Run 11 Beam Background Follow-up

In the presentation I gave last week on the Spin PWG phone there were some questions about whether we know that beam backgrounds are the cause of only ~70% of L2BW triggered events having a valid reconstructed vertex.  Figure 1 (top) shows the 7-bit bXing dist for all L2BW triggered events (white) and L2WB triggered events where a rank > 0 vertex was reconstruted (blue).  The fraction of L2WB events with reconstructed vertices is shown in the bottom plot.  The dataset I'm using is the L2W stream from day 42-52 produced on the cloud at Nersc Magellan.

Figure 1:  7-bit bunchX for L2WB events (white) and L2WB events with reco vertex (blue)

To test whether these events without a reco vertex come from beam backgrounds, I plotted the L2WB algo seed towers (ET > 5 GeV) in eta-phi space to see if the L2WB trigger clusters were localized in some area where we might expect beam background to contribute to the trigger.  These eta-phi distrubtions for trigger towers are also separated by the 7-bit bXing to see the effects in the abort gaps separately.  From left to right the plots below are for 7-bit bXing = [0,30] , [31,39] , [40,110] , [111,119].

Figure 2:  L2WB algo seed towers in eta-phi for events with a valid reconstructed vertex

Figure 3:  L2WB algo seed towers in eta-phi for events with no valid reconstructed vertex

For another test of whether these events without a valid reco vertex come from beam backgrounds, I plotted the "hit" BEMC towers (ADC-ped > 10) vs eta and phi to look for localizations in energy deposition in L2BW triggered events.  They are sorted into similar 7-bit bXing bins as above.

Figure 4:  Hit towers eta-phi for events with a valid reconstructed vertex

Figure 5:  Hit towers eta-phi for events with no valid reconstructed vertex

Figure 2 and 4 above show nicely that when a reconstructed vertex is found the eta-phi distribution of trigger barrel towers and "hit" barrel towers is ~uniform in the collision bunches, and there aren't many hit towers in the abort gaps, as expected.  The one caveat here is a few "warm" towers that have since been masked from the trigger.

Figure 3 and 5 above show that both in the collision bunches and in the abort gaps there is definite localization of the trigger towers and "hit" towers, in regions where we might expect beam backgrounds, when no reconstructed vertex is found. 

Summary: The large differences in the trigger and "hit" tower distribution for events with and without a valid reconstructed vertex in the plots above, are consistent with beam background.  So it seems the fraction of L2WB triggered events with a valid reconstructed vertex is at least a reasonable qualitative measure of the beam background.