March 31, 1997
Research Update: Particle physics
SCIPP researchers skeptical about hints of a new particle
By Robert Irion
In a famous experiment more than 80 years ago, physicist Ernest Rutherford fired particles into an ultrathin layer of gold. Most particles zipped through, but some ricocheted sideways and backward, as if they had struck immovable objects. This amazing result parted the curtain of mystery around the atom and proved that most of an atom's mass lies in a tiny hard ball at its center: the nucleus.
Since then, scientists have found protons and neutrons within the nucleus and still smaller bits of matter, called quarks, inside of protons and neutrons. Now, physicists at UCSC and elsewhere have updated Rutherford's experiment--at unimaginably higher energies--to unveil tantalizing hints that the Chinese puzzle of the nucleus may yet contain even more layers.
The new results hail from two detectors at the Deutsches Elektronen-Synchrotron (DESY), a particle accelerator in Hamburg, Germany. Researchers at the Santa Cruz Institute for Particle Physics are among several hundred physicists working at ZEUS, one of the detectors. SCIPP postdoctoral researcher David C. Williams visited the campus from Germany recently to brief his colleagues on the curious data.
Inside ZEUS, a beam of protons slams into a beam of positrons (the antimatter counterparts to electrons). Most of the collisions create unremarkable fireworks of particles. But in a few, the positrons rebound in clean, high-energy U-turns. Those encounters, physicists believe, probe incisively into the quarks within each proton. Indeed, some researchers interpret the collisions as the first good evidence for an entirely new particle inside the quark--or even a weird electron-quark hybrid, dubbed a "leptoquark."
The Standard Model, today's theoretical edifice of particle physics, would crack wide open if either of those claims holds up. However, Williams and others at SCIPP aren't yet willing to shimmy out on that daring limb.
"It's too early to say," Williams said. "Some think the data can represent only new physics or an extraordinary statistical fluke. I would consider a third possibility: We are not applying the Standard Model correctly to understand the results." The DESY experiments will continue through August, Williams noted, which should double the precious cache of data.
The ZEUS research team will publish its results in the German journal Zeitschrift fuer Physik. Other UCSC contributors are SCIPP director Abraham Seiden, Professor Clemens Heusch, Adjunct Professor Hartmut Sadrozinski, researchers Bill Lockman and Tim Dubbs, and several graduate students.
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