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									                           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



                     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, n  p  e   e
                                  




• Lepton-hadron scattering (semi-leptonic processes)




                   Andrei Afanasev, Lepton Scattering…
       Electroweak Nucleon Form Factors
•   For q2<<M2W,Z scattering matrix element for  n    p
                  GF
        M CC 
          fi           cosc  p( p2 ) | J | n( p1 )   (k2 ) |   (1   5 ) |  (k1 ) 
                                          CC

                    2
                          q                           q             q
         CC
        J       FV1    FV2     5 FA  i 5   FT   5 FP
                          2mN                          mN              mN

•   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




                                       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, http://nuint09.ifae.es
        http://www-boone.fnal.gov/slides-talks/conf-talk/dschmitz/schmitz-nufact09-plenary.pdf
                               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 VNN 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?
     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)

B. 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)

C. 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 Q2 extensions)
       A1n, g2n w/ 12 GeV follow-on (Hall A)        A1p (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       0.1 — 1 fm                             < 0.1 fm
  Nucleons    Constituent quarks                     “bare” quarks
              and glue                               and glue

      S=1/2                      S=1/2                      S=1/2



     Q=1                       Q=1                         Q=1



               Andrei Afanasev, Lepton Scattering…
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
                                  d (  1)  d (  1)
                            APV                            ~ 10  5  10  4
                                  d (  1)  d (  1)
                               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



As is the case for GEn, the                      Q=1                            Q=1
strangeness distribution is
very sensitive to the
nucleon’s properties


                              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 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) 122003
    R.D. Young et al. Phys. Rev. Lett. 97 (2006)
    102002
    D. S. Armstrong et al. (G0 Collaboration), Phys.
    Rev. Lett. 95 (2005) 092001
    A. Acha et al. (HAPPEX Collaboration), Phys.
    Rev. Lett. 98 (2007) 032301


                                 Andrei Afanasev, Lepton Scattering…
    The QpWeak 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                           • Electroweak radiative corrections
       (Scale dependence in MS scheme)                sin2W varies with Q
         0.25

                              Weak Mixing Angle
                      Scale dependence in MS-bar scheme

                                                                                                                         + 
                Uncertainties shown include statistical and systematic

    0.245
                        Semi-Leptonic Sector (published)
                                                                                                       +
                         Semi-Leptonic Sector (proposed)
                                                                          SM
                         Pure Leptonic Sector
                                                                                      • Extracted values of sin2W must agree with
                                                                                        Standard Model or new physics is indicated.
                        E-158 Runs I + II
         0.24             (Preliminary)                    NuTeV

                                                                                                 Q weak  1  4 sin 2 W ~ 0.072
                                                                                                   p
sin2W




                                                                               MSSM
                 QW (APV)
                 (Moves around                                                        • A 4% QpWeak measurement probes for new physics
    0.235        every year or so!)
                                                                                          at energy scales to:
                                                                                                            
                                                                                                                    4.6 TeV
                                                                                                           GF DQ W
                                                                         Z-pole                                   p
         0.23
                                                                                                 g
                     QeWeak
                                            Qweak
                     E-158                  (4% Qp Weak)
                                                                                      • Qpweak (semi-leptonic) and E158 (pure leptonic)
                        Ant icipat ed Final Errors
                                                                                        together make a powerful program to search for
    0.225                                                                               and identify new physics.
                0.001    0.01         0.1        1         10      100      1000

                                      Q (GeV)                             Andrei Afanasev, Lepton Scattering…
                                  DVCS




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

                         Andrei Afanasev, Lepton Scattering…
                     N->Delta transition
•   The ratio E2/M1 as a function of
    Q2




•   CLAS data on M1 at high                  The pion cloud probed at long
    transferred momenta                      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]




                         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)
    192001
    V. Tadevosyan et al., Phys. Rev. C 75
    (2007) 055205
    J. Volmer et al., 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)
014313 and Phys. Rev. Lett. 96 (2006)         •   Illustration of the 12C(e,e'pN)
082501.                                           reaction. The incident electron
R. Subedi et al., Science 320 (2008) 1476         couples to a nucleon-nucleon pair via
and R. Shneor et al., Phys. Rev. Lett. 99         a virtual photon. In the final state,
(2007) 072501.
                                                  the scattered electron is detected
M. M. Sargsian et al., Phys. Rev. C 71
(2005) 044615. and R. Schiavilla et al.,          along with the knocked-out proton,
Phys. Rev. Lett. 98 (2007) 132501                 as well as the correlated partner


