The CKM matrix has this property, allowing the namesake triangle to be constructed.
In the scattering of weak gauge bosons in the Standard Model it is perturbatively violated unless there is a relatively light Higgs boson. As a property of the S-matrix, it implies the optical theorem. The CKM matrix has this property, allowing the namesake triangle to be constructed. "Wave-function collapse," which occurs in some interpretations of quantum mechanics, does not obey it, but normal quantum processes do as a consequence of the Hamiltonian being hermitian. FTP, what is this basic property of quantum mechanics which implies that the sum of all possible probabilities is 1?
In the scattering of weak gauge bosons in the Standard Model it is perturbatively violated unless there is a relatively light Higgs boson.
unitarity