Vortex Particle Beams Enhance Angular Momentum and Collision Observables in Accelerators

The quest to accelerate particles to ever-higher energies drives innovation in beam physics, and a new study explores the potential of using particles carrying orbital angular momentum – known as vortex particles – in future accelerators. Researchers at ITMO University, led by D. Karlovets, D. Grosman, and I. Pavlov, investigate how these particles behave as they circulate within accelerator rings. The team demonstrates that vortex particles possess significantly more decisive magnetic moments than conventional beams, potentially unlocking new avenues for high-energy collisions and previously inaccessible observations. However, the research also reveals that maintaining the orbital angular momentum of these particles presents unique challenges, as they are prone to resonances that can disrupt beam stability – a problem far more pronounced than with standard spin-polarized beams. The findings suggest that linear accelerators may be better suited for vortex particle acceleration, and that specialised devices, such as ‘Siberian snakes’, could prove essential for controlling their angular momentum.

Beyond Spin: Harnessing Orbital Angular Momentum for Future Accelerators For decades, controlling the spin of particles has been crucial in accelerator physics, enabling experiments that probe the fundamental structure of matter. However, a growing body of research suggests that harnessing another form of angular momentum – the orbital angular momentum (OAM) – could unlock entirely new possibilities for high-energy physics. Unlike spin, an intrinsic property of particles, OAM arises from the particle’s motion and can, in principle, be arbitrarily large.

This translates to a significantly stronger magnetic moment than that associated with spin alone, potentially opening doors to experiments inaccessible with conventional particle beams. The core idea involves moving beyond using standard plane waves for particle beams and instead employing vortex beams, which carry this well-defined OAM. These beams twist in space like a helix, and the magnitude of their angular momentum can be precisely controlled.

While spin-polarized beams have long been used to investigate particle interactions, vortex beams offer a complementary approach. They could allow scientists to generate spin-polarized particles in novel ways and explore strong interactions at low energies with unprecedented precision, potentially providing new insights into quantum phenomena like entanglement and coherence, which are typically difficult to observe in high-energy collisions. However, accelerating vortex beams presents unique challenges, primarily related to maintaining the OAM during acceleration.

Researchers have demonstrated that while the OAM is generally stable, it is susceptible to depolarization due to the emission of twisted photons, though calculations show this loss is remarkably slow – far slower than the typical acceleration process. A more significant hurdle arises from the way OAM precesses – wobbles – within the accelerator’s magnetic fields. This precession occurs at a different frequency than that of spin, and can lead to resonances that disrupt the beam’s OAM, particularly in circular accelerators.

Analysis reveals that these OAM-depolarizing resonances occur more frequently than those affecting spin, demanding careful control to maintain beam quality. Techniques already used for spin control, such as “Siberian snakes,” could be adapted to manipulate and preserve the OAM of vortex beams. Interestingly, linear accelerators (linacs) are inherently better suited for accelerating vortex beams than circular accelerators, due to the reduced risk of OAM depolarization, suggesting that linacs could become key facilities for exploring the potential of vortex beams and enabling a new generation of experiments in particle physics and beyond.

Research has investigated the dynamics of twisted electron beams, focusing on how their orbital angular momentum behaves during acceleration. A generalized equation describing the precession of an electron’s OAM has been derived, building upon existing models of particle spin. This equation reveals that the anomalous magnetic moment, which significantly influences spin precession, has a much larger effect on OAM precession.

The team explored how this difference impacts acceleration in linacs and storage rings, demonstrating that the longitudinal electric field within a linac does not affect either the spin or OAM, allowing for potential acceleration of twisted electrons without disturbing their OAM, provided radiative effects are negligible. Analysis of magnetic fields shows that the ratio between OAM and spin precession remains constant regardless of energy in a uniform field. However, in transverse magnetic fields, the precession rates differ, leading to potential resonance conditions that could disrupt the OAM.

Researchers also examined the radiative losses of OAM during acceleration, calculating the lifetime of twisted electron states in magnetic fields. They found that the lifetime is surprisingly long, ranging from 10^2 to 10^3 seconds in fields of 0.1 to 10 Tesla, significantly longer than the typical acceleration time in a linac. Calculations for heavier ions, like carbon ions, predict even longer lifetimes, scaling with the particle mass.

This suggests that maintaining the OAM of twisted beams during acceleration is feasible, opening possibilities for novel beam-based applications. Current research focuses on developing more sophisticated models to account for quantum effects and further refine these predictions. Investigations have also explored both radiative and non-radiative dynamics of angular momentum for vortex particles within accelerator fields.

While the emission of twisted photons carrying away angular momentum is infrequent under relevant parameters, non-radiative dynamics are more significant for preserving angular momentum during acceleration. Both orbital angular momentum and spin precess at different angular velocities, and orbital angular momentum can experience resonances, suggesting that Siberian snakes are nearly unavoidable for unpolarised vortex beams in circular accelerators. Accelerating these beams in linacs appears more feasible, although the highest current at which space charge effects would degrade the quantum states of particles remains unknown.

While space charge typically diminishes at higher energies for conventional beams, this problem requires dedicated study for a few MeV vortex electrons. Further research is focused on understanding the interplay between radiation, space charge, and the preservation of quantum states in vortex particle beams, with implications for future accelerator designs and advanced radiation sources. Studies continue to refine theoretical models and explore experimental verification of predicted phenomena.