Production mechanisms for a new neutral particle below 2 MeV

Recently observed $e^{+}$ kinetic-energy peaks at 336±40 keV in several heavy-ion systems have suggested the existence of a particle φ decaying to $e^{+}$ $e^{\mathrm{-}}$. We construct a simple model of φ production by the electromagnetic field of the ions. It is not possible to produce φ at rest as the data require. If $m_{\phi }$ is just above 2$m_e$, then a sharp peak in the $e^{+}$ kinetic energy can be obtained if the φ velocity is sharply peaked and nonzero. Our model suggests a mechanism for this via the formation of a resonance in the ionic system for which there is independent evidence.