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Physicists Report Unusual Particle Collisions

International Team Led by Columbia Researchers
May Have Seen New Interaction Between Positrons and Quarks

An international physics collaboration led by Columbia researchers has reported findings that raise new questions about how quarks, among the tiniest particles known, interact with other particles.

Working at the HERA high-energy particle accelerator in Hamburg, Germany, the physicists observed hundreds of thousands of collisions between protons, or hydrogen nuclei, and positrons, electrons with a positive charge. Over three years of observations, from 1994 to 1996, physicists saw a much greater number than expected of very high energy interactions, a result that had only about a one percent chance of occurring.

If the results are not a statistical fluke, new physics has been observed, said Frank Sciulli, professor of physics at Columbia, who drafted one of the papers reporting the observations. "One possibility is that our understanding of what's inside the proton is somehow wrong," he said. "The other is that some new interaction between positrons and quarks has taken place. That would mean some completely new mechanism is at work, a new particle or a new force." The observations at HERA (Hadron-Electron Ring Accelerator) were described in papers submitted Monday (Feb. 24) to the German physics journal Zeitschrift fÄr Physik by the 430-member collaboration, which includes physicists from 12 nations. The collaboration built and operates the ZEUS particle detector, which will run through the year 2004 and has only collected about 5 percent of the data researchers expect to find. The project is headed by Allen Caldwell, associate professor of physics at Columbia. Steven Ritz, associate professor of physics, and John Parsons, assistant professor of physics, are also participating.

In the collisions between positrons and protons, at energy levels never before attempted, an incoming positron interacts with a single quark within a proton. The positron is deflected and a jet of particles that contain quarks emerges from the proton. Researchers measure the angles of deflection and energy of both the deflected positrons and the jet of particles, and can predict from accepted physics theory and previous observations what the statistical distribution of those angles and energies should be, Professor Ritz said.

But more collisions than expected resulted in huge energy and momentum transfer between positrons and quarks, leading the researchers to speculate that something never before seen might have occurred. "We're seeing more home runs than we expected," Professor Ritz said.

Researchers expected to see 0.9 events that meet certain high-energy requirements, but instead saw four; they expected a certain threshold of another variable to be exceeded 0.15 times but it was exceeded twice. Since such predictions are statistical probabilities, the observations could be statistical flukes, but that is not likely, Professor Ritz said.

What makes the ZEUS results even more intriguing is that another team of physicists, also at HERA, is reporting a similar result in the same physics journal. That team of 400 physicists from a dozen countries operates another particle detector, dubbed H1 and located exactly opposite ZEUS on HERA's circular beam path, 6.3 kilometers or about four miles in circumference beneath suburban Hamburg. Both particle detectors obtain results from the same colliding beams of protons and positrons.

"The fact that both experiments see something unusual is quite interesting," Professor Ritz said. Both collaborations will continue taking data and hope to have more information by the end of the year, he said.