deeply virtual compton scattering on the neutron slides by malek mazouz june 21 st 2007 physics case...
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Deeply Virtual Compton Scattering on the neutron
Slides by Malek MAZOUZ
June 21st 2007
Physics case
n-DVCS experimental setup
Analysis method
Results and conclusions
For JLab Hall A & DVCS collaborations
Hall A Meeting
Deeply Virtual Compton Scattering
DVCS is the simplest hard exclusive process involving GPDs
kk’
q’
GPDsp p’
Factorization theorem in the Bjorken regime
222
2222
)'(
)'(
Qppt
MkkqQ
Non perturbative description by GPDs
1
1
1 1( , ,0) ( , ,0)
2 2q
qq L xdJ xH Ex x
GPDs give an access to quark angular momentum (Ji’s sum rule)
less constrained GPD No link to DIS
DVCS and Bethe-Heitler
The polarized cross-section difference accesses the Imaginary part of DVCS and therefore GPDs at x=±ξ
2 25 5 m( .2 BH DVCS DVCSDVCST )d σ d σ T T T
����
Purely real and fully calculable Small at JLab energies (twist-3 term)
The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x
5 5 2( , , , ) sin BId σ d σ x Q t m C
��If handbag dominance
1 1 2 22( ) ( ) ( ) ( ) ( )
4I t
C F F t F t F t F tM
H H E
1 1 2 22( ) ( ) ( ) ( )
2 4
m 0.03 0.01 0. 3
m
1
B
B
I
I x tF t F t F t F t
M
C
Cx
H H E
1 1 2 22( ) ( ) ( ) ( )
2 4m I B
B
x tF t F t F tC F t
x M
H H E
0.3 -0.91 -0.04 -0.17 -0.07
Neutron Target
2 ( )nF t 1 ( )nF t 1 2( ) ( ) /(2 )n nB BF t F t x x 2
2( / 4 ) ( )nt M F t t
Target
neutron 0.81 -0.07 1.73
H H E(Goeke, Polyakov and Vanderhaeghen)
Model:2 2
2
2 GeV
0.3
0.3 GeV
B
Q
x
t
n-DVCS experiment
s (GeV²)
Q² (GeV²)
Pe (Gev/c)
Θe (deg)
-Θγ* (deg)
4.22 1.91 2.95 19.32 18.25 4365
4.22 1.91 2.95 19.32 18.25 24000
An exploratory experiment was performed at JLab Hall A on hydrogen target and deuterium target with high luminosity (4.1037 cm-2 s-1) and exclusivity.
Goal : Measure the n-DVCS polarized cross-section difference which is mostly sensitive to GPD E (less constrained!)
E03-106 (n-DVCS) followed directly E00-110 (p-DVCS) which shows strong indications of handbag dominance at Q2 about 2 GeV2.
Ldt (fb-1)xBj=0.364
Hydrogen
Deuterium
(C. Muñoz-Camacho et al., PRL 97 (2006) 262002)
Experimental apparatus
LH2 / LD2 targetPolarized Electron Beam
Scattered Electron
N
Recoil nucleon detector
Left HRS
Electromagnetic Calorimeter
The experimental resolution is good enough to identify DVCS events with MX
2
Beam energy = 5.75 GeV
Beam polarization = 75%
Beam current = ~ 4 μA
Luminosity = 4. 1037 cm-2.s-1
charged particletagger
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)
2
0( ) ( )hS N N d N N d
Helicity signal and exclusivity
After :
-Normalizing H2 and D2 data to the same luminosity
-Adding Fermi momentum to H2 data
2 principle sources of systematic errors :
-The contamination of π0 electroproduction on the neutron (and deuteron).
- The uncertainty on the relative calibration between H2 and D2 data
n-DVCS
d-DVCS
Extraction results
Exploration of small –t regions in future experiments might be interesting
d-DVCS extraction results
Deuteron moments compatible with zero at large -t
F. Cano & B. Pire calculationEur. Phys. J. A19, 423 (2004).
PRELIMINARY
Extraction results
Results can constrain GPD models (and therefore GPD E)
n-DVCS extraction results
Neutron contribution is small and compatible with zero
VGG Code : M. Vanderhaeghen, P. Guichon and M. Guidal
GPD model : LO/Regge/D-term=0 Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401.
PRELIMINARY
VGG CodeGPD model : LO/Regge/D-term=0 Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401.
Systematic errors of models are not shown
n-DVCS experiment results
n-DVCS is sensitive to Jd
p-DVCS is sensitive to Ju
Complementarity between neutron and
transversally polarized proton measurements
Summary and conclusion
n-DVCS is mostly sensitive to GPD E : the less constrained GPD and which is important to access quarks orbital momentum via Ji’s sum rule.
Our experiment is exploratory and is dedicated to n-DVCS. n-DVCS and d-DVCS contributions are obtained after a subtraction of Hydrogen data from Deuterium data (no recoil detectors needed).
First measurements of n-DVCS and d-DVCS polarized cross-sections difference.
