Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
1993-12-27
Prog.Theor.Phys. 91 (1994) 927-948
Physics
High Energy Physics
High Energy Physics - Phenomenology
30 pages, plain TeX (using `phyzzx' macro), 17 Postscript Figures are included as uuencoded files (need `epsf.tex'), KUNS-1236
Scientific paper
10.1143/PTP.91.927
A systematic method is given for obtaining consistent approximations to the Schwinger-Dyson(SD) and Bethe-Salpeter(BS) equations which maintain the external gauge invariance. We show that for any order of approximation to the SD equation there is a corresponding approximation to the BS equations such that the solutions to those equations satisfy the Ward-Takahashi identities of the external gauge symmetry. This formulation also clarifies the way how we can calculate the Green functions of current operators in a consistent manner with the gauge invariance and the axial anomaly. We show which type of diagrams for the $\pi^0\rightarrow\gamma\gamma$ amplitude using the pion BS amplitude give result consistent with the low-energy theorem. An interesting phenomenon is observed in the ladder approximation that the low energy theorem is saturated by the zeroth order terms in the external momenta of the pseudoscalar BS amplitude and the vector vertex functions.
Bando Masako
Harada Masayasu
Kugo Taichiro
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