LHC reports pentaquark sightings

Quarks, the elementary units of matter found in every atomic nucleus, are surprisingly comfortable in large crowds.

Two particles discovered at the Large Hadron Collider, or LHC, near Geneva are each composed of five quarks, researchers report online July 13 at arXiv.org. Until recently, quarks had only been found in pairs or trios. Now, in just over a year, an experiment called LHC beauty has exposed a four-quark particle (SN: 5/17/14, p. 12) and two five-quark ones.

The fresh evidence for the existence of five-quark particles, or pentaquarks, is “far more compelling” than a controversial claim made a decade ago (SN: 5/14/05, p. 318), says Curtis Meyer, a particle physicist at Carnegie Mellon University in Pittsburgh. He says the new study will help physicists better understand the strong nuclear force, which binds quarks together.

For decades, physicists have had trouble finding quark-based matter that isn’t a three-quark baryon (like protons and neutrons) or a two-quark meson, even though the strong force could seemingly shape larger particles.

Sheldon Stone, a particle physicist at Syracuse University in New York, says he and his LHC beauty experiment colleagues did not set out to find pentaquarks. But after carefully analyzing about 26,000 decays of the bottom lambda particle, the researchers found that roughly 8 percent produced a previously unknown particle nearly 4.7 times as massive as a proton; another 4 percent generated a slightly heftier particle. The new particles’ masses, decay products and other properties indicated a composition of a charm quark, its antimatter counterpart, two up quarks and a down quark.

Stone is confident that the new particles are bound sets of five quarks, although it’s possible they are “molecules” of tightly packed two- and three-quark clusters. The relatively high mass of the new particles (charm quarks are the third heaviest of the six quark types) suggests that heftier quarks may be better at binding into big groups, Stone says.