Doubly Strange Nuclei

"Doubly strange" nuclei, each containing two strange quarks, have been produced by an international team at Brookhaven National Laboratory (Bob Chrien, Brookhaven, 631-344-3903, chrien@bnl.gov). Doubly strange nuclei are presumed to play an important part in the role of strange matter in neutron stars and in the formation of the early universe. They also promise to deliver new information on the forces that hold a nucleus together.

Previously, various groups have reported single candidates for a nucleus containing two strange quarks. However, these candidates were inconsistent with one another and never independently confirmed, according to the Brookhaven team, which has now observed hundreds of these novel particles.

A "doubly strange" nucleus, or lambda-lambda hypernucleus as it's more formally known, consists of the usual protons and neutrons plus two lambda particles, each made of an up, a down, and a strange quark. In effect, a lambda is a neutron with one of its quarks, a down quark, replaced by a strange quark. The doubly strange nuclei produced at BNL can be thought of as a sort of heavy hydrogen nucleus consisting of a deuteron (proton and neutron) and two lambdas.

To create the doubly strange nuclei, the researchers aimed a beam of K^- mesons at a beryllium target surrounded by appropriate detection equipment. (The K mesons, each containing an up quark and an anti-strange quark, were created by colliding protons at a tungsten target in BNL's Alternating Gradient Synchrotron.) The absorption of a K^- meson by a beryllium nucleus resulted in a reshuffling of quarks in which two nucleons were converted into two lambda particles, each of which contains a strange quark.The doubly strange nuclei subsequently decayed into lighter particles by emitting pi mesons (the lightest kind of quark-antiquark pair) which were detected by the experimenters, and identified by their characteristic energies.

Having created many of these nuclei, the Brookhaven researchers believe that they have a reliable production technique. They hope to learn more about the force that binds lambdas together and thereby round out knowledge of nuclear forces.

In addition, the production of doubly strange nuclei constitutes a strong argument against the existence of the "H particle," a hypothesized bag of six quarks instead of the usual two or three. That's because bringing two lambdas together in a nucleus would make it energetically favorable for them to decay into an H, as long as the lambdas are not too tightly bound; however, the researchers did not observe any evidence of such a decay. (Ahn et al., upcoming article in Physical Review Letters; also see Brookhaven press release.)