1. BigCal Reconstruction and Elastic Event Selection for GEp-III Andrew Puckett, MIT on behalf of the GEp-III Collaboration

2. Introduction Experiment E04-108 will measure the proton form factor ratio G E /G M to Q 2 of 8.5 GeV 2 using the polarization transfer method. Scattered protons are detected in the HMS using parts of the standard detector package—drift chambers and S1 scintillators. New scintillator S0 forms custom trigger. Transferred polarization is measured using a new FPP built by the collaboration (Dubna). BigCal, a large solid-angle electromagnetic calorimeter, detects the electron in coincidence with the proton and is part of the trigger. Timing and kinematic correlations between BigCal and HMS are used to reject inelastic backgrounds

3. HMS Detector Package for GEp Scintillators S1 and S0 (new): Trigger and timing HMS Drift Chambers: Track protons FPP Drift Chambers: Track scattered protons CH 2 Analyzer HMS Shower Counter

4. BigCal—Detect Scattered Electron 1744 lead-glass blocks equipped with PMTs 4” Al absorber in front reduces radiation damage Light source-- Lucite plate illuminated by LED via fiber

5. Floor Layout of BigCal

6. HMS Trigger Nominal Settings: 1.Require PMT at both ends of paddle to fire 2.Require S1X and S1Y for “S1” trigger 3.Require S1 and S0 for HMS trigger 4.Two different trigger types for HMS at T.S.—one for each paddle of S0 Different logic was used at different times to check efficiency Non-standard triggering affects TOF calibration

7. BigCal Trigger Apply high threshold to the analog sum of 64 PMT signals. Summed groups overlap vertically, improving efficiency To get best efficiency for this trigger, phototube gains must be fairly well-matched— calibrate HV using elastic ep.

8. Coincidence Trigger Trigger signals are timed so that BigCal trigger arrives first, about 15-20 ns before HMS trigger This way, the HMS scintillators determine the timing of all ADC gates and TDC stops(or starts) for true coin. events. Width of coincidence timing window is  50 ns.

9. Trigger Rates Rates in this table in kHz

10. Trigger Rates, cont. Accidental coincidence rate estimate for kin. 5: 11.6 kHz HMS2 triggers (elastic paddle of S0) 621 kHz BigCal triggers True elastic rate < 1 kHz << HMS/BigCal rate Poisson Statistics—probability of random BigCal trigger given HMS trigger:

11. BigCal Reconstruction Three main tasks for GEp: 1. Energy reconstruction 2. Position reconstruction 3. Timing  Energy calibration can be updated continuously for elastic ep—straight- forward linear system.  Position requires shower shape determination  Timing—offsets and walk corrections

12. Cluster Finding Strategy 1.Find largest maximum 2.Build a cluster by adding nearest neighbors with hits 3.Work our way outward—allow clusters to expand freely in any direction 4.“Zero” hits in the current cluster 5.Repeat 1-4 with remaining hits until no more “maxima” are found

13. Energy Reconstruction Electron energy is known to within ~1% from HMS momentum/elastic kinematics Chi-squared minimization gives a system of linear equations in the calibration constants—determine as often as needed for GEp. Have to solve system of 1,744 equations!

14. BigCal Position Reconstruction Observable quantities are shower “moments”: energy-weighted mean block positions Moments vary with distance of electron impact point from center of max. block.

15. Shower Shape Determination Distance from block center varies non- linearly with measured moment Fit “S” correction to the distribution of impact point vs. cluster moment. Tracks incident at large angles have distorted shower shape

16. Position Resolution Using BigCal monte-carlo developed at Protvino, coordinate resolution betwen 4 mm and 1 cm is demonstrated Determination of true shower shape considerably more complicated This example has 4” absorber, ~1.2 GeV electrons

17. BigCal Timing Blocks are timed in groups of 8: 32x56/8 = 224 TDC channels The major correction to the measured time is an offset for the slightly (or very) different cable lengths. There is also a significant pulse- height dependence to the measured time that can be corrected for. Timing information is also available from TDCs of the sums of 64 used to form the trigger.

18. Cable Length Offset Hit times relative to BigCal trigger TDC hits come in at a nearly constant time relative to the trigger Find peak position in TDC spectrum to determine offset

19. Walk Correction Hit time has a significant pulse-height dependence Determine for each group of 8, do simple fit Apply correction to hit times Sample time-walk profiles for groups of 8

20. Cluster Timing Throw away TDC hits outside a window of about 150 ns (  ± 75 ns of BigCal trigger time). Such hits won't have corresponding ADC hits within the gate. Within clusters, find all TDC hits in corresponding groups of 8. If multiple hits, take the hit which best agrees with the maximum. Compute energy-weighted mean and rms times. Timing resolution ~3 ns

21. Elastic Event Selection HMS measures proton momentum and angles. With BPM and raster info, we can correct reconstructed target quantities to determine IP Correct BigCal angles using the ray from the HMS vertex to the reconstructed BigCal position In the case of multiple clusters, use HMS to pick the best cluster assuming elastic kinematics:

22. HMS momentum-angle correlation We can select elastic events by looking at  vs  in the HMS by itself. Some kinematics still have substantial inelastic backgrounds under elastic peak. To put FPP in HMS hut: –No PID capability (no gas/aerogel Cerenkov) –Limited timing resolution (no S2) Need BigCal to clean things up: –See effect of various BigCal cuts in figure-->

23. HMS momentum-angle correlation

24. HMS-BigCal Correlation

25. Remaining Tasks Use survey data to fine-tune geometry definition Check BPM/raster corrections Optimize cluster finding parameters/improve the code Improve/optimize parameter database for large-scale analysis Determine shower shape parameters from the data Write  0 reconstruction code for multi-cluster events

26. Conclusion BigCal is successfully serving its purpose as electron detector for GEp-III Some work remains to be done on analysis code (clustering/pions/shower shape/etc) but things looking good so far Clean elastic event selection for high Q 2 GEp-III and low-ε GEp-2g