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Brookhaven physicists produce 'doubly strange nuclei'

Physicists from Brookhaven National Laboratory report the first large-scale production of nuclei containing two strange quarks

August 27—Strange science has taken a great leap forward at the U.S. Department of Energy's Brookhaven National Laboratory. There, physicists have produced a significant number of "doubly strange nuclei," or nuclei containing two strange quarks.

Studies of these nuclei will help scientists explore the forces between nuclear particles, particularly within so-called strange matter, and may contribute to a better understanding of neutron stars, the superdense remains of burnt-out stars, which are thought to contain large quantities of strange quarks.

The 50 physicists collaborating on the experiment, who represent 15 institutions in six countries, describe their findings in an upcoming issue of Physical Review Letters.

"This is the first experiment to produce large numbers of these doubly strange nuclei," said Brookhaven physicist Adam Rusek, a co-spokesperson for the collaboration. Four previous experiments conducted over the past 40 years in the U.S., Europe, and Japan have produced one such nucleus each, with varying degrees of certainty. In the current publication, which is based on data taken in 1998, the Brookhaven collaboration describes 30 to 40 events out of several hundred produced. "That's enough events to begin a study using statistical techniques," Rusek said.

To create the nuclei, the scientists aim the world's most intense proton beam—produced at one of Brookhaven's particle accelerators, the Alternating Gradient Synchrotron—at a tungsten target. From the particles produced in those collisions, the scientists separate out an extremely intense beam of negatively charged kaons, which are each composed of one "strange" quark and one "up" antiquark. When these negative kaons then strike a beryllium target and interact with its protons, some of the energy is converted into new strange quarks and strange antiquarks.

These quarks then regroup to form a variety of particles, some of which continue to interact. Occasionally, a structure containing a proton, a neutron, and two lambda particles (each composed of one up, one down, and one strange quark) is formed. This double-lambda structure, with its two strange quarks, is the observed doubly strange nucleus.

Detecting the formation of this strange species is no easy task. It's more like finding a subatomic needle in a particle-soup haystack. For one thing, many other species are produced in the collisions. Plus, the scientists can't "see" the double lambda structure directly. Instead, they look for pions, a subatomic product the lambdas emit as they decay in less than one billionth of a second. Furthermore, in order to infer that the pions came from a nucleus containing two lambdas, there must be two pion decay signals at very specific energies.

Sophisticated computers and careful analyses helped narrow the search from 100 million potentially interesting events, to 100,000 where two strange quarks were produced, to the 30 to 40 where those two strange quarks existed for a fleeting instant inside the same nucleus. "The most important part is eliminating all the other possible explanations for these events," said Sidney Kahana, a theoretical physicist at Brookhaven. "We're left with this double lambda species as the only explanation," he said.

Now that they believe they have a reliable method for producing the double lambda species, the scientists would like to produce more so they can get better measurements of the binding energy, or force of interaction, between the two lambda particles. "We can use this nucleus as a laboratory in which the two lambdas can be held together long enough to study," Kahana said.

Based on the current data, the interaction between lambdas appears to be rather weak—possibly too weak for the two particles to merge to produce a postulated, six-quark structure called an H particle. But further experiments are necessary, the scientists say.

The interaction between lambdas may also offer insight into the properties of neutron stars, which are thought to contain vast numbers of strange particles, including lambdas. Neutron stars are the only place in the universe scientists believe such strange matter exists in a stable form.

With the ability to produce appreciable numbers of doubly strange nuclei, "Brookhaven is now the best place in the world to study strange matter," said Morgan May, who leads the strangeness nuclear physics program at Brookhaven.—by Karen McNulty Walsh

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