Transcript Brief overview of the theory

Deeply Virtual Compton Scattering on
the neutron
Malek MAZOUZ
LPSC Grenoble
EINN 2005
September 23rd 2005
GPDs properties, link to DIS and elastic form factors
Link to DIS at =t=0
H q ( x,0,0)  q( x)  q ( x)
~
H q ( x,0,0)  q( x)  q ( x)
Link to form factors (sum rules)
Generalized Parton distributions
~ ~
H q , E q , H q , E q ( x,  , t )
1
 dxH
q
( x,  , t )  F1q (t )
1
1
 dxE
q
( x,  , t )  F2q (t )
1
1
 dx H
q
( x,  , t )  g (t ) ,
q
A
1
1
 dx E
q
( x,  , t )  hAq (t )
1
Quark correlations !
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
Brief overview of the theory
k’
k
X. Ji, Phys. Rev. DS56 (1997) 5511
A. Radyushkin, Phys. Lett. B380 (1996) 417
q’
Simplest hard exclusive
process involving GPDs
p’
p
GPDs
pQCD factorization
theorem
Q 2  q 2  (k  k ' ) 2  M 2
t  ( p  p' )    Q
2
2
Perturbative description
(High Q² virtual photon)
Non perturbative description by
amplitude
Bjorken
regime
Q2
Q2
xB 

2 pq 2 M
xB

2  xB
  x  fraction of longitudinal
Generalized Parton Distributions
DVCS  
2
dx
GPD( x,  , t )  
x    i
momentum
What can be done at JLab Hall A
Using a polarized electron
beam:
Asymmetry appears in Φ


d 5σ  d 5σ  BH. Im(DVCS)
Purely real
Direct handle on the imaginary
part of the DVCS amplitude
Enhanced by the full magnitude of
the BH amplitude
d 5  BH  2. Re( DVCS ). BH
2
-High luminosity
-High precision measurement
K. Goeke, M.V. Polyakov and M. Vanderhaeghen
cross-section difference in the handbag dominance
d
d
2



