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XVIII th International Symposium on Spin Physics

Eric Voutier. Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France. GPD s, The Hunt for Q uark O rbital M omentum. ?. The nucleon spin puzzle Generalized parton distributions Deeply virtual Compton scattering Experimental access to E(Q 2 ,x, x ,t)

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XVIII th International Symposium on Spin Physics

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  1. Eric Voutier Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France GPDs, The Hunt for Quark Orbital Momentum ? • The nucleon spin puzzle • Generalized parton distributions • Deeply virtual Compton scattering • Experimental access to E(Q2,x,x,t) • Perspectives • Experimental strategy • Conclusions XVIIIth International Symposium on Spin Physics Charlottesville, October 6-11, 2008

  2. Helicity distributions describing the longitudinal polarization of quarks and antiquarks (known) Gluon polarization (first data) 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 The nucleon spin puzzle Eric Voutier Terra Incognita Longitudinal spin sum rule Orbital Momentum of gluons (unknown) Generalized parton distributions (GPDs) procure the cement of the nucleon spin puzzle, and more generally of the hadron structure. Orbital Momentum of quarks and antiquarks (first glimpse) 1/1 SPIN2008, Charlottesville, October 6-11, 2008

  3. Generalized parton distributions Eric Voutier Parton Imaging • 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. • GPDs = GPDs(Q2,x,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 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. 1/6 SPIN2008, Charlottesville, October 6-11, 2008

  4. GPDs Generalized parton distributions Eric Voutier The GPDs Framework D. Müller et al., FP 42 (1994) 101 A.V. Radyushkin, PRD 56 (1997) 5524 X. Ji, PRL 78 (1997) 610 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 (+8gluon GPDs). P. Hoodbhoy, X. Ji, PRD 58 (1998) 054006 M. Diehl, EPJC 19 (2001) 485 x Initial longitudinal momentum fraction -2x Transferred longitudinal momentum fraction (skewness) (-t)1/2 Momentum transfer to the nucleon • In the forward limit (t  0, x  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. E’s are new and unknown distributions 2/6 SPIN2008, Charlottesville, October 6-11, 2008

  5. GPDs Generalized parton distributions Eric Voutier Form Factors • 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. x independence from Lorentz invariance Transverse spin-flavor dipole moment in an unpolarized nucleon Energy Momentum Tensor 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. Correlation between quark spin and angular momentum in an unpolarized nucleon GPDsunify in the same universal framework parton distributions, form factors, and the spin of the nucleon. 3/6 SPIN2008, Charlottesville, October 6-11, 2008

  6. GPDs Generalized parton distributions Eric Voutier Deep Exclusive Scattering 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 factorization theorem allows to express the cross section for deep exclusive processes as a convolution of a knownhard scattering kernel with an unkownsoft matrix element related to the nucleon structure (GPDs). Factorization Interaction with elementary partons (Q2>>M2) Separation of perturbative and non-perturbative scales (-t<<Q2) Hardness ? GPD(Q2,x,x,t) Probe tagging (x) Production of one additional particle (D) Final stateidentification Exclusivity The key requirementsfor the experimental study of GPDs are luminosityandresolution. 4/6 SPIN2008, Charlottesville, October 6-11, 2008

  7. DVCSDeeply Virtual Compton Scattering DVMPDeeply Virtual Meson Production DA Hard gluon GPDs GPDs Generalized parton distributions Eric Voutier GPDs can be accessed via exclusive reactionsin the Bjorken kinematic regime. Longitudinal response only • 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. • The DVCS process is identified via double (eg) or triple (egN) coincidences, allowing for small scale detectors and large luminosities. • Additionalamplitudes contribute to the reaction process and interfere with the DVCS amplitude. 5/6 SPIN2008, Charlottesville, October 6-11, 2008

  8. GPDs Generalized parton distributions Eric Voutier GPDs enter the cross section of hard scattering processes via Compton form factors, that are integralsover the intermediate parton longitudinal momenta. Q2 >> M2-t << Q2 6/6 SPIN2008, Charlottesville, October 6-11, 2008

