Signs of mass-giving particle get stronger

LEP’s L3 detector. CERN

Signals from a particle collider near Geneva suggested in September that scientists might have sighted a long-sought particle thought to be the source of mass itself. Those hints of the so-called Higgs boson were enough to postpone the permanent shut-down of the 11-year-old Large Electron Positron (LEP) collider until Nov. 2 (SN: 9/23/00, p. 196: Most-Wanted Particle Appears, Perhaps). A new hint of the Higgs has cropped up in that extended run.

The new data have bolstered the possibility that the particle is within reach. Besides its allure as the origin of mass, the Higgs is also a prominent experimental target because it’s the last particle predicted by the central theory, or standard model, of particle physics that hasn’t yet been found.

The finding has rekindled excitement and tension at the European Laboratory for Particle Physics (CERN), where LEP is located. A huge accelerator project called the Large Hadron Collider (LHC) can’t move forward until LEP’s 27-kilometer tunnel is available for the new tenant.

“It was pretty depressing most of the month,” says Princeton University physicist and LEP researcher Christopher Tully. No new events suggestive of the Higgs appeared, and one of LEP’s prior clues lost its luster under reanalysis. The new event on Oct. 16 “changed the whole picture,” he says.

“It’s a very good event, a very clean event,” comments Marcela Carena of Fermi National Accelerator Laboratory in Batavia, Ill.

The just-observed hint of the Higgs boson is a collision recorded by the so-called L3 detector. The fireball of electron-positron annihilation spat out a particle that promptly decayed into a bottom quark and its antimatter counterpart. Researchers suspect that the short-lived particle was a Higgs boson in part because some energy escaped the detector. The missing energy suggests, as expected, that a Z boson was created along with the presumed Higgs boson and then decayed into a pair of neutrinos, which LEP’s detectors can’t pick up.

The event may have caught a Higgs decaying in a scenario, or channel, less common than the so-called four-jet events seen until now at LEP. “What people are excited about is that you can see it from a different decay channel,” says Sau Lan Wu of the University of Wisconsin-Madison, a member of the team that found the three strongest of the previous four events indicating a Higgs’ presence.

As of press time, the four LEP experimental teams were debating whether to ask CERN’s managers on Nov. 3 to extend LEP’s life another year—at the cost of millions of Swiss francs and a delay of the LHC—to give the scientists a shot at firmly discovering the Higgs.

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