BigCal Reconstruction and Elastic Event Selection for GEp-III

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 GE/GM to Q2 of 8.5 GeV2 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 Shower Counter HMS Drift Chambers: Track protons FPP Drift Chambers: Track scattered protons CH2 Analyzer

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: • Require PMT at both ends of paddle to fire • Require S1X and S1Y for “S1” trigger • Require S1 and S0 for HMS trigger • 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: Energy reconstruction Position reconstruction 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 • Find largest maximum • Build a cluster by adding nearest neighbors with hits • Work our way outward—allow clusters to expand freely in any direction • “Zero” hits in the current cluster • 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 • TDC hits come in at a nearly constant time relative to the trigger • Find peak position in TDC spectrum to determine offset Hit times relative to BigCal trigger

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 Q2 GEp-III and low-ε GEp-2g