Window Opens into Strange Nuclei

Some of the strangest atomic nuclei ever observed have made fleeting appearances in a recent accelerator experiment. Whereas ordinary nuclei contain protons and neutrons, so-called hypernuclei produced in an experiment at Brookhaven National Laboratory in Upton, N.Y., also contain exotic particles quite different from those in ordinary matter. Although such particles, known as lambdas, have been spotted in nuclei before, this is the first time that nuclei with pairs of these exotic particles have been generated by the dozen, scientists say.

 The experiment also offers new evidence that nature is conservative in how it packages quarks, which scientists say are the building blocks of much of the matter in the universe.

 Moreover, with a means for essentially mass-producing two-lambda nuclei, experimenters now look forward to determining whether lambda particles repel or attract each other-interactions not measurable before. Those results, in turn, could deepen astrophysicists' understanding of supernovas and neutron stars, whose extreme conditions presumably could generate lambdas.

 Since there's no way to study extreme conditions on Earth, researchers have looked for other ways to get lambdas together. "When we put two lambdas in the same nucleus, you might regard the nucleus as a laboratory in which we can study their interactions," says Brookhaven's Robert E. Chrien, a member of the experimental team.

 Lambda particles are "strange" because they incorporate so-called strange quarks. Although lambdas each contain an up, a down, and a strange quark, they're not the same kind of strange matter that some people feared might trigger the destruction of Earth if an accelerator that opened at Brookhaven last year were to produce it.

 In the new hypernuclei experiment, a team of 50 scientists from six countries used a Brookhaven accelerator known as the Alternating Gradient Synchrotron to direct the world's most intense proton beam at a piece of tungsten. That yielded a powerful plume of strange-quark-containing particles called kaons. These, in turn, impinged on a beryllium target, which, on occasion, released a hypernucleus containing a proton, a neutron, and two lambdas.

 The Brookhaven-based scientists detected fewer than 40 of these "doubly strange" hypernuclei, but they say they actually produced hundreds of others whose trajectories veered away from the setup's detector. The team will report its findings in an upcoming issue of Physical Review Letters.

 In prior experiments during the past 40 years at Brookhaven and elsewhere, researchers detected only traces of single hypernuclei after painstaking examinations of particle tracks in filmlike emulsions, says Brookhaven's Adam Rusek, also a team member.

 In the Brookhaven study, the team verified the presence of doubly strange hypernuclei by using a cylindrical detection chamber to recognize pairs of particles called pions, which are produced when lambdas decay. The disintegration of lambdas takes a mere fraction of a nanosecond because the strange quarks in the particles are unstable.

 The experiment's findings could have been different, however, if nature were as creative in packaging quarks as some theorists have proposed. A theory developed in 1977 suggests that lambdas would readily fuse together into 6-quark particles called H's, each composed of two strange, two up, and two down quarks.

 If H's had formed in the experiment, lambdas wouldn't have disintegrated into detectable pions, because lambda fusions would have happened a hundred million times faster than lambda decays, Rusek explains.

 So for now, the data still show that nature deals its quarks in twos and threes. Says Frank Wilczek of the Massachusetts Institute of Technology, that's "a very profound result."