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/
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