A Precision Measurement of the π0 Lifetime
An effect of color confinement in quantum chromodynamics (QCD) is that traditional perturbation theory breaks down at large distances and low energies. A quantitative understanding of the strong interaction in this region remains one of the greatest intellectual challenges in physics. The symmetries of QCD in the chiral limit (in which the quark mass vanishes) are an important element in resolving this problem.
As the lightest particle in the hadron spectrum, the neutral pion represents the most sensitive platform to study fundamental symmetry issues in QCD at low energy. Spontaneous chiral symmetry breaking gives birth to the π0 as one of the Goldstone particles, and the chiral axial anomaly primarily determines the π0 lifetime. As such, a precision measurement of the lifetime of the π0 will provide a fundamental test of QCD at the confinement scale. The present experimental uncertainty of the π0 decay width is about an order of magnitude greater than the theoretical uncertainties, so a measurement of the π0 lifetime with a precision comparable to these calculations will provide an important test of the fundamental QCD predictions.
The PrimEx collaboration at Jefferson Lab developed an experimental program to measure the π0 lifetime with high precision using the small angle coherent photoproduction of pions in the Coulomb field of a nucleus (the Primakoff effect). It uses the high intensity and high resolution photon tagging facility in Hall B and a newly developed and novel high-resolution electromagnetic hybrid calorimeter (HYCAL). The first experiment on 12C and 208Pb targets was performed in 2004. A preliminary result on the π0 decay width gives Γ(π0→ γγ) =7.93 eV± 0.18 eV (stat) ± 0.13 eV (sys), with a total error of 2.8%, which is about a two and half times improvement over the Particle Data Book average value. The experimental systematic errors on the cross section measurement are controlled at the 1.3% level and verified by Compton scattering and pair-production cross section measurements. Further results are forthcoming.