Abstract
In this talk I will review the recently published results by the A2 and CB-TAPS Collaborations at MAMI on neutral pion photoproduction in the near-threshold region. The combined measurement of the differential cross section and the photon beam asymmetry with low statistical errors allowed for a precise determination of the energy dependence of the real parts of the S- and P-wave amplitudes for the first time, providing the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.
Highlights
The dynamical consequences of the spontaneous breaking of chiral symmetry in Quantum Chromodynamics (QCD) and the appearance of the meson as a pseudoscalar Nambu–Goldstone boson are well known and many predictions are available in the literature [1]
The combined measurement of the differential cross section and the photon beam asymmetry with low statistical errors allowed for a precise determination of the energy dependence of the real parts of the S- and P-wave amplitudes for the first time, providing the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment
To parametrize the multipoles we have employed three different approaches which we have fitted to the data: (i) Empirical fit [5,6,7] which depends on eight parameters and serves as a consistency test to single-energy multipole extraction; (ii) Heavy Baryon Chiral Perturbation Theory (HBCHPT) [8] which depends on five parameters; and (iii) Relativistic Baryon Chiral Perturbation Theory (RBCHPT)
Summary
The dynamical consequences of the spontaneous breaking of chiral symmetry in Quantum Chromodynamics (QCD) and the appearance of the meson as a pseudoscalar Nambu–Goldstone boson are well known and many predictions are available in the literature [1]. The high quality of these data allows to extract the S-wave and P-waves energy dependence and to use the data to test current CHPT and assess the energy range where the theory is accurate for this particular process [8, 9]. The red area at the top of the E0+ figure represents the uncertainty in the single-energy multipoles due to D waves
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