PHOBOS LRP: Should we fill the holes?! What happens to flow as the silicon gets blasted? J. Hamblen, S. Manly, I.C. Park

Do we fill the spec holes with silicon? The gain: Improved reaction plane resolution Less complex flow/reaction plane analysis Flat/defensible psi2 vs. phi for flow/rxn plane analysis The pain: Depends on type of module, may depend on what fraction of hole is filled Affect on tracking (multiple scattering thought to be small, affect of additional secondaries on pattern recognition to be studied).

We must be able determine the reaction plane using data in the mid-z region due to need to use vertex trigger for many analyses. This can be done now using hole filling techniques or using the symmetric regions of the OCT and RINGs. Hole filling: good resolution psi2 vs. phi distribution has structure that we have not learned to remove results for v2 seem to be less robust and tends to fluctuate in ways that are not understood Symmetric detector: Well-behaved, but poor resolution if implemented at mid-z due to fact that subevents are in regions of eta with low multiplicity and low flow.

Ideally fill the spec holes with new type of silicon: OCT module sized, SPEC layer 1 segmentation Fine for flow/rxn plane Improves tracking with reduced lever arm for multiple scattering and additional hit, assuming the additional material does not create problems with pattern recognition in the rest of spec. New type of silicon Many channels We assume this is either practically impossible or painful enough that we must study the case of using OCT modules instead.

Can we use OCT silicon modules?   RingsN Octagon RingsP Two possibilities: one module (reduce size of hole) three modules (fill hole) 1 module, no new support structure, less of an impediment to tracking, only use one module, but less of a gain for flow and mult 3 modules, more material

Discussion we have heard to date, OCT module filling of hole not a huge deal Bigger issue is: What is effect on tracking? That is to say, is this something we are willing to do? Earlier studies by Inkyu (not summarized here) showed that the multiple scattering induced by the additional layer of silicon was not a big deal in terms of the tracking. However, the additional material will increase the background and this might affect the pattern recognition. This requires simulations. We have requested three minimal MC samples with events generated using the same seeds. One is current geometry. One has 1 module filling hole. One has 3 modules filling hole. We will study affect on tracking.

OCT module replacement Suppose we don’t bother. What is the effect on flow? Remember: flow analysis is robust. If we are missing a pad, we reflect the information from the pad that is 180  opposite. If we are missing both pads, we remove that small slice in eta.

Josh’s mid-z analysis using symoct regions and rings for subevents: comparison of pre and post blast for same statistics No big change. Low statistics makes it hard to draw much of a conclusion

Inkyu’s blast study See look at “blast study” Inkyu’s dead/hot channel masking: For a given z-position, 8 Octagon sensors are attatched with each other. This makes 32(rows) x 30 (cols) pixel map. Define hit with some reasonal hit energy cut, and profile the histogram. Fit the profile histogram with a line fit (pol0) Any pad which deviate more than 20% is marked as dead/hot.

Look at difference between PR00 and PR01 masks to determine effect of blast

PR00 weight matrix

Weight matrix after three hypothetical blasts: note removed regions in eta Actual effect depends on exact distribution of blast damage

One blast Three blasts Little change … is good

One blastThree blasts Measurement is robust, lose a little coverage Where one loses coverage is dependent on region of damage

Scaling of error with blast damage Flow value is robust with damage, can lose regions of eta coverage and error grows due to loss in number of hits and sensitivity