In the isobar runs at 200 GeV, the BBC high voltage was set low, in order to maximize its dynamic range.  This means that it may miss single MIPs that pass through it.  We are reverting to normal values for the 27 GeV run.

Here, I look at hits in the EPD, when a BBC inner tile reports no signal at all.  We see plenty of hits in EPD tiles that mostly overlap silent BBC tiles.  Now, the overlap is not perfect, and of course we cannot tell where a particle hits in an EPD tile (I plot the points at random positions within the tile), but still there are some EPD tiles which are completely covered by BBC tiles, and we see more hits than I'd naively expect.

In this study, I treat the EPD as a hit/no-hit detector.  A "hit" corresponds to an nMIP value greater than 0.9.  (This is WAY above the noise.)

This is about 6.6 M minbias events.

Some observations:

• the position mapping of EPD tiles relative to BBC tiles is correct.  A nice check, though we had already seen it before in cosmic runs.  (That pdf from the 5 March update of this page)
• there is significant EPD hit rate even when the (partially) overlapping BBC tile is silent.  This may or may not be surprising.  It will be interesting to see whether there is more suppression when the BBC HV are back to their nominal values.
• It seems to me that PMT #7 on the West BBC is nearly blind, at least with this current HV setting.

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