The SELEX experiment at Fermilab has announced three candidates for doubly charmed baryons. In a presentation at the US laboratory at the end of May, collaboration co-spokesman Jim Russ of Carnegie Mellon University described the painstaking analysis of data recorded in 600 GeV proton, pion and sigma hyperon collisions with a diamond target in 1996. The observation comes as a surprise to the collaboration, which did not expect to see doubly charmed baryons at all, and there remain uncertainties over the conclusion to be drawn. The SELEX candidates have the right characteristics to be up-charm-charm and down-charm-charm combinations, but the mass difference between the two states is larger than expected. Like the proton and neutron, the candidate particle pair is related by the replacement of an up by a down quark, so the mass difference was naively expected to be similar. SELEX, however, sees a mass difference 60 times larger.
The collaboration chose to make its announcement in the hope that doing so would spur the broader community to take up the search. In particular, data from the BaBar and Belle collaborations at the SLAC and KEK B-factories in the US and Japan would be good places to look for doubly charmed baryons.
Multiply charmed baryons are a natural consequence of the quark model of hadrons, and it would be surprising if they did not exist. Whether or not the high-mass states that SELEX reports turn out to be the first observation of doubly charmed baryons, studying their properties is important for a full understanding of the strong interaction between quarks.