessively delay the LHC, Cashmore says. The extension would also cost millions of unbudgeted Swiss francs. "It's a price we're not willing to pay," he says.

Carena and other physicists find LEP's few Higgs-like events particularly heartening because they suggest the particle can be created at relatively low energies. Such a low-energy Higgs dovetails with a popular, though unverified, theory known as supersymmetry.

Supersymmetry includes the standard model but also goes beyond it by proposing a menagerie of new particles that are companions to the ones already known. The theory also predicts multiple Higgs bosons, and the least energetic of them falls into the energy range that the LEP events spotlight. For physicists hoping that supersymmetry will take the field beyond the standard model, the LEP events are "a very encouraging hint," Carena says.

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