The last meson

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The last meson
The last meson

The Last Meson

Physicists have finally discovered the latest member of a family of subatomic particles they call mesons. They expect that the new particle's mass and lifetime will lead to a better understanding of the force "that holds our world together." 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 "periodic table" of these particles.

Quarks are the elementary building blocks of protons and neutrons, which form the nucleus of an atom. These quarks are held together by the so-called strong force. They can also pair up to form mesons, but these are extremely short-lived. Physicists who collect cosmic rays with their measuring devices or collide particles in particle accelerators in order to then examine the debris have so far found 14 of the 15 conceivable mesons, which are formed by various combinations of the five (out of six) different quarks that are relatively easy to produce be able. However, no experiment produced enough Bc mesons to get a glimpse of the particle.

This year, the experiment of around 450 physicists at Fermilab's Collider Detector (CDF) finally provided enough data to detect the shy building block. Two students - Prem Singh of the University of Pittsburgh in Pennsylvania and Jun-Ichi Suzuki of the University of Tsukuba in Japan - dug through data from trillions of collisions and hunted for the remains of Bc: a lighter meson and another particle. They found what looked like a trace of Bc about 20 times.

The lifetime of the Bc meson - half a trillionth of a second - is about half the time span predicted by some theories. According to these considerations, the strong force binds the two quarks so tightly that the meson has a hard time decaying and should live longer. The measured short lifetimes, says Jonathan Lewis, physicist at Fermilab, will help physicists solve this open puzzle.

However, the small number of observations leaves room for doubt about the existence of the new meson. "I'm skeptical," says Sheldon Stone, a physicist at Syracuse University in New York. But Lewis of the CDF, who oversaw the research, says there's about a one-in-a-million chance that the observations were just a result of random fluctuations. "The chance of being struck by lightning in North America is just as great," he says. "And that's an acceptable risk by my personal definition."

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