CAMBRIDGE – Fifty years ago, particle physicists faced an unexpected challenge. Their best mathematical models could account for some of the natural forces that explain the structure and behavior of matter at a fundamental level, such as electromagnetism and the weak nuclear force responsible for radioactive decay. But the models worked only if the particles inside of atoms had no mass. How could huge conglomerations of such particles – proteins, people, planets – behave as they do if their constituent parts weighed nothing at all?