Coherent production of the long-lived pionium nP states in relativistic nucleus-nucleus collisions

Abstract

The coherent production of the nP states of the $\pi^+\pi^-$ atoms ($A_{2\pi}$) in relativistic nucleus-nucleus collisions is considered as a possible source of the $A_{2\pi}(nP)$ beam for the pionium Lamb-shift measurement. A general expression for estimation of the $A_{2\pi}(nP)$ yield is derived in the framework of the equivalent photon approximation.