Polarization tests of one-particle-exchange mechanisms

Since one-particle-exchange (OPE) mechanisms are predominant in all aspects of elementary-particle dynamics, a novel class of polarization tests is proposed for such mechanisms. They test whether a single particle of total angular momentum $J$ is exchanged ("$J$ constraints") and whether the process can be factorized into two vertices ("factorization constraints"), but the tests are independent of more detailed dynamical features such as the exact nature of the coupling at the vertices. Except for a restricted type of processes containing some low spin values, the constraints reduce the number of reaction amplitudes and offer tests of OPE which are independent of the value of $J$. The tests have a particularly simple form in a "magic" formalism in which the quantization directions of the particles are in the reaction plane and are rotated from the helicity directions by a "magic" angle which can be easily specified for a given $s$ and $t$. The tests consist of measuring whether a certain polarization quantity vanishes or not, thus providing sensitive "null experiments" for the exploration of particle dynamics. The results are illustrated on the popular reaction ½+½→½+½, which is embodied, for example, in elastic nucleon-nucleon scattering. The tests can be used either for one single-exchange mechanism or for a combination of such mechanisms (even if they involve different $J$ exchanges), as long as they all have the same type of parity.