The off-shell axial anomaly via the {gamma}*{pi}{sup 0}{yields}{gamma} transition

Technical Report · Tue Aug 01 04:00:00 EDT 1995

DOI:https://doi.org/10.2172/166445· OSTI ID:166445

Roberts, C. D.; Frank, M. R.; Mitchell, K. L.; Tandy, P. C.

The {gamma}*{pi}{sup 0} {yields} {gamma} form factor, F{sup {pi}0{gamma}{gamma}}(s), including the extension off the pion mass-shell, is calculated in generalized impulse approximation within the Dyson-Schwinger Equation framework used to provide an excellent description of the pion charge form factor, described above. This anomalous process is a fundamentally important characteristic of the quantum field theoretical structure of QCD because it signals the breaking of the U{sub A}(1) symmetry by quantization. This form factor was measured by the CELLO collaboration at the PETRA storage ring using the process e{sup +}e{sup -} {yields} e{sup +}e{sup -} {pi}{sup 0}. There is a letter-of-intent at CEBAF to remeasure this form factor in virtual Compton scattering from a proton target. In this case a (virtual) pion is supplied by the target and a final real photon selected through the excellent missing mass spectrometry available at CEBAF. An extrapolation to the pion mass shell will be needed to deduce the physical transition form factor. Our calculation shows that the dependence on the virtual-pion momentum is smooth and well described by a simple suppression factor, which is qualitatively independent of the details of the pion interpolating field. The correct mass-shell value of this form factor is naturally generated in our approach and the q{sup 2} dependence is in accord with the available CELLO data. No parameters are adjusted to achieve this; the are fixed at the values derived in the study of F{sub {pi}}(q{sup 2}). A significant result of our study is that for this anomalous process, soft nonperturbative effects remain significant for Q{sup 2} < 20 GeV{sup 2}. A paper describing this work was submitted for publication.

• Research Organization: Argonne National Lab., IL (United States)
• OSTI ID: 166445
• Report Number(s): ANL--95/14; ON: DE96000985
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
DYSON REPRESENTATION
ELECTROMAGNETIC FORM FACTORS
ELECTRON-POSITRON INTERACTIONS
PHOTON-MESON INTERACTIONS
PIONS
PROGRESS REPORT
QUANTUM CHROMODYNAMICS
SCHWINGER FUNCTIONAL EQUATIONS