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

Andrei Afanasev, Lepton Scattering… 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

Andrei Afanasev, Lepton Scattering… Electroweak Form Factors of a Nucleon Weak process: β-decay, Lepton-hadron scattering (semi-leptonic processes)

Andrei Afanasev, Lepton Scattering… Electroweak Nucleon Form Factors For q 2 <<M 2 W,Z scattering matrix element for GF- Fermi constant; CVC relates vector-current form factors F1V, F2V to nucleon EM form factors: F 1,2 V =F 1,2p -F 1,2n ; relations for neutral current involve weak mixing angle and may be non-trivial in presence of isospin-zero constituents (strange quarks) Scattering cross section

Andrei Afanasev, Lepton Scattering… 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, MiniBooNE Collab are active at Fermilab Further reading: NuINT’09 Proceedings,

Andrei Afanasev, Lepton Scattering… Highlights of Jefferson Lab’s research on electron-hadron scattering

Andrei Afanasev, Lepton Scattering… Jefferson Lab is Located in Newport News, Virginia, USA

Andrei Afanasev, Lepton Scattering… Accelerator: CEBAF

Andrei Afanasev, Lepton Scattering… Experimental Hall A

Andrei Afanasev, Lepton Scattering… Experimantal: CLAS Detector (Hall B)

Andrei Afanasev, Lepton Scattering… Experimental Hall C,G0,HKS

Andrei Afanasev, Lepton Scattering… 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?

Andrei Afanasev, Lepton Scattering… JLab Scientific “Campaigns” The Structure of the Nuclear Building Blocks 1.How are the nucleons made from quarks and gluons? 2.What are the mechanism of confinement and the dynamics of QCD? 3.How does the NN Force arise from the underlying quark and gluon structure of hadronic matter? The Structure of Nuclei 4.What is the structure of nuclear matter? 5.At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident? Symmetry Tests in Nuclear Physics 6.Is the “Standard Model” complete? What are the values of its free parameters?

Andrei Afanasev, Lepton Scattering… How are the Nucleons Made from Quarks and Gluons? Why are nucleons interacting via V NN such a good approximation to nature? How do we understand QCD in the confinement regime? A.What are the spatial distributions of u, d, and s quarks in the hadrons? G E p /G M p (3 techniques); G E n (2 expts in Hall C; higher Q 2 coming) G M n (Hall A; CLAS to high Q 2 ) G M n to high Q 2 (CLAS) HAPPEX, G0 forward angle, w/ G0 backward angle & HAPPEX II F  (new data to 5.75 GeV; w/ future extension at 12 GeV) B.What is the excited state spectrum of the hadrons, and what does it reveal about the underlying degrees of freedom?  (All three halls) Higher resonances (CLAS :     production) Missing resonance search (CLAS e1 and g1:  production VCS in the resonance region (Hall A) C.What is the QCD basis for the spin structure of the hadrons? Q 2 evolution of GDH integral and integrand for: proton (CLAS) and neutron (Hall A) (w/ low Q 2 extensions) A 1 n, g 2 n w/ 12 GeV follow-on (Hall A) A 1 p (Hall C, CLAS) D.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)

Andrei Afanasev, Lepton Scattering… The Proton (and Neutron) are the “Hydrogen Atoms” of QCD What we “see” changes with spatial resolution >1 fm Nucleons 0.1 — 1 fm Constituent quarks and glue Q = 1 S=1/2 Q = 1 < 0.1 fm “bare” quarks and glue S=1/2 Q = 1

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

Andrei Afanasev, Lepton Scattering… 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 Q 2 ). 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) R.D. Young et al. Phys. Rev. Lett. 97 (2006) D. S. Armstrong et al. (G0 Collaboration), Phys. Rev. Lett. 95 (2005) A. Acha et al. (HAPPEX Collaboration), Phys. Rev. Lett. 98 (2007) Phys. Rev. Lett. 99 (2007) Phys. Rev. Lett. 97 (2006) Phys. Rev. Lett. 95 (2005) Phys. Rev. Lett. 98 (2007)

Andrei Afanasev, Lepton Scattering… Electroweak radiative corrections  sin 2  W varies with Q + +  Extracted values of sin 2  W must agree with Standard Model or new physics is indicated. A 4% Q p Weak measurement probes for new physics at energy scales to: Q p weak (semi-leptonic) and E158 (pure leptonic) together make a powerful program to search for and identify new physics. The Q p Weak Experiment 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) Q (GeV) sin 2  W

Andrei Afanasev, Lepton Scattering… DVCS DVCS cross section results for one of twelve kinematics bins measured in Hall A E 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)

Andrei Afanasev, Lepton Scattering… N->Delta transition The ratio E2/M1 as a function of Q 2 CLAS data on M1 at high transferred momenta The pion cloud probed at long wavelengths. b) The nucleon core probed at high Q 2 (high resolution) M.Ungaro et al, PRL 97:112003,2006 K. Joo et al, PRL 88:122001,2002]

