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HEP School, January 11-13, 2010, Valparaiso, Chile

Part 3. Lepton Scattering as a Probe of Hadronic Structure. Andrei Afanasev Jefferson Lab/Hampton U USA. HEP School, January 11-13, 2010, Valparaiso, Chile. Plan of the talk. Electroweak form factors and parity-violating electron scattering JLAB research highlights

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HEP School, January 11-13, 2010, Valparaiso, Chile

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  1. Part 3 Lepton Scattering as a Probe of Hadronic Structure Andrei Afanasev Jefferson Lab/Hampton U USA HEP School, January 11-13, 2010, Valparaiso, Chile

  2. Plan of the talk • Electroweak form factors and parity-violating electron scattering • JLAB research highlights • Form factors of mesons • Spin structure of a nucleon: DIS, SIDIS, VCS • Excitation of baryon resonances • Short-range nucleon-nucleon correlations • Primakoff production of mesons • Search for mesons with exotic quantum numbers • Conclusions and outlook

  3. Electroweak Form Factors of a Nucleon • Weak process: β-decay, • Lepton-hadron scattering (semi-leptonic processes)

  4. Electroweak Nucleon Form Factors • For q2<<M2W,Z scattering matrix element for • GF- Fermi constant; CVC relates vector-current form factors F1V, F2V to nucleon EM form factors: F1,2V=F1,2p-F1,2n ; relations for neutral current involve weak mixing angle and may be non-trivial in presence of isospin-zero constituents (strange quarks) • Scattering cross section

  5. Data on Neutrino-Nucleon (Quasi)Elastic Scattering • Experiments at ANL,BNL,FNAL,CERN,IHEP: Lyubushkin et al (NOMAD Collab), Eur.Phys.J.C 63, 355 (2009) and references therein; MINERVA, MiniBooNECollab are active at Fermilab Further reading: NuINT’09 Proceedings, http://nuint09.ifae.es http://www-boone.fnal.gov/slides-talks/conf-talk/dschmitz/schmitz-nufact09-plenary.pdf

  6. Highlights of Jefferson Lab’s research on electron-hadron scattering

  7. Jefferson Lab is Located in Newport News, Virginia, USA

  8. Accelerator: CEBAF

  9. Experimental Hall A

  10. Experimantal: CLAS Detector (Hall B)

  11. Experimental Hall C ,G0,HKS

  12. JLab’s Scientific Mission • How are the hadrons constructed from the quarks and gluons of QCD? • What is the QCD basis for the nucleon-nucleon force? • Where are the limits of our understanding of nuclear structure? • To what precision can we describe nuclei? • To what distance scale can we describe nuclei? • Where does the transition from the nucleon-meson to the QCD description occur? To make progress toward these research goals we must address critical issues in “strong QCD”: • What is the mechanism of confinement? • Where does the dynamics of the q-q interaction make a transition from the strong (confinement) to the perturbative (QED-like) QCD regime? • How does Chiral symmetry breaking occur?

  13. JLab Scientific “Campaigns” The Structure of the Nuclear Building Blocks • How are the nucleons made from quarks and gluons? • What are the mechanism of confinement and the dynamics of QCD? • How does the NN Force arise from the underlying quark and gluon structure of hadronic matter? The Structure of Nuclei • What is the structure of nuclear matter? • At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident? Symmetry Tests in Nuclear Physics • Is the “Standard Model” complete? What are the values of its free parameters?

  14. How are the Nucleons Made from Quarksand Gluons? Why are nucleons interacting via VNN such a good approximation to nature? How do we understand QCD in the confinement regime? • What are the spatial distributions of u, d, and s quarks in the hadrons? GEp/GMp(3 techniques); GEn(2 expts in Hall C; higher Q2 coming) GMn (Hall A; CLAS to high Q2) GMn to high Q2 (CLAS) HAPPEX, G0 forward angle, w/ G0 backward angle & HAPPEX II F (new data to 5.75 GeV; w/ future extension at 12 GeV) • What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom? ND (All three halls) Higher resonances (CLAS : , 0,  production)Missing resonance search (CLAS e1 and g1: ,  production VCS in the resonance region (Hall A) • What is the QCD basis for the spin structure of the hadrons? Q2 evolution of GDH integral and integrand for:proton (CLAS) and neutron (Hall A) (w/ low Q2extensions)A1n, g2n w/ 12 GeV follow-on (Hall A) A1p(Hall C, CLAS) • What can other hadron properties tell us about ‘strong’ QCD? VCS (Hall A) Separated Structure Functions (Hall C) DVCS (CLAS, Hall A & CLAS coming)Single Spin Asymmetries (CLAS, Hall A)Compton Scattering (Hall A)

