A future n-DVCS experiment in Hall A Malek MAZOUZ October 15 th 2007 What was done in E03- 106 Motivations Experimental upgrades Projected Results DVCS meeting Faculté des Sciences de Monastir – Tunisia
Mar 27, 2015
A future n-DVCS experiment in Hall A
Malek MAZOUZ
October 15th 2007
What was done in E03-106
Motivations
Experimental upgrades
Projected Results
DVCS meeting
Faculté des Sciences de Monastir – Tunisia
Deeply Virtual Compton Scattering
1
1
( , , )DVCS GPD x tT dx
x i
1
1 ...
( , )( , , )
,i G
GPD x tP
xPDx x td
The GPDs enter the DVCS amplitude as an integral over x :
GPDs appear in the real part through a PP integral over x
GPDs appear in the imaginary part at the line x=±ξ
kk’
q’
GPDsp p’
factorization
Simplest hard exclusive process involving GPDs
Mueller, Radyushkin, JiCollins, Freund, Strikman
What could be done at JLab Hall A
2 25 5 Im2 ( .B DVCS DH DV VCSCST T )d σ d σ T T
����
2 25 Re2 . ( )BH BH D SS VCDVCTd T T T
The cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=±ξ
The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x
Purely real and fully calculable
Twist-3 term
Kroll, Guichon, Diehl, Pire …
Twist-2 term Bilinear combinations of GPDs
Expression of the cross-section difference
Combination to be extracted from the data
I5 5
12 21 2
sin( )
1 1
2
m C( ) ( )
twist-3 term sin( , 2 )s
I
B e B e
d d
dQ dx dtd d dQ dx dtd d P P
��
1 1 2 22( ) ( ) ( ) ( )
4I t
C F t F t F t F tM
H H E
2 ( , , ) ( , )m ,qq
q
qE te E t E
GPDs
n-DVCS experiments provide different linear combinations of GPDs than p-DVCS experiments.
n-DVCS experiments have different flavor sensitivity than p-DVCS experiments.
A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).
E03-106 experimental apparatus and kinematics
LH2 / LD2 targetPolarized Electron Beam
Scattered Electron
Left HRS
Electromagnetic Calorimeter
DVCS events are identified with MX
2
Beam energy = 5.75 GeV
Beam polarization = 75%
Beam current = ~ 4 μA
Luminosity = 4. 1037 cm-2.s-1 nucleon-1
2 2
2 2
2
1.9 GeV
0.36
4.22 GeV
0.1 0.5 GeV
B
Q
x
W
t
I.A. and DVCS on coherent deuteron
, ' , ' , ' , 'D e e X d e e d n e e n p e e p Impulse approximation :
With a Deuterium target one can have 3 different DVCS processes
p
nd
n
dd dd
p-DVCS d-DVCSn-DVCS(incoherent) (coherent)(incoherent)
Access deuteron GPDs
overlap < 3%
Final state interaction effects between a pn pair should be small
(GeV/c)p
prob
abili
ty
eD e X
p-DVCS and n-DVCS
accidentals
MN2
MN2 +t/2
d-DVCS
Analysis method
0e X XeD e Contamination by
Mx2 cut = (MN+Mπ)2
N + mesons(Resonnant or not)
Extraction of observables
2
exp exp
2 2
2 2
2
1
2
sin( , , , ) ( ,m s, , ) inmI In
B
B
e
B d
B
d e
e
e nx x
d d
d
C C
Q dx d d d dQ dx d d d
��
exp exp
( )
( ) .sin .sinm m
e e
e e
I In n
Expe i i
Mdi d
Ce x x i
N i N N
N i L Acc A cC cC
MC sampling MC samplingLuminosity
220 12 7 bins in , ,e Xi M t with
Under the MX2 cut :
A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).A. Kirchner, D. Muller, Eur. Phys. J. C32, 347 (2004).
, ' , ' , ' , 'D e e X H e e X n e e n d e e d
Fit of :
by :
What was done : E03-106 results
Deuteron moments compatible with zero at large |t|
Neutron moments are small and compatible with zero
Results can constrain GPD models (and therefore GPD E)
- Large systematic error due to the uncertainty on the relative calibration between the LH2 and the LD2 data.
- Large systematic error due to the uncertainty on the contamination of the DVCS-like π0 channel.
M. Mazouz et al., submitted to PRL.arXiv0709.0450
n-DVCS is sensitive to Jd
p-DVCS is sensitive to Ju
Complementarity between neutron and proton measurements
What was done : E03-106 results
Model dependent extraction of Ju and Jd
What is interesting to do now
kk’
q’
GPDsp p’
factorization
Already determined in E03-106
Sensitive to x=±±…
1
1
( , , )DVCS GPD x tT dx
x i
1
1 ...
