A2 – Real Photons

A photon incident on a nucleon couples to the nucleon electromagnetic current causing it to radiate mesons if the photon energy is sufficiently high. Such reactions induced by circularly and linearly polarized real photons up to energies of 1.5 GeV are being studied by the international A2 collaboration at MAMI.


A beam of photons with known energy and flux is produced via bremsstrahlung using a dedicated tagging spectrometer provided by the University of Glasgow. A polarized electron beam produces circularly polarized photons. Linearly polarized photons can be obtained from coherent bremsstrahlung in an oriented crystal radiator. The central part of the detector system is a hermetic calorimeter consisting of the Crystal Ball detector (672 NaJ crystals) in combination with the TAPS detector (384 BaF2 crystals) in forward direction. For charged-particle tracking and identification two layers of coaxial multi-wire proportional chambers and a barrel of 24 scintillation counters surrounding the target are installed inside the cavity of the Crystal Ball sphere. A frozen-spin target for polarized protons and deuterons is of particular importance for studying spin degrees of freedom.

The main physics goals are:

  • Protons and neutrons are excited when they absorb a photon. If the photon energy is sufficiently high, mesons are emitted. The probabilities for such meson production reactions as well as their angular and spin dependence incorporate indispensable information about nucleon excited states and meson-nucleon dynamics.
  • Electric and magnetic polarizabilities are well known concepts in classical physics, which describe the influence of static electric and magnetic fields on composite systems. In case of protons and neutrons scalar and spin-dependent polarizabilities can be measured in low energy Compton scattering.
  • At MAMI η and η' mesons are photoproduced with high rate. With the Crystal Ball detector, decay modes of these mesons can be studied in an almost background-free environment.
  • The charge distributions inside nuclei has been measured with high accuracy in electron scattering experiments. Information about the matter distribution can be obtained from the coherent photoinduced production of pions from nuclei, where photon and pion interact coherently with all protons and neutrons inside a nucleus.

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