First large-scale production of nuclei containing two strange quarks
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 super dense 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.
This work was funded by the U.S. Department of Energy, which supports
basic research in a variety of scientific fields.
