XVIII th International Symposium on Spin PhysicsCharlottesville, October 6-11, 2008 GPDs, The Hunt...
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Transcript of XVIII th International Symposium on Spin PhysicsCharlottesville, October 6-11, 2008 GPDs, The Hunt...
XVIIIth International Symposium on Spin Physics Charlottesville, October 6-11, 2008
GPDGPDs,s,
The Hunt for The Hunt for QQuark uark OOrbital rbital MMomentumomentum
(i) The nucleon spin puzzle(ii) Generalized parton distributions(iii) Deeply virtual Compton scattering(iv) Experimental access to E(Q2,x,,t)(v) Perspectives(vi) Experimental strategy(vii) Conclusions
Laboratoire de Physique Subatomique et de CosmologieGrenoble, France
Eric Voutier
?
zg
q
zq LGL 2
1
2
1
Orbital Momentum
of gluons(unknown)
Orbital Momentum of quarks and
antiquarks(first glimpse)
Helicity distributionsdescribing the longitudinal
polarization of quarks and antiquarks(known)
Gluon polarization(first data)
Eric VoutierThe nucleon spin puzzle
1/1SPIN2008, Charlottesville, October 6-11, 2008
Transversity distributions describing the transverse
polarization of quarks and antiquarks
(first experimental attempts)
B.L. Bakker, E. Leader, T.L. Trueman, PRD 70 (2004) 114001
gqq
sq
TLxqdx,,,
)(2
1
2
1
Terra Incognita
Longitudinal spin sum rule
Generalized parton distributions (GPDs) procure the cement of the
nucleon spin puzzle, and more generally of the hadron structure.
Generalized parton distributions
1/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
GPDs are the appropriate framework to deal with the partonic structure of hadrons and offer the unprecedented possibility to access the spatial distribution
of partons.
Parton Imaging
M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223
GPDs can be interpreted as a 1/Q resolution distribution in the
transverse plane of partons with longitudinal momentum x.
GPDs = GPDs(Q2,x,,t) whose perpendicular component of the momentum transfer to the nucleon is Fourier conjugate to the transverse position of partons.
GPDs encode the correlations between partons and contain information about the dynamics of the system like the angular momentum or the distribution of the strong forces experienced by quarks and gluons inside hadrons.
X. Ji, PRL 78 (1997) 610 M. Polyakov, PL B555 (2003) 57
A new light on hadron structure
The GPDs Framework
Generalized parton distributions
2/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
GPDs
x x
t
Coherence between quantum states of different helicity, longitudinal momentum and transverse position.
At leading twist, the partonic structure of the nucleon is described by 4 quark helicity conserving and chiral even GPDs and 4 quark helicity flipping and chiral odd GPDs (+8 gluon GPDs).
qqqq EHEH~
,~
,, qT
qT
qT
qT EHEH
~,
~,,
D. Müller et al., FP 42 (1994) 101 A.V. Radyushkin, PRD 56 (1997) 5524 X. Ji, PRL 78 (1997) 610
P. Hoodbhoy, X. Ji, PRD 58 (1998) 054006 M. Diehl, EPJC 19 (2001) 485
E’s arenew and unknown
distributions
)()0,0,( xqxH q )()0,0,(~
xqxH q )()0,0,( xqxH qT
)()0,0,( xgxxH g )()0,0,(~
xgxxH g 0)0,0,(~
,~
,, xEHEHgT
In the forward limit (t 0, 0), H’s reduce to the forward parton distributions (density, helicity, transversity).
E’s, which involve nucleon helicity flip, do not have a DIS equivalent.
x Initial longitudinal momentum fraction -2 Transferred longitudinal momentum fraction (skewness)
(-t)1/2 Momentum transfer to the nucleon
Generalized parton distributions
3/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
Form Factors
GPDs unify in the same universal framework parton distributions, form factors, and the spin of the nucleon.
t
GPDs
The first Mellin moments relate GPDs to Dirac (Hq), Pauli (Eq), axial ( ), and pseudo-scalar ( ) nucleon form factors.
