Neutral Pion Decay

Abstract

The study of π 0 decay has played an important role in the development of particle physics: The π 0 → γ γ decay provides key insights into the anomaly sector of quantum chromodynamics. In this review, the historical progression of π 0 discovery, lifetime measurements, and theory are presented. A new measurement of the π 0 radiative width via the Primakoff effect has been made at JLab. The result, (π 0 → γ γ ) = 7.82 ± 0.14 (stat.) ± 0.17 (syst.) eV, is a factor of 2.1 more precise than the currently accepted value, and it is in agreement with the chiral anomaly prediction and with next-to-leading-order chiral perturbation theory calculations. Primakoff experiments at higher energies to measure the η and η radiative widths are also discussed.

Incoherent photoproduction of pseudoscalar mesons off nuclei at forward angles

“…OFF NUCLEI In the Glauber multiple scattering theory [15] the amplitude of the process as a result of which the nuclear wave function in the initial state Φ i (r 1 , r 2 , …, r A ) transforms to the wave function of the final state Φ f (r 1 , r 2 , …, r A ) is (3) Here the two dimensional vectors b and s i are the impact parameter and the nucleon's transverse coor 2 The photoproduction of pseudoscalar mesons off the nucleon is described by four independent amplitudes [10]. In the coherent photoproduction off spinless nuclei only the spin nonflip ampli tude survives.…”

“…As for the incoherent photoproduction, this amplitude is the dominant one [11], though all four amplitudes are involved in the process. 3 For instance, the incoherent cross section would be strongly suppressed as a result of the Pauli exclusion principle, which is not the case in [14] (see, for instance, expression (33b)). …”

“…γ A Ps A Ps = π 0 η η' , , ( ) , + → + been completed at Jefferson Lab [2,3]. The energy upgrade of the CEBAF accelerator to 12 GeV will pro vide a great opportunity to extend the current program to the η and η' mesons.…”

“…3 The Pauli suppression is at work for produc tion at small angles and its correct accounting is cru cial for the process under consideration.…”

Forward-angle incoherent photoproduction of pseudoscalar mesons off nuclei

“…Detailed explanations regarding * tulio@if.usp.br the theoretical and experimental aspects of the radiative decay of pseudoscalar mesons can be found in Refs. [9,10].…”