For the third time in 14 months, physicists have unearthed an odd new particle. The newcomer is even more curious than its unusual cousins, and together the three have theorists puzzling over the strong force that binds nuclear matter.
The strong force is nature's craziest of crazy glues. It binds particles called quarks to make the protons and neutrons in everyday atomic nuclei, and produces weirder stuff out of other kinds of quarks. The so-called up and down quarks that form protons and neutrons have two sets of heavier cousins--the charm and strange, and the top and bottom quarks. Quarks also have antimatter twins, and a quark and an antimatter antiquark can stick together through the strong force to form a composite particle called a meson. In fact, a particular quark-antiquark combination can form a variety of slightly different mesons with various masses. For example, a heavier charm quark and a lighter strange antiquark form a particle called a Ds meson, and physicists had known of four versions. Then last year, experimenters created two new ones, which turned out to be significantly less massive than expected.
Now, a third new Ds has even more peculiar properties, report researchersworking with the Segmented Large X baryon Spectrometer (SELEX) at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. In 1996-97, the team of 125 physicists blasted high-energy protons into targets of copper and diamond to produce showers of short-lived, exotic particles. Poring over their data, the researchers spotted about five dozen copies of the new particle, they report in a paper submitted to the journal Physical Review Letters. The particle lived longer before splitting into other particles than researchers expected. Most surprisingly, it rarely decayed into a pair of particles known as a Do meson and a K+ meson, the combination that ought to be most common.
If confirmed, the new meson may mark the limits of one of physicists' favorite analogies, says Estia Eichten, a theoretical physicist at Fermilab. In the past, theorists have predicted the properties of mesons containing heavy quarks by assuming they vaguely resemble an electron and a proton bound to make a hydrogen atom. The new meson shows where that analogy breaks down, Eichten says. William Bardeen, also a theorist at Fermilab, says it's possible that the new particle could actually be an exotic new combination of two quarks and two antiquarks, although that's a long shot.