Similar relations relate chiral odd GPDs to tensor form factors.
tFtxEdx qq2
1
1,,
tGtxEdx q
Pq
,,
~1
1
qH~ qE
~
qT
qT
qT xExHdx
1
10,0,0,0,
~2
independence from Lorentz invariance
Transverse spin-flavor dipole moment in an unpolarized nucleon
Energy Momentum Tensor
1
10,,0,,
2
1
2
1 xExHxdxLJ qqqqq
X. Ji, PRL 78 (1997) 610 M. Polyakov, PLB 555 (2003) 57 M. Burkardt, PRD 72 (2005) 094020
The second Mellin moments relate GPDs to the nucleon dynamics, i.e. parton angular momentum and strong forces distributions.
1
10,0,0,0,
~20,0,
4
1xExHxHxdxJ q
TqT
qT
q
Correlation between quark spin and angular momentum in an unpolarized
nucleon
Generalized parton distributions
4/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
GPDs
x x
k
k
'kkq
?
p
J.C. Collins, L. Frankfurt, M. Strikman, PRD56 (1997) 2982 X. Ji, J. Osborne, PRD 58 (1998) 094018 J.C. Collins, A. Freund, PRD 59 (1999) 074009
The key requirements for the experimental study of GPDs are luminosity and resolution.
GPD(Q2,x,,t)Probe tagging ()
Production of one additional particle ()
Final state identification
Exclusivity
p
t
22 'kkQ 22' pptqp
QxB
2
2
FactorizationInteraction with elementary partons (Q2>>M2)
Separation of perturbative and non-perturbative scales (-t<<Q2)
Hardness
Deep Exclusive Scattering
The factorization theorem allows to express the cross section for deep exclusive processes as a convolution of a known hard scattering kernel with an unkown soft matrix element related to the nucleon structure (GPDs).
Generalized parton distributions
5/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
GPDs
x x
t
DVCSDeeply Virtual
Compton Scattering
Longitudinal response only
GPDs
xx
DA
L
DVMPDeeply Virtual
Meson Production),(
),(
t
Hard gluon
GPDs can be accessed via exclusive reactions in the Bjorken kinematic regime.
The DVCS process is identified via double (e) or triple (eN) coincidences, allowing for small scale detectors and large luminosities.
Additional amplitudes contribute to the reaction process and interfere with the DVCS amplitude.
The identification of the DVMP process requires the detection of the meson disintegration products in large acceptance detectors.
Factorisation applies only to longitudinally polarized virtual photons whose contribution to the electroproduction cross section must be isolated.
Meson production acts as a GPD and a flavor filter.
x
)0,,( xH
GPDs enter the cross section of hard scattering processes via Compton form factors, that are integrals over the intermediate parton longitudinal momenta.
t
GPDs
x x
Q2 >> M2 -t << Q2
6/6
Generalized parton distributions Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
),,(),,(),,( 1
1
1
1txi
x
txdx
ix
txdx
GPD
GPDGPDP
B
B
x
x
2
Photon Electroproduction
Eric Voutier
The Bethe-Heitler (BH) process where the real photon is emitted either by the incoming or outgoing electron interferes with DVCS.
DVCS & BH are indistinguishable but the BH amplitude is exactly calculable and known at low t.
The relative importance of each process is beam energy and kinematics dependent.
Deeply virtual Compton scattering
SPIN2008, Charlottesville, October 6-11, 2008 1/5
leptonic plane
e-’
p
e- *
hadronic plane
Out-of-plane angle entering the harmonic development of the reaction amplitudeA.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323
Polarization observables help to single-out the DVCS amplitude.
P0 = 6 GeV/c
γpepe
EHHF )(4
~)()()()( 22211 tF
M
ttFtFtFC I
Polarized Beam Cross Section Difference
Eric VoutierDeeply virtual Compton scattering
SPIN2008, Charlottesville, October 6-11, 2008 2/5
DVCSDVCSDVCSBHeBeB dddtdxdQ
d
dddtdxdQ
dTTTT mem
2
5
2
5
2
1J
The sDVCS,I are bilinear and linear combinations of GPDs at ±
C. Muñoz-Camacho et al., PRL 97 (2006) 262002
Twist-2
Twist-3
)2sin()sin()()(
),,()sin(),,(
2
1
2121
23
12
2
2
5
2
5
IIBDVCSB
eBeB
ssPP
tQxstQx
dddtdxdQ
d
dddtdxdQ
d
J
γpepe
Q2 = 2.3 GeV2
xB = 0.36Q2 = 2.3 GeV2
xB = 0.36
DVCS / Bethe-Heitler interference amplitudeDVCS amplitude
Imaginary part The DVCS process is associated with a non-zero polarized beam
signal.
