Generalized Parton Distributions studies @ JLab
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Transcript of Generalized Parton Distributions studies @ JLab
Generalized Parton Distributions studies @ JLab
Introduction
DVCS in JLab/Hall A
DVCS in JLab/Hall B
Comparison with models
0 electroproduction in Halls A/B
Franck SabatiéCEA Saclay
Franck SabatiéCEA Saclay
IWHSS’08 - TorinoMarch 31st, 2007
IWHSS’08 - TorinoMarch 31st, 2007
Collins, Freund
GPDs from Theory to Experiment
Theory
x+ x-
t
GPDs
Handbag Diagram
Physical process
Experiment
Factorization theorem states:In the suitable asymptotic limit, the handbag diagram is the leading contribution to DVCS.
Q2 and largeat xB and t fixed
but it’s not so simple…
1. Needs to be checked !!!1
1
1
1
( , , ) +
( , ,
- i + ( , )
, )
DVCS
GPD x t
GPD x tT dx
x
GPD x tdx
i
P x
2. 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=
Experimental observables linked to GPDs
Experimentally, DVCS is undistinguishable with Bethe-Heitler
However, we know FF at low t and BH is fully calculable
Using a polarized beam on an unpolarized target, 2 observables can be measured:
42
2
4 4
2
2
m2 I
ReBH BH D
DVC
B
BH
C
S
V S
B
dT T
dx dQ dtd
d dT
dx dQ d
T
tdT
At low energy,|TDVCS|2 supposed small
42 2
2
4 42 2
2
2
Im2
Re DVCBH BH DVCS
B
BH DVCS DVCSDVCS
B
SdT T T
dx dQ dtd
d dT T T
dx dQ
T
tT
d d
Ji, Kroll, Guichon, Diehl, Pire, …
e-’
pe-*
hadronic plane
leptonic plane
0 1
42
1
2 3
1
10 1 22
2
2
1 22
4
21 2
24
2
1( , , ) cos cos 2
1 (
( ) ( )
( ) (, , ) cos cos 2 cos3
( , , )sin sin 2
)
( ) ( )
BH BH BHB
B
B
B
B
I I I I
I I
c c c c
dx Q t c c c
dx dQ dtd
x Q t
x Q td d
dxs
dQ ds
dt
Into the harmonic structure of DVCS
|TBH|2
Interference term
1 2( ) (
1
)
BH propagators dependenceBelitsky, Mueller, Kirchner
Special case of the asymmetry
4 42
4 4 0 1
1 2
0 1
sin sin 2( , , )
( ) cos ...
A B BH BH
I I
I I
d dx Q t
c cd
s s
cd c
The asymmetry can be written as:
Pro: easier experimentally, smaller Radiative corr., smaller systematics
Con: direct extraction of GPDs is model- (or hypothesis-) dependent (denominator complicated and unknown)
It was naturally the first observable extracted from non-dedicated experiments…
Potentiallymany more terms
0
20
40
60
80
100
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
# o
f p
aper
s
Theory GPD
Total GPD
Exp. GPD
Timeline
Invention of GPDs my Müller, who needed the theoretical object for his thesis work
Ji and Radyushkin show how the GPDs are interesting, and how to access them. Collins and Freund prove the factorization, Diehl, Pire and Gousset point out the rich harmonic structure of DVCS
First experimental paper usingH1-ZEUS non-dedicated data: Observation of DVCS at very low x
First dedicated DVCS experiment proposed and accepted by a PAC:E00-110, proton DVCS in Hall A Jefferson Lab.
HERMES and CLAS publish non-dedicated data on DVCS in the same PRL issue. They both show a large sine wave in the BSA, proving they indeed deal with the interference of DVCS and BH.
Dedicated DVCS experiment with CLAS proposed/accepted:E01-113, proton DVCS in Hall B
Intense theoretical work begings: higher twist effects, corrections, models, interpretation of various F.T., full harmonic description, …
Dedicated DVCS experiment in Hall A proposed/accepted:E03-106, neutron DVCS in Hall A
The three JLab experiments take data within 1 year (2004-2005), about 4 years after the first proposition to the JLab PAC.
