Transcript Camacho.ppt

Deeply Virtual Compton Scattering
on the neutron
Slides by Malek MAZOUZ
For JLab Hall A & DVCS collaborations
 Physics case
 n-DVCS experimental setup
 Analysis method
 Results and conclusions
Hall A Meeting
June 21st 2007
Deeply Virtual Compton Scattering
GPDs give an access to quark angular momentum (Ji’s sum rule)
1
1
1
J q   q  Lq   xdx  H q ( x,  , 0)  E q ( x,  , 0) 
2
2 1
less constrained GPD
No link to DIS
DVCS is the simplest hard exclusive process involving GPDs
k’
k
q’
Factorization theorem
in the Bjorken regime
p’
p
GPDs
Q 2  q 2  (k  k ' ) 2  M 2
t  ( p  p' ) 2  2  Q 2
Non perturbative
description by GPDs
DVCS and Bethe-Heitler
The total cross-section
accesses the real part of
DVCS and therefore an
integral of GPDs over x
The polarized cross-section difference accesses the Imaginary
part of DVCS and therefore GPDs at x=±ξ
d σ  d σ  2 m(T
5
5
Purely real and fully calculable
BH
.T
DVCS
2
2
DVCS
DVCS


) T
T


Small at
JLab energies (twist-3 term)
If handbag
dominance
d 5σ  d 5σ  ( xB , Q 2 , t ,  ) m  C I  sin 
C I ( F )  F1 (t ) H    F1 (t )  F2 (t )  H 
t
F2 (t )E
2
4M
Neutron Target
Model:
(Goeke, Polyakov
and Vanderhaeghen)
Target
H
neutron
0.81
E
H
-0.07
1.73
Q 2  2 GeV 2
xB  0.3
t  0.3 GeV 2
xB
t
m  C   F1 (t )  H 
  F1 (t )  F2 (t )   H 
F (t )  E
2 2
2  xB
4M
I
t
F2n (t )
F1n (t )
0.3
-0.91
-0.04
m  C I   F1 (t )  H 
m  C I   0.03 
F
n
1
(t )  F2n (t )   xB /(2  xB )
-0.17
(t / 4M 2 )  F2n (t )
-0.07
xB
t
  F1 (t )  F2 (t )   H 
F2 (t )  E
2
2  xB
4M
0.01
 0.13
n-DVCS experiment
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.
(C. Muñoz-Camacho et al., PRL 97 (2006) 262002)
xBj=0.364
s
(GeV²)
Q²
(GeV²)
Pe
(Gev/c)
Θe
(deg)
-Θγ*
(deg)
Hydrogen
4.22
1.91
2.95
19.32
18.25
4365
Deuterium
4.22
1.91
2.95
19.32
18.25
24000
 Ldt
(fb-1)
Experimental apparatus
Left HRS
Beam energy = 5.75 GeV
Beam polarization = 75%
Beam current = ~ 4 μA
Luminosity = 4. 1037 cm-2.s-1
Polarized Electron Beam
LH2 / LD2 target
g
The experimental
resolution is good
enough to identify
DVCS events with MX2
charged particle
tagger
N
Recoil nucleon
detector
Electromagnetic Calorimeter
Analysis method
eD  eg X
Mx2 cut = (MN+Mπ)2
p-DVCS and
n-DVCS
Contamination by
eD  e 0 X  eg X
MN2
d-DVCS
MN2 +t/2
N + mesons
(Resonnant or not)
accidentals
Helicity signal and exclusivity
After :
-Normalizing H2 and D2 data
to the same luminosity
-Adding Fermi momentum to
H2 data

d-DVCS
2

Sh   ( N n-DVCS
 N  )d   ( N   N  )d
0

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
Extraction results
d-DVCS extraction results
PRELIMINARY
F. Cano & B. Pire calculation
Eur. Phys. J. A19, 423 (2004).
Deuteron moments compatible with zero at large -t
Exploration of small –t regions in future experiments might be interesting
Extraction results
n-DVCS extraction results
PRELIMINARY
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.
Neutron contribution is small and compatible with zero
Results can constrain GPD models (and therefore GPD E)
n-DVCS experiment results
Systematic errors
of models are not
shown
n-DVCS is sensitive to Jd
p-DVCS is sensitive to Ju
VGG Code
GPD model : LO/Regge/D-term=0
Goeke et al., Prog. Part. Nucl. Phys 47 (2001), 401.
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 crosssections 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
1
d
d

 2
2  dQ dxB d  2 de dgg dQ 2 dxB d  2 de dgg







 n ( xB , e ,  2 ,  )  m CnI exp sin    d ( xB , e ,  2 ,  )  m CdI exp sin 
N Exp (ie )  N ie  N ie
N MC (ie )  L  m  CnI exp    n .sin   Acc  m  CdI exp    d .sin   Acc 


xie
xie
Luminosity
MC sampling
MC sampling
MC includes real radiative corrections (external+internal)
2  
ie
 N
Exp
(ie )  N

