The humble protons and neutrons that make up atomic nuclei are but the two best-known members of a vast family of particles. Now a team of physicists says it has spotted for the first time two exotic cousins of the proton and neutron that each contain two copies of a fundamental particle called the charm quark. But researchers from a competing experiment say they've seen no evidence for the new particles.
According to the standard model of particle physics, quarks come in six flavors: up, down, charm, strange, top, and bottom. Three quarks can combine to form a particle called a baryon, and the most common baryons by far are protons--which contain two ups and a down--and neutrons--which contain two downs and an up. However, the other, heavier quarks can also form baryons. Physicists have long predicted that two charms should also join either with a down quark to make a particle called a cascade-c-c-plus or with an up quark to make a cascade-c-c-double-plus. So far, though, no one has seen such beefy doubly charmed baryons.
Now, researchers working at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, report 16 particles that look like the cascade-c-c-plus and seven that look like the cascade-c-c-double-plus. To make them, physicists working with the Segmented Large X Baryon Spectrometer (SELEX) smashed a beam of ?-minus baryons--each of which contains a strange and two down quarks--into a series of copper and diamond targets. The researchers sifted through more than a billion particle interactions for evidence of the doubly charmed particles. The results were announced at Fermilab on 31 May at a seminar by Jim Russ, a physicist at Carnegie Mellon University in Pittsburgh, Pennsylvania, and spokesperson for the SELEX team. "We're comfortable that the signals we're seeing are real," Russ says. "The interpretation of them as double charm is pretty strong."
However, others are skeptical. Fermilab's Harry Cheung says that if the particles are really there, experimenters working on the independent FOCUS experiment should also have seen lots. "We don't see any of these, and we've looked," Cheung says. He also notes that the masses of the two putative kinds of particles are quite different, whereas physicists expect them to be nearly the same. Still, the SELEX signals are quite strong, says Roy Briere, a physicist at Carnegie Mellon who works on the CLEO experiment at Cornell University in Ithaca, New York. "It looks nice. It smells funny," Briere says. "I don't know what to think."