\midheading{$Z+b{\bar b}$ production ($m_b=0$), processes 51--53}
\label{subsec:zbbmassless}

Process {\tt 51} is identical to {\tt 50} except for the fact
that the bottom quark mass is neglected. This allows the calculation to be
performed up to NLO. The other processes {\tt 52} and {\tt 53},also with massless $b$-quarks
account for the decays into
neutrinos ({\tt nproc=52}) and bottom quarks ({\tt nproc=53}). Note that
the NLO calculation of process {\tt 53} does not currently
include radiation from the
bottom quarks produced in the decay.

When {\tt removebr} is true in process {\tt 51}, the $Z$ boson does not decay.