Doubly Strange Particles Debut

Xi no more. An emulsion at KEK captures the demise of a X- and the creation of a LL6He.

WAILEA, HAWAII--For the first time, scientists are confident that they have created doubly strange nuclei: atoms that contain two particles bearing "strange" quarks. This has allowed them to get the first rough estimate of the attractive force between these strange particles, filling a gap in their basic understanding of matter.

The key to the discovery is an exotic particle known as L. Whereas ordinary neutrons and protons are combinations of well-understood up and down quarks, a L particle is made from an up quark, a down quark, and a more mysterious particle called a strange quark. To better understand how strange matter interacts, nuclear physicists have been scrambling to get two L particles together to measure how tightly they are bound. Toward this end, scientists have inserted single L particles into nuclei such as beryllium-7, creating "hypernuclei" (ScienceNOW, 6 March). But they had failed to coax two L particles into the same nucleus--until now.

Last week, Ken'ichi Imai of Kyoto University provided the first solid evidence of the creation and destruction of LL6He (helium-6-lambda-lambda): an ensemble of two protons, two neutrons, and two L particles. At the KEK high-energy accelerator in Tsukuba, Japan, Imai and his colleagues smashed kaons--two-quark particles with a strange component--into diamond, creating X-particles, which each contain two strange quarks. Then the X- particle was apparently absorbed by a carbon atom, creating LL6He. From this first glimpse LL6He, "the lambda-lambda interaction energy was determined for the first time," says Imai.

Another glimpse of the interaction was caught by Robert Chrien's team at Brookhaven National Laboratory in Upton, New York. They reported evidence of a different doubly strange hypernucleus, LL6H (hydrogen-4-lambda-lambda). According to these experiments--presented here at a joint meeting of the nuclear physics divisions of the Japanese Physical Society and the American Physical Society--the L-L attractive energy is about 1 million electron volts (MeV), much less than an earlier, dubious estimate of 4 or 5 MeV. The new number is more in line with expectations. "One MeV will make a lot of theorists happy," says Chrien.

Both groups stress that scientists still need to produce more doubly strange hypernuclei to be certain about the binding energy. But Ed Hungerford of the University of Houston says the recent advances in strange physics are extremely encouraging. So for once, physicists can be forgiven their extra dose of strangeness.

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