6 High Tower ADC-pedestal 10 15 20 25 30 35  equivalent -1 Counts, scaled to 1 pb 2 10$^{3}$ 10  Trigger I, with Trig II cuts  Trigger II /dy = 100 pb): Simulation (BR d  500 (1S) in acceptance   500 (1S) triggered  FIG. 3: The L0 DSM-ADC distribution ( ∝ E$_{⊥}$ ) for the high- est EMC tower of a candidate pair. We show Trigger I data after applying Trigger II thresholds (red circles) and Trigger II data (black squares). The yields are normalized by the integrated luminosity. The histograms are from simulation of Υ(1S), showing the corresponding distribution for electron daughters satisfying acceptance (solid line) and trigger (dot- ted line) requirements. The simulation histograms are nor- malized assuming B × dσ/dy = 100 pb, times a factor of 500 for clarity. The vertical line is the Trigger II threshold of 16 counts. The histograms have the bin centers set at integer values to match the integer nature of ADC counts. that satisfies both of the pairwise conditions in the event, the algorithm issues an L2 trigger. In order to evaluate the trigger performance, we ap- plied the Trigger II cuts to the Trigger I data. To do this, the exact same trigger condition was applied offline on the recorded values of the original trigger input data. Since events satisfying Trigger II cuts also satisfy Trigger I cuts, applying the tighter set of Trigger II cuts to the Trigger I data offline allows us to make a single dataset with uniform properties for the entire 2006 run. The in- tegrated luminosity for Trigger I was L = 3 . 12 ± 0 . 22 (syst.) pb − $^{1}$, and for Trigger II it was = 4 . 76 0 . 33 (syst.) pb − $^{1}$, giving a total integrated luminosity of 7.9 ± 0.6 pb − $^{1}$, where the 7% uncertainty originates from the uncertainty in the BBC measurement of the cross section as determined by a van der Meer scan [22]. Figure 3 shows the ADC distribution of the tower with the largest E$_{⊥}$ for each candidate pair seen at the trigger level in an event (in very few cases we find more than one tower above the L0 threshold, so most events have only one candidate pair). The two triggered data sets are shown, with Trigger II displayed as squares and Trigger I (after applying Trigger II cuts) displayed as circles. The rejection factor achieved with Trigger II, defined as the number of minimum bias events counted by the trigger scalers divided by the number events where the trigger was issued, was found to be 1 . 8 × 10 $^{5}$. The distributions seen at the trigger level only include information from the BEMC. This causes the trigger rate seen in the experi- ment to be dominated by background from di-jet events with two nearly back-to-back π $^{0}$s. This di-jet background is removed offline when including tracking information from the TPC. The trigger distributions are scaled by the overall luminosity in order to compare the relative nor- malization of the two datasets. The scale is chosen such that the counts in each triggered dataset correspond to an integrated luminosity of 1 pb − $^{1}$. The relative luminosity normalization between the datasets agrees to a level of ≈ 1%. The solid-line histogram is from simulated Υ(1S) after acceptance selection, requiring both electrons to de- posit at least 1 GeV of energy in the BEMC. It shows the spectrum for the daughter with the highest ADC count. The spectrum for the simulated Υ(1S) events that satisfy all the trigger requirements is shown as the dashed-line histogram. The vertical line at 16 ADC counts represents the L0 ADC threshold for Trigger II. In order to compare the size of the expected Υ signal relative to the trigger background from di-jets, the histograms from the simula- tions for the Υ(1S) are scaled with two factors. The first factor corresponds to normalizing to an integrated lumi- nosity of 1 pb − 1 assuming a cross section times branching ratio into e $^{+}$e − for the Υ(1S) at midrapidity of 100 pb, chosen because it is of the expected order of magnitude. From this, we expect one upsilon every ≈ 3000 triggers, so we use a second multiplicative factor of 500 for dis- play purposes. The spectral shape of the data in Fig. 3 is therefore dominated by di-jet background. The same normalization and scale factors are used in Figs. 4–6 for comparing the trigger-level background distributions and the expected Υ(1S) signal. Figure 4 shows the distribution of the L2 Cluster-1 energy for all clusters that include a tower above the HT DSM-ADC threshold of 16 ADC and that have their corresponding Trigger Patch DSM-ADC sum above the threshold of 19 ADC counts. It can be seen that the peak of the L2-cluster distribution near 6 GeV is not right at the threshold of 4 GeV (vertical line). This must happen because another trigger selection that is correlated to the measurement of the L2-cluster is being applied. In our case it is the L0 selection, which consists of both the HT and the TP requirement. Once we see a tower with energy above the HT threshold that is also in a trigger patch with energy above the TP threshold, the energy in the 3-tower cluster for that tower will likely already be above the L2 E1 threshold. The distribution for the L2 opening angle θ L 2 12 is shown in Fig. 5. It is highly peaked towards back-to-back topologies, much more so than the distribution from sim- C. Trigger Performance