Study of the LHCb trigger performance robustness

Several ``Robustness Scenarios'' have been studied with the latest versions of the trigger algorithms, as described in the Trigger TDR~\cite{TDR}. The Level-1 Event Size and Execution Time are particularly sensitive to the event multiplicity. Changing the PYTHIA settings within reasonable values changes the event multiplicity by $\pm30\%$, hence the uncertainty on the size of the CPUs farm and event building network is of this order of magnitude. In the ``worse case'' scenario, denoted as ``Global Test'' in this note, the trigger performance may be degraded by up to $~25\%$ for the hadronic channels.