Interactions News Wire #18-04
23 March 2004
http://www.interactions.org

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Source: Brookhaven National Laboratory
Content: Press release
Date Issued: 23 March 2004
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Physicists See Golden Needle in a Micro-Cosmic Haystack

UPTON, NY -- An international team of physicists examining an
extremely rare form of subatomic particle decay -- a veritable golden
needle in a micro-cosmic haystack of 7.8 trillion candidates -- has
discovered evidence for the highly sought process, which could be an
indication of new forces beyond those incorporated in the Standard
Model of particle physics. That long-standing theory of all particle
physics precisely predicts the rate of such decays to be half that
observed by the experimenters although it is still too soon to say if
a deviation has occurred. The innovative experiment, which uses the
most comprehensive particle detector ever built, is located at the
U.S. Department of Energy's Brookhaven National Laboratory. The
result is being presented at a colloquium at Brookhaven Lab today and
has been submitted to Physical Review Letters.

The experiment detects the disintegration of an unstable subatomic
particle called a K meson, which can decay, or break apart, in a
variety of ways. One particular decay -- in which the K meson turns
into other particles, a positively charged pion, a neutrino, and an
antineutrino -- is extremely important due to the internal subatomic
processes involved and its sensitivity to new physical effects not
accounted for in the Standard Model. The decay is so rare that it was
predicted to happen only once in all the decays ever observed by all
of the experiments that have searched for it since the 1960s.

The latest evidence of the long-sought process was found in
just-analyzed data. It followed two earlier sightings at Brookhaven
in 1997 and 2002 (see:
http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr011002.htm). The new
data were obtained using improved apparatus that exploited higher
beam intensities and achieved greater efficiency of detection than
any previous experimental setup.

The current result indicates that this particular rare K meson decay
occurs once in every 7 billion decays. The improved result continues
to suggest a possible discrepancy with the Standard Model, although
with only 3 events, the result is still consistent with this model's
prediction of one in 13 billion decays.

"It is very important to establish whether these first few events
represent a statistical fluke or an important breakthrough," said
Douglas Bryman, Professor of Physics at the University of British
Columbia, one of the experiment's spokespersons. "This can only be
done with an enhanced event sample, which could be obtained by
further running of the experiment.

"Additional running would resolve the issue and firmly establish
whether we are seeing an extremely significant departure from
standard theory," Bryman said. Such further running would require
program funding not presently planned.

The long trek leading to discovery

The experimental collaboration -- now composed of 70 scientists from
Canada, Japan, Russia and the United States, (see:
http://www.phy.bnl.gov/E949/, Collaboration List) -- has been
conducting the search for the past decade at the Alternating Gradient
Synchrotron, a particle accelerator at Brookhaven Lab that produces
the world's most intense beams of K mesons. K mesons are elusive
particles that exist for only 12 billionths of a second before
decaying into other forms. So, to catch the fleeting events and
identify the rare decay, the scientists built a state-of-the-art
particle detector the size of a small house, capable of examining 1.6
million decays every second. Interesting events get recorded on tape,
with several tens of thousands of gigabytes of data stored so far.
The physicists then use sophisticated software to pore over the data
to find the most interesting events and examine them in exquisite
detail.

Although a neutrino and an antineutrino are also emitted in this K
meson decay, these particles interact too weakly to be detected.
Thus, evidence that one positive pion -- and only a positive pion --
was produced by the K meson decay must be proven beyond a reasonable
doubt, eliminating the possibility that other detectable particles
are present. To establish the validity of the observations, the
scientists must reject all background cases where a K meson decays in
other ways, usually involving a charged particle or a neutral pion.
In order to achieve the unprecedented level of filtering required,
the group developed the most efficient particle detector system ever
built.

The hard part here is that neutral pions immediately decay into two
high-energy gamma rays (photons), and the experiment must not miss
them more than once in every million decays. To do this, the detector
stops the K mesons in their tracks -- in a scintillating fiber target
-- before they decay. The decay products then travel through a
particle-tracking chamber surrounded by a huge magnet and plastic
scintillation counters, so their momentum, trajectories, and energy
can be precisely measured to positively identify the types of
particles detected. Events that emit photons are picked up by
sensitive detectors and rejected, leaving only the rarest decays as
candidates for the process the scientists are seeking.

Out of all the data analyzed, the scientists have now seen three
events explicable by the rare K meson decay they've been searching
for. Their goal is to increase the experimental exposure by five
times. If their findings continue at the current pace, 20 or more
events would be observed. Such a result could profoundly alter our
current picture of particle physics, forcing an expanded view of the
fundamental constituents of the universe and their interactions since
the 'Big Bang.'

This research was funded by the Office of High-Energy Physics within
the Department of Energy's Office of Science, with additional support
from the Natural Sciences and Engineering Research Council and the
National Research Council of Canada, and through agreements with the
Japanese and Russian governments to support research at Department of
Energy facilities.

For technical background, go to: http://www.phy.bnl.gov/E949/

If you'd like a PDF version of this press release using the
scientific characters, contact: genzer@bnl.gov.

One of the ten national laboratories overseen and primarily funded by
the Office of Science of the U.S. Department of Energy (DOE),
Brookhaven National Laboratory conducts research in the physical,
biomedical, and environmental sciences, as well as in energy
technologies and national security. Brookhaven Lab also builds and
operates major scientific facilities available to university,
industry and government researchers. Brookhaven is operated and
managed for DOE's Office of Science by Brookhaven Science Associates,
a limited-liability company founded by Stony Brook University, the
largest academic user of Laboratory facilities, and Battelle, a
nonprofit, applied science and technology organization.

Visit Brookhaven Lab's electronic newsroom for links, news archives,
graphics, and more: http://www.bnl.gov/newsroom

Contacts:
Mona S. Rowe, 631 344-5056, mrowe@bnl.gov
Peter Genzer, 631 344-3174, genzer@bnl.gov
Karen McNulty Walsh, 631 344-8350, kmcnulty@bnl.gov