T3B Trigger System

The T3B experiment can acquire events in two different trigger modes: CALICE sync mode and standalone mode. Figure 4.11 shows a simplified sketch of the most relevant beam line instrumentation during the T3B test beam phases and essential parts of the trigger system. A more detailed elaboration can be found in [79]. Peculiarities of the various test beam phases will be discussed in Section 4.3.3. In the sketch, the T3B experiment is triggered in CALICE sync mode. First, the particle beam traversed two Cerenkov counters. Their signals were discriminated and then recorded by the CALICE and the T3B DAQ. This allows for an offline determination of the incoming particle type corresponding to the event.

Downstream of the Cerenkov counters was a veto scintillator positioned. This is a large scintillator of30×30cm2 with a circular hole of8cm in diameter in the center and an attached PMT. If a beam particle propagates far from the beam axis, it will traverse the veto scintillator and trigger a veto signal that is recorded. Rejecting such events in the offline analysis can improve the beam collimation of the analyzed data set.

For hadron runs, the triggering of an event is performed by two10×10cm2 _{scintillators}

located in the beam center and in front of the calorimeter. If both trigger scintillators detect light simultaneously, the coincidence signal is fed into the CALICE trigger electronics and induces the readout of all calorimeter cells. For muon runs two large

80×80cm2 _{scintillators located in front and behind the calorimeter were used to trigger}

the events. This assured nearly full coverage of the lateral area of the HCAL and the total penetration of minimum ionizing muons which is relevant for the calibration of all calorimeter cells. For a timing experiment like T3B, the size of the trigger scintillators is a crucial parameter. Usually a PMT is attached to one side of the scintillator. So the trigger signal of a particle that traverses in maximal distance of the PMT has a delay

Cerenkov A Cerenkov B Veto 10x10 10x10 Beam CALICE T3B TCMT 80x80 80x80 Calorimeter CALICE Trigger Electronics Muon Run Hadron Run CALICE Trigger 15 T3B Cells Ext CH 5 PS6403 1 PS2203 Spill Signal & &

Figure 4.11: Systematic sketch of the beam line and the trigger setup of T3B together with the CALICE AHCAL (not to scale). TCMT stands for Tail Catcher and Muon Tracker. Not all elements were available for all run periods.

of >2ns with respect to a particle traversing close to it for a scintillator of 80cm edge length. Furthermore, the time jitter increases since scintillation photons are emitted in random directions. For T3B, this results in a trade off between the coverage of a large cross section of the beam for larger scintillators and the capability of small scintillators for fast timing. For muons runs, good coverage is favoured whereas the CALICE team chose the smaller trigger scintillators for the triggering of the usually highly collimated hadron beam.

The CALICE electronics exhibits a certain dead time for the processing of an event (O(1ms)) and has a limited event buffer size. Therefore, the number of coincidence signals is significantly larger than the number of events acquired by the CALICE DAQ (see Figure 4.12). The T3B DAQ is not capable of assigning an individual timestamp to each of the recorded events. Thus, one needs to assure that the T3B DAQ records exactly the same events as the CALICE DAQ for a successful offline matching. Fortunately, the CALICE DAQ has a live trigger output. In the CALICE sync mode this output was multiplied and used to trigger the oscilloscopes of the T3B experiment simultaneously. So a successful synchronization relies on a spill-by-spill matching of the event count of T3B and CALICE. This is further complicated by the output of timeout triggers by the CALICE DAQ. In test beam operation, the T3B DAQ switches to capturing mode when the spill start is detected and stops it with the end of the spill signal. In this process, the capturing mode of the five PS6403 oscilloscopes is finished consecutively

Scintillator Coincidence CALICE Trigger Timeout Trigger Spill Trigger Stop Capturing Delay Area

Figure 4.12: Sketch of the trigger signals needed for the test beam operation of the T3B experiment. A scintillator coincidence signal is created when a particle traverses two trigger scintillators simultaneously (blue). A fraction of these signals triggers the CALICE DAQ. The trigger output of CALICE is characterized by particle triggers (red) and timeout triggers (green) which can be distinguished since T3B records the scintillator coincidence signal of particle events explicitly. The event triggers occur within the spill. The spill start and the spill stop are indicated by a spill signal (black) supplied by the accelerator.

which takes a few milliseconds. If a trigger signal arrives within this time (see Figure 4.12), the number of captured events differs among the oscilloscopes. In addition to the beam trigger, the CALICE DAQ has a so-called timeout trigger which probes if the spill signal is still HIGH, meaning within the spill. The timeout trigger releases a trigger signal with a periodicity usually set in the order of 1s. This can occur exactly when the oscilloscopes stop capturing consecutively and are the most probable reason for event mismatches. But since this case cannot be distinguished from a true mismatching of particle triggers, such spills have to be rejected completely. This effect reduces the statistics of T3B by approximately5−10 %.

When T3B is in standalone mode the oscilloscopes are triggered directly by the coin- cidence signal of the trigger scintillators. This increases the acquisition rate of T3B significantly, but excludes the possibility to combine the data of CALICE and T3B in the later analysis.