Quantum Bull’s-Eye: Particle-mass prediction hits the mark

Quarks, the basic constituents of much of matter, are so complicated that scientists have been unable to apply fundamental theory to precisely predict the mass of a quark-containing particle. Until now.

In the May 6 Physical Review Letters, researchers report their theoretical prediction of the mass of the rare particle known as the Bc meson, and that prediction agrees to within a few tenths of a percent with a not-yet-published experimental determination of the mass. The unprecedented match suggests that after a quest of more than 30 years, physicists may have finally fine-tuned a computational tool known as lattice quantum chromodynamics (QCD) so that it’s equal to the challenge of quark physics.

“This is by far the most dramatic confirmation to date that [lattice QCD] can deliver the long-promised precision,” comments experimentalist Ian P. Shipsey of Purdue University in West Lafayette, Ind.

Last year, lattice-QCD investigators reported that a refinement of the tool had enabled them to compute numerous previously measured properties of quark-containing particles with extraordinary precision (SN: 8/7/04, p. 90: Starting from Square One). However, as a physicist, “you don’t really trust something calculated after the result,” says Saverio D’Auria of the University of Glasgow in Scotland. A much more credible achievement is to calculate in advance a property that is later measured in an experiment, he says.

D’Auria worked on the experiment to measure the mass of the Bc meson at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. An announcement on the lab’s Web site (http://www.fnal.gov/pub/today/archive_2005/today05-05-11.html) unveiled the final experimental result on May 11.

Quarks are the building blocks of protons, neutrons, and other particles known generally as hadrons. A meson is a kind of hadron containing one quark, which is a constituent of matter, and one antiquark, a constituent of antimatter. The Bc meson contains a charm quark and a bottom antiquark. Its mass, the new findings reveal, is about six times that of a proton.

Calculating properties of hadrons is a daunting proposition, even with a supercomputer. The scientists worked for 14 months to calculate the mass in the new prediction, says coauthor and Fermilab theorist Andreas S. Kronfeld. That’s partly because the rules of quantum mechanics permit countless additional quarks and other particles to continually flit in and out of existence inside each hadron.

Lattice-QCD theorists impose a simplifying, gridlike framework onto reality. The framework confines that sea of particles to specific locations in space and time, but at the cost of introducing errors into the result. The refinement of lattice QCD described last year substantially minimized those errors.

Still, some lattice-QCD specialists suspect that the refinement remains subtly flawed in ways not apparent in the Bc meson calculation. “Eventually, the results will have to be checked in another way,” contends theorist Michael J. Creutz of Brookhaven National Laboratory in Upton, N.Y.

A host of new tests is coming. Among them are calculations focused on D mesons, which also contain charm quarks. Shipsey and other experimentalists are now measuring such mesons at Cornell University with mounting precision.