January 10, 2002

Electronic newsroom

02-05

Physicists See Once-in-a-Trillion Event — Again!

UPTON, NY — After careful study of six trillion subatomic particle decays, an international collaboration of physicists announced that they have spotted one of the rarest occurrences in the subatomic world — for a second time. The 50 researchers, from the United States, Canada, and Japan, have spent 12 years searching for this rare decay at the U.S. Department of Energy's Brookhaven National Laboratory. In this time, they have processed enough data to fill 50 million CD-ROMs.

The collaboration, called "E787" for its experiment number, reported the first-ever example of this rare decay four years ago. The second observation, which will be published online by Physical Review Letters on January 11, 2002, and in print on January 28, represents an important confirmation of that discovery.
 

K+ => pi+,nu,nubar event in the E787 apparatus. Click for description of this graphic.

 
The rare decay involves an unstable particle called a kaon, which can decay, or break apart, in a variety of ways. One particular decay mode, in which the kaon turns into a positively charged pion, a neutrino, and an antineutrino (written), is so rare that theory predicts it will only happen several times in every hundred billion kaon decays.

Because of difficulties in distinguishing this type of decay from the many others that look like it, the physicists had to search through many times that number of events, just to have a chance of spotting one.

How to search for a needle in a universe of haystacks

To search through 6 trillion kaon decays, the E787 collaboration first needed to get a hold of 6 trillion kaons. For this they chose Brookhaven’s Alternating Gradient Synchrotron, a particle accelerator capable of producing the world’s most intense beam of kaons.

Since kaons only exist for about 12 billionths of a second, the E787 collaboration then had to build a state-of-the-art particle detector the size of a small house in order to capture these fleeting decays in detail. This machine is capable of examining one million decays every second.

Those decays that the machine decided were promising were short-listed to magnetic tape. This “short” list, which was actually thousands of gigabytes in size, was pored over in detail by the physicists as they reconstructed what really happened inside the detector.

“Out of all that data, we’ve now found two events explicable only by the rare kaon decay we were searching for,” said co-spokesperson Brookhaven physicist Laurence Littenberg.

Strange science

Co-spokesperson Doug Bryman, of the University of British Columbia, says that this rare decay is one of the keys to understanding the universe’s most elemental forces and building blocks. “This is a decay that physicists have been looking for since the 1960s, but nobody knew for sure if we would see it,” said Bryman.

The reason this decay is worth so much careful study is that it involves some of the more exotic aspects of the Standard Model, the theory that describes the subatomic world. Kaon decays in general have proved a rich and often surprising source of information on fundamental questions in particle physics, largely due to the kaon’s “strange” quark, a heavy relative of the quarks that comprise ordinary matter such as atomic nuclei.

When a K+, the lightest particle to contain a strange quark, decays to a pi+, which is comprised of ordinary quarks only, the strange quark is converted into a “down” quark. This is forbidden in any direct process by the Standard Model. K+ -> pi+,nu,nubar, however, still has a small chance of occurring by means of an indirect two-step process involving two very massive gauge boson force carriers and other quarks — in particular, the massive “top” quark, an exotic particle discovered in 1995 at Fermi National Accelerator Laboratory.

“Understanding such complex forms of decay is especially important to physicists attempting to learn how matter behaves at the most fundamental level,” said Bryman.

You ain’t seen nothing . . . yet

Over the course of the 12-year experiment, the E787 team, led by Bryman, Littenberg, and, at an earlier phase, Stewart Smith of Princeton University, upgraded its detector by adding new instruments and components to track particles and observe them decay in nanosecond detail.

Now that it has proven to have the sensitivity to see the rare Kaon decay, a follow-up experiment, E949, will attempt to gather ten times as much data so the decay can be studied in greater detail.

“We will also test the possibility that the events we’ve seen might instead involve entirely new particles or forces,” added Littenberg.

An expanded collaboration called KOPIO, led by Bryman, Littenberg, and Michael Zeller of Yale University will continue the study of rare kaon decays for the next several years at the AGS. KOPIO 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 chance of glimpsing the still-mysterious phenomenon of charge conjugation and parity violation (CP-violation). KOPIO is awaiting congressional approval of funding through the National Science Foundation, and has already garnered support from Canadian and Japanese counterparts.

Experiments on these rare kaon decays are complementary to the large worldwide program to study CP-violation in the B meson system. The BABAR experiment at the Stanford Linear Accelerator Center and the BELLE experiment at the KEK Laboratory in Japan have recently reported observations of CP-violation. If the Standard Model is correct, the kaon and B experiments are two very different approaches to measuring the same fundamental quantities. If the results of these two approaches disagree, that will be a dead giveaway that the Standard Model is wrong, and new theories will need to be developed.

This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields; the Ministry of Education, Culture, Sports, Science, and Technology of Japan; the Natural Sciences and Engineering Research Council; and the National Research Council of Canada.

Note to local editors: Laurence Littenberg lives in Aquebogue, New York.