BigCal Reconstruction and Elastic

Event Selection for GEp-III

Andrew Puckett, MITon behalf of the GEp-III Collaboration

Introduction• Experiment E04-108 will measure the proton form factor ratio

GE/G

M 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

HMS Detector Package for GEp

Scintillators S1 and S0 (new):

Trigger and timing

HMS Drift Chambers:

Track protons

FPP Drift Chambers:

Track scattered protons

CH2 Analyzer

HMS Shower Counter

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

Floor Layout of BigCal

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

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.

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.

Trigger Rates

Rates in this table in kHz

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:

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

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

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!

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.

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

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

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.

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

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

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

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:

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-->

HMS momentum-angle correlation

HMS-BigCal Correlation

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

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