                             Andrei Afanasev, Lepton Scattering…
               Spin Structure of a Nucleon




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


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




                        12                                   208Pb
                        C




      A. Gasparian
                     Andrei Afanasev, Lepton Scattering…
                PrimEx-I Result




                                       () = 7.93eV2.3%1.6%




A. Gasparian

               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…
                        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  VM transition (same spin
configuration)            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:                          f1   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…
                      Finding an Exotic Wave
An exotic wave (JPC = 1-+) was generated at level of 2.5 % with 7 other
waves. Events were smeared, accepted, passed to PWA fitter.
                                                   50 0
                                                   50 0

       X(exotic )    3                                 events/20 MeV
                                                                                      generated

Mass
                                                   40 0
                                                   40 0
                                                                                       PWA fit
Input: 1600 MeV
Output: 1598 +/- 3 MeV
                                                   30 0
                                                   30 0


Width
                                                   20 0
                                                   20 0
Input: 170 MeV
Output: 173 +/- 11 MeV
                                                   10 0
                                                   10 0



  Statistics shown here correspond
  to a few days of running.                          0
                                                     0
                                                          1.2
                                                          1.2      1.4
                                                                   1.4       1.6
                                                                             1.6       1.8
                                                                                       1.8
  Double-blind M. C. exercise                                       Mass (3 pions) (GeV)

                                     Andrei Afanasev, Lepton Scattering…
               Generalized Parton Distributions


                         Transverse
                         momentum
 Quark angular           of partons
 momentum                                   Quark spin
                                            distributions


                                                   Form factors
Pion
distribution              GPDs                     (transverse quark
                                                   distributions)
amplitudes




                 Pion             Quark longitudinal
                 cloud            momentum
                                  distributions


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




                                    qq distribution




                         Andrei Afanasev, Lepton Scattering…
          Proton Properties Measured
           in Different Experiments




Elastic Scattering                 DIS                          DES (GPDs)
 transverse quark            longitudinal                    The fully-correlated
    distribution in                                         Quark distribution in
                           quark distribution
                                                            both coordinate and
 Coordinate space        in momentum space                   momentum space
                      Andrei Afanasev, Lepton Scattering…
                                    DVCS
Physics issue:
constrain GPD’s from DVCS                                         XB = 0.45
measurement
           e
           ’       
   e                                                rate low



    p      GPD’s           p

                                                                                    XB =
                                                                                    0.15

Experimental issue:
isolate small DVCS cross
                                                                              Q2
section                                                                       low
Solution for CEBAF Upgrade:
- detect all final state particles
- observe interference term DVCS-BH                   CLAS acceptance for
                                                      DVCS
                           Andrei Afanasev, Lepton Scattering…
           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
                         Andrei Afanasev, Lepton Scattering…
           Hard Meson Electroproduction (o)
Physics issue: map out
GPD’s (need to isolate L)
            e
                    
            ’
   e



       p    GPD’s           p                                      L ~ Q
                                                                   -6



Technique: determine L
from   decay angle
distribution

                                                                   T ~ Q
CLAS at 11 GeV                                                     -8



  400 hrs at L = 1035 cm-
2s-1
                             Andrei Afanasev, Lepton Scattering…
               PrimEx Project @ 12 GeV
 Experimental program
 Precision measurements of:

   Two-Photon Decay Widths:
    Γ(0→), Γ(→),
    Γ(’→)

   Transition Form Factors at          Input to Physics:
    low Q2 (0.001-0.5 GeV2/c2):          precision tests of Chiral
    F(*→ 0), F(* →),                 symmetry and anomalies;

    F(* →)
                                        determination of quark
                                          mass ratio
                                        -’ mixing angle
                                        0, and ’ interaction
                                          electromagnetic radii
                                        is the ’ an approximate
                                           Goldstone boson?



A. Gasparian                Jan 27, 2009
                     PAC34, Afanasev, Lepton Scattering…
                     Andrei
                                                           38
                            Pion Form Factor
  Physics issue:
   electromagnetic structure,
  can be predicted in pQCD


Experimental technique:
isolate *   vertex

           e
           ’
 e
                  

     p            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




                     Andrei Afanasev, Lepton Scattering…

								
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