Neutron results can constrain GPD models (GPD E parametrization)
Neutron experiments are mandatory complements to proton ones.
- PRL draft will circulate in the next few weeks
- New proposal for PAC-33 (6 GeV) in preparation (collaborators welcome…!)
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 sampling
MC includes real radiative corrections (external+internal)
2
22
( ) ( )
( )e
Exp MCe e
Expi
e
N i N i
i
Luminosity
exp
exp
m
m
In
Id
C
C
Analysis method
eD e X eH e X
accidentals accidentals
( , ' ) ( , ' ) ( , ' ) ( , ' )D e e X p e e p n e e n d e e d
p-DVCS events
n-DVCS events
d-DVCS events
Mesons production
Mx2 cut = (MN+Mπ)2 Mx
2 cut = (MN+Mπ)2
Double coincidence analysis
Hydrogen data
Deuterium data
Deuterium- Hydrogen
simulation
Nb
of C
ount
s
MX2 (GeV2)
Mx2 cut
Helicity signal and exclusivity
After :
-Normalizing H2 and D2 data to the same luminosity
-Adding Fermi momentum to H2 data
2 principle sources of systematic errors :
-The contamination of π0 electroproduction on the neutron (and deuteron).
- The uncertainty on the relative calibration between H2 and D2 data
n-DVCS
d-DVCS
2
0( ) ( )hS N N d N N d
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).
π0 contamination subtraction
Unfortunately, the high trigger threshold during Deuterium runs did not allow to record all exclusive π0 events (MX
2<1.15 GeV2)
But : according to the procedure of π0 contamination subtraction, we must have :
H2 data
0
0
( )0. 0.5
( )
e e n
e p
n
p e
with MX
2<1.15 GeV2
0
0
( )0.95 0.06
( )
e e Xsys
e e X
d
p
Actually, we find :
by comparing two samples of high energy π0 in each case
0 0
00.5 1
h
h
n dS e en e ed
S pe ep
0ep ep0 0 0( ) ( )ed ep n ed en p ed ed
Exclusive π0 asymmetry
Well known from H2 data
D2 data H2 data
sin(φ) and sin(2φ) moments
Results are coherent with the fit of a single sin(φ) contribution
Test of the handbag dominance : E00-110
Twist-2 contribution dominates the total cross-section and the cross-section difference.
No Q2 dependence of twist-2 and twist-3 terms
p-DVCS experiment resultsC. Muňoz-Camacho et al., to appear in PRL (2007)
Strong indications for handbag dominance
VGG parametrisation of GPDs
11
1 1
( ,( , ), , ) qq qF tx
d d x x DH x t
D-term
Non-factorized t dependenceVanderhaeghen, Guichon, Guidal, Goeke, Polyakov, Radyushkin, Weiss …
2 2
2 12
'
1 2
1
12 2
2 1 1
( , , ) ,
,
b
qt
bb
F
hb
qh
b
t
Double distribution :
Profile function :
Parton distribution
for , the spin-flip parton densities is used :G PD qE e
Modelled using Ju and Jd as free parameters
-2' 0.8 GeV for quarks
n-DVCS polarized cross-section difference
d-DVCS polarized cross-section difference
Prediction from F. Cano and B. Pire.
Eur. Phys. J. A19, 423 (2004)
Experimental results
+
π0 electroproduction on the neutron
Pierre Guichon, private communication (2006)
03( , ) ,3 NT N T T i T
Amplitude of pion electroproduction :
α is the pion isospin
nucleon isospin matrix
π0 electroproduction amplitude (α=3) is given by :
0
0
2 1,3
3 31 2
,33 3
T T T
T
p
T Tn
u d
u d
Polarized parton distributions in the proton
,3 ,3 3 31.15
,3
/
/2
dpT
d
nT u
T up
Triple coincidence analysis
Identification of n-DVCS events with the recoil detectors is impossible because of the high background rate.
Many Proton Array blocks contain signals on time for each event .
Proton Array and Tagger (hardware) work properly during the experiment, but :
Accidental subtraction is made for p-DVCS events and gives stable beam spin asymmetry results. The same subtraction method gives incoherent results for neutrons.
Other major difficulties of this analysis:
proton-neutron conversion in the tagger shielding.Not enough statistics to subtract this contamination correctly
The triple coincidence statistics of n-DVCS is at least a factor 20 lower than the available statistics in the double coincidence analysis.
Triple coincidence analysis
One can predict for each (e,γ) event the Proton Array block where the missing nucleon is supposed to be (assuming DVCS event).
Triple coincidence analysis
Rel
ativ
e a
sym
me
try
(%)
Rel
ativ
e a
sym
me
try
(%)
PA energy cut (MeV)
PA energy cut (MeV)
After accidentals subtraction
neutrons selection
protons selection
-proton-neutron conversion in the tagger shielding
- accidentals subtraction problem for neutrons
p-DVCS events (from LD2 target) asymmetry is stable
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.
Differences between the results of the 2 methods introduce a systematic error of 1% on the calorimeter calibration.
Nb
of
coun
ts
Invariant mass (GeV)
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