The Large Electron Positron (LEP) collider at the European Centre for Particle Physics (CERN) was a wonder of both scale and delicacy. The great circular tunnel that housed it was 27 kilometers long, yet the high-energy particle beams within it were so exquisitely balanced that they responded to changes in the water level of nearby Lac Leman and trains leaving Geneva Station. For the past 11 years this sensitive monster provided physicists with fundamental new facts about the world; this fall it was scheduled to be shut down so that an even mightier machine, the Large Hadron Collider (LHC), could be installed in the same tunnel. But over the summer, something happened: scientists using LEP started to see hints of the Higgs boson, for almost 20 years the most-sought-after particle in physics.

Setting aside the more egregious hype–it was once dubbed the “God Particle”–the discovery of the Higgs would be a Big Thing in particle physics. So the LEP scientists asked CERN’s management to keep the great machine running until the end of October to see if the hints would firm up into bankable evidence. The management agreed, and LEP set off on its last hurrah.

LEP was designed to mass-produce the Z particles discovered at CERN in the early 1980s. If the Higgs is the Next Big Thing, the Z was the Last Big Thing (its sibling, the plausibly presidential W particle, just never got people excited the same way). The existence of the Z particle was predicted by a theory that sought to unify the electromagnetic force and the weak nuclear force. Unifying fundamental phenomena–finding ways of describing them as different aspects of even more fundamental phenomena–is what particle physicists are endlessly striving to do. (The most exciting thing about the Higgs–which by its nature interacts with almost every other known particle–is that studying it should open the way to further such unifications.) Europe’s success with the Z rattled American physicists and politicians into planning a scarily vast machine called the Superconducting Super Collider (SSC) on the basis that it would find the Higgs.

To produce its Zs, LEP sent electrons and positrons (the electron’s antimatter twin) racing in opposite directions round its circular track until they were moving at a hair’s breadth of the speed of light, and then smacked them into each other. The energy released in these collisions brought new particles into existence: the more energy the electrons were given to begin with, the more massive the particles created. Physicists started LEP off with enough energy in its beams just to make Zs, then stepped up the energy to produce Ws. And once they had learned as much as they could about the tail end of the alphabet, they cranked up the machine to produce even more massive particles, should they exist. It was at the peak of these energies that scientists saw signs of Higgs particles being produced alongside the more usual Zs.

After an extra month at these highest energies, the experimenters thought their evidence for the Higgs, though still inconclusive, was good enough to ask that LEP be started up again next spring (high energy costs keep CERN’s machines shut down during the winter, come what may). Unfortunately, running LEP for another year meant delaying the LHC, and the various CERN committees simply couldn’t decide whether that was worthwhile. If the Higgs had the mass predicted by the LEP results, the LHC would be able to study it with ease, whereas it would always remain at the edge of LEP’s capabilities. CERN has partners in the LHC–notably America, which decided that the SSC was simply too expensive to bear, even in the righteous cause of Higgs-hunting–with no stake in LEP, and they would have good reason to object to a delay. And if the hints of a Higgs turned out to be just a plausible delusion, a delay would be unrewarded. That said, discovering the Higgs would be immensely exciting and prestigious–and would deny the honor to an American machine called the Tevatron, which has just been upgraded and might well produce undeniable evidence of the Higgs before the LHC gets turned on in 2005.

In the end, CERN’s director-general came down in favor of keeping the LHC on schedule. No one doubts that the LHC will eventually produce exciting new discoveries and paths to unification. But if the Higgs turns up at the Tevatron in the meantime, and has the mass that the CERN experimenters have been talking about, then LEP will always be remembered for the one that got away.