Gluons
Twist-2
Twist-3
DVCSBHDVCSBHeB dddtdxdQ
dTTTT e 2
222
5
The cDVCS,I are bilinear and linear combinations of Compton form factors.
Unpolarized Cross Section
Eric VoutierDeeply virtual Compton scattering
SPIN2008, Charlottesville, October 6-11, 2008 3/5
C. Muñoz-Camacho et al., PRL 97 (2006) 262002
EEHHFF )(4
)()()()()( 22212
1 tFM
ttFtFtFCC II
Q2 = 2.3 GeV2
xB = 0.36Q2 = 2.3 GeV2
xB = 0.36
γpepe
The magnitude of the DVCS process is seen as a deviation from the pure BH cross section.
Bethe-Heitler amplitude
DVCS amplitude DVCS / Bethe-Heitler interference amplitude
)3cos()2cos()cos(
)()(
),,(
)2cos()cos(),,(
)2cos()cos()()(
),,(
321021
23
2102
2
21021
21
2
5
IIIIB
DVCSDVCSDVCSB
BHBHBHB
eB
ccccPP
tQx
ccctQx
cccPP
tQx
ddφdtdxdQ
σd
Real part
E00-110 p-DVCSP.Y. Bertin, C.E. Hyde, R. Ransome, F. Sabatié et al.
Twist-two contributions dominate the DVCS cross section and are Q2 independent in the measured kinematics range. Twist-three contributions to the cross section are small and are Q2 independent within error bars.
C. Muñoz-Camacho et al., PRL 97 (2006) 262002
Deeply virtual Compton scattering Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 4/5
Experimental results support the dominance of the handbag diagram at Q2 as low as 2
GeV2.
Beam Spin Asymmetry
Deeply virtual Compton scattering Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 5/5
F.-X. Girod et al., PRL 100 (2008) 162002
2coscos1
sin)(
)(55
55
tdtc
tadd
ddA
JJ
@ CLAS (5.77 GeV)γ pepe
GPDs Twist-2
JML / Regge
GPDs Twist-3
Calculations based on hadronic degrees of freedom, within a Regge approach, are in fair agreement with data up to 2.3 GeV2.
GPD based calculations reproduce reasonably well the main features of the data.
The angular dependence of the asymmetries is compatible with expectations from leading-twist
dominance.
F.-X. Girod talk in GPD parallel session
F. Ellinghaus,W.-D. Nowak, A.V. Vinnikov, Z. Ye EPJ C46 (2006) 729
Z. Ye, Doctorat Thesis, Universität Hamburg (2006)
)~
,,(~
4
4)(
2212
122
HEHE
EHF
OFFM
t
FFM
tC ITP
)
~,
~,,(
~
4
~~
4)(
22212
122
EHEHEE
EHF
OFFFM
t
FFM
tC ITP
DVCS Transverse Target Asymmetry
Experimental access to E(Q2,x,,t) Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 1/6
Twist-2
A.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323 M. Diehl, S. Sapeta, EPJC 41 (2005) 515
B. Zihlmann talk in GPD parallel session
)sin()cos()()cos()sin()(
),,(,,
,,, 2
455
55
SITPS
ITP
BSS
SSSUT
CC
tQxdd
ddA
FF
2/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
Experimental access to E(Q2,x,,t)
DVMP Transverse Target Asymmetry
pρpγ LL
Tra
nsv
ers
e T
arg
et
Asy
mm
etr
y
xB
2Ju+Jd
pωpγ LL
Tra
nsv
ers
e T
arg
et
Asy
mm
etr
y
xB
2Ju-Jd
The s-channel helicity conservation (SCHC) allows to extract the longitudinal/transverse ratio from the angular distribution of the decay products of the vector mesons, avoiding a Rosenbluth separation.
The longitudinal polarization of the vector meson is obtained from the angular distribution of the decay products.
Transversally polarized proton targets are of primary importance for the determination of E(Q2,x,,t).
