Abstract
The COMPASS experiment at CERN studies with high precision pion-photon induced reactions on nuclear targets via the Primakoff effect. This offers the possibility to test chiral perturbation theory (ChPT) in various channels: Pion Compton scattering allows to clarify the longstanding question of the pion polarisabilities, single neutral pion production is related to the chiral anomaly, and for the two-pion production cross sections exist as yet untested ChPT predictions.
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
Summary
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.
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