Limit on the muon beam divergence at a ν-factory

The effects of the divergence of an unpolarized muon beam at a ν-factory on the resulting neutrino flux at a distant detector are calculated. It is shown that the muon beam divergence should be <15γμ in order to keep the loss of the neutrino flux below the 10% level w.r.t. the one generated by a perfectly parallel muon beam.