Preliminary results fromJLab experiments
0
20
40
60
80
100
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
# o
f p
aper
s Theory GPD
Total GPD
Pentaquark
Exp. GPD
Published non-dedicated results on ALU and AUL
PRL 97, 072002 (2006)
JLab/Hall B - E1 & HERMES
JLab/Hall B - Eg1
Both results show, with a limited statistics, a sin behavior(necessary condition for handbag dominance)
In the ALU result, DD models (VGG) tend to over-estimate the data
ALU
AUL
CLAS: PRL 87, 182002 (2001) HERMES: PRL 87, 182001 (2001)
E00-110 experimental setup
• 75% polarized 2.5uA electron beam• 15cm LH2 target• Left Hall A HRS with electron package• 11x12 block PbF2 electromagnetic calorimeter• 5x20 block plastic scintillator array
50 days of beam time inthe fall 2004, at 2.5A
113294 fbLu dt
Designing the Hall A DVCS experiment
Measuring cross-sections differential in 4 variables requires:Measuring cross-sections differential in 4 variables requires:
A good knowledge of the acceptance
Scattered electronThe HRS acceptance
is well known
Emitted photonThe calorimeter has a simple
rectangular acceptance
e p → e (p)
Perfect acceptancematching by design !Virtual photon « acceptance »placed at center of calorimeter
R-functioncut
*
Simply:t: radius: phase
Analysis by missing mass (e)X
HRS: Cerenkov, vertex, flat-acceptance cut with R-functions
Calo: 1 cluster in coincidence in the calorimeter above 1 GeV
With both: subtract accidentals, build missing mass of (e,) system
Analysis – o subtraction effect on missing mass spectrum
Using 0→2 events in the calorimeter,the 0 contribution is subtracted bin by bin
After0 subtraction
Analysis – Exclusivity check using Proton Array and MC
Normalized (e,p,)triple coincidence events
Using Proton-Array, we compare the missing mass spectrum of the triple and double-coincidence events.
Monte-Carlo(e,)X – (e,p,)
2 cutXM
The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%.
Difference of cross-sections
2 22.3 GeV
0.36B
Q
x
Corrected for real+virtual RCCorrected for efficiencyCorrected for acceptanceCorrected for resolution effectsChecked elastic cross-section @ ~1%
Twist-2Twist-3
Extracted Twist-3contribution small !
PRL97, 262002 (2006)
Q2 dependence and test of scaling
<-t>=0.26 GeV2, <xB>=0.36
No Q2 dependence using BMK separation:strong indication for scaling behavior and handbag dominance
Twist-2Twist-3
Twist 4+ contributions are smaller than 10%
PRL97, 262002 (2006)
1 1 2 22()
2 4( )
B
I BCx t
F F Fx
F FM
Supposedly mostly H (~80%)
Total cross-section
2 22.3 GeV
0.36B
Q
x
Corrected for real+virtual RCCorrected for efficiencyCorrected for acceptanceCorrected for resolution effects
Extracted Twist-3contribution small !
PRL97, 262002 (2006)
but impossible to disentangle DVCS2
from the interference term !
DVCS on the neutron in JLab/Hall A: E03-106
LD2 target24000 fb-1
xB=0.36, Q2=1.9 GeV2
MODEL-DEPENDENTJu-Jd extraction
E1-DVCS with CLAS : a dedicated DVCS experiment in Hall B
~50 cm
Inner Calorimeter+ Moller shielding solenoid
Beam energy: ~5.8 GeVBeam Polarization: 75-85%Integ. Luminosity: 45 fb-1
Next half of data in 2008
M(GeV2)
E1-DVCS kinematical coverage and binning
W2 > 4 GeV2
Q2 > 1 GeV2
Integrated over t
E1-DVCS : Asymmetry as a function of xB and Q2
<-t> = 0.18 GeV2 <-t> = 0.30 GeV2 <-t> = 0.49 GeV2 <-t> = 0.76 GeV2
Accurate data in a large kinematical domain
E1-DVCS : ALU(90°) as a function of -t + models
JM Laget
VGG twist-2
VGG twist-2+3
Accepted by PRL
0.09<-t<0.2
0.2<-t<0.4
0.4<-t<0.6
0.6<-t<1
1<-t<1.5
1.5<-t<2
PRELIMIN
ARY,
Analysis in
Progre
ss
PhD Thesis H.S. Jo
E1-DVCS : Cross-sections over a wide kinematical range
New developments in GPD modeling : minimal dual model
Central quantities are t-dependent parton distributions q and e(x,t), obtained as →0 limits of the nucleon GPDs:
q and e are estimated using a dual representation (parton distributions as an infinite series of t-channel exchanges).
