Using the HERA - B detector, the
production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The
production was tagged via inclusive bottom quark decays into
by exploiting the longitudinal separation of
decay vertices from the primary proton-nucleus interaction. Both
e
+
e
-
and
channels have been reconstructed and the combined analysis yields the cross section
.
Received: 6 June 2002 / Revised version: 2 September 2002 / Published online: 8 November 2002