Multipolar transitions excited by twisted photons in heavy quarkonia

TL;DR

This paper explores how twisted photons can excite higher multipole transitions in heavy quarkonia, providing new spectroscopic tools and detailed theoretical expressions for transition probabilities.

Contribution

It derives explicit formulas for multipole transition amplitudes induced by twisted photons in quarkonium, highlighting their potential to isolate higher multipoles and impose new selection rules.

Findings

Twisted photons enable excitation of higher multipoles like octupole in quarkonia.

Selection rules depend on the wave packet parameters of the quarkonium center of mass.

Comparison shows twisted photons can enhance excitation probabilities over plane waves.

Abstract

In this study, we investigate the excitation of multipole transitions in quarkonium systems by high-energy photons carrying orbital angular momentum. We derive explicit expressions for the amplitude and probability of photoexcitation incorporating the dynamics of the quarkonium center of mass. It is shown that the leading contribution to the transitions induced by twisted photons comes from the multipole $j=|m_\gamma|$ and $m_\gamma=l'-l$ in the long-wave approximation. Despite the fact that similar transitions can be excited by plane-wave photons, twisted photons with $|m_\gamma|>1$ enable the isolation of higher multipoles offering a unique spectroscopic tool. The fulfillment of the derived selection rules imposes constraints on the parameters of the wave packet of the quarkonium center of mass, since, in general, the angular momentum can be transferred to the center of mass. As a case study, we examine the octupole transition $1^{3}G_3 \rightarrow 3^{3}P_0$ in the charmonium system. We also compare the probabilities of excitation of this multipole transition by plane-wave and twisted photons with different polarizations.