Post on 01-Jan-2016
description
Deeply Virtual Compton Scattering in JLAB Hall A
Malek MAZOUZ
LPSC Grenoble : mazzouz@lpsc.in2p3.fr5th ICPHP May 22nd 2006
For JLab Hall A & DVCS collaborations
Form Factors via Elastic scaterring
Parton distribution via Deep inelastic scatering
Generalized parton distributions: GPDs
Two independent informations about the nucleon structure
Generalized parton distribution via Deep exclusive scaterring
LinkMueller, Radyushkin, Ji
Generalized parton distributions: GPDs
xProbability |Ψ(x)|2 that a quark carries a fraction x of the nucleon momentum
Parton distributions q(x), Δq(x) measured in inclusive reactions (D.I.S.)
x+ x-
t
GPDs measure the Coherence Ψ*(x+ξ) Ψ(x-ξ) between a initial state with a quark carrying a fraction x+ξ of the nucleon momentum and a final state with a quark carrying a fraction x-ξ
4 GPDs : , , , ( , , )H H E E x tFor each quark flavor
Dependence in t : new wealth of physics to explore
Mueller, Radyushkin, Ji
GPDs properties, link to DIS and elastic form factors
),,(~
,~
, , txEHEH qqqq Generalized Parton distributions
Link to DIS at =t=0
)()()0,0,(~
)()()0,0,(
xqxqxH
xqxqxHq
q
Link to form factors (sum rules)1
1
1
1
2
1
1 1
1 1
( , , ) ( )
( , , ) ( )
( , , ) ( ) , ( , , ) ( )
q q
q q
q q q qA A
dxH x t F t
dxE x t F t
dx H x t g t dx E x t h t
1
1
1 1( , ,0) ( , ,0)
2 2q q
q q qJ L xdx H x E x
Access to quark angular momentum (Ji’s sum rule)
How to access GPDs: DVCS
222
2222
)'(
)'(
Qppt
MkkqQ
k k’ q’
GPDsp p’
Simplest hard exclusive process involving GPDs
pQCD factorization theorem
Perturbative description
(High Q² virtual photon)
Non perturbative description by
Generalized Parton Distributions
Bjorken regime
x
2
22
22
B
B
B
x
x
M
Q
pq
Qx
fraction of longitudinal momentum
Collins, Freund, Strikman
Deeply Virtual Compton Scattering
1
1
( , , )DVCS GPD x tT dx
x i
1
1 ...( , ,
( , , ))
GPDx
x tP
xGPDd ix t
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 but at the line x=ξ
What is done at JLab Hall A
2 25 5 Im2 ( .B DVCS DH DV VCSCST T )d σ d σ T T
����
But using a polarized electron beam: Asymmetry appears in Φ
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
Small at Jlab enegies
Kroll, Guichon, Diehl, Pire …
cross-section difference in the handbag dominance
2 22 2 2 2
2
( , , , ). ( , , , ).
with / 2 and ' ,
and the angle between the leptonic and photonic
si
pl
n si
e
n
an s
2A B e B B eB e B e
B
d dx x
dQ dx d d d dQ dx d d d
x Q p q
A B
p p
��
Γ contains BH propagators and some kinematics
B contains twist 3 terms
A contains twist 2 terms and is a linear combination of three GPD imaginary part evaluated at x=ξ
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
Pire, Diehl, Ralston, Belitsky, Kirchner, Mueller
Test of the handbag dominance
-Twist-2 contribution(Γ.A.sinφ) dominate the total cross-section and cross-section difference.
-Twist-2 term (A) and twist-3 term (B) have only log(Q2 ) dependence.
Test of the handbag dominance
To achieve this goal, an experiment was initiated at JLab Hall A on hydrogen target with high luminosity (1037 cm-2 s-1) and exclusivity.
Another experiment on a deuterium target was initiated to measure DVCS on the neutron. The neutron contribution is very interesting since it will provide a direct measure of GPD E (less constrained!)
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
Neutron Target
Neutron
2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2
2( / 4 ) ( )pt M F t t
t=-0.3
Target
Proton 0.81 -0.07 1.73
H H E
Goeke, Polyakov and Vanderhaeghen
Model:
2 22 GeV
0.3
0.3B
Q
x
t
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
1 2( ) ( ) !!!n nF t F t
1 1 2 22( ) ( ) ( ) ( )
2 4
0.03 0.01 0.12
B
B
x tA F t F t F t F t
x M
A
H H E
neutron
Experiment kinematics
s (GeV²)
Q² (GeV²)
Pe (Gev/c)
Θe (deg)
-Θγ* (deg)
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
proton
neutron
xBj=0.364
Beam polarization was about 75.3% during the experiment
E00-110 (p-DVCS) was finished in November 2004 (started in September)
E03-106 (n-DVCS) was finished in December 2004 (started in November)
Ldt (fb-1)
Left High Resolution Spectrometer
LH2 or (LD2) target
Polarized beam
Electromagnetic Calorimeter (photon detection)
Scintillator Array(Proton Array)
Experimental method
(proton veto)Scintillating paddles
scattered electron
( )
e p e p
e D e n p
photon
Proton:(E00-110)
Neutron:(E03-106)
Only for Neutron experimentCheck of the recoil nucleon position
recoil nucleon
Reaction kinematics is fully defined
Calorimeter in the black box
(132 PbF2 blocks)
Proton Array
(100 blocks)
Proton Tagger
(57 paddles)
Analysis - Selection of DVCS events
ep e X
DVCS
Mp2
Mx2 cut = (Mp+Mπ)2
Contamination by0e p pep e N + mesons
(Resonnant or not)
π0 contamination subtraction
One needs to do a π0 subtraction if the only (e,γ) system is used to select DVCS events.