Only the leading function is kept for simplicity. Only the GPD H is kept, supposedly dominant in DVCS.
It is a simple one-parameter model with strong predictive content.If part of the data cannot be fit, it will mean higher twists/genuine non-forward effects need to be included: first step to understand DVCS data.
0 0( , ) lim ( , , ), ( , ) lim ( , , )q x t H x t e x t E x t
arXiv:0803.1271v2Polyakov, Vanderhaeghen
Minimal dual model, confrontation with data
DD model (VGG)Minimal dual model
Data: Hall A polarized cross section data at Q2=2.3 GeV2
Difference of cross sections (imaginary part of interference term)
Data are perfectly described by minimal dual model
Minimal dual model, confrontation with data (2)
Beam Spin Asymmetries(Hall B data)
Total cross sections(Hall A data)
DD model (VGG)Minimal dual model
e’
e
0
leptonic plane
hadronicplane
p’
• Q2= - (k-k’)2
• xB = Q2/2M=Ee-Ee’
• t = (p-p’)2
• (angle between leptonic and hadronic plane)
= (Q2,xB)d4
dQdxBddtd2
ddtreduced cross section for *p→p0
dTT
dt+
d2ddt
=1
2(
dT
dt
dL
dt+
dLT
dt+√ 2+1) cos cos2
+h√ 2-1)dL’T
dtsin
0 electroproduction cross section
VGGLaget
-Low-t cross-section largelyovershoots GPD & JML models !!!
-TT term is large, suggesting a large transverse component.
-But 0 cross-section ratio for proton and deuteron targets is found ~ 1
More data needed !
Submitted soon
0 cross sections in Hall A – a puzzle !
0 cross sections in Hall B
Arbitrary unitsStatistical errors only
T+L vs. –t (GeV2/c2)
S. Niccolai
PRELIMIN
ARY,
Analysis in
Progre
ss
Summary
Scaling test of DVCS cross section difference
→ DD models over-estimate it→ Minimal dual model provide a good description
Asymmetry measured in a large kinematical range,badly described by models.
Cross section measured both accurately and(soon) in a large kinematical range, also badly described by models: suggest genuine off-forward contribution and/or higher twist at work.
0 cross sections (unseparated) measured: not well described by either Regge model or GPD model: a challenge for theory !
Like Stefano said: mode data are needed, but modelling is only in its first phase and shows interesting progress recently.
JLab 6 & 12 GeV,HERMES recoil,COMPASS + upcomingTheory/Phenomenology work
Use of base CLAS12 equipment, including Inner Calorimeter (IC)
Detection of the full (e,p,) final state
Perform 2 experiments for the extraction of the BSA and the TSA
Experimental Setup and proposed experiments at 11 GeV
IC tungsten shielding
Distance, target-IC: ~1.75m currently(note: this is NOT optimized for DVCS!)
Beam Spin Asymmetry
From 1% statistical error on extracted Twist-2 coefficient
to 10% statistical errorat high xB
IC in standard position – 80 days – 10^35 Lum – VGG model
The cross-section difference accesses the imaginary part of DVCS and therefore GPDs at x =
1
1
1
1
( , , ) +
( , ,
- i + ( , )
, )
DVCS
GPD x t
GPD x tT dx
x
GPD x tdx
i
P x
The total cross-section accessesthe real part of DVCS and therefore an integral of GPDs over x
Observables and their relationship to GPDs
CLAS 6 GeV
DVCSBSAProton
ep→epπo/η
Hall A 6 GeV
DVCSprotonneutron
ep→epπo
CLAS 5.75 GeV
DVCS
DDVCS
ΔDVCS
D2VCS
PolarizedDVCS
ep→epρL
ep→epωL
ep→epπ0/η
ep→enπ+
ep→epΦ
HERMES 27 GeV
DVCS – BSA + BCA
+ nuclei BSA d-BCA
+ Polarized target DVCS
ep→epρσL + DSA
CLAS 4-5 GeV
DVCSBSA
HERMES
DVCS
BSA+BCA
With recoil detector
COMPASS
DVCS
+BCA
With recoil detector
Published or Finalized Published or Analysis in 2009 –
2001 close to be progress 2010 2010+ 2014?
JLab@
12GeV
(low/medium energy) Deep Exclusive experimentsE
VE
RY
TH
ING
, with
more statistics than ever before
JLab
Hall A
DVCS²
CLAS
BSA+TSA
In red: selected topics for JLab