Baryon Resonance Form Factors _ CLAS

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					Baryon Resonance Form Factors
           @ CLAS

          Maurizio Ungaro
      University of Connecticut
        Jefferson Laboratory



               Menu 2007, Jülich, Germany, September 10-14, 1007
Overview

N* Program @ CLAS

Exclusive Processes: results, ongoing analysis

Conclusions
    Why excited baryons are important



 Since the discovery of the (1232) in 1951, many N∗’s
  have been identified (mainly through amplitude
  analyses of pion-nucleon elastic scattering).

 This effort peaked in the 1970’s with fairly consistent
  results from several independent groups.

 A longstanding challenge in hadronic physics is to
  understand the spectroscopy of these resonant
  states, including their electromagnetic and strong decays,
  within a framework consistent with QCD.
Quark orbital angular momentum
                                    SU(6)SF x O(3) Classification of Baryons


                                                           F15(1680)




                                         S11(1535)

                                                             D13(1520)



                                 P33(1232)             P11(1440)




                                             Harmonic Oscillator-Potential - Principal Energy Levels
CLAS     Inclusive Electron Scattering
       p(e,e’)X

                                                       (GE, GM)




                                                               N(1680)
                                                     N(1440)
                                           D(1232)




                                                               D(1620)
                                                               N(1520)
                                                               N(1535)
    In contrast to elastic scattering, resonances cannot
   be uniquely separated in inclusive scattering → need to
   measure exclusive processes.
CLAS   Exclusive Electron Scattering

              p(e,e’p)X
       Exclusive Processes in N*
CLAS
                Studies

                         p(e,e’)X




                       Hadronic mass
 Electromagnetic Excitation of N*’s
The experimental N* Program has two major components:

1) Accurate measurements of transition form factors (A3/2, A1/2,
S1/2) of known states as photon virtuality (Q2) to probe their
internal structure and confining mechanism
          e’
                                                   p, h, pp,..
      e            γv
                                                                      lgp=1/2
                              N*,△
                                                                 gv             N
               N                              N’
                        A3/2, A1/2, S1/2                              lgp=3/2
                        Ml+/-, El+/-, Sl+/-


2) Search for undiscovered new baryon states.

Both parts of the program are being pursued in various decay
channels, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using cross sections
and polarization observables.
       CEBAF at Jefferson Lab

                                Emax          ~ 6 GeV
                                Imax          ~ 200 mA
                                Duty Factor   ~ 100%
                                 sE/E          ~ 2.5 10-5
                                Beam P         ~ 80%
                                Eg(tagged)      ~ 0.8- 5.5
                                GeV




CLAS
                                  A      B         C
CEBAF Large Acceptance Spectrometer
               (CLAS)



                                 •Six identical sectors
                         /
                     e           •5 T toroidal B-field
                                 •Δθ=15-140 degrees
                                 •Δφ = 0-50 degrees
                                 •Δp/p = 10-2-10-3


      p

              q = e e       /
Event Reconstruction


                     e




                 p
      The g*NΔ(1232) Quadrupole Transition


              SU(6): E1+= S1+=0
                                                      Shape at low Q2

                           ~ -0.03 -0.1                                     pQCD
                                                                            limit




                                                                            pQCD
                                                                            limit



Non-zero values at higher Q2 reveal intrinsic quadrupole charge distribution.
               g*NΔ with CLAS
7,200 data points




                         • Highest in Q2 so far
                         • Full coverage of cm angles
           g*NΔ with CLAS


W = 1.25 GeV

Q2 = 4.2 GeV2
g*NΔ Multipole Ratios REM, RSM


                      REM= -2 to -4% at 0 ≤ Q2
                       ≤ 6 GeV2.

                      RSM < 0, increasing in
                       magnitude.

                      REM < 0 favors oblate
                       shape of Δ(1232).

                      Pion contributions
                       needed to explain shape,
                       magnitude.

                      No trend towards
                      asymptotic behavior
                      REM→+100%.
        γ*pΔ+ - Magnetic Transition Form Factor G*M
                  p
e                          p0
         g   *


                                                                    T.-S. H. Lee
                                                                     N. Sato
e                       Pion cloud
                       contribution


    e
             g*
                       Quark core
                       contribution

    e


Large pion contribution needed
to explain NΔ transition.


