1. 15 Dec 2010 CERN Sept 2010 beam test: Sensor response study Chris Walmsley and Sam Leveridge (presented by Paul Dauncey) 1Paul Dauncey

2. 15 Dec 2010 CERN 2010 beam test data Taken with EUDET telescope Two arms, each of three layers of MIMOSA-26 silicon pixel sensors Ran with and without tungsten sheets mounted between the arms MIMOSA-26 sensors Digital CMOS pixel sensors, fabricated with 0.35  m AMS 700  m thick (TBC) with 15  m epitaxial layer. 1152×576 regular (i.e. no gaps) pixel array Pixel pitch 18.4  m, total active area 21.2×10.6mm 2 Rolling shutter readout; each step is a group of 72 pixels in single row so 16 steps per row and 9216 steps for whole sensor Rolling shutter time ~110  s total, so ~100ns per step Data from two full rolling shutters around trigger are kept 2 Paul Dauncey

3. 15 Dec 2010 Sensor study motivation Overall aim of beam test is to measure EM shower density Form tracks in front three sensor layers Project through tungsten to back three sensors layers Measure particle density w.r.t. track projection position Clusters of hits are used for two purposes In front three layers to form track hits In back three layers to count particles and know their position For both need to know how a single particle forms a cluster Determine by studying rates of various patterns of hits in clusters Want average particle position within cluster for a given pattern Want resolution of particle position for a given pattern 3 Paul Dauncey

4. 15 Dec 2010 Cluster patterns Cluster pattern depends on where the particle passes through If near centre, one pixel in cluster If near centre of edge, two pixels in cluster If near corner, four pixels in cluster 4 Paul Dauncey Classify by number of pixels and pattern Some specific patterns are “physical”; all can be rotated/reflected Examples of other patterns which are not physical

5. 15 Dec 2010 Cluster patterns in data Form track in the five “other” layers Project into layer being studied and look for nearby clusters Look at rates of clusters with various patterns See non-negligible rate of a pixel firing twice in one event Presumably this is due to real particle hit Integrate over both time frames Only count number of different pixels, not number of times fired, when characterising the clusters Look at runs 20021-26 All are 120GeV hadrons with no tungsten Different threshold per run; 6.0-10.5 (in some arbitrary threshold units) 5 Paul Dauncey

6. 15 Dec 2010 Distance of cluster from track projection Clean; almost no background even at threshold=6 5-hit tracks difficult to fake Alignment looks OK; all centred on zero 6 Paul Dauncey

7. 15 Dec 2010 Cluster simulation 7 Paul Dauncey 2D simulation of charge spread and threshold Standalone model of 5×5 pixel array Put “charge” with Landau distribution at point; units MPV=1 Initial position varied uniformly over 100×100 array in central pixel Parameterise diffusion by spreading with Gaussian Charge absorbed per pixel is integral of Gaussian over pixel area Add noise per pixel and apply threshold Tune parameters Diffusion spread Gaussian width Threshold units-to-MPV conversion factor Pixel noise Minimise chi-squared comparison with data cluster pattern distribution

8. 15 Dec 2010 Tuning example 8 Paul Dauncey Chi-sq versus two parameters Noise fixed to 2.4 threshold units X axis is spread Gaussian width in % of pixel pitch Y axis is conversion factor Layer 0, threshold = 10.0 units Data Sim 8% inefficiency

9. 15 Dec 2010 Layer 0, threshold=6 9 Paul Dauncey

10. 15 Dec 2010 Layer 0, threshold=8 10 Paul Dauncey

11. 15 Dec 2010 Layer 0, threshold=9 11 Paul Dauncey

12. 15 Dec 2010 Layer 0, threshold=10.5 12 Paul Dauncey

13. 15 Dec 2010 Each layer tuned to all runs 13 Paul Dauncey

14. 15 Dec 2010 Tuning results 14 Paul Dauncey Parameters reasonable consistent for all thresholds and for five of the six layers Spread Gaussian width ~ 25-30% ~ 5  m Threshold unit-to-MPV conversion ~ 0.03-0.04 E.g. threshold = 10 units ~ 0.3-0.4MPV; when charge split between four pixels, efficiency will be low Noise ~ 2.4 threshold units ~ 0.08MPV Layer 3 significantly different in conversion parameter Spread width and noise similar Threshold unit-to-MPV conversion ~ 0.09 E.g. threshold = 9 units ~ 0.8MPV; generally will have low efficiency) Annoying as this is closest layer to tungsten in shower runs

15. 15 Dec 2010 Layer 3, threshold=9 15 Paul Dauncey 42% inefficiency!

16. 15 Dec 2010 Simulation positions for patterns 16 Paul Dauncey Not layer 3

17. 15 Dec 2010 Cluster-track positions for N=0 17 Paul Dauncey Left: Standalone simulation (no track, perfect position) Middle: Data (including track resolution) ~ 100k events Right: First go at full simulation (including tracking resolution) ~ 100k events

18. 15 Dec 2010 Cluster-track positions for N=1, 2 line 18 Paul Dauncey

19. 15 Dec 2010 Cluster-track positions for N=3 L, 4 square 19 Paul Dauncey

20. 15 Dec 2010 Cluster resolution 20 Paul Dauncey Cross-check simulation using resolution Each cluster pattern results from particle hitting particular position within pixel Look at track projection positions for each cluster pattern Look at resolution from simulation and data (subtracting track resolution) for each pattern Resolve resolution along symmetry axes a b a b a b

21. 15 Dec 2010 Layer 1 cluster-track residuals 21 Paul Dauncey Layer 1 has smallest track resolution Histograms show a (solid) and b (dashed) residuals in each category

22. 15 Dec 2010 Layer 1 resolutions 22 Paul Dauncey Left: Data, right: full simulation; a and b resolutions in each category Red is raw RMS, blue is with track resolution subtracted Qualitative agreement, except category 4 (N=3L) is quite different Not understood.... Quantitative agreement to ~20% level for others

23. 15 Dec 2010 One other oddity not understood... 23 Paul Dauncey Rolling shutter readout with two frames per event Cluster hits should appear in frame 0 if in high rows and frame 1 if in low rows; depends on trigger (“pivot”) time Data,  high rate Full simulation Data, e low rate

24. 15 Dec 2010 Conclusions and future work Sensor simulation seems to give good agreement Gives right cluster shapes and reasonable resolutions Chris and Sam will finish at the end of March Some tweaking of simulation parameter tuning Obtain standalone simulation resolutions Next steps (by me...) Interface fully to GEANT4 simulation Model positron showers as taken in real data Measure apparent density of shower hits in layers 3,4,5 Correct using simulation to true density at back of tungsten block 24 Paul Dauncey