Threshold effects in high-energy vortex state collisions

Collisions of particles prepared in non-plane-wave states with a nontrivial phase structure, such as vortex states carrying an adjustable orbital angular momentum (OAM), offer novel opportunities in atomic, nuclear, and high-energy physics unavailable for traditional scattering experiments. Recently, it was argued that photoinduced processes such as yd & RARR; pn and y p & RARR; A+ initiated by a high-energy vortex photon should display a remarkable threshold shift and a sizable cross-section enhancement as the impact parameter b of the target hadron with respect to the vortex photon axis goes to zero. In this work we explore theoretically whether this effect exists within the quantum-field-theoretic treatment of the scattering process. We do not rely on the semiclassical assumption of a pointlike nonspreading target particle and instead consider the toy process of heavy-particle pair production in collisions of two light particles prepared as Laguerre-Gaussian and compact Gaussian wave packets, paying particular attention to the threshold behavior of the cross section. We observe threshold smearing due to nonmonochromaticity of the wave packets, but we do not confirm the near-threshold enhancement. Instead we find an OAM-related dip at b & RARR; 0 compared with the two-Gaussian-wave-packet collision.