(
x
,
y
,

,  )   A sin   B sin 2 
B
2
2
dxB dyd  d dxB dyd  d
with xB  Q 2 / 2 p  q , y  q  p / k  p ,   p ' p ,
and  the angle between the leptonic and photonic planes
Γ contains BH propagators and some kinematics
B contains higher twist terms
A is a linear combination of three GPDs evaluated at x=ξ
xB
t
A  F1 (t )  H 
  F1 (t )  F2 (t )   H 
F2 (t )  E
2
2  xB
4M
Proton Target
xB
t
A  F1 (t )  H 
  F1 (t )  F2 (t )   H 
F2 (t )  E
2
2  xB
4M
Proton
t
F2p (t )
F1 p (t )
0.1
1.34
0.81
0.38
0.04
0.3
0.82
0.56
0.24
0.06
0.5
0.54
0.42
0.17
0.07
0.7
0.38
0.33
0.13
0.07
Model:
Q 2  2 GeV 2
xB  0.3
t  0.3
F
p
1
(t )  F2p (t )   xB /(2  xB )
(t / 4M 2 )  F2p (t )
Target
H
H
E
Proton
1.13
0.70
0.98
Goeke, Polyakov and Vanderhaeghen
t=-0.3
xB
t
A  F1 (t )  H 
  F1 (t )  F2 (t )   H 
F (t )  E
2 2
2  xB
4M
A  0.64 
0.17
+ 0.06
Neutron Target
xB
t
A  F1 (t )  H 
  F1 (t )  F2 (t )   H 
F2 (t )  E
2
2  xB
4M
Neutron
t
F2p (t )
F1 p (t )
0.1
-1.46
-0.01
-0.26
-0.04
0.3
-0.91
-0.04
-0.17
-0.06
0.5
-0.6
-0.05
-0.12
-0.08
0.7
-0.43
-0.06
-0.09
-0.08
F1n (t )  F2n (t ) !!!
Model:
Q 2  2 GeV 2
xB  0.3
t  0.3
F
1
p
(t )  F2p (t )   xB /(2  xB )
(t / 4M 2 )  F2p (t )
Target
H
H
E
Proton
neutron
0.81
-0.07
1.73
Goeke, Polyakov and Vanderhaeghen
xB
t
A  F1 (t )  H 
  F1 (t )  F2 (t )   H 
F (t )  E
2 2
2  xB
4M
t=-0.3
A  0.03 
0.01
 0.12
Experiment status
E00-110 (p-DVCS) was finished in November 2004 (started in September)
E03-106 (n-DVCS) was finished in December 2004 (started in November)
xBj=0.364
proton
neutron
Θe
(deg)
-Θγ*
(deg)
 Ldt
s
(GeV²)
Q²
(GeV²)
Pe
(Gev/c)
4.94
2.32
2.35
23.91 14.80
5832
4.22
1.91
2.95
19.32 18.25
4365
3.5
1.5
3.55
15.58 22.29
3097
4.22
1.91
2.95
19.32 18.25
24000
(fb-1)
Beam polarization was about 75.3% during the experiment
Experimental method
Proton:
(E00-110)
Neutron:
(E03-106)
e pe  p
Left High
Resolution
Spectrometer
e D  e  n (p)
Polarized
beam
LH2 or (LD2)
target
photon
Scintillating paddles
(proton veto)
Only for Neutron experiment
Scintillator Array
(Proton Array)
recoil
nucleon
Reaction kinematics
is fully defined
Check of the recoil
nucleon position
Electromagnetic Calorimeter
(photon detection)
Proton
Array
(100 blocks)
Calorimeter in the
black box
(132 PbF2 blocks)
Proton
Tagger
(57 paddles)
High luminosity measurement
Up to
2 1
Lnucleon  4.10 cm s
37
37
At ~1 meter from target
(Θγ*=18 degrees)
L  10 cm2 s1
Low energy electromagnetic background
PMT
x10
G=104
Requires good electronics
electronics
Electronics
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.
Electronics
Not all the calorimeter channels are read for each event
Calorimeter trigger
Following HRS trigger, stop ARS.
30MHz trigger FADC digitizes all
calorimeter signals in 85ns window.
- Compute all sums of 4 adjacent blocks.
- Look for at least 1 sum over threshold
- Validate or reject HRS trigger within 340 ns
Not all the Proton Array channels are read for each event
Analysis Status - Preliminary
ep  e X
0
Invariant mass of 2 photons in
the calorimeter
Good way to control the
calorimeter calibration
ep  e X
Missing mass2
with LH2 target
Sigma=9.5 MeV
LH2 target
0.5 GeV2 < missing mass 2 < 1.5 GeV2
α (N+ - N-)
Analysis Status – Very preliminary
LD2 target
0.5 GeV2 < missing mass 2 < 1.5 GeV2
α (N+ - N-)
φ
φ
α
Absolute cross sections
necessary to extract helicity
dependence of neutron
(N+
Possible neutron signal !
N-)
LD2 – LH2
φ
Signal
1.5 GeV2 < missing mass 2 < 2.5 GeV2
φ
α (N+ - N-)
0.5 GeV2 < missing mass 2 < 1.5 GeV2
α (N+ - N-)
Analysis Status – Very preliminary
φ
2.5 GeV2 < missing mass 2 < 3.5 GeV2
α (N+ - N-)
No signal
φ
Conclusion
With High Resolution spectrometer and a good calorimeter, we
are able to measure:
•Helicity dependence of the neutron using LD2 and LH2 target.
Work at precisely defined kinematics: Q2 , s and xBj
Work at a luminosity up to
2 1
Lnucleon  4.10 cm s
37
Coming soon:
Polarized cross sections to extract GPD E
Relative asymmetry considering Proton Array and Tagger.
Proton preliminary results tomorrow morning
Analysis status – preliminary
Time difference between
the electron arm and the
detected photon
Sigma = 0.6ns
2 ns beam
structure
Selection of events in
the coincidence peak
Determination of the missing
particle (assuming DVCS
kinematics)
Check the presence of the missing
particle in the predicted block (or
region) of the Proton Array
Time spectrum
in the predicted
block (LH2 target)
Sigma = 0.9ns
Analysis – preliminary
Triple coincidence
Missing mass2 of H(e,e’γ)x
for triple coincidence events
Background subtraction
with non predicted blocks
Proton Array and Proton Veto are used to check the exclusivity and
reduce the background
π0 electroproduction - preliminary
Invariant mass of 2 photons
in the calorimeter
Sigma = 9.5 MeV
Good way to control
calorimeter calibration
Missing mass2 of epeπ0x
Sigma = 0.160 GeV2
2π production
threshold
2 possible reactions:
epeπ0p
epenρ+ , ρ+ π0 π+
Missing mass2 with LD2 target
Time spectrum in the tagger
(no Proton Array cuts)