  15. The Proton (and Neutron) arethe “Hydrogen Atoms” of QCD What we “see” changes with spatial resolution 0.1 — 1 fm Constituent quarks and glue < 0.1 fm “bare” quarks and glue >1 fm Nucleons S=1/2 S=1/2 S=1/2 Q = 1 Q = 1 Q = 1

  16. Measurements of the Strange Quark Distribution Provide a Unique New Window into Hadron Structure Spatial parity is violated due to Z-exchange Parity-violating spin asymmetry Unlike GEn, the ss pairs come uniquely from the sea; there is no “contamination” from pre-existing u or d quarks S=1/2 S=1/2 Q = 1 Q = 1 As is the case for GEn, the strangeness distribution is very sensitive to the nucleon’s properties

  17. Parity-Violating Electron Scattering • The worldwide program of parity violating electron scattering data that constrain the contributions of strange quarks to the proton’s charge and magnetism at large spatial distances (low Q2). The solid ellipse represents a fit to the data shown, incorporating a theoretical prediction for the proton’s axial form factor (GA), which is not yet well-constrained experimentally. The dashed ellipse incorporates more data at shorter spatial distances and removes the theoretical constraint on the axial term. • R. D. Young, R. D. Carlini, A. W. Thomas and J. Roche, Phys. Rev. Lett. 99 (2007) 122003R.D. Young et al. Phys. Rev. Lett. 97 (2006) 102002D. S. Armstrong et al. (G0 Collaboration), Phys. Rev. Lett. 95 (2005) 092001A. Acha et al. (HAPPEX Collaboration), Phys. Rev. Lett. 98 (2007) 032301

  18. The QpWeakExperiment The First Measurement of the Weak Charge of the Proton; a Precision Test of the Standard Model via a 10 Measurement of the Predicted Running of the Weak Coupling Constant, and a Search for Evidence of New Physics Beyond the Standard Model at the TeV Scale Weak Mixing Angle (Scale dependence in MS scheme) • Electroweak radiative corrections • sin2Wvaries with Q +  + • Extracted values of sin2W mustagree with • Standard Model or new physics is indicated. • A 4% QpWeak measurement probes for new physics at energy scales to: • Qpweak (semi-leptonic) and E158 (pure leptonic)together make a powerful program to search for and identify new physics. sin2W Q (GeV)

  19. DVCS • DVCS cross section results for one of twelve kinematics bins measured in Hall A E00-110. • A model-dependent extraction of up- and down-quark contributions (orbital angular momentum plus spin) to the spin of the proton (Hall A E03-106)

  20. N->Delta transition • The ratio E2/M1 as a function of Q2 • CLAS data on M1 at high transferred momenta The pion cloud probed at long wavelengths. b) The nucleon core probed at high Q2 (high resolution) M.Ungaro et al, PRL 97:112003,2006 K. Joo et al, PRL 88:122001,2002]

  21. Pion Form Factor • Pion form factor results from the two JLab Hall C experiments. Also shown are e-pi elastic data from CERN and earlier pionelectroproduction data from DESY. The curves are from a Dyson-Schwinger equation (Maris and Tandy, 2000), QCD sum rule (Nesterenko, 1982), constituent quark model (Hwang, 2001), and a pQCD calculation (Bakulev, 2004). • T. Horn et. al., Phys. Rev. Lett. 97 (2006) 192001V. Tadevosyan et al., Phys. Rev. C 75 (2007) 055205J. Volmer et al., Phys. Rev. Lett. 86 (2001) 1713 • The pion form factor in leading order pQCD

  22. NN Short Range Correlations The nucleus can often be approximated as an independent collection of protons and neutrons confined in a volume, but for short periods of time, the nucleons in the nucleus can strongly overlap. This quantum mechanical overlapping, known as a nucleon-nucleon short-range correlation, is a manifestation of the nuclear strong force, which produces not only the long-range attraction that holds matter together, but also the short-range repulsion that keeps it from collapsing. K. S. Egiyanet al., Phys. Rev. C 68 (2003) 014313 and Phys. Rev. Lett. 96 (2006) 082501.R. Subediet al., Science 320 (2008) 1476 and R. Shneoret al., Phys. Rev. Lett. 99 (2007) 072501.M. M. Sargsianet al., Phys. Rev. C 71 (2005) 044615. and R. Schiavillaet al., Phys. Rev. Lett. 98 (2007) 132501 • Illustration of the 12C(e,e'pN) reaction. The incident electron couples to a nucleon-nucleon pair via a virtual photon. In the final state, the scattered electron is detected along with the knocked-out proton, as well as the correlated partner

  23. Spin Structure of a Nucleon • Improvement on the gluon polarization ∆. Solid (dashed) lines: uncertainty on ∆ before (after) the JLab data. • Large-x JLab data on quark polarizations. The solid lines include quark orbital anglar momentum while the dashed lines do not.

