Z BOSON PAIR PRODUCTION IN HIGH-ENERGY PHOTON PHOTON COLLISIONS AND THE HIGGS SIGNAL

The complete standard model calculation for the reaction gammagamma--> ZZ has been done. The W boson loop contribution is found to be substantial. It leads to a large value of the transversely polarized Z(T)Z(T) pair production cross section of 200 fb at square-root s(gammagamma) = 500 GeV for unpolarized initial photons. The Z(T)Z(T) pair production would represent a severe irreducible background for the observation of the longitudinal Z(L)Z(L) pair production. So, the observation of the Higgs signal for M(H) greater-than-or-equal-to 500 GeV at the Photon Linear Collider seems to be a problem unless discrimination between transverse and longitudinal Z polarization is possible.