Strange but charming new particle found

Scientists deploy leaky bucket analogy

Scientists working on Fermilab's SELEX experiment have found a odd new sub-atomic particle in the 'heavy-light' meson family. The researchers are intrigued because the meson lives longer than it should, weighs more than it should, and decays into unexpected particles.

Typically, the heavier the meson, the faster it decays. However, this meson - a combination of a strange quark and a charm antiquark - is the heaviest meson ever observed in this family, yet it lasts three times as long as its lighter relatives.

Fermilab's Peter Cooper said this kind of contradiction is "just not supposed to happen. If this meson played by the normal rules of the strong interaction," Cooper said, "it should fall apart quickly and we never would have seen it."

The lifetime of a typical meson is so short, they almost don't exist at all: about 10-24 seconds, or the time it would take light to cross a proton. This makes them tricky to locate. In fact, these mesons have only just been identified after extended analysis of data from an experiment that ended in 1997.

A meson is made up of a quark and an antiquark, held together by the strong force. The family name, 'heavy-light', refers to the fact that these mesons are made up of a massive quark and a relatively light quark.

Relative to one another, the heavy quark sits still and researchers need only track the motion of the lighter companion. This makes them relatively easy to study, and an ideal place for scientists to investigate the strong force. This is the force that keeps atomic nuclei from flying apart. It also controls the decay rates of particles, which are generally fairly predictable, the researchers say. This result implies other undiscovered decay patterns lurking in nuclei.

If that isn't weird enough, when it finally does decay, it does so strangely. Researchers found that the meson decayed into an eta particle six times more often than theory predicts. Scientists were expecting it to decay into a K-meson.

The scientists think the results indicate that a new (as in, new to science) dynamical aspect of the strong force is at work. Fermilab's James Russ says this is like watching someone pour water into a bucket with a small hole and a large hole in the bottom: "For some reason, the water is pouring out the small hole six times faster than it's coming out of the large one. Something unusual must be going on inside the bucket." ®

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