( , )( , , )
,i G
GPD x tP
xPDx x td
2 25 Re2 . ( )BH BH D SS VCDVCTd T T T
Expression of the unpolarized cross section
5
2
0 1 221 2
1
( ) ( )
twi
e e
st-3 terms gluon terms
cos(
)
DVCS
B e
dBH
dQ dx dC C C
td d PC
P
1 1 2 22( ) ( ) ( ) ( )
4I t
C F t F t F t F tM
H H E
21 1 2 22( ) ( ) ( ) ( )
4I I t
C C F t F t F t F tM
H H E E
2
2 22 22 2
4(1 ) 2 e1
2 24 4
B BDVCS
B B B B
x x
C t tx x x x
M M
HH HH HE HE
EE EE
1
2
1
1 1e , , f
ffe dx
xH x
xt
H GPDs
In the nucleon case :
A. V. Belitsky, D. Muller, A. Kirchner, Nucl. Phys. B629, 323 (2002).
What was NOT done in E03-106 and why
5 exp2
0 1 221 2
2
0 1 21 2
e e
1cos( )
( ) ( )
1cos( )
( ) ( )e e
I I I DVCS
I I I DVCS
n n n n n nB
n n
d d d d d d d
e
d
dBH
dQ dx dtd d P P
BH
C C C C
C C C CP P
Cannot be separated with a φ analysis
Six twist-2 coefficients to be extracted from the data
The large systematic error (>50%) of E03-106 did not allow a significant extraction of unpolarized cross sections.
The DVCS2 and the interference terms have a similar φ dependence and therefore cannot be separated.
We propose to perform a new n-DVCS experiment at the same kinematics than E03-106 (Q2=1.9 GeV2 and xB=0.36) but with 2 different beam energies (4.82 GeV and 6 GeV) and with 5% systematic errors.
Interference and DVCS2 Separation
0 21 2
1( ) ( ) n nP P
The Γ factors depend on the beam energy while the GPD combinations do not.
Different beam energies allow to separate the DVCS2 and the interference terms
C. Muňoz Camacho et al., E07-007.
Accessible observables
Neutron GPDs integral
Determine twice as accurately. I In dandm C m C
Measure the polarized cross sections of exclusive π0 electroproduction on the neutron and coherent deuteron.
n nI
nI Iande C e C C Linear combinations :
Bilinear combinations :S
nDVCC
Deutron GPDs integral ?
d dI
dI Iande C e C C Linear combinations :
Bilinear combinations :S
dDVCC
Model Calculation for n-DVCS
DVCSnC
with GPD E
without GPD E
VGG model : Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401.M. Vanderhaeghen, P.A.M. Guichon, M. Guidal, Phys. Rev. D 60 (1999), 094017.
4.82 GeVk
6.00 GeVk
GPD Ě contribution ≈ 30%
In VGG model, the DVCS2 contribution is larger than the interference one.
DVCS2/BH2 depends on the beam energy
Experimental upgrades
We will use the experimental apparatus of the previous DVCS experiments with a few modifications and upgrades ( already planed for the approved p-DVCS experiment)
50% expanded calorimeter (13 x 16 blocks instead of 11 x 12 blocks) Improve the π0 contamination subtraction at high -t
No recoil detectors will be used Reduction of the experimental dead time
Faster electronics to improve the data record and transfer
New trigger logic (higher threshold + specific trigger for π0)Detection of enough π0 to subtract correctly the contamination and determine the π0 electro-production polarized cross sections.
Remove the systematic error due to the uncertainty on the contamination of DVCS-like π0 channel.
Calorimeter Calibration
Elastic calibration H(e,e’p) : will be performed periodically.
We have 2 independent methods to continuously check and correct the calorimeter calibration
1st method : missing mass of D(e,e’π-)X reaction2nd method : Invariant mass of 2 detected photons in the calorimeter (π0)
Calorimeter blocks with radiation damage will be cured with blue light.
Will help to keep a good resolution.
Frequent swap of LH2 and LD2 target
Remove the systematic error due to the uncertainty on the relative calibration between LH2 et LD2 data
Projected results
Target Beam energy (GeV)
LH2 4.82 5250
LH2 6.00 1750
LD2 4.82 13500
LD2 6.00 13500
Ldt (fb-1)
27000 fb-1
7000 fb-1
Projected results
previous systematics
new systematics
Ine C I I
n ne C C DVCSnC
Ide C I I
d de C C DVCSdC
The error bars are computed with the E03-106 experimental resolution
Requested kinematics and beam time
Target Beam energy (GeV)
k’ (GeV/c)
q’ (GeV)
W2 (GeV2)
Θe (deg)
-Θγ* (deg)
Requested beam time
(hours)
LH2 4.82 2.01 2.73 4.26 25.60 16.07 90 (approved)
LH2 6.00 3.19 2.73 4.26 18.13 18.45 30 (approved)
LD2 4.82 2.01 2.73 4.26 25.60 16.07 200LD2 6.00 3.19 2.73 4.26 18.13 18.45 200
400Total requested beam time
2 2 an1.9 GeV =0.3 d 6BQ x
Beam current = 4 μA
Summary
The results of the DVCS experiments in Hall A prove that, with our experimental apparatus, we are able to measure the DVCS polarized cross sections and extract GPDs from these quantities.
