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What vertex resolution can we expect for d-Au collisions? Typically BRAHMS triggers on Au-Au events with 10 or more neutrons in each ZDC. For such events the measured ZDC vertex resolution defined as a difference between Z(ZDC) and Z(TPM1) is of the order of 2.5 cm (top plot). In the upcoming d-Au we will expect only a few neutrons in the ZDC which "sees" the d-beam. We expect that the vertex resolution will get worse, because less light is produced and we are more sensitive to the y-position of the neutron that hits the ZDC. In order to simulate d-Au conditions with existing Au-Au data we selected ultra-peripheral events (with only 1 and 2 neutrons in ZDC) in an effort to predict the detector's resolution. Cuts were determined from ZDC EM spectrum for the run #5347. The difference between Z(ZDC) and Z(BB) were measured, since these events produce very few tracks in the TPC. Module #1 in the right ZDC a dead ADC channel for that run, so in the time calculation only times from 2nd and 3rd modules were used. When all modules are used we can expect some improvement in resolution. The distribution was fit with a Gaussian, giving a sigma of about 7 cm. The result is shown in the middle picture. In attempt to improve this result the "second order slew-correction" for 1 - 2 neutron range was applied. This procedure is justified because of the small statistical weight of such events in total neutron spectrum. The third picture shows the effect of this second order slew-correction; sigma Z is about 5 cm. Andrei, Michael.
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