K. Goeke, M. Polyakov, M. Vanderhaeghen, PPNP 47 (2001) 401 S.V. Goloskokov, P. Kroll, arXiv/hep-ph:0809.4126
3/6SPIN2008, Charlottesville, October 6-11, 2008
Eric VoutierExperimental access to E(Q2,x,,t)
DVMP Longitudinal Cross Section
S.A. Morrow et al., arXiv/hep-ex:0807.3834
pρpγ LL @ CLAS (5.74 GeV)
Standard GPD calculations fail to reproduce data in the valence region, while successfull at large W.
Data can be interpreted in terms of hadronic degrees of freedom, following a Regge approach.
Data can be interpreted in terms of partonic degrees of freedom via strongly modified GPDs, via a D-term like adjusted component.
2222 MQWQxB
VGG / GPDs
GK / GPDsJML / Regge
VGG++ / GPDs
The question is asked whether current GPD parametrizations must be revisited or DVMP cannot be interpreted GPD wise in the valence
region ?
Neutron Target
EHHF )(4
~)()()( 22211 tF
M
ttFtFtFC I
n
Suppressed because F1(t) is small
Suppressed because of cancellation between u and d quarks
From polarized beam cross section difference
Neutron targets allow to access the least known and constrained GPD
that appears in the nucleon spin sum rule.
Neutron targets provide new linear combinations of GPDs
tEtEe qq
qq ,,,,2 Em
Neutron and proton targets are sensitive to the u quark flavor and, following isospin symmetry, appear then complementary.
Eric VoutierExperimental access to E(Q2,x,,t)
SPIN2008, Charlottesville, October 6-11, 2008 4/6
E03-106 n-DVCSP.Y. Bertin, C.E. Hyde, F. Sabatié, E. Voutier et al.
deedneenpeepXeeD ),(),(),(),(
S. Ahmad et al., PR D75 (2007) 094003
M. Vanderhaeghen et al., PR D60 (1999) 094017
M. Mazouz et al., PRL 99 (2007) 242501
The twist-2 effective harmonic coefficients of the neutron are small, compatible with zero.
The measured t-dependence can be used to constrain the parametrization of the GPD E, within a particular model.
Impulse approximation
Q2 = 1.9 GeV2 xB = 0.36
M. Mazouz et al., PRL 99 (2007) 242501
Experimental access to E(Q2,x,,t) Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 5/6
)sin()()(
),,()sin(
)()(
),,(
2
11
21
23
121
23
2HD
5
2HD
52222
Id
dd
BdIn
nn
Bn
eBeB
sPP
tQxs
PP
tQx
dddtdxdQ
d
dddtdxdQ
d
J
Twist-2
γ nene
6/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
u
M. Mazouz et al., PRL 99 (2007) 242501
Measurements off neutron are sensitive to
Jd
(u quark in the neutron)
Measurements off proton are sensitive to
Ju
(u quark in the proton)
1
)()(),( 2
min2exp2exp
2,exp
22min
i isystistat
iJJ
VGGIni
In
dutt
tCtCJJ
duFF mm
A. Airapetian et al., arXiv/hep-ex:0802.2499
Model Dependent Quark Angular Momenta
d
A.W. Thomas, arXiv/hep-ph:0803.2775
Neutrons are a mandatory step in the hunt for the quark orbital momentum.
Experimental access to E(Q2,x,,t)
E08-021 -DVCSH. Avakian, V. Burkert, M. Guidal, R. Kaiser, F. Sabatié et al.
Perspectives
1/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
p
G. Thorn talk in Polarized Sources & Targets parallel session
Advantages
The HDice frozen spin target figures a large spin relaxation time (~1 year) at operational temperature (0.5 K) and field (0.9 T).
High T -> high cooling power compensates beam heatingLow B -> ideal for transverse polarization @ CLAS
Pure target -> small physics background & faster data taking
Low Z -> small bremsstrahlung background (e-)
Drawbacks
The many delicate processes between production and operation make the complete system quite complex. Operation with an electron beam is promising but has not yet been established.
The technical feasability of electron experiments with the
HDice target has to be demonstrated.
HD is condensed into the target cell with ~2000 50 µm Al wires soldered to a copper cooling ring
HD (77%) Al(16%) C2ClF3 (7%)HDice Lab @ JLab
A. Sandorfi et al., Proc. of the Workshop on Exclusive Reactions, Edts. P. Stoler and A. Radyushkin, World Scientific (2008) 425.
Eric Voutier
E07-007 & E08-025 p-DVCS & n-DVCSP.Y. Bertin, C.E. Hyde, C. Muñoz Camacho, J. Roche et al.
