1. In nova fert animus mutatas dicere formas corpora; di, coeptis (nam vos mutastis et illas) adspirate meis primaque ab origine mundi ad mea perpetuum deducite tempora carmen! Ante mare et terras et quod tegit omnia caelum unus erat toto naturae vultus in orbe quem dixere chaos: rudis indigestaque moles nec quicquam nisi pondus iners congestaque eodem non bene iunctarum discordia semina rerum. nullus adhuc mundo praebebat lumina Titan, nec nova crescendo reparabat cornua Phoebe, nec circumfuso pendebat in aere tellus ponderibus librata suis, nec bracchia longo margine terrarum porrexerat Amphitrite utque erat et tellus illic et pontus et aer, sic erat instabilis tellus, innabilis unda, lucis egens aer GLAST Background Analysis An examination of events recorded by the LAT prototype detector

2. 11 September 2003GLAST Background Analysis2 The Experiment The minitower was exposed to cosmic rays: Muons Electrons / positrons Photons Protons (Muons are the dominant component of CR flux at the Earth’s surface) Raw data is saved to disk for later analysis Another minitower was delivered a few weeks ago and we did a preliminary analysis on that data

3. 11 September 2003GLAST Background Analysis3 Data Analysis Plan Characterize the signature produced by cosmic ray muons Quantify the effects of a threshold change Investigate a correlation to angle of incidence Do preliminary studies on the second minitower ( if time permits)

4. 11 September 2003GLAST Background Analysis4 Particle Interaction Si Si thickness 0.400 mm Strip pitch 0.228 mm 1536 strips / layer

5. 11 September 2003GLAST Background Analysis5 Particle Interaction Si 1 hit E h+h+ e-e- Strip pitch 0.228 mm

6. 11 September 2003GLAST Background Analysis6 Particle Interaction Si 2 hits h+h+ e-e- h+h+ e-e- E

7. 11 September 2003GLAST Background Analysis7 Particle Interaction Si E Electron/hole pairs can drift to a neighboring strip if the track passes through a “diffusion zone” approx 6  m Strip pitch 0.228 mm

8. 11 September 2003GLAST Background Analysis8 Particle Interaction Si E 1 - 3 hits h+h+ e-e- h+h+ e-e- h+h+ e-e-

9. 11 September 2003GLAST Background Analysis9 Background The detector is not 100% efficient EM showering can occur at any time Scattering may occur within the detector Electrical noise can create “hits” Angular incidence will affect hit clustering

10. 11 September 2003GLAST Background Analysis10 Background The detector is not 100% efficient EM showering can occur at any time Scattering may occur within the detector Electrical noise can create “hits” Background

11. 11 September 2003GLAST Background Analysis11 The First Minitower All coords. in mm z xy We expected to see hits in every SSD…

12. 11 September 2003GLAST Background Analysis12 The Second Minitower New prototype gives much better results! All coords. in mm z xy PRELIMINARY DATA

13. 11 September 2003GLAST Background Analysis13 Threshold Levels A higher threshold yields more one-hit events in lieu of events with multiple hits. This is what we expected. What else is affected by the change? Efficiency! Data from first minitower

14. 11 September 2003GLAST Background Analysis14 Angular correlation A track with a given zenith angle can hit one or multiple strips depending on its azimuthal angle

15. 11 September 2003GLAST Background Analysis15 Angular correlation A track with a given zenith angle can hit one or multiple strips depending on its azimuthal angle Restricting the azimuthal angle to a small “pie slice” we attain events in an approximately two- dimensional plane 

16. 11 September 2003GLAST Background Analysis16 Angular correlation A track with a given zenith angle can hit one or multiple strips depending on its azimuthal angle Restricting the azimuthal angle to a small “pie slice” we attain events in an approximately two- dimensional plane In each event, the bottom layer will experience low hit multiplicity… the top layer will show a correlation between zenith angle and strips hit 

17. 11 September 2003GLAST Background Analysis17 Angular correlation  l t tan  = l / t

18. 11 September 2003GLAST Background Analysis18 Data vs Monte Carlo Cuts on experimental data: Single track events Single cluster per layer Projected track is within 8 degrees from Y axis Number of strips hit in each X layer averaged by zenith angle Discrepancy ~30% difference Data from first minitower Reconstructed direction (deg)

19. 11 September 2003GLAST Background Analysis19 Improved Monte Carlo Features added to Monte Carlo: Path length threshold (128  m = 0.32 MIPS) ~10% difference Data from first minitower Reconstructed direction (deg)

20. 11 September 2003GLAST Background Analysis20 Further improvements Features added to Monte Carlo: Path length threshold (128  m = 0.32 MIPS) Diffusion zones (7  m) Most points fit within error bars! Data from first minitower Reconstructed direction (deg)

21. 11 September 2003GLAST Background Analysis21 Preliminary Comparisons Preliminary data from new tracker yield a much better fit! Threshold in Monte Carlo matches that used during data taking (114  m = 0.29 MIPS) PRELIMINARY DATA Data from second minitower! Reconstructed direction (deg)

22. 11 September 2003GLAST Background Analysis22 Summary Our expectations (threshold, geometry) have been validated A simpler model (no complicated physics!) can be easier to understand A simple model can represent a complicated process Thank you Xin Chen, Tune Kamae and Eduardo do Couto e Silva !!!