Prompt $\nu_{\tau}$ background in Wide Band $\nu_{\mu}$ Beams

To estimate the prompt background for $\nu_{\mu} \rightarrow \nu_{\tau}$ 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 $D_{s}^{\pm}$ mesons to $\tau^{\pm} \nu_{\tau}$ gives rise to $3.3 \times 10^{-6}$ $\nu_{\tau}$ charged-current interactions per $\nu_{\mu}$ charged-current interaction. Over the course of the Chorus experiment, this corresponds to 0.1 detected $\nu_{\tau}$ 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.