Crashing debut of an exotic heavy particle?

A high-speed positron, the antimatter counterpart of an electron, smashes head-on into an onrushing proton. It penetrates the particle, striking one of the proton's constituent quarks; then, picking up an enormous amount of momentum from the struck quark, it rebounds nearly straight back.

Such spectacular events are rare among the thousands of positron-proton collisions monitored at the HERA particle accelerator in Hamburg, Germany. Most of the time, the particles either miss or merely glance off each other.

Nonetheless, collisions in which positrons are sharply deflected seem to occur more frequently than expected, based on the standard model of particle physics. The standard model represents the current theoretical understanding of the fundamental particles and the forces of nature (SN: 7/1/95, p. 10).

Members of the large international team operating the H1 detector at HERA observed 12 high-momentum events in 3 years' worth of data, whereas theory would predict only 4.7 such events. Independently, the physicists operating the ZEUS detector at HERA obtained comparable results. Each group describes its findings in separate papers submitted for publication to Zeitschrift fur Physik.

"It might be that what we are seeing is some statistical fluctuation," says ZEUS team member Malcolm Derrick of Argonne (Ill.) National Laboratory. "On the other hand, both experiments independently see an excess, so we have some confidence that maybe it's a real effect."

The findings, if confirmed, suggest the existence of new particles or forces that are not part of the standard model. One possibility is that a positron may merge briefly with a quark, creating a particle that theorists call a leptoquark. This particle would then decay and spit out a positron.

Alternatively, "the underlying effect might not be the formation of a new particle but the action of a new force that causes a stronger interaction between the positron and the quark than anything in the standard model," says physicist David J. Miller of University College London.

A third possibility is that quarks are themselves made up of still smaller building blocks. Last year, physicists at the Fermi National Accelerator Laboratory in Batavia, Ill., detected a subtle effect hinting at such a quark substructure in debris scattered from high-energy collisions between protons and antiprotons (SN: 2/17/96, p. 102). However, subsequent analysis showed it was possible to explain the results in terms of conventional theory.

Which interpretation, if any, proves correct depends on future research. "As we collect more data, we'll find out whether our initial results hold up," Derrick says. The researchers expect to double the number of observations during 1997.

"The [high-momentum] events are so spectacular that we can see them immediately when they occur," Derrick adds. "We don't have to collect all the data before analyzing them." Stay tuned!