EXOTIC MESON AT BROOKHAVEN? Testing the notable proposition that quarks always bind in groups of two (quark- antiquark objects called mesons) or three (baryons), physicists at BNL send 18-GeV pi mesons into a hydrogen target and then harvest events in which the emergent debris includes eta mesons and pi mesons. In particular, they seek to study the parent particle that decayed into the eta and pi. Most of the time this is the humdrum a2 meson, with a mass of 1320 MeV. But about 3% of the time another particle appears to be the parent, one with a mass of about 1370 MeV. Two things in combination hint that something unusual is happening: first, the relative angle between the outgoing eta's and pi's suggests a rivalry or "interference" between the a2 meson and the mysterious particle, strengthening the argument that the particle exists and is not just a spurious blip in the data. Second, the new particle's quantum numbers (its spin, parity, and charge conjugation number) do not conform to what one would expect for a conventional quark-antiquark meson. Scientists in the Northwestern-Rensselaer-Massachusetts- Notre Dame-BNL-Moscow State-IHEP collaboration (one contact is Neal Cason at Notre Dame, 219-631-6305) speculate that the exotic particle might be either an unprecedented 4-quark state (two quarks and two antiquarks) or a quark-antiquark-gluon state; gluons are the particle-like carriers of the strong nuclear force, but in this case the gluon in question would be an independent particle on an even footing with the quarks. (1 Sept. Physical Review Letters; copies for journalists available from physnews@aip.org.)