The

The $\gamma \gamma \to A^0 A^0 $ process at a $\gamma \gamma $ collider

“…At sufficiently high energies, these contributions are mainly imaginary. But the predominance of the imaginary parts of the amplitudes is not so strong, as the one observed in the gauge boson production cases [13,14].…”

“…The motivation for this stems from the recent observation in [13,14], that at c.m energies 250GeV , out of the many independent helicity amplitudes for the processes (γγ → γγ, γZ, ZZ), only the two helicity conserving amplitudes F ++++ and F +−+− are important, which moreover turn out to be almost purely imaginary 2 . The physical reason for this result is not very clear [13,14]. Therefore, it seems worthwhile to investigate what happens in other processes, like e.g.…”

“…As in [13,14], we employ the non-linear gauge of [24], which implies the gauge fixing and FP-ghost interactions of (A.1, A.2), leading to the conclusion that there are no γW ± G ∓ , ZW ± G ∓ vertices. The diagrams contributing to A 0 -pair production are then given in Figs.1-4.…”

“…The first is induced by the two diagrams in the first line in Fig.1 and describes the (h 0 , H 0 )-pole contributions appearing in (B.19, B.20). The diagrams in 3 For the gauge boson polarization vectors, here and in [13,14], we use the same conventions as in [18]. The only difference is that we use the JW convention [19], which introduces an additional minus to the polarization vector of a longitudinal "Number 2" Z and affects γγ → ZZ, γZ.…”

“…The only difference is that we use the JW convention [19], which introduces an additional minus to the polarization vector of a longitudinal "Number 2" Z and affects γγ → ZZ, γZ. 4 We use the same conventions as in [13,14].…”