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Calorimetry for Deeply Virtual Compton Scattering in Hall A
Alexandre Camsonne
Hall A Jefferson Laboratory
Workshop on General Purpose High Resolution Radiation Hard Electromagnetic Calorimeter at
JLab
October 31st 2008
Outline
•Measurement
•Experimental technique
•Background
•Analysis
•New experiment
•Requirements summary
Workshop on Calorimetry October 31, 2008
Deeply Virtual Compton Scattering
epep
Bethe and HeitlerDVCS
Workshop on Calorimetry October 31, 2008
• High resolution and high precision on a limited kinematic range
• Study of scaling with fixed xbj and variation in Q2
– High resolution calorimeter
– Focus on cross section measurement
– Ensure exclusivity relying on calorimeter
– High luminosity
Deeply Virtual Compton Scattering in Hall A
Workshop on Calorimetry October 31, 2008
Specificity of the experiment
electron
HR
S
PbF2
electron
p
Centralangleup to 14 degrees
Workshop on Calorimetry October 31, 2008
Background issues
Workshop on Calorimetry October 31, 2008
0 contamination
• Symmetric decay: minimum angle in lab of 4.4° at max o energy
• Asymmetric decay: sometimes one high energy cluster can be misindentified for a DVCS event
•Interesting : study of Deep Exclusive 0 production could access to another linear combination of GPDs
Workshop on Calorimetry October 31, 2008
Experimental setup
Workshop on Calorimetry October 31, 2008
Cross sections measurement
2
55
22
55
Re
)(
Im
DVCS+
DVCS)(BH+BHdd
DVCSDVCS+
(DVCS)BHdd
2
Electron helicity dependent cross sections of photon electroproduction
Workshop on Calorimetry October 31, 2008
Background reduction
•Scattering chamber 1 cm Al as shielding
15 cm
5 cmbeam dump
Liquid H2
target110 cm
High luminosity running possible by
• reducing secondary background source
PbF2
Workshop on Calorimetry October 31, 2008
Data analysis
0 subtraction done using the 0 sample recorded in the
calorimeter
Subtracted data fits exactly the simulation and the shape of the
exclusive events: good understanding of the detectors
Exclusivity in two arms
Cut im Mx2
Xeep
Workshop on Calorimetry October 31, 2008
nDVCS – deuterium Data analysis
XeepXeed )',()',( Xeep )',(
π0 electroproduction
Invariant mass of 2 photons in the calorimeter
Missing mass2 of epeπ0x 2π production
threshold
Sigma = 0.160 GeV2
Sigma = 9.5 MeV
π0
Separation of photons for 3 GeV π0 : 4.4 degrees = 8.4 cm at 110 cm
Workshop on Calorimetry October 31, 2008
New experimental setup• Double arm experiment
– Increase acceptance
– Increase in luminosity
• Calorimeter
– Increased size of the calorimeter from 132 to 208 blocks for larger acceptance in t
– 11x12 blocks = 33 cm x 36 cm =
= 30 mrd x 32 mrd= 98 msrd
to 13x16 blocks = 39 cm x 48 cm =
= 32.5 mrd x 40 mrd = 120 msrd
Covers full acceptance of HRS at 110 cm
from target
– Improved trigger for optimal 0 detection
– Data transfer improvement to accommodate lower threshold
Workshop on Calorimetry October 31, 2008
Kinematics 6 GeV
Workshop on Calorimetry October 31, 2008
Kinematics 12 GeV
Workshop on Calorimetry October 31, 2008
Radiation hardness
Curing planned every two weeks for new experiments when more than 20 % attenuation
Workshop on Calorimetry October 31, 2008
Requirements summary
• Lead fluoride performance were sufficient• Energy resolution
– 2.4% at 4.2 GeV – 2 % + 3.2 %*E^1/2 , DVCS photons 2 to 3 GeV– Position resolution of 3 mm– 120 msrd = 32 mrd x 40.5 mrd – Radiation Hardness to preserve resolution : around 20% change for
750 kRad.• 2000 kRad for 6 GeV experiment, 8400 kRad for 12 GeV• Curing of blocks about every 2 weeks• Cerenkov to reduce hadronic background and fast to reject background
( coincidence time 0.6 ns )• Reach forward angles around centered around 14 degrees