I have investigated how various trigger conditions affect the asymmetry observed in the ZDC SMD. The data for this study are two good fills at the end of 2011 transverse period, fills 15409 and 15419.

During data taking, our standard ZDC polarimetry trigger has been the coincidence: ZDC front & ZDC back & BBC minbias. In Run 11, we had four inner small BBC tiles routed to the ZDC polarimetry scaler boards, and I tried to see if using these tile signals as a trigger can change the ZDC asymmetry.

The best effect is achieved when these inner small tiles are used as a veto signal. The following figures show the raw ZDC SMD asymmetry with the standard polarimetry trigger (ZDC & BBC coincidence), and with the additional requirement that the inner tiles are not hit (ZDC & BBC coin. & !BBC inner small). The exclusion of these BBC inner tiles increases the ZDC signal by 10% of its value.

There are four BBC tiles available in the ZDC scaler boards: tiles 12,13,14, and 15. The following figures show the azimuthal modulation of ZDC asymmetry caused by requiring only one individual tile to be hit. The horizontal axes are an approximation of the azimuthal angle. The vertical axis is the ZDC asymmetry with one exclusive BBC tile requirement normalized to the asymetry with "no BBC tile" requirement.

There are several runs that cause outliers, but most of the data show an azimuthal modulation, about 10% deep. This correlation between ZDC and BBC hits can be caused by a real physics effect (correlated forward neutron + soft pion production), or by an azimuthally asymmetric beam background (it would cause an azimuthal modulation of the asymmetry dilution).