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 packagedrift
  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
BigCalDetect 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-matchedcalibr
  ate 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 lt 1 kHz ltlt HMS/BigCal rate
• Poisson Statisticsprobability 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 epstraight-forward linear system.
• Position requires shower shape determination
• Timingoffsets 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 outwardallow 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 constantsdetermine
  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--gt

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-e GEp-2g