On the radiative decays of light vector and axial-vector mesons

TL;DR

This paper investigates the nature and radiative decay processes of light vector and axial-vector mesons, proposing a novel counting scheme and demonstrating that certain axial-vector mesons are dynamically generated as molecular states.

Contribution

It introduces a new counting scheme based on large-N_c QCD constraints and systematically studies the properties and decay amplitudes of light axial-vector mesons as dynamically generated molecular states.

Findings

Axial-vector mesons like f_1(1282) are generated as molecular states.

The proposed counting scheme successfully describes vector meson properties.

Radiative decay amplitudes are computed with gauge invariance, emphasizing loop contributions over tree-level processes.

Abstract

We study the light vector and axial-vector mesons. According to the hadrogenesis conjecture the nature of the two types of states is distinct. The axial-vector mesons are generated dynamically by coupled-channel interactions based on the chiral Lagrangian written down in terms of the Goldstone bosons and the light vector mesons. We propose a novel counting scheme that arises if the chiral Lagrangian is supplemented by constraints from large-N_c QCD in the context of the hadrogenesis conjecture. The counting scheme is successfully tested by a systematic study of the properties of vector mesons. The spectrum of light axial-vector mesons is derived relying on the leading order interaction of the Goldstone bosons with the vector mesons supplemented by a phenomenology for correction terms. The f_1(1282), b_1(1230), h_1(1386), a_1(1230) and K_1(1272) mesons are recovered as molecular states. Based on those results the one-loop contributions to the electromagnetic decay amplitudes of axial-vector molecules into pseudo-scalar or vector mesons are evaluated systematically. In order to arrive at gauge invariant results in a transparent manner we choose to represent the vector particles by anti-symmetric tensor fields. It is emphasized that there are no tree-level contributions to a radiative decay amplitude of a given state if that state is generated by coupled-channel dynamics. The inclusion of the latter would be double counting. At present we restrict ourselves to loops where a vector and a pseudo-scalar meson couple to the axial-vector molecule. We argue that final and predictive results require further computations involving intermediate states with two vector mesons. The relevance of the latter is predicted by our counting rules.