Wigner distributions and quark orbital angular momentum

transcript

Wigner distributions and quark orbital

angular momentum

Cédric Lorcé

May 14 2012, JLab, Newport News, VA, USA

Outline Parton distribution zoo

PDFs, FFs, GPDs, TMDs, GTMDs Partonic interpretation

Proton spin puzzle Decompositions Relations with observables

Model results Overlap representation Wigner distributions

Parton distribution zooCharges

Charges

Partonic interpretation (twist-2)

Vector

Tensor

0D Picture

Parton distribution zoo

Parton Distribution Functions

Charges

1D Picture

Form Factors

Charges

2D Picture

Impact parameter

Transverse center of

momentum

Generalized PDFs

Charges

PDFs FFs

3D Picture

[Soper (1977)][Burkardt (2000,2003)]

[Diehl, Hägler (2005)]

Transverse-Momentum dependent PDFs

No direct connection

Position space

Momentum space

Mean momentu

Displacement

Mean position

Momentum

transfer

gauge Charges

PDFs FFs

3D Picture

Generalized TMDs

[Wigner (1932)][Belitsky , Ji, Yuan (2004)]

[C.L., Pasquini (2011)]

Quasi-probabilistic interpretation

??????

Charges

5D Picture

Parton distribution zooComplete set

TMDs[C.L., Pasquini, Vanderhaeghen

(2011)]

Charges

FFsTMCs

4D Picture 2D Picture

Twist-2 structureGPDs TMDs

Quark polarization Quark polarization

Monopole Dipole Quadrupole

Naive T-odd

Twist-2 structureGTMDs

Quark polarization

Naive OAM

[Meißner, Metz, Schlegel (2009)]

Twist-3 structureGPDs TMDs

Naive T-odd

• Does not satisfy canonical relations• Incomplete decomposition

Proton spin puzzleJi Jaffe-Manohar

[Ji (1997)] [Jaffe, Manohar (1990)]

• Gauge-invariant decomposition• Accessible in DIS and DVCS

Kinetic Canonical

• Satisfies canonical relations• Complete decomposition

• Gauge-variant decomposition• Missing observables for the OAM

Improvements:

[Wakamatsu (2009,2010)]• Complete decomposition

Improvements:

[Chen et al. (2008)]

• Gauge-invariant extension

• OAM accessible via Wigner distributions [C.L., Pasquini (2011)]

[C.L., Pasquini, Xiong, Yuan(2011)]

[Hatta (2011)]

Quark spin and OAMGPDs TMDs GTMDs

Quark spin

Quark kinetic OAM

AUU+AU

[Penttinen et al. (2000)]

Pure twist-3!

[Ji (1997)]

ALL ALLQuark spin Quark spin

Quark canonical OAM

Quark canonical OAMATT

• Model-dependent• Not intrinsic!

[C.L., Pasquini (2011)][Hatta (2011)]

[C.L. et al. (2012)]

[Burkardt (2007)][Efremov et al.

(2008,2010)][She, Zhu, Ma (2009)][Avakian et al. (2010)][C.L., Pasquini (2011)]

AULTwist-2

Twist-3

OAM and origin dependenceRelative IntrinsicNaive

Transverse center of

momentumPhysical interpretation ?

Depends on proton

position

Equivalence

Intrinsic RelativeNaive

Momentum conservation

Momentum

Fock expansion of the proton state

Fock statesSimultaneous eigenstates

Light-front helicity

Overlap representation

Light-front wave functions

Proton state

Eigenstates of parton light-front helicity

Eigenstates of total OAM

Probability associated with the N, Fock state

Normalization

Fock-state contributions

[C.L. et al. (2012)]

GPDsKinetic OAM

Naive canonical OAM

Canonical OAM

Model calculationsLight-front 3Q

models[C.L., Pasquini

(2011)]

Models are not QCD

Truncation of Fock space spoils Lorentz covariance

In model calculations, one would expect but

[Carbonell, Desplanques, Karmanov, Mathiot (1998)]

Model calculationsUnpold quark in unpold

nucleon [C.L., Pasquini (2011)]

favored

disfavored

Left-right symmetry No net quark OAM

Model calculationsUnpold quark in longitudinally pold

nucleon

Proton spin

u-quark OAM

d-quark OAM

Model calculations

Proton spin

u-quark OAM

d-quark OAM

[C.L. et al. (2012)]Unpold quark in longitudinally pold

nucleon

Quark spin

u-quark OAM

d-quark OAM

Model calculationsLongitudinally pold quark in unpold

Proton spin

u-quark spin

d-quark spin

Model calculationsLongitudinally pold quark in longitudinally pold

Emerging pictureLongitudinal

Transverse

[Burkardt (2005)][Barone et al.

(2008)]

Summary Parton distribution zoo

Twist-2 phase-space distributions

Proton spin puzzle Decompositions

Kinetic versus canonical

Relations with observables

Consistent emerging picture

Incoherent scatteringDVCS vs. SIDIS

DVCS SIDIS

GPDs TMDs

Factorization

Compton form factor

Cross section • process dependent• perturbative

• « universal »• non-perturbative

hard soft

GPDs vs. TMDsGPDs TMDs

Correlator

Dirac matrix

Wilson line

Off-forward! Forward!

FSIISI

e.g. SIDISe.g. DY

LC helicity and canonical spin

LC helicity Canonical spin

Interesting relationsModel

relations

Flavor-dependent

Flavor-independent

Linear relations Quadratic relation

BagLFQSMLFCQM

S DiquarkAV DiquarkCov. Parton

Quark Target

[Jaffe, Ji (1991), Signal (1997), Barone & al. (2002), Avakian & al. (2008-2010)][C.L., Pasquini, Vanderhaeghen (2011)][Pasquini & al. (2005-2008)][Ma & al. (1996-2009), Jakob & al. (1997), Bacchetta & al. (2008)][Ma & al. (1996-2009), Jakob & al. (1997)] [Bacchetta & al. (2008)][Efremov & al. (2009)][Meißner & al. (2007)]

*=SU(6)

Geometrical explanationPreliminaries

Quasi-free quarks Spherical symmetry

Conditions:

Light-front helicity

Canonical spin

Wigner rotation

(reduces to Melosh rotation in case of FREE

quarks)

Geometrical explanationAxial symmetry about

Wigner distributions and quark orbital angular momentum

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