  9. Out-of-plane angle entering the harmonic development of the reaction amplitude A.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323 hadronic plane j g e-’ • 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. g* e- leptonic plane p Deeply virtual Compton scattering Eric Voutier Photon Electroproduction P0 = 6 GeV/c Polarization observables help to single-out the DVCS amplitude. 1/5 SPIN2008, Charlottesville, October 6-11, 2008

  10. Twist-2 Twist-3 Deeply virtual Compton scattering Eric Voutier Polarized Beam Cross Section Difference C. Muñoz-Camacho et al., PRL 97 (2006) 262002 DVCS amplitude DVCS / Bethe-Heitler interference amplitude Q2 = 2.3 GeV2 xB = 0.36 Q2 = 2.3 GeV2 xB = 0.36 • The DVCS process is associated with a non-zero polarized beam signal. Imaginary part The sDVCS,I are bilinear and linear combinations of GPDsat±x. 2/5 SPIN2008, Charlottesville, October 6-11, 2008

  11. Twist-2 Twist-3 Gluons Deeply virtual Compton scattering Eric Voutier Unpolarized Cross Section C. Muñoz-Camacho et al., PRL 97 (2006) 262002 Bethe-Heitler amplitude Q2 = 2.3 GeV2 xB = 0.36 Q2 = 2.3 GeV2 xB = 0.36 DVCS amplitude DVCS / Bethe-Heitler interference amplitude • The magnitude of the DVCS process is seen as a deviation from the pure BH cross section. Real part ThecDVCS,I are bilinearandlinearcombinations of Compton form factors. 3/5 SPIN2008, Charlottesville, October 6-11, 2008

  12. Deeply virtual Compton scattering Eric Voutier E00-110  p-DVCS P.Y. Bertin, C.E. Hyde, R. Ransome, F. Sabatié et al. C. Muñoz-Camacho et al., PRL 97 (2006) 262002 • Twist-twocontributionsdominatethe DVCS cross section and areQ2 independent in the measured kinematics range. • Twist-three contributions to the cross section aresmalland areQ2 independentwithin error bars. Experimental results support the dominance of the handbag diagram at Q2 as low as 2 GeV2. 4/5 SPIN2008, Charlottesville, October 6-11, 2008

  13. Deeply virtual Compton scattering Eric Voutier Beam Spin Asymmetry F.-X. Girod et al., PRL 100 (2008) 162002 @ CLAS (5.77 GeV) GPDs Twist-3 JML / Regge • 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. GPDs Twist-2 F.-X. Girod talk in GPD parallel session The angular dependence of the asymmetries is compatible with expectations from leading-twist dominance. 5/5 SPIN2008, Charlottesville, October 6-11, 2008

  14. j Experimental access to E(Q2,x,x,t) Eric Voutier DVCS Transverse Target Asymmetry A.V. Belitsky, D. Müller, A. Kirchner, NPB 629 (2002) 323 M. Diehl, S. Sapeta, EPJC 41 (2005) 515 Twist-2 F. Ellinghaus,W.-D. Nowak, A.V. Vinnikov, Z. Ye EPJ C46 (2006) 729 Z. Ye, Doctorat Thesis, Universität Hamburg (2006) B. Zihlmann talk in GPD parallel session 1/6 SPIN2008, Charlottesville, October 6-11, 2008

  15. Transverse Target Asymmetry 2Ju-Jd xB Experimental access to E(Q2,x,x,t) Eric Voutier DVMP Transverse Target Asymmetry K. Goeke, M. Polyakov, M. Vanderhaeghen, PPNP 47 (2001) 401 S.V. Goloskokov, P. Kroll, arXiv/hep-ph:0809.4126 • 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. Transverse Target Asymmetry 2Ju+Jd xB Transversally polarized proton targets are of primary importance for the determination of E(Q2,x,x,t). 2/6 SPIN2008, Charlottesville, October 6-11, 2008

