The electric απ and magnetic βπ charged pion Compton polarizabilities are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). They are directly linked to the phenomenon of spontaneously broken chiral symmetry within QCD and to the chiral QCD lagrangian. The combination (απ-βπ) was measured by: (1) CERN COMPASS via radiative pion Primakoff scattering (Bremsstrahlung) in the nuclear Coulomb field, π-Z → π-Zγ, (2) SLAC PEP Mark-II via two-photon production of pion pairs, γγ→π+π-, and (3) Mainz Microtron MAMI via radiative pion photoproduction from the proton, γp → γπ+π. COMPASS and Mark-II agree with one another: (1) απ-βπ = (4.0±1.2stat±1.4syst)×10-4fm3, (2) απ-βπ = (4.4±3.2stat+syst) ×10-4 fm3. The Mainz value (3) απ-βπ = (11.6 ± 1.5stat ± 3.0syst ± 0.5model) ×10-4fm3 is excluded on the basis of a dispersion relations calculation which uses the Mainz value as input, and gives significantly too large γγ→π0π0 cross sections compared to DESY Crystal Ball data. COMPASS and Mark-II polarizability values agree well with the two-loop chiral perturbation theory (ChPT) prediction απ-βπ = (5.7±1.0)×10-4 fm3, thereby strengthening the identification of the pion with the Goldstone boson of QCD.
|State||Published - 2017|
|Event||2017 Meeting of the APS Division of Particles and Fields, DPF 2017 - Batavia, United States|
Duration: 31 Jul 2017 → 4 Aug 2017
|Conference||2017 Meeting of the APS Division of Particles and Fields, DPF 2017|
|Period||31/07/17 → 4/08/17|