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Mo, colleagues find evidence of submicroscopic particles

By Sally Harris

Spectrum Volume 19 Issue 30 - May 1, 1997

Analysis of data taken at an electron-proton collider in Germany from 1994 to 1996 has revealed five "events" that may change the standard model of matter, according to Luke Mo, professor of physics at Virginia Tech, who is a member of the executive group for the ZEUS experiment.

While some scientists have taken to space shuttles to explore the outer frontiers of our existence, others have gone to miles-long underground facilities to force the collision of submicroscopic particles in an effort to discover another final frontier--the smallest building block of all matter, the particle that cannot be further split into parts.

Much of matter cannot be observed because of the energy and mass form in which it exists. Until a few decades ago, the electron, proton, and neutron, which make up the atom, were considered the basic building blocks of all matter. Now scientists know there are subparticles, such as quarks, and that the electron is not the only member of the family of particles called leptons.

Back in the mid-1960s, Luke Mo had been part of a group at the Stanford Linear Accelerator Center that discovered quarks, particles so small they cannot be seen, but can be sensed only by the tracks they leave when larger particles such as protons and electrons explode when they are made to collide with one another with great force. Three from that research group won the Nobel Prize in Physics in 1990 for the work, and Mo was invited to the ceremonies in Sweden because of his contributions to the project.

In the 1970s, physicists had found evidence of five quarks, then thought to be the basic building blocks of all matter. Having theorized from the relationship between quarks and leptons that there were six quarks, physicists searched for the last one (which was found only last year at Fermilab, Batavia, Ill.). In the meantime, a question had already started burning in their minds: "What are quarks made of?"

The events discovered from the data collected during the 1994-96 experiments appear to be evidence that quarks are not the smallest particles. "If it's true," Mo said, "it means we have a much greater number of species of particles than previously thought. It means the quark is made up of something else." And, he said, if the quark has a substructure, maybe electrons, too, are made up of other particles.

Two independent groups of scientists from all over the world got the same results. One group was conducting the ZEUS experiment and another the H1 experiment, both at the collider HERA at the DESY laboratory in Hamburg, Germany. HERA, some 200 feet underneath Hamburg, is the only facility in the world that forces the collision of electrons or positrons and protons. There, scientists can observe collisions at energy levels previously unavailable, allowing them to study structures inside the proton down to one part in a thousand of the size of the proton itself, according to a press release from DESY on the World Wide Web.

Inside HERA, the press release says, the two particle beams circulate in opposite directions in separate storage rings and are brought to head-on collision at two interaction points equipped with the detectors H1 and ZEUS.

In the ZEUS experiment, about 450 scientists use the accelerator to slam electrons with 30 GeV (one billion electron volts) of energy into protons with 820 GeV's. The result of the shattering of the particles is a scattering of tracks of particles "We're examining a distance shorter than 10-16 centimeters," Mo said.

This year, the researchers combined the data from 1994-96 and found five events whose rate of occurrence was four to five times greater than expected from the standard model, Mo said. Scientists on the H1 experiment found similar evidence.

Both groups compared their measurements with the standard model and found the events that did not fit. They presented their results in a DESY Seminar Feb. 19, and both papers were submitted to the German physics journal Zeitschrift fur Physik. Both papers can be found on the WWW.

"We're not sure what it's about," Mo said. It could mean the quark is not the smallest particle or it could be merely a statistical fluctuation. The researchers, excited by the new finding, will begin work three months earlier this year, in March, to do more experiments to see if the data backs up the discovery.

"We'll collect more data than the past three years," he said. "Then we can see if it confirms or not."

If it confirms the findings, the researchers will need more energy than they have now to probe further and further into the subparticles (the smaller the distance, the greater the energy needed). Such energy requires a large hadron collider, which smashes proton against proton. Two such experiments have been approved, the ATLAS and the CMS at CERN, Geneva, Switzerland. Researchers at Virginia Tech are members of the latter.