The problem of a relativistic massive scalar particle trapped in an infinite potential spherical
well is pedagogically addressed in this paper. The wave function solutions and probability density
of the Klein–Gordon equation in spherical coordinates are derived, as well as the energy levels. The
results are compared with the non-relativistic solutions of the Schrödinger equation for different
values of the particle’s mass. As expected, for very large masses the non-relativistic results are
recovered. For illustration, these results are discussed in the specific case of the standard model
Higgs field constrained inside a proton, in the quadratic approximation of the Higgs potential
around the expectation value.