In order to cope with high luminosities and large background rates, the trigger system is composed of four successive levels (T1, T2, T3 and T4) of increasing selectivity.
The first two trigger levels (T1 and T2) are synchronous with respect to the Beam Cross Over signal (BCO). T1 acts as a loose pre-trigger while a positive T2 decision triggers the acquisition of the data collected by the front-end electronics. T1 and T2 have been active since LEP startup.
With four bunches of electrons and positrons circulating at equal distances in the machine, the LEP bunch-crossing interval is 22 microsecond. The T1 and T2 trigger decisions are taken 3.9 microsecond and 39 microsecond after the BCO respectively. The dead-time introduced is then typically 3%, with 2% due to T1 and 1% to T2 for a typical readout time of 3 ms per event. The inputs to T1 are supplied by individual detectors, namely by:
and combinations of signals from different subdetectors.
The last two trigger levels (T3 and T4) are software filters performed asynchronously with respect to the BCO.
T3 halves the background passing T2 by applying the same logic as T2 but using more detailed information. It was implemented in 1992 with the aim of maintaining the data logging rate below 2 Hz.
T4 was implemented in 1993 to tag, and in 1994 to reject, about half of the background events remaining after T3.
Each subdetector contributes to the trigger decision with data generated by the respective subtrigger processors. Those with low counting rates produce their own triggers while ones most affected by background are grouped in level 2 majorities, i.e. at least one
acceptable coincidence of 2 signals out of the n inputs forming the majority is required. This is an effcient way to correlate detectors in a "quasi" single-track or single-cluster configuration that avoids the background typical of single detector triggers. The T1 and T2 decisions are taken by OR-ing a number of "in time" combinations of signals.
The overlapping geometrical acceptance of the different detectors provides substantial redundancy between the different trigger conditions. This feature of the DELPHI trigger ensures high and stable efficiency over long running periods.
Track elements give trigger signals in the TPC, FCA/FCB, ID, OD and TOF.
A transverse momentum cut pt>1 GeV/c for polar angles from 29 to 151 degrees(TPC) and pt>1.6 GeV/c in the forward/backward region from 11 to 33 degrees and from 147 to 169 (FCA/FCB) is applied.
Muons also give trigger signals in the barrel region, for polar angles from 50 to 130 degrees with a 1 degree hole at 90 degrees (MUB) and in the forward and backward regions, from 15 to 41 degrees and from 139 to 165 degrees , in the HOF and in the MUF.
Electromagnetic energy deposition gives trigger signals in the barrel region in the HPC and in the forward/backward regions in the FEMC. Energy depositions above 2 GeV and 2.5GeV respectively are demanded. A lower threshold is applied in the FEMC(1.2 GeV) when it is correlated with other detectors.
Hadronic energy deposition gives trigger signals in the
Hadron Calorimeter, both barrel (HAB) and forward (HAF). Energy deposition thresholds of 0.5, 2 and 5 GeV -this last is referred as High Threshold- are used.
The redundancy between the different trigger components also makes it possible to determine both the trigger effciency and its maximal error with good precision. The global trigger effciency for electron and muon pairs is consistent with 1 at the level of 0.0001
for polar angles between 20 and 160 degrees. Even for single tracks, provided their momentum transverse to the beam exceeds 1 GeV/c, the effciencies in the barrel (between 42 and 138) and forward (between 10 and 32) and backward (between 148 and 170) regions exceed 95%. In the barrel region, single photons are triggered by the HPC scintillators and also (at T2) by the charge pattern recorded in the HPC: the single photon effciency rises linearly from 5% for photons between 1 and 2 GeV to 60% for photons above 4 GeV. Due to their high nal state multiplicity, hadronic events
(Z0 --> hadrons) are triggered with an e ciency hardly distinguishable from 1 over nearly the full solid angle.