Application of Nuclear Coherence Properties to Elementary-Particle Reactions

We consider the use of nuclei as targets in low-momentum-transfer elementary-particle reactions. At low momentum transfer the nucleus acts coherently and may be used to enhance or suppress various exchange mechanisms for the elementary particles. We use the impulse approximation and optical-model wave-functions representing the absorption of the particles passing through the nucleus. For production reactions in which $\pi$ exchange on hydrogen is dominant, coherent nuclear reactions may be used to eliminate $\pi$ exchange and enhance $\omega -\varphi$-type exchanges, thus allowing study of the behavior of these exchanges. Calculations are given for production on ${\mathrm{He}}^4$, giving a crude estimate of 35 μb for $\pi +{\mathrm{He}}^4\to \rho +{\mathrm{He}}^4$. We also consider the general properties of coherent production reactions, including the $A$ dependence of coherent reactions, the suppression of incoherent mechanisms at low momentum transfer, and the competition with production in the Coulomb field. We also consider "semicoherent" reactions in which the nucleus is excited to special states as a means of isolating particular components of the two-body scattering matrix.