If the radioactive decay constant for a particular material is zero (λ = 0), how does the concentration of that material change as a function of time? Which one of the four forces of nature is...

If the radioactive decay constant for a particular material is zero (λ = 0), how does the concentration of that material change as a function of time?

Which one of the four forces of nature is responsible for the process of radioactive decay, and what are the particle(s) that carry this force?