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

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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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