Belle found the new state while searching for missing states of charmonium (bound states of a charmed quark and a charmed antiquark) in B-meson decays, and X(3872) may turn out to be a particularly interesting example of the species. But the mass of X, 3872 MeV (million electron volts), so nearly equals the sum of two charmed meson masses, that X(3872) might turn out to be something entirely new: a fragile molecule binding a D⁰meson to an anti D^*0 meson. The same terrain is also considered a hunting ground for "hybrid" mesons made not of a charmed quark and antiquark, but of a charmed quark and antiquark plus a gluon. A puzzle for the quarkonium interpretation is that Belle has looked for, but not found, the "radiative transitions" to other charmonium states, like the transitons by which ordinary atoms radiate light.

Acting on Belle's compelling signal, physicists using the Collider Detector at Fermilab (CDF) quickly found X(3872) in 2-TeV proton-antiproton collisions in Fermilab's Tevatron, the world's most powerful accelerator. CDF announced its result at a Quarkonium Workshop held at Fermilab in mid-September. CDF's observation strongly suggests that X is produced not only in weak decays of B-mesons, but also directly through the strong interaction. The character of the Belle and CDF signals illustrates the complementary strengths of the B-factory and the Tevatron: Belle observed about sixty X(3872) events with little background, whereas CDF has seen about seven hundred X-events on a background of about 6000.

The two observations have sparked a flurry of theoretical activity, including proposals for follow-up measurements and searches. Fermilab theorist Estia Eichten devoted his "Future Opportunities in Theory" talk at the Quarkonium Workshop entirely to X(3872). If X turns out to be the missing charmonium state, its position just on the threshold for decay into charmed mesons will provide a superb laboratory for charmonium studies. If X is something else--a charm molecule or a hybrid meson--then it is the first hint of a whole new spectroscopy. In any event, the discovery of X(3872) leaves us in a no-lose situation, with plenty to do and plenty to learn, whatever it turns out to be.

A selection of X-theorists
Charmonium. Estia Eichten (Fermilab); Chris Quigg (Fermilab).
Charm Molecules. Nils Törnqvist (University of Helsinki); Misha Voloshin (University of Minnesota).
Hybrid States. Steve Godfrey (Carlton University).
General Diagnostics. Frank Close (Oxford University); Philip Page (Los Alamos); Sandip Pakvasa (University of Hawaii); Mahiko Suzuki(University of California).