Particle experiment produces abundant hypernuclei
18:38 30 January 04

A new particle physics experiment has created as many "hypernuclei" in its first three months of operation as have been produced in the 50 years since the exotic particle was first discovered. Scientists announced the achievement on Friday at a meeting in Bormio, Italy.

Hypernuclei are atomic nuclei combining not just the usual protons and neutrons but also rare particles called hyperons. About 100,000 have been created in an experiment called FINUDA, on the Dafne accelerator at the National Laboratory of Frascati.

Hypernuclei are incredibly short-lived, surviving for less than a billionth of a second. But by studying them scientists hope to learn more about the weak force, one of nature's four fundamental forces, as well as the first moments of the Universe's existence.

The FINUDA experiment has so far churned out 35 varieties of hypernuclei already known to science. But in future months, scientists hope to fashion completely new hypernuclei, such as a hydrogen-7-lambda, comprised of one proton, five neutrons, and one exotic lambda particle, a hyperon that includes a strange quark. Normally, hydrogen contains zero, one, or two neutrons, but the massive lambda particle allows the nucleus to bind to additional neutrons.

"There is a general consensus that we had abundant production of strange quarks at the Big Bang," says FINUDA spokesman and nuclear physicist Tullio Bressani. "If we demonstrate objects like hydrogen-7-lambda are stable, it would be a bridge to answer questions about the strange nuclear matter in the early Universe."

Rare fish
Hypernuclei are only rarely produced in the natural world, when a high-energy cosmic ray strikes a nucleus on Earth under just the right conditions. "It's like you are trying to catch a rare fish," Bressani told New Scientist. But the accelerator provides a steady supply of hypernuclei.

The FINUDA experiment has produced many more hypernuclei than other attempts around the world because its detectors were designed specifically to create and study hypernuclei.

However, creating hypernuclei is a difficult, multi-stage process in which researchers must swap a lambda particle for a proton or neutron in a nucleus. "It isn't easy to do this," says Peter Meyers, a nuclear physicist at Princeton University, New Jersey, US. "You have to make the exotic particle, get it into the nucleus, have it stick there, and know all this has happened."

First the researchers smash an electron and its antimatter counterpart, a positron, together to produce a medium-weight elementary particle called a phi meson. This then decays into charged kaons. Shooting the kaons at paper-thin targets of lithium, carbon, vanadium, or aluminium produces the hypernuclei.

Researchers know they have created a hypernucleus when they measure the energy of one of its decay products, a negatively charged particle called a pion.