Particles inside a nucleus weigh slightly less than the same particles in free space, new research shows. The experiment is a step toward understanding what determines the masses of particles.
Most of these particles' masses come not from the quarks of which they're made, but from the strong forces that hold those quarks together. Within a nucleus, a particle's internal forces are weakened by interference from forces exerted by its neighbors, so the particle has been expected to have slightly less mass there.
Attempts to measure this difference have yielded ambiguous results. But the new research, performed by Hideto En'yo and his colleagues at the KEK accelerator in Tsukuba, Japan, detected the mass lost within a nucleus by the phi meson, which consists of two tightly bound quarks.