ok, you mean why there is need of gauge bosons?
The simplest answer is first that the bosons are VIRTUAL, they are a mathematical tool/term which shows up when you calculate the interaction by means of perturbation theory. They are virtual particles since they do not fulfil E^2 = p^2 + m^2. So as an example the W boson which 'is responsible' for beta decay, is not there, it exists in calculations. The description why they are needed can not be straightforward presented since it is graduate level physics.
Now all gauge bosons have physical counterparts, real observable particles. e.g in top-quark decay, it decays into a botton quark and a W boson. Those particles are real, they are 'on shell' i.e they fulfill E^2 = p^2 + m^2.
But the W-boson is unstable, and decays into several things, and it is those things which are caught in the detector. One does not find the mass of particles by putting them on a weight-scale, one reconstruct/calculates the mass by looking at the four-momentum vectors of the detected particles.