Eur. Phys. J. A (2008) 37: 15-32
https://doi.org/10.1140/epja/i2008-10600-0

Regular Article — Theoretical Physics

Two photons into π⁰π⁰

Departamento de Física, Universidad de Murcia, E-30071, Murcia, Spain

^* e-mail: oller@um.es
^** e-mail: luisroca@um.es

Received: 2 April 2008
Revised: 30 May 2008
Published online: 26 June 2008

Abstract

We perform a theoretical study based on dispersion relations of the reaction γγ → π ⁰ π ⁰ emphasizing the low-energy region. We discuss how the ƒ ⁰(980) signal emerges in γγ → ππ within the dispersive approach and how this fixes to a large extent the phase of the isoscalar S-wave γγ → ππ amplitude above the threshold. This allows us to make sharper predictions for the cross-section at lower energies and our results could then be used to distinguish between different ππ isoscalar S-wave parameterizations with the advent of new precise data on ππ → π ⁰ π ⁰. We compare our dispersive approach with an updated calculation employing the unitary chiral perturbation theory (U _gC PT). We also pay special attention to the role played by the σ-resonance in γγ → ππ and calculate its coupling and width to γγ, for which we obtain Γ(σ → γγ) = (1.68 ± 0.15) keV.