Angular momentum dynamics of vortex particles in accelerators

Abstract: Experiments with spin-polarized beams of leptons and hadrons typically employ plane-wave states with definite momenta and energies. In contrast, vortex states represent cylindrical waves carrying a well-defined orbital angular momentum projection along the propagation direction. This projection can be arbitrarily large, endowing such particles with magnetic moments orders of magnitude greater than those of plane-wave states. Consequently, vortex particles could complement - or even replace - spin-polarized beams in high-energy collisions, enabling access to observables beyond the reach of the conventional states. Although relativistic vortex beams have yet to be realized, we investigate the radiative and non-radiative dynamics of angular momentum for vortex particles in accelerators. We compute the timescale for angular momentum loss via photon emission, finding it significantly longer than typical acceleration times. The non-radiative dynamics is governed by precession, with the orbital angular momentum precessing at a frequency markedly different from that of spin. Similar to spin tunes in circular accelerators, this can induce resonances that disrupt the beam's orbital momentum - occurring far more frequently for vortex beams than for spin-polarized ones. Thus, vortex particle acceleration can be more feasible in linacs, while Siberian snakes could serve as a tool for angular momentum manipulations.