The Leptoquark

David Pacchioli
September 01, 1997

While Hamburg goes about its business, physicists working beneath that German city are tracking an elusive particle that could change our understanding of the world. Or maybe it's nothing at all.

The particle accelerator at the Deutsches Elektronen-Synchrotron, or DESY, is a circular underground tunnel four miles around, where subatomic particles are revved up to nearly light speed and set loose, speeding in opposite directions until they crash. These high-energy collisions produce strange new particles, which decay in an instant, but leave behind a tell-tale pattern of energy on a super-sensitive detector. Reading the scattered debris, physicists can learn about the fundamentals of matter.

According to the Standard Model, the current overarching theory of subatomic physics, all the matter in the universe is made up of only two kinds of particles: leptons and quarks. Quarks are the particles which make up protons and neutrons, the two larger particles that revolve in the nucleus of an atom. Electrons, those negatively charged particles that orbit outside the nucleus, and neutrinos, hard-to-detect particles that rarely encounter other matter, are both leptons. Both quarks and leptons come in six "flavors." To confuse things further, all of these particles have their mirror counterparts in so-called anti-matter—identical particles that carry the opposite electrical charge.

The problem with the Standard Model, says Penn State physicist James Whitmore, "is that every once in a while we get a piece of data that doesn't fit." Plainly put, the model has gaps. It can't predict the masses of the particles, for instance, or explain why there are only six flavors of quark. At DESY, two large international teams of physicists, each with over 400 members from a dozen or more countries, have been smashing particles since 1992, looking for answers to such questions. Whitmore, of Penn State's Laboratory for Elementary Particle Science, belongs to one of these teams.

Last February, after reviewing their results of the past four years, both teams independently noticed something strange. Among the millions of collisions were a handful that could not be explained. In these cases, the crash pattern left behind in the detector (a house-sized block of instruments) suggested a particularly violent event, in which the lepton, meeting up with a quark, was knocked clear back in the direction from whence it came.

It happened only a few times: less than ten for each team, according to Whitmore. But still too many times, he adds, to be explained by the Standard Model.

One of three things must have happened, Whitmore suggests. It could be just a statistical blip. "This is a small number of events we're talking about," he says. "The rate of statistical fluctuation at this level is about one percent." Alternatively, their odd results could mean that physicists don't know the inner workings of the Standard Model as well as they thought. "We don't completely understand the distribution of quarks and gluons within a proton," Whitmore acknowledges.

But both teams saw the same thing, he notes, which leads to a third possibility: "Maybe there's some new physics out there." The strange results could indicate the split-second existence of a whole new type of particle: a leptoquark.

The leptoquark, a hybrid of lepton and quark, has been imagined by particle theorists for at least two decades, often as a feature in various unfinished versions of the "Grand Unified Theory," a kind of physics Holy Grail. Such a particle, in most estimates, would require great energy to create. It would be massive, almost the weight of a lead atom, and would decay very quickly to some more stable form.

Up until now, however, despite the speculation, no one has ever seen evidence that the leptoquark does exist. Nor, Whitmore emphasizes, is it at all clear that he and his colleagues are seeing such evidence now. "We need more events to make any kind of case," he says. "We'll have to wait for another year's data at least."

James Whitmore, Ph.D., is professor of physics in the Laboratory for Elemantary Particle Science (LEPS), Eberly College of Science, 303 Osmond Laboratory, University Park, PA 16802; 814-863-3314, For more on the activities of LEPS, visit the laboratory's Web site.

Last Updated September 01, 1997