    Pion contribution predicted to drop more rapidly with Q2 than the quark core.
    Probe core at sufficiently high Q2.
Quark orbital angular momentum
                                    SU(6)SF x O(3) Classification of Baryons


                                                           F15(1680)




                                         S11(1535)

                                                             D13(1520)



                                 P33(1232)             P11(1440)




                                             Harmonic Oscillator-Potential - Principal Energy Levels
                             ep→ e’p(n)
ds
    = s T  s L  s TT sin 2  * cos(2 * )  2 L (  1)s LT sin  * cos *
d*



                                    y = a  b cos  c cos2

                                                   s T  s L = a
                                                              b
                                              s LT =
                                                      sin  2 ( T  1)
                                                             c
                                                  s TT = 2
                                                          sin  T
CLAS                            Legendre Moments

       Q2=3GeV2                   σT +εσL for g*p→π+n

            ~const.                    ~cosΘ               ~ (a + bcos2Θ)

          with      S11(1535)
          Roper     D13(1520)                   with
     Δ
                                                Roper



                                        no
                                        Roper
                                                          Δ(1232)
         no Roper
                                                                     D13(1520)




             W(GeV)                     W(GeV)                      W(GeV)


 The Roper P11, S11 and D13 states become dominant contributions at high Q2
CLAS Nature of the Roper N(1440)P11 ?
        r |Q3>LC              nr |Q3>
                                                                     preliminary
                                                            nr|Q3>




                 quark
              |Q3G>
                 Core?                           r|Q3>LC

      zero               preliminary
      crossing
                                                    |Q3G>
           meson
           cloud?
   LC Models: S. Capstick & B. Keister; S. Simula; I. Aznauryan

   Roper is not a gluonic excitation Q3G.
   At short distances consistent with Q3- radial excitation.
   At large distances meson couplings may be important.
CLAS                                    N(1535)S11
       What is the nature of the N(1535) ?


                                                                    N(1535) in the CQM is
                                                                    a L3Q = 1, P=-1 state. It
                                                                    has also been
                                                                    described as a bound
                                                                    (KΣ) molecule with a
                                                                    large coupling to pη.

                                                                    The slow falloff of the
                                                                    A1/2 amplitude seen in
                                                                    pη and Nπ suggests a
                                                                    small Q3 system rather
                                                                    than a large KΣ
                                                                    molecule.

pη     Shaded area shows range of results from ep  eph analysis
CLAS                                 N(1520)D13
                                                             preliminary




                       preliminary



                 Q2(GeV2)                               Q2(GeV2)

 Transition from A3/2 dominance to A1/2 dominance seen for Q2 > 0.5 GeV2
 A1/2 is dominant amplitude at high Q2 as expected from asymptotic helicity
 conservation.

 A1/2 amplitudes P11, S11, D13 appear to behave similarly at high Q2.
Quark orbital angular momentum
                                    SU(6)SF x O(3) Classification of Baryons


                                                           F15(1680)




                                         S11(1535)
                                                                                        Predicted
                                                             D13(1520)                  states



                                 P33(1232)             P11(1440)




                                             Harmonic Oscillator-Potential - Principal Energy Levels
       Discover new baryon states
          SU(6) symmetric quark model |Q3> predicts many
         states that have not been seen in elastic πN scattering
         analysis.
|Q3>
          The diquark-quark model |Q2Q> has frozen degrees of
         freedom → fewer states. It accommodates all observed
         **** states.

