Much of the material in this section is similar to what has previously been described by P.V. Landshoff, J.C. Polkinghorne: Phys. Rev.D18, 153 (1978) or I. Schmidt, R. Blankenbecler: Phys. Rev.D16, 1318 (1977)
In more old-fashioned language, this result is equivalent to saying that atq 2=0 the form factor is dominated by its triangle singularity represented by Fig. 1: See R.J. Eden et al.:The analytic S-matrix, Sect. 2.3. Cambridge: Cambridge University Press 1966
The vertex functionV does have an anomalous threshold, but this is harmless (see [2])
For a recent review of such processes, with references to earlier work, see W. Celemaster: 1980 Madison Conference talk, preprint ANL-HEP-CP-80-60
Because of the zero mass of the photon, the branch points in Fig. 3 now coalesce with the poles giving a cut starting atk 22 =m 2 and a cut startingk 21 =m 2. Neark 22 =m 2 the cut can be parametrised by (k 22 −m 2)β−1 where β=αv 2 (see [6]). In the region of interest the velocity ν∼α and so the discontinuity across the cut is strongly peaked near the beginning and the dominant contribution comes from setting β=0 and approximating the cut by the original pole
T.W.B. Kibble: Phys. Rev.173, 1527 (1968)
G.P. Lepage, S.J. Brodsky: SLAC-PUB-2478
E.C. Poggio, H.J. Schnitzer: Phys. Rev.D20, 1175 (1979)
C. Michael, F.P. Payne: Phys. Lett.91B, 441 (1980)
T. Appelquist, M. Dine, I.J. Muzinich: Phys. Rev.D17, 2074 (1978)
A. Martin: Phys. Lett.93B, 338 (1980)
—: J. Dias de Deus: Phys. Lett.98B 301 (1981)