Prompt ντ background in wide band νμ beams

To estimate the prompt background for νμ → ντ oscillation search in the Chorus and Nomad experiments, two possible sources of tau-neutrinos were considered. The first is the production and (semi-)leptonic decay of heavy flavour mesons. Secondly, the Drell-Yan mechanism can lead to tau-pairs. It is shown that this last contribution is negligible. However, the decay of Ds± mesons to τ±ντ gives rise to 3.3 × 10−6 ντ charged-current interactions per νμ charged-current interaction. Over the course of the Chorus experiment, this corresponds to 0.1 detected ντ charged-current events. This value is an upper limit and the actual background might be smaller by a factor of 2. However, it should carefully be taken into account, especially in view of future experiments, wishing to explore smaller values of mixing.