          Discovery of new states could have significant
         impact on our understanding of the relevant degrees
         of freedom in baryonic matter.

|Q2Q>
          Search for new states in different final states, e.g.
         Nππ, KΛ, KΣ, pω, pη’. Analyses are more complex and
         channel couplings are likely important.
     Predicted SU(6) x O(3) States

 Examples of states predicted in the symmetric
 quark model with masses near 1900 MeV.
    ( S. Capstick, W. Roberts )


SU(6) x        Partial wave        Mass     Decays
 O(3)             L2J,2I          (MeV)
[N1/2+]4              P11         1880      Δπ, ∑K

[N1/2+]5              P11         1975    Δπ, Nω, Nρ

[N3/2+]2              P13         1870    Nπ, ∑K, Δπ
[N3/2+]3              P13         1910    Δπ, Nω, Nρ
[N1/2-]3              S11         1945    Nρ, Δπ, KΛ*
[N3/2-]3              D13         1960    Δπ, ΛK, Nρ
New N* states in KΛ Photo-production?
CLAS N* candidate at 1720 MeV in pπ+π- ?
                                           no 3/2+ (1720)
                                           full
    photoproduction                                  electroproduction

                             no 3/2+




                        full calculation

                        Background




                        Resonances

                        Interference


               W(GeV)                                                  W(GeV)
                                               M. Ripani et al, Phys.Rev.Lett. 91, 2003
CLAS                     Search for New Baryon States

reactions        beam pol.        target pol. recoil status
_____________________________________________________________

γp→Nπ,pη,pππ,KΛ/Σ          -                       -              Λ,Σ          complete

γp→p(ρ,φ,ω)               linear                   -               -            complete
---------------------------------------------------------------------------------------------

γp→Nπ, pη, pππ, KΛ         lin./circ.              long./trans.   Λ,Σ          2007

γD→KΛ, KΣ                 circ./lin.               unpol.         Λ,Σ          2007/2009

γ(HD)→KΛ,KΣ,Nπ            lin./circ.               long./trans.   Λ,Σ          2009/2010



 This program will, for the first time, provide complete amplitude information on
 the KΛ final state, and nearly complete information on the Nπ final states.
CLAS12                 JLab Upgrade to 12 GeV

 Luminosity > 1035cm-2s-1                Forward Tracker,
• General Parton Distributions           Calorimeter,
• Transverse parton distributions        Particle ID
• Longitudinal Spin Structure
• N* Transition Form Factors
• Heavy Baryon Spectroscopy
• Hadron Formation in Nuclei



            Solenoid, ToF,
            Central Tracker




                                    1m
         NΔ Transition - Future Program


Transition towards
asymptotic behavior?
                          Conclusions
•   Exclusive electroproduction of mesons has become a precise tool to map out the
    intrinsic structure of established baryon states.

•   With large acceptance detectors in use, and the development of highly polarized
    electron/photon beams and polarized targets the search for new baryon states has
    advanced to a much higher level of sensitivity.

•   Planned precision measurements with polarized beams, targets, and recoil
    polarization measurements with CLAS will provide the basis for unraveling the S=0
    baryon spectrum in the critical mass region near 2 GeV.

•   Making full use of the precise data produced by the new equipment requires
    sound theoretical methods in the search for complex resonance structure, and in
    understanding the physics at the core of baryons. This effort is underway with the
    Excited Baryon Analysis Center at JLab and with continuing efforts in Lattice QCD.

•   Jlab @ 12 GeV and CLAS12 allows extension of N* transition form factors to much
    higher Q2, and spectroscopy of heavy strange baryons.
      p experiment overview

                                          E1
                                         M =1
                                         
                  G+ helicity conserving  1
                                          S1 = const
  g    l = 0,1                           M 1
                                         
          g
              g


                  G0, -   Orbital motion of quarks

l
  1
  M
  2
     , l
        l
        l
= D( , =
         1
G  D )P 
       JP
         2
                  S
                  1  log 2
                      2
                        2
                         Q
                                     Idilbi, Ji, Ma (G0)
   N              M    Q
                   
                   1
              JLAB Unitary Isobar Model (UIM)

                     I. Aznauryan, PRC71, 015201 (2005)
Ingredients:         V.Burkert, T.-S. Lee, Int.J.Mod.Phys.E13:1035 (2004 )

 Pion Born terms + ,  exchanges + Regge
exchange at high energy, fitted to photoproduction
multipoles

 Q2-dependence of background amplitudes from
known nucleon and pion form factors

 pN coupling from pN phase-shift analysis

 Baryon resonances as relativistic Breit-Wigner
forms with energy-dependent widths

 Full amplitude (resonance + background) unitarized
using K-Matrix formalism

				
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posted:8/29/2012
language:English
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