  Several particles are not observed directly, but only through their decay
products. We consider the possibility that they might be fakeons, i.e. fake
particles, which mediate interactions but are not asymptotic states. A crucial
role to determine the true nature of a particle is played by the imaginary
parts of the one-loop radiative corrections, which are affected in nontrivial
ways by the presence of fakeons in the loop. The knowledge we have today is
sufficient to prove that most non directly observed particles are true physical
particles. However, in the case of the Higgs boson the possibility that it
might be a fakeon remains open. The issue can be resolved by means of precision
measurements in existing and future accelerators.
