STAR Background Check with the SVT

Wieman 4/14/04

The SVT raw data has been used to check the hit density at the center of STAR to determine possible consequences for the micro vertex detector operation. The check was made using a 1/2 B field run, Au+Au at full energy, run 5062008. About 4000 events were used with a central trigger, cent-bbc-narrow-half, Trigger ID 25105. Clusters were counted in the three SVT layers for the ladders at 3:00 o'clock, barrel 1 ladder 2, barrel 2 ladder 3 and barrel 3 ladder 4. Clusters were required to have at least two anodes and any clusters with greater than 20 time buckets were excluded.

The results are shown in the following plot, hit density as a function of radius. The blue squares show the results for the three SVT layers.

The red curve shows the expected hit density for central collisions with a uniform dN/dh = 700. This is the hit density averaged over the interaction diamond as determined by the trigger condition. This was assumed to be a Gaussian distribution with s = 16 cm. The red circle shows this prediction for the 1.5 cm radius where the inner layer of the proposed vertex detector lies.

This shows that for central collisions there are no surprises, namely in STAR the measured hit density does not show any significant additional background effects.

The measured hit density at the inner SVT layer is about 10% higher if we remove the 2 or more anode constraint. If we use the official SVT hit finder software the result is about 50% higher, this discrepancy is still being investigated.

There is still a question about background accumulation in the micro vertex detector from beam gas showers etc. The zero bias SVT data could potentially provide this information, but so far the cluster finder filter is not up to the task. The SVT noise leak through will overwhelm the zero bias signal.