Prospects of a search for a new massless neutral gauge boson at the ILC

Prospects to search for a new massless neutral gauge boson, the paraphoton, in e{sup +}e{sup -} collisions at center-of-mass energies of 0.5 and 1 TeV are studied. The paraphoton naturally appears in models with Abelian kinetic mixing. A possible realistic model-independent lowest order effective Lagrangian contains magnetic interactions of the paraphoton with the standard model fermion fields. These interactions are proportional to the fermion mass and grow with energy, with however very weak paraphoton couplings to ordinary matter. At the ILC, a potentially interesting process to search for the paraphoton is its radiation from top quarks, so that the event topology of interest is a pair of acoplanar top quarks decaying to jets and missing energy. By combining many discriminating features of signal and background events, efficient paraphoton event selection is achieved, allowing us to set limits for the top-paraphoton coupling. Arguments in favor of the missing energy as the paraphoton with spin 1 are discussed.