The Standard Model cannot explain why a particle has a certain mass. For example, both the photon and the W particle are force carrier particles: why is the photon massless and the W particle massive?

Physicists have theorized the existence of the so-called Higgs field, which in theory interacts with other particles to give them mass. The Higgs field requires a particle, the Higgs boson. The Higgs boson has not been observed, but physicists are looking for it with great enthusiasm.