Charm School For Quarks

The venerable standard model says that all nuclear matter--those particles that can exert an influence among each other via the strong nuclear force--consists of arrangements of the six known quark types, or "flavors."

Clumps of three quarks are called baryons and clumps of two are mesons; other clumpings may yet be found at places like Brookhaven's RHIC machine. The proton (two up quarks and a down quark) and neutron (two downs and one up) are notable examples of conventional baryons.

Subject to the rule that the quark combinations must possess an integral amount of electrical charge, many three-way quark combinations are possible, and among these are several not yet seen yet in experiments, especially those that include the heavier and less stable quarks (strange, charm, bottom, top).

Just as 19th-century chemists, once they had the periodic table in front of them, set out to find all the missing entries and to understand better the relations among elements along rows or columns in the table, so particle physicists strive to find all the quark combinations that can be found as a way of understanding the "chemistry" among the quarks.

What's the latest news from this baryon frontier? The SELEX experiment at Fermilab seems to have discovered the first baryon with two charm quarks.

SELEX steers beams of 600-GeV sigma hyperons, pions, or protons into a thin foil. The interaction vertex is, in effect, a cauldron or factory for producing new charm baryons. One charm quark in a particle is rare enough so having two charms is even more interesting.

Maybe too interesting; the masses of the double-charm "isotopes" seen so far (double-charm-plus-down-quark as opposed to double-charm-plus-up-quark) are more different from each other than they should be. (See the Selex website.)