The Last of the Mesons

Physicists say they have finally unearthed the last member of a set of subatomic particles called mesons. The discovery of the Bc meson, announced recently at a seminar at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, is the final entry in the basic "periodic table" for these particles. Physicists expect that the new meson's mass and lifetime will sharpen their understanding of the force that holds atomic nuclei together.

The basic building blocks of the protons and neutrons that form an atom's nucleus are even smaller particles called quarks, bound together by the so-called strong force. Quarks can also pair up to make mesons, which are extremely short-lived. Physicists studying cosmic rays pummeling Earth or the debris from particles colliding in accelerators have found 14 of the 15 possible mesons formed by different combinations of five kinds of quarks. But no experiment had given birth to enough Bc mesons to glimpse the particle.

This year, the 450-odd physicists of the Collider Detector at Fermilab (CDF) experiment finally had enough data to corner this rare beast. Wading through data from trillions of collisions, two students--Prem Singh of the University of Pittsburgh in Pennsylvania, and Jun-Ichi Suzuki of Tsukuba University in Japan--hunted for the remains of the Bc: a lighter meson and another particle. They found what looked like the tracks of about 20 Bc's.

The Bc meson's lifetime--a half a trillionth of second--is about half as long as some theories predicted. Those theories say the strong force binds the two quarks so tightly that the meson has a hard time decaying and should live longer. The measured short lifetime, says Fermilab physicist Jonathan Lewis, will help physicists resolve this lingering puzzle.

But the small number of observations has left room for doubt over the new meson's existence. "I'm skeptical," says Sheldon Stone, a physicist at Syracuse University in New York. But CDF's Lewis, who helped supervise the research, says the odds of what they saw being random fluctuations are about one in a million. "That's the same chance as being hit by lightning in North America," he says, "which is my personal definition of an acceptable risk."