Electromagnetic Properties of Off-shell Particles and Gauge Invariance
نویسنده
چکیده
Electromagnetic properties of off-shell particles are discussed on the sis of a purely electromagnetic reaction: virtual Compton scattering off a proton. It is shown that the definition of off-shell electromagnetic form factors is not gauge invariant and they cannot be investigated in practice. Using only fundamental requirements of gauge invariance it is demonstrated that off-shell effects are cancelled in the longitudinal components of the total conserved current by the “minimal” contact current, while off -shellness appears only in the transverse (gauge invariant) non-pole part. This provides the possibility to introduce an on-shell extrapolated form factor F 1 in a gauge invariant way for the timelike region 4me < q 2 < 4M2. The form factors F 1,2 are removed from the total conserved current and do not affect observables. PACS numbers: 13.20-v, 13.40.Em, 13.40.Hq, 13.60.Fz
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