A. Camsonne, C.E. Hyde, M. Mazouz et al.
)cos()()(
),,(),,( 10
21
232
22
2
5
In
In
nn
BnDVCSnBnn
eB
n ccPP
tQxctQxBH
dddtdxdQ
d
),,(
),,(
)()(
12
2
23
21 tQx
tQx
PP Bn
Bn
nn
-0.36
0E ~
0E ~
Within the VGG model, the neutron pure DVCS
amplitude is sensitive to E .
E~
DVCSnC
22 GeV9.136.0 QxB
A.V. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323
Perspectives
SPIN2008, Charlottesville, October 6-11, 2008 2/6
The pure DVCS contribution to the cross section is separated from the interference contribution, in the twist-2 approximation, taking advantage of the beam energy dependence of the kinematic factors.
Twist-2
E05-114 -DVCSA. Biselli, L. Elouadrihri, K.Joo, S. Nicolai et al.
Perspectives
3/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
p
A.V. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323
A. Biselli and H. Egiyan talks in GPD parallel session
EEHH~
)(4
)(1
)()(1
2)()(
~)( 22121211
tF
M
ttFtFtFtFtFtFC I
LP
Access to another Compton form factor,that is another linear combination of GPDs
)3sin()2sin()sin()()(
),,(
)2sin()sin(),,(2
1
,3,2,121
25
,2,12
42
5
2
5
ILP
ILP
ILP
B
DVCSLP
DVCSLPB
eBeB
sssPP
tQx
sstQxdddtdxdQ
d
dddtdxdQ
d
Gluons
Twist-2
Twist-3
Access to the imaginary part of H from the target spin
asymmetry.
H~
Access to the real part from double polarization
observable.
55
55
dd
ddULA90GeV3.230.0 22 QxB
Perspectives
4/6
Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
R. Kaiser talk in GPD parallel session
The GPDs World
xB
One may expect to get a reasonable mapping of the GPDs in the
intermediate and valence regions by 2020.
H1, ZEUS, HERMES, JLab 6 GeV are providing the first but limited data sets.
The energy upgrade of the CEBAF accelerator allows access to the high xB region which requires large luminosity.
The DVCS project at COMPASS will explore intermediate xB (0.01-0.10) with a reasonable overlap with the JLab 12 GeV kinematic domain.
Perspectives Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008
GPDs @ JLab 12 GeV
Several experiments will measure DVCS (Hall A & B) and DVMP (Hall B) with (un)polarized beam and target, extending the experimental techniques and methods developed at 6 GeV.
An eventual transversally polarized target program in Hall B is attached to the technical success of the HDice target.
The development of neutron detection capabilities in the central detector region (Hall B) and of polarized neutron targets sustaining high beam currents (Hall A) are investigated.
D(e,e’n)p
5/6
D(e,e’n)p
Perspectives Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 6/6
GPDs @ COMPASS
μ
p’
μ’
The GPDs program is part of the COMPASS Phase II (2010-2015) proposal to be submitted to CERN in 2008-2009.
Step 1 (~2011) to constrain H
d(+, ) + d(-, ) Im(F1 H) sin d(+, ) - d(-, ) Re(F1 H) cos
Step 2 (~2013) to constrain E
d(, S) - d(, S+π) Im(F2 H – F1 E) sin (- S) cos
The first step of this program requires a 4 m long recoil proton detector (RPD) together with a 2.5 m long LH2 target. An additional electromagnetic calorimeter will enlarge the kinematical coverage at large xB.
The second step requires either a transversally polarized NH3 target inserted in the RPD or a new SciFi (?) RPD inserted in the existing NH3
target system. %80μGeV190100 ,
Experimental strategy Eric Voutier
SPIN2008, Charlottesville, October 6-11, 2008 1/1
γpepe
Q2 = 2.3 GeV2 xB = 0.36
Compton Form factors
M. Guidal, arXiv/hep-ph:0807.2355
Within a fitting code relying on a leading order and leading twist description of the DVCS amplitude, it was shown that a reliable and model-independent extraction of the real and imaginary parts of each Compton form factor requires a minimal number of observables.
Cross section (, single and double polarized cross section differences (ij) should be measured; asymmetries are providing additional constraints but are not enough selective by themselves.
