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