Researchers at CERN, the European center for particle physics in Geneva, are trying to make sense of a set of observations that seem to lie outside the boundaries of conventional physics. Over the past year or so, one of the four huge detectors on the Large Electron-Positron Collider (LEP) has picked up 18 events that don't fit into any existing theories. But so far, physicists haven't been able to write them off as experimental artifacts.
In each case, according to a story in tomorrow's issue of Science, the ALEPH detector recorded four jets of particles spraying from high-energy collisions of electrons and antimatter positrons. The total mass of the daughter particles always added up to 106 billion electron volts, but the two pairs of jets made unequal contributions. This pattern could imply that each collision created a pair of unlike particles, which quickly decayed to produce the jets. But existing physics has no candidates for what those particles may be.
Many physicists were inclined to dismiss the first few events, seen in data from 1995, as a fluke. But during later runs at higher energies, "a few more events have come along. When it's all added together, this effect looks more significant, so this is what's now creating the excitement," says ALEPH researcher John Thompson of the Rutherford Appleton Laboratory near Oxford in the United Kingdom. The other three detectors on LEP have seen nothing comparable, however. "It's somewhat bizarre that, if there is anything there, they are seeing nothing," says ALEPH's spokesperson-elect Peter Dornan, an experimentalist based at Imperial College London.
Thompson and his colleagues think it's unlikely that they've made a mistake, but theorists say it's equally hard to accept that ALEPH is seeing new physics where none is expected. What may finally settle the issue is more data, which will come when LEP begins its new runs in May 1997. In the meantime, physicists face a frustrating waiting game. As Frank Close of the Appleton Laboratory puts it, "You can't tell yet whether this is the emergence of a signal, like the tip of an iceberg, or whether it's a small piece of ice that's going to melt away."