Abstract

The COMPASS experiment at CERN accesses pion-photon reactions via the Primakoff effect., where high-energetic pions react with the quasi-real photon field surrounding the target nuclei. When a single real photon is produced, pion Compton scattering is accessed and from the measured cross-section shape, the pion polarisability is determined. The COMPASS measurement is in contradiction to the earlier dedicated measurements, and rather in agreement with the theoretical expectation from ChPT. In the same experimental data taking, reactions with neutral and charged pions in the final state are measured and analyzed in the context of chiral perturbation theory.

Highlights

  • The COMPASS experiment at CERN accesses pion-photon reactions via the Primakoff effect., where high-energetic pions react with the quasi-real photon field surrounding the target nuclei

  • The COMPASS measurement is in contradiction to the earlier dedicated measurements, and rather in agreement with the theoretical expectation from chiral perturbation theory (ChPT)

  • Properties of the pions ( −, 0, +) are of crucial interest in understanding quantum chromodynamics (QCD) and its low-momentum expansion, chiral perturbation theory (ChPT), where the pions are identified as the Goldstone bosons emerging from the spontaneous breaking of chiral symmetry

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Summary

Primakoff technique and the pion polarisability

Interactions of high-energetic hadrons with the nuclear Coulomb field represent a scattering off the quasi-real photon density, referred to as Primakoff reactions. The cross-section for a reaction −A → X−A on a nucleus A reads, in one-photon exchange approximation [1]), d ds dQ2 d EPJ Web of Conferences where d /d is the cross-section for the subprocess − → X− with squared total energy s, Q2 is the momentum transfer to the nucleus, the fine structure constant, m the pion rest mass, Qmin = (s − m2)/2p the minimum momentum transfer at beam momentum p.

Compton scattering
Mark II
Findings
Chiral dynamics in Primakoff pion production processes
Full Text
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