The mystery of the missing mass.

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 threes 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.

En'yo and his colleagues measured the particles masses by firing protons at targets of either carbon or copper, creating showers of particles that included phi mesons. The particles decayed quickly, and by carefully measuring the energies of decay products, the scientists calculated the phi mesons' masses. Those that decayed inside a nucleus had 3.4 percent less mass than phi mesons decaying outside, the team reports in the Jan. 26 Physical Review Letters.

If experiments confirm mass loss for all nuclear particles, "it's a paradigm shift in the way you view nuclear structure," comments Anthony Thomas, a nuclear theorist at the Thomas Jefferson National Accelerator Facility in Newport News, Va. In a sense, the nucleus would no longer be made of protons and neutrons, but rather variants of these particles whose masses have been altered, he says.--B.P.