the UPV OR

Figure 5: UPV OR scaler rate vs. beam current measured at the beginning of the June run period.

Figure 6: UPV OR scaler rate vs. beam current measured at the end of the June run period.

The rates in the upstream veto wall run a factor of 20-40 lower than the tagger OR. The UPV OR is formed by a OR'ing together the outputs from the 8 discriminators connected to the UPV counters. The width $t_{_C}$ of the UPV OR signal is assumed to be the same as that of the CPV. The observed scaler rate $S_{_U}$ for the UPV OR is given in the model by

$$S_{_U} = r_{_U}\,I$$ (5)

$${\cal F}_{Uhit} = 1$$ (6)

$${\cal F}_{Umiss} = 1-e^{-t_{_C}\,r_{_U}\,I}$$ (7)

where $r_{_U}$ is the rate in the UPV OR per nA of beam current. The value of $r_{_U}$ varied by as much as a factor of 5, depending upon the quality of the beam tune and what CLAS was doing. The scan shown in Fig. 5 taken towards the beginning of the June period under average beam conditions should be contrasted with the scan shown in Fig. 6 during which CLAS emptied their hydrogen target. Under no circumstance did the UPV OR rate rise high enough that nonlinear effects from electronics dead time became observable. This implies a restriction on the validity of the model to rates where the UPV OR is below 5-10MHz. Under worst-case conditions this implies a maximum beam current of 360nA, well above our design luminosity that corresponds with this radiator to 250nA. The other restriction of the model is that the veto window width is wide enough to exclude the entire coincidence peak (see Eq. 6), which implies a lower limit on $t_{_C}$ of roughly 15ns (see Fig. 18 below).

In analogy with the classification of events as either trues or accidentals with respect to the tagger, each event is also classified as either a Uhit or Umiss event, depending upon whether or not whatever produced the hit in the RPD shared the same mother photon as something that produced a hit over threshold in the UPV. The quantities ${\cal F}_{Uhit}$ [ ${\cal F}_{Umiss}$] represent the fractions of events in each class which fall within the UPV coincidence window. As before, one assumes in the definition of Uhit events that the causal chain connecting the RPD and UPV hits is a few tens of ns long at most.