#97-96
Contact: Kara Villamil, or Mona S. Rowe
RELEASED 9/22/97
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RAREST PARTICLE DECAY
SEEN AT BROOKHAVEN LAB


UPTON, NY -- After ten years of searching, an international collaboration of physicists working at the U.S. Department of Energy's
Brookhaven National Laboratory (BNL) believes it has seen the rarest decay of a subatomic particle ever detected.

The phenomenon is thought to happen only once or twice in every 10 billion self-destructions of an unstable particle known as a kaon. Instead of producing the usual breakdown products seen when a kaon decays, the rare kaon decay, as it is called, is thought to have released a positively charged pi meson, a neutrino and an anti-neutrino.

Not only is the proccess rare, it is also extremely elusive. The scientists report that to see even one such event using the most sensitive equipment at BNL's Alternating Gradient Synchrotron accelerator, they had to sift through one trillion ordinary decays to achieve the one-in-ten-billion level of sensitivity required.

The team, made up of 50 researchers from Brookhaven, Canada's TRIUMF laboratory and University of Alberta, Japan's KEK laboratory and Osaka University, and Princeton University, describes its findings in a paper in the September 22 issue of Physical Review Letters.

The spotting of the rare kaon decay sheds new light on the universe's most elemental forces and most basic building blocks, as explained by the extraordinarily successful theory of subatomic particles known as the Standard Model. It may also suggest new phenomena that cannot be explained by the Standard Model.

Rare, But Important

Because of its highly unusual nature, the knowledge gained in studying this decay is expected to be exceptionally important to particle physicists.

"This is a phenomenon that physicists have been looking for since the 1960s, but that nobody knew for sure we would see," said Douglas Bryman of TRIUMF, one of three co-spokesmen for the collaboration, which is known as
E787. "Now, after years of searching, we believe we have seen it."

Added co-spokesman and Brookhaven physicist Laurence Littenberg, "From here, it is up to us and others to test that belief through further exploration and experimentation. We plan to collect and analyze ten times more data in order to gauge its consistency with the Standard Model, and to test the possibility that the event we've seen could even involve entirely new particles or forces."

The usual decays (and similar interactions) seen in particles and in radioactive atomic nuclei occur by the transmission of one massive W or Z boson, which are carriers of the weak force in the same way that the particle form of light, known as photons, carries the electromagnetic force.

The Standard Model predicts that the decay of a kaon to a pi meson and a neutrino pair sometimes involves the momentary creation of both a charged W boson and a neutral Z boson (which itself instantly decays into the two neutrinos), rather than the more easily produced exchange of a single W or Z. It can also involve the recently discovered massive top quark, and thus give a window into the relation between that exotic object and the normal quarks which make up our everyday world.

Understanding such complex forms of decay is especially important to physicists attempting to learn how matter behaves at the most fundamental level. The one-in-ten-billion probability of a kaon decaying to a pi meson and a neutrino pair is a remarkable prediction of the Standard Model and one that the experimenters set out to test.

Catching A Shooting Star

Finding the rare kaon decay required an accelerator powerful enough to produce kaons in vast numbers, making BNL's AGS an appropriate choice. Recent upgrades there made the accelerator capable of producing the world's most intense kaon beam.

But equally important was the team's array of detectors sensitive enough to catch the particle equivalent of a shooting star: Kaons last only about 12 billionths of a second before decaying, and they can decay a multitude of different ways, creating showers of particles that can only be seen with specialized equipment.

So, to catch a fleeting pi meson, the E787 team, led by Bryman, Littenberg and A.J. Stewart Smith of Princeton, in 1995 built a new "catcher's mitt," located in a strong magnetic field and made up of sophisticated particle detectors used to measure as much as possible about each pi meson that passed by. These detectors included scintillating fibers, a tracking chamber and several other devices used to determine the energy and momentum of the pi meson and to observe its characteristic decay into other particles.

The improved equipment increased the chances of seeing a rare decay if it occurred, and vastly reduced the chances of confusing it with other phenomena that send out nearly the same signal but happen billions of times more often.

"The experiment's state-of-the-art apparatus is sensitive enough to examine one million decays per second," said Littenberg. "We collected thousands of gigabytes of data, and out of all that data, we saw one event that was completely unexplainable except by the rare kaon decay we were searching for."

A Nod to the Past, a Glimpse of the Future


The discovery ties in to both past and future research at BNL's accelerators.

"It's especially fitting we should see this phenomenon at the AGS, since kaon decays have figured centrally in past important discoveries there," said Bryman. "The most notable example, of course, is the work on CP-violation that won the 1980 Nobel Prize for James Cronin and Val Fitch. That discovery, which among other things may help explain why there's more matter than antimatter in the universe, was an unanticipated rare kaon reaction which had a revolutionary impact."

And, he added, the E787 collaboration plans to continue the present study of rare kaon decays for the next several years, even after the AGS becomes the injector for the Relativistic Heavy Ion Collider (RHIC) when it begins operations in 1999.

There are also plans to study the closely related decay of the long-lived neutral kaon into a neutral pi meson and a pair of neutrinos, a process that may offer the single best window into the still-mysterious phenomenon of CP-violation.

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RARE KAON DECAY GRAPHICS



A pi meson travels swiftly through the detectors in the E787 experiment at BNL's Alternating Gradient Synchrotron after another particle, called a kaon (red striped squares), spontaneously decays. The pi emerges from the stopping target (its path is shown by the blue squares) and enters the detector's central drift chamber (the small circles in the drift chamber are tangent to the path of the pion). It then penetrates the range stack, an array of scintillators (excited ones shown as blue rectangles) and chambers and loses energy until it stops. The signal in the scintillator where it stops is shown at the top right. The double pulse is characteristic of a pion. After examining 1.5 trillion events, the E787 collaboration found the pictured decay in which a pion is completely unaccompanied by other detectable particles, an important find for physics.

Members of the E787 collaboration at Brookhaven National Laboratory's Alternating Gradient Synchrotron pose in front of their apparatus, which allowed them to spot the rarest particle phenomenon ever seen.