Dispersion relation constraints are expected to improve this picture by reducing the number of required observables and/or improving the accuracy of the fitting procedure.
Current and z0 data suggest that VGG predictions underestimate slightly the imaginary part of H and miss the real
part.
jz 00 zj
Imaginary parts Real parts
It remains a theoretical concern to extract GPDs from CFFs.
A.V. Belitsky, D. Müller, arXiv/hep-ph:0809.2890
Conclusions
1/2
Eric Voutier
Summary
GPDs offer the unique opportunity to access the contribution of the quark angular momentum to the spin of the nucleon.
In this effort, neutron and transversally polarized proton targets are mandatory steps.
SPIN2008, Charlottesville, October 6-11, 2008
Special thanks to François-Xavier Girod, Michel Guidal, Nicole d’Hose, Malek Mazouz, Andy Sandorfi
Together with the evolution of lattice QCD calculation capabilities, the road towards a comprehensive and quantitative
understanding of the nucleon structure is OPEN!!
The exploration of the GPD world has been starting at DESY & JLab and will continue at JLab 12 GeV and COMPASS with high precision and exclusive experiments.
Recent phenomenological & theoretical developments tend to indicate that this world should be efficiently mapped with the measurement of a limited set of
observables.
Eric VoutierPerspectives
SPIN2008, Charlottesville, October 6-11, 2008Hall
B
HDice Lab
HDice Lab @ JLab
HDice Lab design and infrastructure – Nov’08 Building construction – (Feb’09, Jun’09)
Cryogenic infrastructure – (Apr’09, Sep’09) Polarized target tests – (Sep’09, Nov’09)
Design & construction of a new In-Beam Cryostat for CLAS – (May’08, Jul’10) Set of polarized targets for experiments – (Mar’10, Aug’10)
Installation in Hall B – (Jul’10, Sep’10) run – (Sep’10, Apr’11)
Electron beam tests – (Apr’11) e run – (Nov’11, Dec’11)
Courtesy of A. Sandorfi
Eric Voutier
InCe DVCSnC
Experimental data are taken for LH2 and LD2 targets at 4.82 GeV/c and 6.00 GeV/c.
The LD2-LH2 yields binned in (k, Mx2, , t) are
fitted with Monte-Carlo simulated yields taking into account experimental and radiative effects.
Unpolarized Cross Section
Projected systematics
Perspectives
SPIN2008, Charlottesville, October 6-11, 2008
P R O J E C T E D D A T A
InIn CC e
Eric Voutier
n-DVCS @ CLAS 12INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble, University of Glasgow …
… European initiative towards the central detector …
Neutron detection at laboratory angles larger than 40° would allow an efficient and fully exclusive study of the n-DVCS process up to the
valence region.
The possibility of developing neutron detection capabilities in the central region of CLAS12 is forseen.
A. El Alaoui (LPSC Grenoble) (2008)
SPIN2008, Charlottesville, October 6-11, 2008
Perspectives
ToF scintillators
Vertex detector
Neutron detector
5 T @ target
D(e,e’n)p
Perspectives Eric Voutier
Polarized Neutron Target
sin~
41 22
EEHM
ttF m
Longitudinal Target Spin Asymmetry
Transverse Target Spin Asymmetry
HEEHm10DVCSc
E-EEH
HE-E
HHE-E
~1
4M
t
11
2
1~
142
1~
11
2
42
22
2
1
221
22
2
220
m
m
mm
tFs
ξtF
M
tc
tFM
tξ
y
ytF
M
tc
I
I
I
The twist-2 target spin asymmetries are derived below in the case of a polarized neutron, assuming that the Dirac form factor and the non spin-flip polarisation dependent GPD are 0.
The most sensitive coefficient to E appears to originate from the pure DVCS amplitude while the kinematical factors enhance H in the other coefficients.
sincoscossinsin 1100 SI
SI
SIDVCS sccc
A. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323
SPIN2008, Charlottesville, October 6-11, 2008
Eric Voutier
Forward Calorimeter
Forward Cerenkov(LTCC)
Beamline Instrumentation
Preshower Calorimeter
Forward Drift Chambers
Inner Cerenkov(HTCC)
SuperconductingTorus Magnet
Central Detector
Inner Calorimeter
Forward Time-of-Flight
Detectors
CLAS12
SPIN2008, Charlottesville, October 6-11, 2008
Perspectives