5.3 Global Trigger

The RPC trigger chain will produce trigger candidates with a preference for large pT assignment while the chambers trigger chain will try to measure the pT of every detected muon. The two devices will produce independently their trigger candidates. At a first glance it seems that they just create twice the same candidate, but, even if this is true for most of the candidates, looking deeper in the problem we see that the two triggers are complementary.

Their complementarity is given obviously from the fact that one system can cover the other system's inefficiencies, but also from the fact that the two detectors react in different way to different kinds of background. For instance single hits from neutrons and gammas enviromental radiation disturb the RPC trigger, but they do not affect the chambers trigger due to the requirement of multiple hits in a module. On the other hand muon showering does not permit a safe track reconstruction, but RPCs will easily find a high momentum track inside the shower.

It is evident that some combination of the trigger candidates is needed. A logic device called Global Muon Trigger [7] merges the two triggers running an algorithm whose task is the maximization of the trigger efficiency. The development of this device has started and preliminary results in the barrel region are reported in Figure 16. We see that there is an improvement of the trigger efficiency with respect to the single trigger chains. The main drawback to the CMS scheme is a certain amount of false muons (~5%), but we are looking for methods to improve the rejection performance.

Figure 16: Preliminary trigger efficiency from Global Muon Trigger in the barrel region h < 0:8.