  16. @ CLAS (5.74 GeV) Experimental access to E(Q2,x,x,t) Eric Voutier DVMP Longitudinal Cross Section S.A. Morrow et al., arXiv/hep-ex:0807.3834 VGG++ / GPDs • 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. VGG / GPDs JML / Regge GK / GPDs The question is asked whether current GPD parametrizations must be revisited or DVMP cannot be interpreted GPD wise in the valence region? 3/6 SPIN2008, Charlottesville, October 6-11, 2008

  17. Experimental access to E(Q2,x,x,t) Eric Voutier Neutron Target Neutron targets provide new linear combinations of GPDs From polarized beam cross section difference Suppressed because F1(t) is small Suppressed because of cancellation between u and d quarks Neutron and proton targets are sensitive to the u quark flavor and, following isospin symmetry, appear then complementary. Neutron targets allow to access the least known and constrainedGPD that appears in the nucleon spin sum rule. 4/6 SPIN2008, Charlottesville, October 6-11, 2008

  18. M. Mazouz et al., PRL 99 (2007) 242501 S. Ahmad et al., PR D75 (2007) 094003 M. Vanderhaeghen et al., PR D60 (1999) 094017 Experimental access to E(Q2,x,x,t) Eric Voutier E03-106 n-DVCS P.Y. Bertin, C.E. Hyde, F. Sabatié, E. Voutier et al. M. Mazouz et al., PRL 99 (2007) 242501 Impulse approximation Twist-2 Q2 = 1.9 GeV2 xB = 0.36 • 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. 5/6 SPIN2008, Charlottesville, October 6-11, 2008

  19. Experimental access to E(Q2,x,x,t) Eric Voutier Model Dependent Quark Angular Momenta d A. Airapetian et al., arXiv/hep-ex:0802.2499 Measurements off protonare sensitive toJu (uquark in theproton) M. Mazouz et al., PRL 99 (2007) 242501 Measurements off neutron are sensitive to Jd (uquark in theneutron) A.W. Thomas, arXiv/hep-ph:0803.2775 u Neutrons are a mandatory step in the hunt for the quark orbital momentum. 6/6 SPIN2008, Charlottesville, October 6-11, 2008

  20. Perspectives Eric Voutier E08-021  -DVCS H. Avakian, V. Burkert, M. Guidal, R. Kaiser, F. Sabatié et al. A. Sandorfi et al., Proc. of the Workshop on Exclusive Reactions, Edts. P. Stoler and A. Radyushkin, World Scientific (2008) 425. 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 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 heating Low 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. G. Thorn talk in Polarized Sources & Targets parallel session 1/6 SPIN2008, Charlottesville, October 6-11, 2008

  21. Perspectives Eric Voutier E07-007 & E08-025  p-DVCS & n-DVCS P.Y. Bertin, C.E. Hyde, C. Muñoz Camacho, J. Roche et al. A. Camsonne, C.E. Hyde, M. Mazouz et al. A.V. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323 Twist-2 • 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. -0.36 Within the VGG model, the neutron pure DVCS amplitude is sensitive to E . 2/6 SPIN2008, Charlottesville, October 6-11, 2008

  22. Access to the imaginary part of H from the target spin asymmetry. Twist-2 Twist-3 Gluons Perspectives Eric Voutier E05-114  -DVCS A. Biselli, L. Elouadrihri, K.Joo, S. Nicolai et al. A.V. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323 Access to another Compton form factor, that is anotherlinear combination of GPDs Access to the real part from double polarization observable. A. Biselli and H. Egiyan talks in GPD parallel session 3/6 SPIN2008, Charlottesville, October 6-11, 2008

  23. xB Perspectives Eric Voutier The GPDs World • 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. One may expect to get a reasonable mapping of the GPDs in the intermediate and valenceregions by 2020. R. Kaiser talk in GPD parallel session 4/6 SPIN2008, Charlottesville, October 6-11, 2008

