A study of the effects of the noise in the SIFT threshold values (due to the discriminator) on the FPS cluster multiplicities

There is electronics noise due to the discriminator affecting the actual value of the threshold that will be "set" in the SIFT chip for the purposes of the cluster finding in the FPS detector. Currently, this noise is being estimated to be on the order of 2.7 fC. We checked the effect of such noise on the FPS cluster multiplicities in the dijets MC sample which has also been extensively studied elsewhere from a different prospective.

In this study, we used the calibration constant of 30 fC/MIP for the FPS shower layer. Thus the Low SIFT threshold of 3 mips corresponds to about 90 fC of charge. Therefore, noise of 2.7 fC is on the order of 3% in the low SIFT threshold value. So, we smeared the Low 3 mip threshold by Gaussian with a width equal to 3% of the threshold value. This was done independently for each FPS shower layer strips and the smeared threshold values were used for FPS cluster reconstruction.

The study was carried out for the highest Et (40 - 500 GeV) dijet sample with 6 MB events overlaid (the worst case from the cluster multiplicity point of view). The cluster multiplicities in the V-strips are shown in this plot (to be compared to Fig. 4 of the DØ Note in preparation).

Finally, we compare the distributions of the total number of U and V clusters with smeared threshold value to that without smearing. Here is this direct comparison plot .

Based on this study, we therefore conclude that the effect of such noise on FPS cluster multiplicity is entirely negligible. However, trigger rates need to be checked carefully as well although no large effects should be expected with this level of electronics noise due to the discriminator in the SIFT chip threshold values.