Interactions of light mesons with photons

Stefan Leupold,A Wrońska,H Ströher,C Guaraldo,A Magiera

doi:10.1051/epjconf/20148105020

Abstract

Interactions between hadrons and photons constitute an important tool to ex- plore the intrinsic structure of hadrons. At the same time, high-precision determinations of electromagnetic processes, as in particular g − 2 of the muon, constitute a promising tool to look for the impact of beyond-standard-model physics. Here hadronic contribu- tions constitute the main uncertainty for the standard-model prediction. In this talk results for electromagnetic transition form factors and two-gamma amplitudes are presented. 1 Motivation As is well-known by now hadrons are not elementary objects but have an intrinsic structure. To explore a complicated object it is certainly a good idea to use a probe that one understands much better than the object which one wants to study. This qualifies electromagnetism, i.e. real and virtual photons, as an excellent tool to study the structure of hadrons. Quantities of central importance are electromagnetic elastic and transition form factors of hadrons and two-photon amplitudes, the latter being related, e.g., to polarizabilities. Amusingly the logic to explore the less known hadrons by well-known electromagnetism gets another twist in the high-precision searches for low-energy traces of physics beyond the standard model. The required high-precision predictions of the standard model receive contributions from hadronic fluctuations. Examples for such an interplay can be observed for the rare pion decay into e + e − and for the gyromagnetic ratio of the muon; see, e.g., (1-3) and references therein. The key quantities are again the same: form factors and two-photon amplitudes. In the last few years my collaborators and me have explored such quantities with two different tools: a Lagrangian and a dispersive approach. The results from the former approach are already well documented in the literature (4-11). Therefore I will use the rest of these proceedings to discuss our recent work on the pion transition form factor based on dispersion theory (12).

Highlights

I will use the rest of these proceedings to discuss our recent work on the pion transition form factor based on dispersion theory [12]
In his talk at this conference Peter Stoffer has explained in detail how the hadronic light-by-light contribution to g − 2 of the muon involves the pion transition form factor; see Peter Stoffer’s contribution to these proceedings and [3]
From the hadronic point of view one interesting aspect is to figure out to which extent vector-meson dominance holds for the pion transition form factor [2, 7, 9, 11]

Summary

Introduction

Quantities of central importance are electromagnetic elastic and transition form factors of hadrons and two-photon amplitudes, the latter being related, e.g., to polarizabilities. The key quantities are again the same: form factors and two-photon amplitudes. I will use the rest of these proceedings to discuss our recent work on the pion transition form factor based on dispersion theory [12].

Results

Conclusion