Physicists have good reasons to believe in the reality of magnetic monopoles--the postulated carriers of single magnetic "charges." But efforts to catch monopoles floating through the lab or trapped in ancient rock have failed. Now physicists have looked with the world's highest energy accelerator and, again, have come up spectacularly empty. The nondiscovery, now in press at Physical Review Letters, puts some new limits on the mass of this aspiring particle.
Most searches have looked for monopoles that may have been drifting through the universe since the beginning of time. "There's every reason to expect they were produced in the big bang," says Jeffrey Harvey, a physicist at the University of Chicago. The traditional way to look for these relic monopoles is with a loop of wire. If a monopole passes through, it induces a distinct blip of current in the wire, which can be easily detected. But after repeated failures, physicists began looking for hints of monopoles produced in the "little bangs" created in particle colliders.
Ilya Ginzburg, a theorist at the Institute of Mathematics in Novosibirsk, Russia, speculated that it should be possible to observe monopoles in the collisions of protons and antiprotons in Fermi National Accelerator Laboratory's (Fermilab's) Tevatron accelerator. The monopoles, he and a colleague calculated, could boost the energy of photons produced in the collisions. "The idea was so new and interesting," says Fermilab's Greg Landsberg, "I got really excited."
So Landsberg and colleagues went back and sifted through data from millions of collisions. They looked for pairs of high-energy photons emerging at large angles to the collision. Sadly, Landsberg says, "we found none." The effort wasn't all for naught, however. To have escaped detection, certain types of monopoles would have to have a relatively large mass--in one case, greater than 1580 times that of a proton.
This new technique of trolling for monopoles has drawn some criticism. Kimball Milton and his colleagues at the University of Oklahoma, Norman, contend that monopole theory isn't far enough along to reliably calculate whether monopoles would really have this kind of effect.