Symmetric decay: two distinct photons are detected in the calorimeter No contamination
Asymmetric decay: 1 photon carries most of the π0 energy contamination because DVCS-like event.
π0 to subtract
- accidentalsep e X
π0 contamination subtraction
Still to check the exclusivity under the missing mass cut !
Mx2 cut =(Mp+Mπ)2
Check of the exclusivityOne can predict for each (e,γ) event the Proton Array block where the missing proton is supposed to be (assuming DVCS event).
Missing mass2 with PA check
Contamination < 3%
Extraction of observables
22 2
22 2
( , , , sin sin) ( 2, , , )B e B e
A B e B B e
dx
d
dQ dx d d d dQ dx d d dBxA
��
Q2 independent
( )
( ) .sin .sin 2
e e
e e
Expe i i
MCAe x i x Bi
N i N N
N i L AcA c cB Ac
MC sampling MC sampling
MC includes real radiative corrections (external+internal)
2
22
( ) ( )
( )e
Exp MCe e
Expi
e
N i N i
i
A
B
Extraction of observables
Acceptance included in fit
2 bins in (Q2,t) out of 15
5.48 0.
6.46 6.9
41
4B
A
2.17 0.
3.13 0.8
16
6B
A
- π0 Contribution is small
- twist-3 contribution is small
Check of the handbag dominance
A (twist-2) and B (twist-3) for a full bin <t>=0.25 GeV2
Strong indication for the validity of twist-3 approximation and the handbag dominance
ed e X
ep e X
ed e X ep e X
By subtracting proton contribution from deuterium, one should access to the neutron and coherent deuteron contributions.
Q2= 1.9 GeV2
<t>= -0.3 GeV2
DVCS on the neutron and the deuteron
Same exclusivity check as before
The number of detected π0 with hydrogen and deuterium target (same kinematics) shows that:
0
0
( )1.0
( )
e e X
e e X
d
p
In our kinematics π0 come essentially from proton in the deuterium
No π0 subtraction needed for neutron and coherent deuteron
π0 asymmetry is small
DVCS on the neutron and the deuteron - Preliminary2 2( ) resolution effects
2X p
tM coherent M Q2= 1.9 GeV2
<t>= -0.3 GeV2
Mx2 upper cut
1st cut2nd cut
It is clear that there are two contributions with different sign : DVCS on the neutron and DVCS on the deuteron
The same extraction method (with an additional binning on the Mx2)
will be applied on this data to have at least the twist-2 terms.
Conclusion
With High Resolution spectrometer and a good calorimeter, we are able to measure the Helicity dependence of the nucleon.
Work at precisely defined kinematics: Q2 , s and xBj
Work at a high luminosity
All tests of Handbag dominance give positive results :
No Q2 dependence of twist-2 and twist-3 terms.
Twist-3 contribution is small.
Accurate extraction of a linear combination of GPDS (twist-2 terms)
High statistics extraction of the total cross-section (another linear combination of GPD!)
Analysis in progress to extract the neutron and deuteron contribution
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
Proton Target
Proton
2 ( )pF t 1 ( )pF t 1 2( ) ( ) /(2 )p pB BF t F t x x 2
2( / 4 ) ( )pt M F t t
t=-0.3
Target
Proton 1.13 0.70 0.98
H H E
Goeke, Polyakov and Vanderhaeghen
Model:
2 22 GeV
0.3
0.3B
Q
x
t
1 1 2 22( ) ( ) ( ) ( )
2 4B
B
x tA F t F t F t F t
x M
H H E
1 1 2 22( ) ( ) ( ) ( )
2 4
0.34 0.17 + 0.06
B
B
x tA F t F t F t F t
x M
A
H H E
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
Calorimeter trigger
Not all the calorimeter channels are read for each event
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
Calorimeter resolution and calibration
-Time resolution < 1ns for all detectors
-Energy resolution of the calorimeter :
2.5% at 4.2E
GeVE
- Photon position resolution in the calorimeter: 2mm
Invariant mass of 2 photons in the calorimeter
σ= 9MeV
Detecting π0 in the calorimeter checks its calibration
High luminosity measurement
2 13710L cm s
123710.4 scmLnucleon
Up to
At ~1 meter from target (Θγ*=18 degrees)
Requires good electronics
Low energy electromagnetic background
PMT
G=104
x10 electronics
Analysis status – preliminary
Sigma = 0.6ns
2 ns beam structure
Time difference between the electron arm and the detected photon
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
Sigma = 0.9ns
Time spectrum in the predicted block (LH2 target)
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
Good way to control calorimeter calibration
Missing mass2 of epeπ0x
2 possible reactions:
epeπ0p
epenρ+ , ρ+ π0 π+
2π production threshold
Sigma = 0.160 GeV2
Sigma = 9.5 MeV
Missing mass2 with LD2 target
Time spectrum in the tagger (no Proton Array cuts)