  24. PrimEx-I Experiment: Γ(0) Decay Width Nuclear targets: 12C and 208Pb; 6 GeV Hall B tagged beam; experiment performed in 2004 12C 208Pb

  25. PrimEx-I Result () = 7.93eV2.3%1.6%

  26. 12-GeV Upgrade at JLab

  27. Experimental Halls

  28. Search for Exotic Mesons: Basic idea Color field: due to self interaction, confining flux tubes form between static color charges Original idea by Nambu, now verified by Lattice QCD calculations Excitation of the flux tube can lead to exotic quantum numbers

  29. Excited Flux Tube Quantum Numbers Normal mesons:JPC = 0-+1+-2-+ First excited state of flux tube has J=1 combined with S=1 for quarks JPC = 0-+ 0+-1+- 1-+2-+ 2+- exotic (mass ~ 1.7 – 2.3 GeV) Photons couple to exotic mesons via g VM transition(same spin configuration)

  30. Strategy for Exotic Meson Search • Use photons to produce meson final states • tagged photon beam with 8 – 9 GeV • linear polarization to constrain production mechanism • Use large acceptance detector • hermetic coverage for charged and neutral particles • typical hadronic final states:f1h KKh KKppp b1p wp pppp rp ppp • high data acquisition rate • Perform partial-wave analysis • identify quantum numbers as a function of mass • check consistency of results in different decay modes

  31. An exotic wave (JPC = 1-+) was generated at level of 2.5 % with 7 other waves. Events were smeared, accepted, passed to PWA fitter. Mass Input: 1600 MeV Width Input: 170 MeV Output: 1598 +/- 3 MeV Statistics shown here correspondto a few days of running. Double-blind M. C. exercise Output: 173 +/- 11 MeV Finding an Exotic Wave

  32. Generalized Parton Distributions Transverse momentum of partons Quark angular momentum Quark spin distributions GPDs Form factors (transverse quark distributions) Pion distribution amplitudes Pion cloud Quark longitudinal momentum distributions

  33. GPDs Contain Much More Information than DIS Quark distribution q(x) DIS only measures a cut at =0 Antiquark distribution q(x) qq distribution

  34. Proton Properties Measured in Different Experiments Elastic Scattering transverse quark distribution in Coordinate space DIS longitudinal quark distribution in momentum space DES (GPDs) The fully-correlated Quark distribution in both coordinate and momentum space

  35. DVCS Physics issue: constrain GPD’s from DVCS measurement XB = 0.45 e’ g rate low e p GPD’s p XB = 0.15 Experimental issue: isolate small DVCS cross section Q2 low Solution for CEBAF Upgrade: - detect all final state particles - observe interference term DVCS-BH CLAS acceptance for DVCS

  36. DVCS Single-Spin Asymmetry Q2 = (2.9 – 3.1) GeV2 W = (2.65 – 2.95) GeV -t = (0.2 – 0.4) GeV2 CLAS experiment E0 = 11 GeV Pe = 80% L = 1035 cm-2s-1 Run time: 500 hrs

  37. Hard Meson Electroproduction (ro) Physics issue: map out GPD’s (need to isolate sL) e’ r e p GPD’s p sL ~ Q -6 Technique: determine sL from r pp decay angle distribution sT ~ Q -8 CLAS at 11 GeV 400 hrs at L = 1035 cm-2s-1

  38. PAC34, Jan 27, 2009 Experimental program Precision measurements of: Two-Photon Decay Widths: Γ(0→), Γ(→), Γ(’→) Transition Form Factors at low Q2 (0.001-0.5 GeV2/c2): F(*→0), F(* →), F(* →) PrimEx Project @ 12 GeV Input to Physics: • precision tests of Chiral symmetry and anomalies; • determination of quark mass ratio • -’ mixing angle • 0, and ’ interaction electromagnetic radii • is the ’ an approximate Goldstone boson?

  39. Pion Form Factor Physics issue: p electromagnetic structure, can be predicted in pQCD Experimental technique: isolate g* p pvertex e’ e p n p JLab Upgrade: - use HMS to detect e’ - use SHMS to detect p

  40. Even longer-term future: Electron-Ion Collider

  41. Summary and Outlook • Presented a comprehensive program on hadronic structure studies with lepton probes (see also J.Soffer’s lectures on Deep-Inelastic Scattering) • Very active research program at Jefferson Lab • JLAB 12-GeV upgrade will extend physics reach and provide new info on hadronic structure and strong interaction dynamics • A longer-term future project: Electron-Ion Collider under discussion

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