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Hard Exclusive Pseudo-Scalar Meson Production in CLAS
Valery KubarovskyJefferson Lab
Partons in Nucleons and NucleiMarrakech, Morocco, September 26-30, 2011
Outline
• Physics motivation• CLAS data on pseudoscalar meson
electroproduction • Hard exclusive electroproduction of p0 and h
with CLAS12 • Conclusion
Description of hadron structure in terms of GPDs
Nucleon form factors transverse charge &
current densitiesNobel prize 1961- R. Hofstadter
Structure functions quark longitudinal
momentum (polarized and unpolarized) distributions
Nobel prize 1990 –J.Friedman, H. Kendall, R. Taylor
GPDs correlated quark momentum distributions (polarized and unpolarized) in transverse
space
Generalized PartonDistributions
• There are 4 chiral even GPDs where partons do not transfer helicity H, H, E, E
• H and E are “unpolarized” and H and E are “polarized” GPD. This refers to the parton spins.
• 4 chiral odd GPDs flip the parton helicity HT, HT, ET, ET. HT is connected with transversity
~
~
~
~
~~
Basic GPD properties
• Forward limit
• Form factors
• Angular Momentum
DVCS and DVMPin leading twist
DVCS:• the clearest way to access the GPDs• Only gT photons participate in DVCS• Interference with BH processDVMP:• Factorization proven only for sL
sL~1/Q6, sT/sL~1/Q2
• Meson distribution amplitude• Gluon exchange required• Vector and pseudoscalar meson production allows to separate flavor and
separate the helicity-dependent and helicity independent GPDs.
• Factorization theorem• Access to fundamental degrees of
freedom
Transversity in hard exclusive electroproduction of pseudoscalar
mesonsS. Goloskokov, P. Kroll, 2011, arXiv:1106.4897v1
M 0-,++~ HT
• The data clearly show that a leading-twist calculation of DVMP within the handbag is insufficient. They demand higher-twist and/or power corrections• There is a large contribution from the helicity amplitude M 0-,++. Such contribution is generated by the the helicity-flip or transversity GPDs in combination with a twist-3 pion wave function• This explanation established an interesting connection to transversity parton distributions. The forward limit of HT is the transversity
HT(x,0,0)=h1(x)
Nucleon Tensor Charge from Exclusive p0 Electroproduction
Ahmad, Goldstein, Luiti, Phys. Rev. D 79, 054014 (2009), arXiv:1104.5682v1
• The quantum numbers and Dirac structure of π0 electroproduction restrict the possible contributions to the 4 chiral odd GPDs, one of which, HT , is related to the transversity distribution and the tensor charge. • This differs from DVCS and both vector and charge p+/- electroproduction, where the axial charge can enter the amplitudes. • Contrary the tensor charge enters the p0 process.
partonic degrees of freedom interpretation;
t-channel exchange diagram
0* pp
JLab Site: The 6 GeV Electron Accelerator
3 independent beams with energies up to 6 GeV Dynamic range in beam current: 106
Electron polarization: 85%
Hall-BCLAS
Hall-A
Hall-C
CEBAF Large Acceptance Spectrometer CLAS
424 crystals, 18 RL, Pointing geometry, APD readout
CLAS Lead Tungstate Electromagnetic Calorimeter
Pseudoscalar mesons
CLAS6: lots of data, cross sections, beam-spin asymmetriesCLAS12: Exp. # E12-06-108
Vector mesons
CLAS DVMP program
4 Dimensional Grid
Rectangular bins are used. Q2- 7 bins(1.-4.5GeV2) xB- 7 bins(0.1-0.58) t - 8 bins(0.09-2.0GeV) φ - 20 bins(0-360°) p0 data ~2000 points h data ~1000 points
xB
Q2
0* pp
Monte Carlo
• Empirical model for the structure cross sections was used for the MC simulation and radiative corrections
• This model is based on CLAS data• MC simulation included the radiative effects and
used empirical model for the Born term.• 100 M events were simulated with GSIM program.
Radiative Corrections
• Radiative Corrections were calculated using Exclurad package with structure cross sections described by our empirical cross section.
Q2 = 1.15 GeV2 xB = 0.13 -t = 0.1 GeV2
f
p0
h
Structure FunctionssU=sT+esL sTT sLT
)cos)1(22cos(2
1),,,( 2
dt
d
dt
d
dt
d
dt
dtxQ
dtd
d LTTTLT
0* pp
GM Laget Regge modelf distribution-t
sU=sT+esL W dependence
sU~1/W1.5-2
• sU decreases with W at Jlab kinematics
• This behavior is typical for Regge model
• Difficult to get such dependence with conventional GPD models
sU=sT+esL xB dependence
• Another way to view the cross section as a function of xB
• sU increases with xB
• W=Q2(1/x-1)
dsU/dt
GeV2
-t
t-slope parameter: xB dependence
The slope parameter is decreasing with increasing xB. Looking to this picture we can say that the perp width of the partons with x1 goes to zero.
Transversity in p0 electroproductionS. Goloskokov, P. Kroll, 2011, arXiv:1106.4897v1
Q2=2.71 GeV2
xB=0.34
Transvers cross section dominates in this model
Theory: Goloskokov&KrollData: CLAS (preliminary)
Transversity in p0 electroproductionS. Goloskokov, P. Kroll, 2011, arXiv:1106.4897v1 Q2=1.6 GeV2
xB=0.19
Q2=2.2 GeV2
xB=0.28
Q2=3.2 GeV2
xB=0.43
• Include transversity GPDs HT and
. Dominate in CLAS kinematics.
