1. The problem statement, all variables and given/known data Show that for real valued E a particle at rest can not decay. 2. Relevant equations Width of resonance. [itex]\Gamma[/itex]=[itex]\hbar[/itex]/[itex]\tau[/itex] Relativistic Breit-Wigner distribution P_rel_=[itex]\Gamma[/itex]/((E^2-M^2)^2+M^2[itex]\Gamma[/itex]^2) Wave function of particle at rest. ψ(t)=ψ(0)e^(-iEt/[itex]\hbar[/itex]) 3. The attempt at a solution I guess Lambda would be 0 if tau (the lifetime of the particle) were infinite but I have no idea what that has to do with real valued E.