Graduates of Particle Physics 101 know that baryons are made of three quarks. An excited state of the Lambda baryon, Λ(1405), might, however, defy this simple description: the particle behaves like a “molecule” made of a quark pair and a quark triplet.
What’s puzzling about the Lambda baryon—a bound state of an up, down, and strange quark—is that much less energy is needed to excite it than expected for three bound quarks. Theoretical calculations haven’t been able to convincingly predict Λ(1405)’s structure because they approximated the quantum field theory that describes quarks.
The team tackled the problem with lattice quantum chromodynamics, which uses supercomputers to simulate quark theory. The authors calculated the strange quark’s contribution to the Λ(1405) magnetic moment and found it was zero. The finding is a strong sign that Λ(1405) has an antikaon-nucleon molecular structure: in other configurations, the authors say, the strange quark would have a nonzero magnetic moment.