charged particle veto

The analysis of tagging coincidences would be simple if a sample of fully-reconstructed events could be formed based on the information from the Radphi detector alone. Because the detector does not reconstruct charged tracks in the forward region, this would require a reliable method for extracting neutral events from the much larger background of events containing charged pions and kaons. For this purpose the design of the experiment incorporated a forward scintillator wall to veto charged particles, the cpv, but the rates in these counters were too high to permit an efficient cp veto in the online trigger. In order to pass the higher levels of the online trigger the total measured energy in the forward calorimeter had to be greater than about 3 GeV, which tends to favor reactions with several gammas in the final state, but the task of separating the mixed events from those with only gammas falls to the offline analysis.

Figure 1: Time difference between any hit in the cpv and the bsdAND signal which generated the event trigger, normalized to $10^6$ events. The continuum measures the background rate in the cpv, mainly coming from electromagnetic conversions in the target. Bin width is 1 ns.

Before a correspondence can be found between the measured energy and tagger channel a way must be found to effectively use the cpv to remove charged events. Fig. 1 shows the time difference between the bsdAND and a hit in any one of the cpv paddles. There are $10^6$ bsdAND triggers in this sample. From the level of the continuum in Fig. 1 one deduces that the inclusive rate in the cpv was about 80 MHz. This rate was spread more or less evenly across the 30 scintillator paddles in the cpv so that the rate in a single counter was only a few MHz. A veto around the entire cpv coincidence peak in Fig. 1 would contain an average of 2 accidentals, which would imply a prohibitive loss of good all-neutral events if such a cut were applied.

A much tighter coincidence peak is obtained if the time correlation is taken between the tagger and the cpv. In Fig. 2 is shown the time difference between any hit in the cpv and any hit in the tagger. The tagger hits were selected outside the tagger coincidence window so that the cpv-tagger correlation in Fig. 2 is unbiased by the trigger. Part of the improvement in the coincidence timing in Fig. 2 over Fig. 1 comes from the use of times for each tagger channel that have been corrected for individual channel offsets, rather than using the electronic OR. It will be shown below that a careful selection of events with a single hit in the bsd and suitable corrections for time-of-flight in the barrel leads to a cpv-bsd timing peak that is nearly as well-defined as that shown in Fig. 2, but the fact remains that the tagger provides the single best time reference for this experiment.

Figure 2: Time difference between any hit in the cpv and an arbitrary hit in the tagger, normalized to $10^6$ tagger hits. The 2 ns microstructure of the CEBAF beam is visible in the spectrum, together with the main peak coming from tagged photons producing charged particles which strike the cpv. Bin width is 1 ns.