Demolition Derby of Physics Jars Loose Clues on Subatomic Glue
(Page 2 of 2)
In an episode that has become almost nonexistent among latter-day
particle collaborations, usually consisting of hundreds of physicists
working in antlike synchrony, that challenge began this year when a
single scientist noticed something odd in the data flowing from BaBar.
Like many experiments now, when physicists have no hope of passing the
high-energy frontier in collisions, BaBar uses a different strategy:
creating vast numbers of particles that are closely monitored when they
decay into other products, with an eye to measuring their properties
precisely and finding exotica.
Scientists like to call this the luminosity frontier, said Dr. Stephen
L. Olsen, a physicist at the University of Hawaii, drawing an analogy
between intense particle beams and bright light beams. BaBar, for
example, has created some 150 million B-meson pairs, particles made of
a bottom quark and an up or down one.
In January, Dr. Antimo Palano, a collaborator on the experiment from
Bari University in Italy, was checking a decay process and saw a small
bump in the data sample. He added more data, and the signal kept
getting bigger. The announcement of a new particle was made by the
collaboration in April.
"I was looking for something new in the data, and I was lucky to see
it," Dr. Palano said in a telephone interview from his office in Italy.
But what was the particle? Dr. Palano and some colleagues believe that
it was a long-sought type of D-meson that contains a charm quark and a
strange quark. In fact, Dr. Estia Eichten, a theorist at Fermilab, said
he and others had predicted an incorrect mass for the particle - a
reason no one had found it before.
If that interpretation turns out to be right, it will shed light on the
workings of the strong force, Dr. Eichten said: his faulty calculations
assumed that the two quarks whirled around each other like the proton
and electron of a hydrogen atom. Improved calculations suggest the
mesons are tethered as if by a rubber band, with one of the quarks
behaving as if it were nearly massless.
But other physicists, like Dr. Close of Oxford, suggested that the
particle's surprising mass could be explained more easily if it were a
kind of molecule of two other mesons, whirling about each other and
exchanging still other particles that help them stick together.
Dr. Close said that debate remained unresolved. But by July, the Thomas
Jefferson National Accelerator Facility in Newport News, Va., had
presented data that might have clinched the case that agglomerations of
five quarks had been seen: two up, two down, and one strange quark
swirling about in a confraternity that had never been seen before.
"At the present moment," said Dr. Lawrence Cardman, an associate
director at the accelerator, "there is to the best of my knowledge no
model that explains all of the data."
In one view, the up quarks bind relatively tightly to the down quarks,
with the strange quark standing alone, and the whole contraption
tumbles about like a three-atom molecule. Another says a vibrating,
rotating clump forms, consisting of a two-quark and a three-quark
nugget.
Finally, in the paper in tomorrow's issue of Physical Review Letters, a
collaboration at KEK says that it has come up with another startling
find, which it calls X(3872). A number of theorists believe the
particle is a kind of charmonium - the term for a pair of charm quarks
orbiting each other. Some scientists, including Dr. Olsen, the
collaboration's spokesman, believe they have turned up another quark
molecule.
This time, the molecule would contain a pair of mesons, each consisting
of a charm quark and an up quark. Somehow the whole seemingly fragile
collection would be sticking together long enough to be detected. Even
with those questions, Dr. Olsen said, "we're really leaning toward this
molecular point of view."
Add it all up, and it has been a hot time for those who study the
strong force, said Dr. Close, who added that scientists had moved
nearer to understanding the mysterious glue of the atomic nucleus.
|