• The model was optimized for low xB and high Q2. The corrections t/Q2 were omitted
• Nevertheless the model successfully described CLAS data even at low Q2
• Pseudoscalar meson production provides unique possibility to access the transversity GPDs.
Q2=1.2 GeV2
xB=0.13
Q2=1.8 GeV2
xB=0.22
Q2=2.7 GeV2
xB=0.34
Goldstein and Liuti GPDT modelData – CLAS6
We are looking forward to extend the comparison with GPD-based model inthe full kinematic domain of CLAS
Beam Spin AsymmetryAhmad, Goldstein, Luiti, 2009
0* pp
• Data CLAS• Blue – Regge model• Red – GPD predictions• tensor charges δu=0.48, δd =-0.62• transverse anomalous
magnetic moments κu
T = 0.6,
κdT = 0.3.
/h p0 Ratio• The dependence on the xB and Q2 is very week. • The ratio in the photoproductionis near 0.2-0.3 (very close to what we have at our smallest Q2).• Conventional GPD models predict this
ratio to be around 1 (at low –t).• KG model predicts this ratio to be~1/3 at CLAS values of t
-t=0.14 GeV2
-t=0.50 GeV2
-t=0.30 GeV2
-t=0.50 GeV2
_Indication of large contributions from the GPD ET
with the same sign for u and d-quark parts
Data: CLAS preliminary
CLAS12Luminosity 1035 cm 2 s-1
Forward Detector- TORUS magnet- Forward SVT tracker- HT Cherenkov Counter- Drift chamber system- LT Cherenkov Counter- Forward ToF System- Preshower calorimeter- E.M. calorimeter
Central Detector- SOLENOID magnet - Barrel Silicon Tracker- Central Time-of-Flight-Polarized target (NSF)
Proposed upgrades- Micromegas (CD)- Neutron detector (CD)- RICH detector (FD)- Forward Tagger (FD)
E12-06-108 Hard Exclusive Electroproduction
of p0 and h with CLAS12
• Cross sections of the reactions ep ➝ epp0, ep ➝eph• Extract structure functions sT, sL, sTT, sLT sLT’ vs. Q2, xB, t
– Fourier decomposition of the reduced cross section sT+esL, sTT, sLT
– Beam-spin asymmetry sLT’
– Rosenbluth separation sT, sL, at energies 11, 8.8 and 6.6 GeV
• Handbag - 3D nucleon tomography – transversity GPDs HT and data. – Backward pion production (high-t, low-u). Transition distribution
amplitudes.
CLAS6 data
CLAS12 Kinematic Coverage
CLAS-6
Statisticst-distributionDQ2= 2 GeV2
W=2.75 +/- 0.75 GeVQ2-distributionDt= 2 GeV2
Example of the Simulated Cross Section and Asymmetry
A=4.5E5+/-6.6E2B=0.047+/-0.002C=0.200+/-0.002
A=0.100+/-0.002B=0.049+/-0.030C=0.210+/-0.030
Simulated beam-spin asymmetrySimulated cross section
— The discovery of Generalized Parton Distributions has opened up a new and exciting avenue of hadron physics that needs exploration in dedicated experiments.
— Moderate to high energy, high luminosity, and large acceptance spectrometers are needed to measure GPDs in deeply virtual exclusive processes.
— The JLab 12 GeV Upgrade provides the tools to do this well and explore the nucleon at a much deeper level.
Summary
The Fin
Structure Functions
Lines – Regge model
Data: CLAS preliminary
Source Error
Acceptance 2.5 %Beam Charge 0.2 %
Particle ID 1.0 %Radiative Corrections
1.O %
sU=sT+esL 4.0 %
sL, sT 10-30 %
Kinematic coverage
W=2.75 +/- 0.75 GeV
t-distributionQ2= 5 GeV2
Q2-distributiont= 1 GeV2
Examples of the p0 MC simulation
Simulated beam-spin asymmetry
0 f 2p
0 f 2p
Anticipated systematic errors
Q2=2 GeV2,t=1 GeV2Q2=2 GeV2,t=1 GeV2
Simulated cross section
A=4.5E5+/-6.6E2B=0.047+/-0.002C=0.200+/-0.002
A=0.100+/-0.002B=0.049+/-0.030C=0.210+/-0.030
GeV2 GeV2
CLAS-6
DVCS and DVMP
DVCS:• the clearest way to access the GPDs• Only gT photons participate in DVCS• Interference with BH processDVMP:• Factorization proven only for sL
sL~1/Q6, sT/sL~1/Q2
• Meson distribution amplitude• Gluon exchange required• Vector and pseudoscalar meson production
allows to separate flavor and separate the helicity-dependent and helicity independent GPDs.
EH~,~
EH ,
• Factorization theorem• Access to fundamental degrees of
freedom
Transition from “hadronic” to the partonic degrees of freedom
?R
p p’
g*M
p p’
g*
L
M
Regge Model
(a) Regge poles (vector and axial vector mesons)(b) and (c) pion cuts
J.M. Laget 2010
Vector meson cuts
0* pp
dsU/dt
GeV2-t
nb/GeV2
JML Regge model
Q2 = 2.25 GeV2
xB = 0.34
-t
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