1. GE1/1-III GEM Cluster Size and Resolution Studies with the FNAL Beam Test Data Aiwu Zhang, Vallary Bhopatkar, Marcus Hohlmann Florida Institute of Technology CMS GEM workshop IX 14-18/07/2014

2. Outline Current data analysis status from the FNAL beam test Cluster size (number of strips in a cluster) studies Resolution results Summary and next plans 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 2

3. FNAL beam test data analysis status Finished basic characteristics: cluster charge distributions, gain curve, detection efficiency, etc. Implemented spatial resolution study method in polar coordinates. This talk presents the cluster size studies and resolution results, with HV scan data in middle-sector 5 and position scan data in middle position of each sector. Note analog pulse information are decoded from APV chips. 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 3

4. Mean cluster sizes with different thresholds HV scan in middle sector 5 Position scan in middle sectors at 3250V 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 4

5. Fraction of different cluster sizes – HV scan in middle-sector 5 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 5 3σ threshold 4σ threshold 5σ threshold

6. Fraction of different cluster sizes – HV scan in middle-sector 5 N=1N=2N=3 N=4N>=5 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 6

7. Fraction of different cluster sizes – middle-sector position scan (3250V) 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 7 3σ threshold 4σ threshold 5σ threshold Majority of the events have cluster size 2 and 3.

8. Fraction for different cluster sizes – middle-sector position scan (3250V) N=1N=2N=3 N=4 N>=5 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 8

9. Spatial resolution study -- method We study resolution in polar coordinates (φ) due to the trapezoidal shape and radial strips. We use the vertex as origin and align the trackers by finding the X, Y offsets. (rotation of the GEM is also taken into account, not shown here.) X offset Eta5 vertex 9.94° Y offset tracker Step 1. Fix an X offset, loop over Y offset with a step 0.2mm; we get a pair of (X,Y) by requiring residual mean equals to 0. Step 2. Fix a Y offset, loop over X offset with a step 1mm; we find another pair of (X,Y) from the minimal of the parabola of residual sigma. 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 9 We assume that any misalignment will shift peak of residual distribution away from zero and give larger residual width (or make distribution wider)

10. GE1/1-III GEM Residual distributions σ=138μradσ=110μrad Exclusive residual Inclusive residual Exclusive (inclusive) residuals at 3250V in sector 5 (from HV scan data) Cluster size in GEM and trackers all >0. 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 10

11. Resolution results – behavior in detail Resolution vs. HV in middle-sector 5Resolution vs. sector at 3250V 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 11

12. Strip non-linear response study -- motivation A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 7/15/2014 12 From the centroid position distributions of fixed cluster sizes N=2 and 3, we observe that they have apparent bumps around each strip. For N=2, the positions in the middle of two neighboring strips; for N=3 the positions in the strip. (note we define the position of a strip in its center.) This brings us to study the non-linear strip response of charge distribution on position reconstruction, and hence to flat these distributions. Centroid position distribution from COG method (in middle-sector 5). N=2N=3

13. Strip non-linear response study -- method h(η 2 ) distribution h(η 3 ) distribution Correction function for 2-strip events Correction function for 3-strip events A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 7/15/2014 13

14. Cluster position distributions before and after correction N=2 N=3 HV scan data @ 3250V Before correctionAfter correction 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 14

15. Strip response functions N=2 N=4N=3 HV scan on middle sector 5 N=2 N=4N=3 Position scan at 3250V 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 15

16. Resolution results (HV scan) before and after position correction Resolution is not improved after position correction with HV scan data! All valid events with cluster sizes 2,3,4 N=2 N=3 N=4 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 16

17. Resolution results (position scan) before and after position correction Resolution is not improved after position correction with position scan data! All valid events with cluster sizes 2,3 and 4 N=2 N=3 N=4 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 17

18. Summary  Most of the clusters have sizes no more than 5.  Spatial resolution is on the order of 120μrad.  We don’t observe improvement on resolution after applying cluster position correction. What else can be done with these data (HV scan, position scan in middle sectors):  Study details of cluster size distributions using VFAT-like strip hits.  Apply position correction a second time to flatten position distributions even more and to see if resolution could be improved.  Study resolution with different thresholds. What needs to be done with other data:  We also performed two position scans with the detector tilted by 7degrees from normal incidence. The effect of the inclination needs to be studied. 7/15/2014 A. Zhang et al., GE1/1-III GEM Studies with FNAL Beam Test Data, GEM workshop IX 18