  24. Perspectives Eric Voutier GPDs @ JLab 12 GeV • Several experiments will measure DVCS (Hall A & B) and DVMP (Hall B) with (un)polarizedbeam 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’gn)p D(e,e’gn)p 5/6 SPIN2008, Charlottesville, October 6-11, 2008

  25. μ’  μ p’ Perspectives Eric Voutier GPDs @ COMPASS • The GPDs program is part of the COMPASS Phase II (2010-2015) proposal to be submitted to CERN in 2008-2009. • 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 polarizedNH3 target inserted in the RPD or a new SciFi (?) RPD inserted in the existing NH3 target system. Step 1 (~2011) to constrain H d(+, ) +d(-, )  Im(F1H) sin  d(+, ) -d(-, )  Re(F1H) cos  Step 2 (~2013) to constrain E d(, S) -d(, S+π)  Im(F2H – F1E) sin (- S) cos  6/6 SPIN2008, Charlottesville, October 6-11, 2008

  26. Experimental strategy Eric Voutier 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 (s), single and double polarized cross section differences (Dsij) 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. Imaginary parts Real parts Q2 = 2.3 GeV2 xB = 0.36 Current s and Dsz0 data suggest that VGG predictions underestimate slightly the imaginary part of H and miss the real part. It remains a theoretical concern to extract GPDs from CFFs. A.V. Belitsky, D. Müller, arXiv/hep-ph:0809.2890 1/1 SPIN2008, Charlottesville, October 6-11, 2008

  27. Conclusions 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. • The exploration of the GPD world has been starting at DESY & JLaband will continue at JLab12 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. Together with the evolution of lattice QCD calculation capabilities, the road towards a comprehensive and quantitative understanding of the nucleon structure is OPEN!! Special thanks to François-Xavier Girod, Michel Guidal, Nicole d’Hose, Malek Mazouz, Andy Sandorfi 1/2 SPIN2008, Charlottesville, October 6-11, 2008

  28. Perspectives Eric Voutier 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 g experiments – (Mar’10, Aug’10) • Installation in Hall B – (Jul’10, Sep’10) • g run – (Sep’10, Apr’11) • Electron beam tests – (Apr’11) • e run – (Nov’11, Dec’11) HDice Lab Hall B Courtesy of A. Sandorfi SPIN2008, Charlottesville, October 6-11, 2008

  29. Perspectives Eric Voutier Unpolarized Cross Section • Experimental data are taken for LH2 and LD2 targets at 4.82 GeV/c and 6.00 GeV/c. • The LD2-LH2yields binned in (k, Mx2, j, t) are fitted with Monte-Carlo simulated yields taking into account experimental and radiative effects. P R O J E C T E D D A T A Projected systematics SPIN2008, Charlottesville, October 6-11, 2008

  30. Perspectives Eric Voutier n-DVCS @ CLAS 12 INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble, University of Glasgow … … European initiative towards the central detector … • The possibility of developing neutron detection capabilities in the central region of CLAS12 is forseen. 5 T @ target D(e,e’gn)p ToF scintillators Vertex detector Neutron detector A. El Alaoui (LPSC Grenoble) (2008) • 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. SPIN2008, Charlottesville, October 6-11, 2008

  31. j Perspectives Eric Voutier Polarized Neutron Target A. Belitsky, D. Müller, A. Kirchner, NP B629 (2002) 323 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. Longitudinal Target Spin Asymmetry Transverse Target Spin Asymmetry The most sensitive coefficient to Eappears to originate from the pure DVCS amplitude while the kinematical factors enhance H in the other coefficients. SPIN2008, Charlottesville, October 6-11, 2008

  32. Perspectives Eric Voutier CLAS12 Forward Calorimeter Preshower Calorimeter Forward Cerenkov (LTCC) Forward Time-of-Flight Detectors Forward Drift Chambers Superconducting Torus Magnet Inner Cerenkov (HTCC) Central Detector Beamline Instrumentation Inner Calorimeter SPIN2008, Charlottesville, October 6-11, 2008

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