Köln 2025 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 18: Hadron Structure and Spectroscopy II

HK 18.1: Group Report

Tuesday, March 11, 2025, 15:45–16:15, HS 3 Physik

Forward angle coherent photoproduction off the deuteron - a puzzle of unexpectedly large cross section measurements — •Thomas Jude for the BGOOD collaboration — Physikalisches Institut, Universität Bonn

The BGOOD photoproduction experiment at the ELSA facility is uniquely designed to explore kinematics where a charged particle is identified in a forward spectrometer and a recoiling hadronic system is reconstructed in the central calorimeter at low momentum transfer. The setup enables studies of coherent reactions off the deuteron where the deuteron takes the majority of the beam momentum. Due to the small deuteron binding energy this kinematic regime is expected to be heavily suppressed, however measurements of the reactions, γ d → π⁰π⁰ d, γ d → π⁰η d and γ d → π⁰π⁰π⁰ d exhibit forward differential cross sections an order of magnitude higher than phenomenological model calculations.

The γ d → π⁰π⁰ d reaction is consistent with a scenario of intermediate dibaryon formation, including the proposed d^*(2380) hexaquark. Other mechanisms however, such as pion exchange in final state interactions may yet prove to play dominant roles in all three measured reactions.

Keywords: BGOOD; Coherent photoproduction