New Measurement of theπ0Radiative Decay Width

Abstract

High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst) eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.

Forward-angle incoherent photoproduction of pseudoscalar mesons off nuclei

“…[3] can be consistently assigned to the mechanism of incoherent photoproduction with a fitted parameter (C NI = 1.35 ± 0.03) a factor two higher than the value found in Ref. [8]. This result shows that the scaling of the MCMC model (NI process) with the PrimEx data at larger angles is strongly correlated with the coherent contribution [2,3].…”

“…[8]) found C NI = 0.69 ± 0.05 for carbon when fitting the MCMC results with a slightly different pion elasticity (ε π 0 0.92). Despite to the fact that the present analysis refers to the data from Group I [19] (Figs.…”

“…Our goal is to provide a consistent interpretation for the cross section excess at larger angles found for π 0 photoproduction in Ref. [3] when comparing their results with PrimEx data [8] and associate this difference to the mechanism of incoherent production; which was not taken into account in Ref. [3].…”

“…From the experimental scenario, the most recent and high precision measurement was carried out by the PrimEx Collaboration at the Thomas Jefferson Laboratory (JLab) [8]. Detailed explanations regarding * tulio@if.usp.br the theoretical and experimental aspects of the radiative decay of pseudoscalar mesons can be found in Refs.…”

“…The excess is obtained by the subtraction of the coherent contribution (dashed blue lines) [3] from PrimEx data [8] and the error in C NI is only statistical. The cascade results consider a meson elasticity (ε π 0 = p 2 + μ 2 /k) greater than 0.982 in accordance with [8], reproducing the data with reasonable accuracy (χ 2 /d.o.f. = 1.47).…”