Andrei Afanasev, Lepton Scattering… 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 pion electroproduction 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) V. Tadevosyan et al., Phys. Rev. C 75 (2007) J. Volmer et al., Phys. Rev. Lett. 86 (2001) 1713Phys. Rev. Lett. 97 (2006) Phys. Rev. C 75 (2007) Phys. Rev. Lett. 86 (2001) 1713 The pion form factor in leading order pQCD

Andrei Afanasev, Lepton Scattering… 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. Egiyan et al., Phys. Rev. C 68 (2003) and Phys. Rev. Lett. 96 (2006) R. Subedi et al., Science 320 (2008) 1476 and R. Shneor et al., Phys. Rev. Lett. 99 (2007) M. M. Sargsian et al., Phys. Rev. C 71 (2005) and R. Schiavilla et al., Phys. Rev. Lett. 98 (2007) Phys. Rev. C 68 (2003) Phys. Rev. Lett. 96 (2006) Science 320 (2008) 1476Phys. Rev. Lett. 99 (2007) Phys. Rev. C 71 (2005) Phys. Rev. Lett. 98 (2007) Illustration of the 12 C(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

Andrei Afanasev, Lepton Scattering… 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.

Andrei Afanasev, Lepton Scattering… PrimEx-I Experiment: Γ(  0  ) Decay Width Nuclear targets: 12 C and 208 Pb; 6 GeV Hall B tagged beam; experiment performed in C 208 Pb A. Gasparian

Andrei Afanasev, Lepton Scattering… A. Gasparian PrimEx-I Result  (  ) = 7.93eV  2.3%  1.6%

Andrei Afanasev, Lepton Scattering… 12-GeV Upgrade at JLab

Andrei Afanasev, Lepton Scattering… Experimental Halls

Andrei Afanasev, Lepton Scattering… 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

Andrei Afanasev, Lepton Scattering… First excited state of flux tube has J=1 combined with S=1 for quarks Photons couple to exotic mesons via  VM transition (same spin configuration) Excited Flux Tube Quantum Numbers J PC = exotic (mass ~ 1.7 – 2.3 GeV) Normal mesons: J PC =

Andrei Afanasev, Lepton Scattering… 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: f 1  KK  KK  b 1    high data acquisition rate Perform partial-wave analysis identify quantum numbers as a function of mass check consistency of results in different decay modes

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

Andrei Afanasev, Lepton Scattering… G eneralized P arton D istributions GPDs Transverse momentum of partons Quark spin distributions Form factors (transverse quark distributions) Quark longitudinal momentum distributions Pion cloud Pion distribution amplitudes Quark angular momentum

Andrei Afanasev, Lepton Scattering… GPDs Contain Much More Information than DIS Quark distribution q(x) qq distribution Antiquark distribution q(x) DIS only measures a cut at  =0

Andrei Afanasev, Lepton Scattering… 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

Andrei Afanasev, Lepton Scattering… DVCS e e’e’ p p Physics issue: constrain GPD’s from DVCS measurement Experimental issue: isolate small DVCS cross section Solution for CEBAF Upgrade: - detect all final state particles - observe interference term DVCS-BH X B = 0.45 X B = 0.15 CLAS acceptance for DVCS rate low Q 2 low GPD’s 

Andrei Afanasev, Lepton Scattering… DVCS Single-Spin Asymmetry Q 2 = (2.9 – 3.1) GeV 2 W = (2.65 – 2.95) GeV -t = (0.2 – 0.4) GeV 2 CLAS experiment E 0 = 11 GeV P e = 80% L = cm -2 s -1 Run time: 500 hrs

Andrei Afanasev, Lepton Scattering… Hard Meson Electroproduction (  o ) e’e’ p p  GPD’s e Physics issue: map out GPD’s (need to isolate  L ) Technique: determine  L from  decay angle distribution CLAS at 11 GeV 400 hrs at L = cm - 2 s -1  L ~ Q -6  T ~ Q -8

Andrei Afanasev, Lepton Scattering… A. GasparianPAC34, Jan 27, PrimEx 12 GeV Experimental program Precision measurements of:  Two-Photon Decay Widths: Γ(  0 →  ), Γ(  →  ), Γ(  ’ →  )  Transition Form Factors at low Q 2 ( GeV 2 /c 2 ): F(  * →  0 ), F(  * →  ), F(  * →  ) 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?

Andrei Afanasev, Lepton Scattering… Pion Form Factor Physics issue:  electromagnetic structure, can be predicted in pQCD Experimental technique: isolate  *  vertex e’e’ p  e n JLab Upgrade: - use HMS to detect e’ - use SHMS to detect 

Andrei Afanasev, Lepton Scattering… Even longer-term future: Electron- Ion Collider

Andrei Afanasev, Lepton Scattering… 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