Monte Carlo Trigger Studies

With the modifications required for the trigger due to the new geometry of HYCAL, a new Monte Carlo study was performed to estimate the acceptance efficiencies and trigger rates. The new study had several improvements over the original including energy smearing, a more accurate beamline definition, more realistic detector geometry, and realistic bremstrahlung angular distribution.

The first part of the study focused on the trigger's acceptance efficiency for Primakoff $\pi ^o$ events. The programming of the C542 MLU units was emulated in software so that the algorithm could be fully tested. Figure 3 shows the event viewer used to analyze simulated data. The acceptance efficiency is expected to be nearly 100% for $\pi ^o$ decays whose photons each have a given minimum energy (determined by discriminator threshold settings) and have trajectories pointing to the fiducial part of HYCAL. The few trigger failures expected are due to energy leakage through the cracks between detectors in HYCAL. Figure 4 shows a plot of the trigger efficiency vs. the minimum decay photon energy if the trigger were set to accept all 500MeV or greater photons. This plot shows that the triggering efficiency is quite insensitve to the discriminator threshold.

Figure 3: PrimEx event viewer for Monte Carlo data. This is used to diagnose individual events and the trigger's response.

Figure 4: Plot of the trigger acceptance efficiency (estimated from simulation) vs. discriminator threshold. The acceptance effciency is based on accepting 500MeV or greater photons. The arrow indicates where the threshold should be set for accepting 500MeV photons. The curve shows that the acceptance effciency is very insensitive to the discriminator threshold.

Figure 5: Trigger rate (upper limit) due to background vs. the minimum decay photon energy. The rate includes accidentals from combining several background events. The discriminator values will initially be set to correspond to the 750MeV decay photon energy point. The final discriminator values will be determined by the deadtime in the DAQ system.

The second part of the Monte Carlo study focused on estimating the trigger and detector rates due to backgrounds and accidentals. Figure 5 shows an upper limit of the PrimEx trigger rate as a function of the decay photon energy, estimated by simulation. This upper limit was calculated assuming a 50 ns coincidence window. In reality, PrimEx will use something closer to 20 ns. A "worst case scenario", however, was employed in order to obtain a believable upper limit. An arrow on the plot indicates the 750MeV photon energy point with an event rate of approximately 1.5kHz. Unlike the triggering effciency, the background rate is very sensitive to the value of the discriminator thresholds. For the actual experiment, the thresholds will be set as low as possible such that the deadtime does not exceed 5%.

The primary contribution to background rates come from two sources: 1.) e+e- pair production upstream of the Helium bag and 2.) Compton scattering from the Helium itself. Figure 6 shows the sources of these backgrounds along the beamline. Backgrounds originating in the beampipe are due to large angle bremstrahlung photons showering in the beampipe. Occasionally, some energetic shower photons will pass down the beamline, through the magnet, and into HYCAL. These types of events will be easily rejected in software.

Figure 6: Location of background sources along Hall-B beamline. The angle of the bremstrahlung generated beam photon is plotted vs. the position along the beamline at which the initial interaction took place. The relevant beamline components included in the simulation are shown.

Figure 7: Discriminator rates for different sections of HYCAL based on simulation study.

Further simulation studies estimated discriminator rates and detector rates. The expected discriminator rates are shown in figure for the PbWO4 detector strips. The rates corresponding to the minumum decay photon energy of 750MeV(initial PrimEx trigger point) for the innermost strips will fire at less than 10kHz.