We point out that the equivalent-photon approximation (
EPA) for processes with massive spin-1 particles in the final state would have validity in a more restricted kinematic domain
than for processes where it is commonly applied, viz., those with spin-1/2 or spin-0 particles in the final state. We obtain
the criterion for the validity of
EPA for the two-photon production of a pair of charged, massive, point-like spin-1 particles
V
±, each of mass
M and with a standard magnetic moment (
κ=1). In a process in which one of the photons is real and the other virtual with four-momentum
q, the condition for the validity of
EPA is |
q
2|≪
M
2, in addition to the usual condition |
q
2|≪
W
2,
W being the
V
+
V
− invariant mass. In a process in which both photons are virtual (with four-momenta
q and
q′), our condition is |
q
2||
q′
2|
W
4 ≪ 16
M
8, in addition to |
q
2| ≪
M
2, |
q′
2| ≪
M
2 and |
q
2| ≪
W
2, |
q′
2| ≪
W
2. Even when these extra conditions permitting the use of
EPA are not fulfilled, convenient approximate expressions may still be obtained assuming merely |
q
2| ≪
W
2 and |
q′
2| ≪
W
2.
We also discuss how the extra conditions are altered when the vector bosons are incorporated in a spontaneously broken gauge
theory. Examples of
W boson production in Weinberg-Salam model are considered for which the condition |
q
2||
q′
2|
W
4 ≪ 16
M
8 is shown to be removed.
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