The 
contamination in the beam is the ultimate irreducible
background for a 
oscillation search.
The largest contribution to this background comes from the prompt tauonic
decay of the 
produced from proton interactions in the
neutrino target and inside the shielding.
Two new estimates of the background have recently been
published [47, 48], providing adequate knowledge of this background.
They include the contribution from
(which is actually dominant), linear A-dependence of
production cross-sections, and a
value for the branching ratio as obtained in recent measurements.
For a proton energy 
of 450 GeV and the CHORUS detector fiducial
volume, the two independent estimates of
the ratio R of the number of charged-current (CC) events to
CC events
are 
from the semi-empirical
parametrisation
of production [47] and 
from the non-perturbative
QCD
approach [48], using the Quark-Gluon String Model (QGSM).
The systematic uncertainties common to both approaches arising from the
leptonic branching ratios, the production cross-section
and the model parameters amount to 
.
This level of the prompt background, still well below one event
for the total duration of the CHORUS and NOMAD experiments, becomes more relevant
for a future neutrino oscillation experiment aiming at
improving the sensitivity by at least one order of magnitude.
An effective way to reduce the prompt background is to lower the primary proton
energy, thus exploiting the steep reduction of the production cross-section
and of the acceptance of the detector for prompt
. The number of CC events per proton drops faster than the number
of CC events. A proton energy in the 300-350 GeV range is found to offer
the best compromise.
Assuming that the present WANF would operate at 350 GeV, the
corresponding value of R is
[47], about 2.5 times lower than at
of 450 GeV.
Additional experimental handles on the prompt background are
provided by the fact that decays would result in an equal number of
prompt and 
and that their energy spectrum
is significantly harder than the one of from the oscillation.