Exp
MC
(ie ) 
2
(ie ) 
2
m  CnI exp 
m  CdI exp 
Analysis method
eD  eg X
eH  eg X
Mx2 cut = (MN+Mπ)2
Mx2 cut = (MN+Mπ)2
accidentals
accidentals
D(e, e ' g ) X  p(e, e ' g ) p  n(e, e ' g )n  d (e, e ' g )d 
p-DVCS
events
n-DVCS
events
d-DVCS
events
Mesons
production
Nb of Counts
Double coincidence analysis
Hydrogen data
Deuterium data
Deuterium- Hydrogen
simulation
Mx2 cut
MX2 (GeV2)
Helicity signal and exclusivity

2
Sh   ( N  N )d   ( N   N  )d



0
After :
-Normalizing H2 and D2 data
to the same luminosity
-Adding Fermi momentum to
H2 data
d-DVCS
n-DVCS
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
π0 contamination subtraction
H2 data
Mx2 cut =(Mp+Mπ)2
π0 to subtract
Subtraction of 0 contamination (1g in the calorimeter) is obtained from
a phase space simulation which weight is adjusted to the experimental 0
cross section (2g in the calorimeter).
π0 contamination subtraction
Unfortunately, the high trigger threshold during Deuterium runs did
not allow to record all exclusive π0 events (MX2<1.15 GeV2)
H2 data
But
: according
to :the procedure of π0 contamination subtraction, we must have :
Actually,
we find
0
 (ed
Xe) 0 n)
(ene
by comparing two samples of
2<1.15 GeV2

0.95

0.06

sys
0. 

0.5
with
M
X
0
high energy π0 in each case
 (ep
) 0 p)
(epeXe
Exclusive π0 asymmetry
ep  ep 0
ed  ep(n) 0  ed  en( p) 0  ed  ed 0
D2 data
H2 data
0.5 
Sh  en  en 0  ed  ed 0 
Sh  ep  ep
0

Well known from H2 data
1
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
p-DVCS experiment results
C. Muňoz-Camacho et al.,
to appear in PRL (2007)
Twist-2 contribution dominates
the total cross-section and the
cross-section difference.
No Q2 dependence of twist-2
and twist-3 terms
Strong indications for
handbag dominance
VGG parametrisation of GPDs
Vanderhaeghen, Guichon, Guidal,
Goeke, Polyakov, Radyushkin, Weiss …
Non-factorized t dependence
1 
 x
H ( x,  , t )   d   d   x      F (  ,  , t )     x  D  
 
1
1 
1
q
q
q
D-term
Double distribution :
 '  0.8 GeV for quarks
-2
F ( , , t ) 
q
1

 't
h   ,  q   
Parton distribution
1      
  2b  2  

h   ,    2b 1 2
2   b  1 1   2b 1
2
Profile function :
2
for GPD E, the spin-flip parton densities is used : eq   
Modelled using Ju and Jd as free parameters
b
n-DVCS polarized cross-section difference
d-DVCS polarized cross-section difference
Experimental
results
+
Prediction from F.
Cano and B. Pire.
Eur. Phys. J. A19, 423 (2004)
π0 electroproduction on the neutron
Pierre Guichon, private communication (2006)
Amplitude of pion electroproduction :
T ( N ,  )    ,3 T    N T 0  i 3  T 
α is the pion isospin
nucleon isospin matrix
π0 electroproduction amplitude (α=3) is given by :
2
1
u  d
3
3
1
2
T  n,3  T   T 0  u  d
3
3
T  p,3  T   T 0 
T  p,3  T  n,3 3  3d / u

 1.15
T  p,3
2  d / u
Polarized parton distributions in the proton
Triple coincidence analysis
Proton Array and Tagger (hardware) work properly during the experiment, but :
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 .
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
After accidentals subtraction
-proton-neutron conversion in
the tagger shielding
- accidentals subtraction
problem for neutrons
Relative asymmetry (%)
neutrons selection
PA energy cut (MeV)
p-DVCS events (from LD2
target) asymmetry is stable
Relative asymmetry (%)
protons selection
PA energy cut (MeV)
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
Mp 2
By selecting n(e,e’π-)p events,
one can predict the energy
deposit in the calorimeter using
only the cluster position.
 2minimisation between the
a
measured and the predicted
energy gives a better
calibration.
Calorimeter energy calibration
2nd method : Invariant mass of 2 detected photons in the calorimeter (π0)
Nb of counts
π0 invariant mass position
check the quality of the
previous calibration for
each calorimeter region.
Corrections of the previous
calibration are possible.
Invariant mass (GeV)
Differences between the results of the 2 methods introduce a
systematic error of 1% on the calorimeter calibration.