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Cédric Lorcé IPN Orsay - LPT Orsay Observability of the different proton spin decompositions June 21 2013, University of Glasgow, UK CLAS12 3rd European Workshop

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The outline Summary of the decompositions Gauge-invariant extensions Observability Accessing the OAM Conclusions [C.L. (2013)] [Leader, C.L. (in preparation)] Reviews: Dark spin Quark spin ? ~ 30 %

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[Wakamatsu (2010)] [Ji (1997)] [Jaffe-Manohar (1990)] [Chen et al. (2008)] CanonicalKinetic The decompositions in a nutshell SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq JgJg LqLq Gauge-invariant extension (GIE) Gauge non-invariant!

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[Wakamatsu (2010)] [Ji (1997)] [Jaffe-Manohar (1990)] [Chen et al. (2008)] CanonicalKinetic The decompositions in a nutshell SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq JgJg LqLq Gauge-invariant extension (GIE)

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[Wakamatsu (2010)][Chen et al. (2008)] The Stueckelberg symmetry Ambiguous! [Stoilov (2010)] [C.L. (2013)] SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq Coulomb GIE [Hatta (2011)] [C.L. (2013)] SqSq SgSg LgLg LqLq Light-front GIE L pot SqSq SgSg LgLg LqLq Infinitely many possibilities!

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Gauge GIE1 GIE2 Gauge-variant operator « Natural » gauges Lorentz-invariant extensions ~ Rest Center-of-mass Infinite momentum « Natural » frames The gauge-invariant extension (GIE) [Ji, Xu, Zhao (2012)] [C.L. (2013)]

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The geometrical interpretation [C.L. (2013)] Parallel transport Non-local! Path dependenceStueckelberg dependence

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The semantic ambiguity Path Stueckelberg Background Observables Quasi-observables Quid ? Measurable, physical, gauge invariant and local « Measurable », « physical », gauge invariant but non-local Expansion scheme E.g. cross-sections E.g. parton distributions dependent but fixed by the process E.g. collinear factorization « measurable »« physical » « gauge invariant » Light-front gauge links

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CanonicalKinetic The observability SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq SgSg LgLg LqLq SqSq JgJg LqLq Not observableObservableQuasi-observable [Wakamatsu (2010)] [Ji (1997)] [Jaffe-Manohar (1990)] [Chen et al. (2008)]

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The gluon spin [Jaffe-Manohar (1990)][Hatta (2011)] Light-front GIE Light-front gauge Gluon helicity distribution Local fixed-gauge interpretationNon-local gauge-invariant interpretation « Measurable », gauge invariant but non-local

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The kinetic and canonical OAM Quark naive canonical OAM (Jaffe-Manohar) [Burkardt (2007)] [Efremov et al. (2008,2010)] [She, Zhu, Ma (2009)] [Avakian et al. (2010)] [C.L., Pasquini (2011)] Model-dependent ! Kinetic OAM (Ji) [Ji (1997)] [Penttinen et al. (2000)] [Kiptily, Polyakov (2004)] [Hatta (2012)] but No gluons and not QCD EOM! [C.L., Pasquini (2011)] Pure twist-3 Canonical OAM (Jaffe-Manohar) [C.L., Pasquini (2011)] [C.L., Pasquini, Xiong, Yuan (2012)] [Hatta (2012)]

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Average transverse quark momentum in a longitudinally polarized nucleon [C.L., Pasquini, Xiong, Yuan (2012)] The orbital motion in a model « Vorticity »

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The conclusions Kinetic and canonical decompositions are physically inequivalent and are both interesting Measurability requires gauge invariance but not necessarily locality Jaffe-Manohar OAM and gluon spin are measurable (also on a lattice) [C.L. (2013)] [Leader, C.L. (in preparation)] Reviews:

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Backup slides

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FSIISI The path dependence Orbital angular momentum [C.L., Pasquini, Xiong, Yuan (2012)] [Hatta (2012)] [Ji, Xiong, Yuan (2012)] [C.L. (2013)] Drell-Yan Reference point SIDIS Canonical [Jaffe, Manohar (1990)][Ji (1997)] Kinetic

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The quark orbital angular momentum GTMD correlator [C.L., Pasquini (2011)] Wigner distribution Orbital angular momentum [Meißner, Metz, Schlegel (2009)] Parametrization Unpolarized quark density

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The emerging picture [C.L., Pasquini (2011)] [Burkardt (2005)] [Barone et al. (2008)] LongitudinalTransverse Cf. Bacchetta

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The Chen et al. approach Gauge transformation (assumed) Field strength Pure-gauge covariant derivatives [Chen et al. (2008,2009)] [Wakamatsu (2010,2011)]

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The gauge symmetry Quantum electrodynamics PassiveActive « Physical » [C.L. (2013)] « Background » Active x (Passive) -1 Stueckelberg

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The phase-space picture GTMDs TMDs FFsPDFs Charges GPDs 2+3D 2+1D 2+0D 0+3D 0+1D

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The phase-space distribution Wigner distribution Probabilistic interpretation Expectation value Heisenbergs uncertainty relations Position space Momentum space Phase space Galilei covariant Either non-relativistic Or restricted to transverse position [Wigner (1932)] [Moyal (1949)]

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[Meißner, Metz, Schlegel (2009)] The parametrization @ twist-2 and =0 Parametrization : GTMDs TMDsGPDs MonopoleDipoleQuadrupole Nucleon polarization Quark polarization

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OAM and origin dependence RelativeIntrinsicNaive Transverse center of momentum Physical interpretation ? Depends on proton position Equivalence IntrinsicRelativeNaive Momentum conservation

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Momentum Fock expansion of the proton state Fock states Simultaneous eigenstates of Light-front helicity Overlap representation

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Fock-state contributions Overlap representation [C.L., Pasquini (2011)] [C.L. et al. (2012)] GTMDs TMDs GPDs Kinetic OAM Naive canonical OAM Canonical OAM

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Cédric Lorcé IPN Orsay - LPT Orsay Orbital Angular Momentum in QCD June 27 2013, Dipartimento di Fisica, Universita’ di Pavia, Italy.

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