Unfortunately, this measurement was not very significant in E03-106 because of very large systematics… But we know how to remove these systematics in a future experiment.
n-DVCS experiments appear as a mandatory step towards a better knowledge of the nucleon structure.
The unpolarized n-DVCS cross section represents a valuable source of information about GPDs (GPDs integral)
A future n-DVCS experiment will also provide interesting results about d-DVCS and π0 electroproduction on the neutron and deuteron.
END Of the first part
Proposed kinematics and luminosity
Target Beam energy (GeV)
LH2 4.82 5250
LH2 6.00 1750
LD2 4.82 13500
LD2 6.00 13500
27000Total LD2 Luminosity
2 2 an1.9 GeV =0.3 d 6BQ x
Same kinematics than E03-106 but with 2 beam energies
Ldt (fb-1)
Extraction of observables
5 exp2
0 1 221 2
2
exp exp exp
0 1 21 2
exp exp exp
e e
1cos( )
( ) ( )
1cos( )
( ) ( )e e
n n n n n n n n
d d
I I I DVCS
I I I DVCS
B e
d d d d d d
dBH
dQ dx dtd d P P
BHP P
C C C C
C C C C
2
22
( ) ( )
( )e
Exp MCe e
Expi
e
N i N i
i
exp expI In d
exp expI I I In n d d
DVCSexp DVCSexpn d
e C , e C
e C C , e C C
C , C
Same extraction procedure than the previous experiment one:Fit of the experimental distribution by a Monte Carlo simulation within a global analysis involving a binning on
( )ExpeN i ( )MC
eN i2
e Xt M ki
Additional binning on the beam energy
Analog Ring Sampler
1 GHz Analog Ring Sampler (ARS)
x 128 samples x 289 detector channels
Sample each PMT signal in 128 values (1 value/ns)
Extract signal properties (charge, time) with a wave form Analysis.
Allows to deal with pile-up events.
Analysis method
Adjust the calibration and the resolution of H2 and D2 data relatively to each other.
Systematic error due to the relative calibration between H2 and D2 data
Add the Fermi motion to the target nucleon for H2 data
Nb
of
coun
ts
Invariant mass (GeV)
Variation of calibration coefficients during the experiment due to radiation damage.
Cal
ibra
tion
varia
tion
(%
)
Calorimeter block number
Solution : extrapolation of elastic coefficients assuming a linearity between the received radiation dose and the gain variation
By selecting n(e,e’π-)p events, one can predict the energy deposit in the calorimeter using only the cluster position.
a minimisation between the measured and the predicted energy gives a better calibration.
2
( , ' )D e e X
Analysis method
Add the Fermi motion to the target nucleon for H2 data
Subtract the π° contamination
Analysis method
Substract H2 data from D2 data
The π° contamination is treated as a systematic error
0
00.5
n n
p p
e e
e e
Exclusivity and helicity signal2
0( ) ( )hS N N d N N d
sin(φ) and sin(2φ) moments
Results are coherent with the fit of a single sin(φ) contribution
n-DVCS results from E03-106
d-DVCS results from E03-106
Prediction from F. Cano and B. Pire.
Eur. Phys. J. A19, 423 (2004)
Experimental results
+E03-106
E03-106 results
π0 to subtract
π0 contamination subtraction
Mx2 cut =(Mp+Mπ)2
H2 data
Subtraction of 0 contamination (1 in the calorimeter) is obtained from a phase space simulation which weight is adjusted to the experimental 0 cross section (2 in the calorimeter).
Model Calculation for n-DVCS
4.82 GeV k
Model Calculation for n-DVCS
6.00 GeV k
Projected results
Projected results
Model Calculation for d-DVCS
Cano-Pire model : F. Cano and B. Pire, Eur. Phys. J. A 10 (2004), 423.
Ide C
I Id de C C DVCS
dC
Idm C
Model Calculation for d-DVCS
4.82 GeV k
Model Calculation for d-DVCS
6.00 GeV k
Projected results (KIN 1)
Projected results (KIN 1)
Projected results (KIN 3)
Projected results (KIN 3)
Projected results
Resolution effect
Calorimeter energy calibration
We have 2 independent methods to check and correct the calorimeter calibration
1st method : missing mass of D(e,e’π-)X reaction
Mp2
By selecting n(e,e’π-)p events, one can predict the energy deposit in the calorimeter using only the cluster position.
a minimisation between the measured and the predicted energy gives a better calibration.
2
Calorimeter energy calibration
2nd method : Invariant mass of 2 detected photons in the calorimeter (π0)
π0 invariant mass position check the quality of the previous calibration for each calorimeter region.
Corrections of the previous calibration are possible.
Nb
of
coun
ts
Invariant mass (GeV)
5 5
1I
DVCS
22 21 2
1
I sin( ) sin(2 )1 1
2 ( ) ( )
m C ( )
m C ( )
m C ( si , n( ) )
I I
B
eff
eff
e B e
DVCS
d d
dQ dx dtd d dQ dx dtdF
F F
dF
P P
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