Big Machines, Tiny Particles. In 1995, 'Standard Model' theorists were elated when this huge detector, located at Fermilab, confirmed predictions of the top quark. Now there may be something more.
Particle physicists are on tenterhooks again. Tentative sightings of an exotic new particle by two separate groups at a laboratory in Hamburg, Germany, have generated tremendous excitement--albeit laced with wariness--among researchers. Over the past two decades, experimentalists have merely confirmed the predictions of the Standard Model--which describes the behavior of all particles and forces except gravity--rather than exposing any truly unexpected phenomena.

When researchers at Fermilab a particle accelerator near Batavia, Illinois, confirmed the existence of the top quark in 1995, for many onlookers the event had all the drama of election night in the former Soviet Union. Quarks are the building bocks of which protons and neutrons are made. The standard model had demanded that the top quark exist--along with five others previously detected--and few theorists had seriously doubted that nature would fail to comply.

More intriguing results emerged from Fermilab a year ago. A preliminary analysis of a few anomalous collisions between protons suggested that their constituent quarks might be made of smaller, more fundamental entities--a direct violation of the Standard Model. After subsequent analysis, however, the "subquarks" vanished; theorists showed that with minor tweaking, the Standard Model could easily account for the data.

Leptoquark's Tracks? The ZEUS detector began showing results that hinted at the leptoquark last fall.

Manyphysicists pray the same will not happen with the new data from the Deutsches Elektronen Synchrotron, or DESY (pronounced "daisy"). DESY houses the Hadron Elektron Ring Anlage, or HERA, an accelerator that smashes protons against positrons, the antimatter versions of electrons. Last fall, teams operating two of HERA's detectors, called ZEUS, independently noticed that a few collisions had generated strange results: the positrons seemed to be merging with the quarks generated in the collision to form, if only for the briefest instant, an entirely new particle.

Although neither result in itself was statistically significant, together they seemed substantial enough to merit the attention of the wider community. Indeed, if it holds up, the discovery would be remarkable. Atoms are made of two elementary particles: leptons, which include electrons and positrons, and quarks. The results from HERA provide the first evidence of a particle combining aspects of both classes of particle--the leptoquark.

The two teams submitted separate papers to the journal Zeitschrift fuer Physik C. The papers have just been accepted and should be published in the same issue shortly, according to a spokesperson for DESY. Meanwhile, in response to burgeoning rumors, the HERA investigators posted reprints of their reports on the Internet in late February. Already dozens of unreviewed papers speculating on the HERA data have also appeared.

Leptoquarks are predicted by certain grand unified theories; called GUTs, they postulate an underlying unity between the electroweak force, which accounts for electromagnetism and nuclear decay, and the strong force, which binds quarks together. But all the protons in the universe would have decayed by now if the leptoquarks actually existed at the energy levels attained in HERA, points out David Miller of University College London.

Leptoquarks also appear in certain versions of supersymmetry; this much loved but still unverified theory posits an underlying unity between particles that transmit forces and particles that bear mass. Unfortunately, the HERA results would support a rather ungainly and thus unpopular version of supersymmetry. Theorists have also proposed that the HERA observations might derive from subquarks or from a new force that couples quarks and positrons at high energies.

HERA researchers expect to have enough additional data to rule out or confirm some of these hypotheses by the end of this year. There is a 1 percent chance that the "leptoquarks" will turn out to be statistical fluctuations that the standard model can accomodate, according to Allen Caldwell, a spokesperson for the ZEUS team. Although that probability seems small, he notes, it must still be considered the most likely outcome, given the history of similar observations.

Frank Wilczek of the Institute for Advanced Study in Princeton, N.J., took the HERA reports seriously enough to co-author a paper on their theoretical implications. But he says those implications, given all the other constraints imposed by previous experiments, evoke explanations that even particle physicists might find too odd to believe. "Whatever is true is good for physics," Wilczek says. "But it's not what anyone wanted." Caldwell, who at the age of 37 has been "living with the Standard Model" for his entire career, demurs. He notes that members of his generation have never experienced the thrill of a totally new and unexpected finding. "Let's hope that this one doesn't go away," he says.