Exotic particle turns out to be quark molecule

Like atoms, subatomic particles can link up to form “molecules.” A long-studied subatomic particle called Lambda (1405) is actually a molecule of two tightly knit particles, researchers report in the April 3 Physical Review Letters. The study reveals a novel arrangement of matter made of quarks, the fundamental constituents of Lambda (1405) and every nucleus of every atom in the universe.

Quark-containing particles are divided into two groups: mesons, which have two quarks, and baryons, which have three. Many physicists considered the particle Lambda (1405) to be a baryon like protons and neutrons. But some researchers proposed that Lambda (1405) could be one part meson and one part baryon, with a total of five quarks. Physicists at the University of Adelaide in Australia ran a supercomputer simulation guided by equations that chart quark interactions to see which interpretation was correct. The simulation showed that Lambda (1405) consists of a meson called an antikaon coupled to a proton or neutron — the first example of a meson-baryon molecule.

The study is the latest to suggest new combinations of quark-based matter. Recent experiments have hinted at the existence of subatomic molecules composed of two mesons (SN: 5/17/14, p. 12). Particle physicist Derek Leinweber  of the Adelaide team says he expects to identify other meson-baryon pairs masquerading as baryons, including a high-energy version of the proton.