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