# Presentation: 648050

## Nucleon Excited States 

**Nucleon Excited States **

- Ralf W. Gothe
- University of South Carolina
- Mini-Workshop on Nucleon Excited States  PAC25
- January 17, 2004
- Introduction
- N      transition
- “Missing” Resonances, Strange Resonances and Two Pion Production
- Pentaquark States
- Outlook

**Notes:**
My talk will be about the electroexcitation of nucleon resonances. 
This conferences is all about nucleons and baryons. 
Nathan Isgur has been a strong supporter of a nucleon research program at this laboratory.  He formulated the importance of studies of the nucleon in 3 remarks he made at the N*2000 conferences  in this auditorium:

## Physics Goals

- Understand QCD in the full strong coupling regime
-  transition form factors
-  mass spectrum, quantum numbers of nucleon excited states
-  relevant degrees-of-freedom
-  wave function and interaction of the constituents

**Notes:**
Pion cloud, constituent quarks, current quarks; transition from constituent to current quarks depends on Q2, W, inclusive and exclusive

##  CLAS Coverage for ep     e’X  at  4 GeV

**2.0**

**1.0**

**1.5**

**2.5**

**2.0**

**4.0**

**0**

**1.0**

**3.0**

**5.0**

- CLAS

**Notes:**
For electron scattering experiments CLAS is usually triggered on electrons only. 
The inclusive electron spectrum at 4 GeV beam energy,
shown here in the Q2 versus invariant mass W kinematics,  exhibits the elastic scattering band, the 3 resonance bands, and the transition to the deep inelastic regime.    
These lines indicate the region where many of the missing states are expected. 
(next slide)
The projection onto the W axis shows the elastic peak and the 
3 resonance bumps

## CLAS Coverage for ep    e’pX  at  4 GeV 

**CLAS Coverage for ep    e’pX  at  4 GeV**** **

**0.**

**0.5**

**1.0**

**1.5**

**1.0**

**1.5**

**2.0**

**missing states**

- CLAS

**Notes:**
...... pi0, eta, or omega in the 
final state. The missing mass vs invariant mass scatter plot shows clear correlations of resonance excitations and specific final states.  
 Strong excitation near 1500 is correlated with the p-eta channel, and there is a lot of strenght in p-omega throughout the higher mass region.
The Delta sticks out in the p-pi0 channel, and can best be studied in this channel. 

## Electromagnetic Probe

- helicity amplitudes are sensitive to the difference in wave functions of N and N*
- can separate electric and magnetic parts of the transition amplitude
- is linearly polarized

- varying Q2 allows to change the spatial resolution and enhances different multipoles

- sensitive to missing resonance states

## N    (1232) Transition Form Factors

- SU(6): E1+=S1+=0

**N    ********(1232) Transition Form Factors**

**Notes:**
B_1+ = -M_1+ + E_1+ = 0 helicity flip amplitude is zero when helicity is conserved, has not to be zero if nonhelicity conserving contribution are taken into account

## Cross Section Decomposition

- example:

- e p     e’ po

**Notes:**
Extracting the multipoles requires first to separate the relevant response function, by measureing the azimuthal and polar angle dependence of the diff. cross section, and fitting it to the expected phi dependence,  (next slide)
 as shown in this graph..... 

## p     p – Response Functions

- p     p **– Response Functions**

- CLAS

**Notes:**
which shows a typical example of such  fit.
(next gaph)
The response function can be expanded in terms of Legendre polynomials which are functions of the multipoles we want 
to measure.  

## Multipole Analysis for *p    p 

**Multipole Analysis for** *p    p

- Q2 = 0.9 GeV2

- |M1+|2

- Re(E1+M1+*)

- Re(S1+M1+*)

- |M1+|2

- CLAS

**Notes:**
and with some approximation analysed in terms of multipoles.
The first terms is dominated by the transverse component 
and strongly dependent on the dominant magnetic dipole transition, while the TT interference is most sensitive to the 
electric quadrupole, and the LT interference to the scalar quadrupole.
(new graph)
  Overlaying the new results on the old data.... 

## Multipole Ratios REM, RSM before 1999

**Multipole Ratios**** ****R****EM****, R****SM**** ****before 1999**

-  Data could not determine sign or Q2 dependence

## Multipole Ratios REM, RSM in 2002

- Sign?

- Q2 dependence !
- Slope < 0

- < 0 !

-  No trend towards
- pQCD behavior is
- observed for Q2 up
- to 4 GeV2.

## N    (1232) Transition Form Factors

- Preliminary results from ELSA and
- Hall A using different techniques confirm CLAS data.

**N    ********(1232) Transition Form Factors**

## N    (1232) Transition Form Factors

**N    ********(1232) Transition Form Factors**

- Lattice QCD indicates that the pion cloud makes E1+ /M1+ more negative at small Q2.
- Data at low Q2 needed to study effects of the pion cloud.

**Notes:**
C. Alexandrou et al, hep-lat/0209074 (2002)

## preliminary

-  Full LQCD results indicate a small oblate deformation of the (1232).
-  Data at high Q2 needed to study the transition to pQCD.

**Preliminary Multipole Ratios**** ****R****EM****, R****SM**

**Notes:**
Q/<R>2: earth -0.0027 oblate, deuteron 0.015 prolate, Delta -0.14 oblate (Buchmann)

## preliminary

- New trend towards pQCD behavior may or may not show up.
- Experiment E-01-002 in Hall C will reach Q2~8 GeV2 and after the energy upgrade REM  and RSM can be measured up to Q2~12 GeV2.

**Very Preliminary Multipole Ratios**** ****R****EM** , **R****SM**

## N    Transition and Background Terms

- systematic uncertainties in extraction of E1+/M1+ from
-     ep   e’p o around 0.5%
- differences in treatment of background terms (models not constrained)
- will become more severe for higher Q**2**  (**** dropping faster)
- more experimental information on hand (polarization and isospin)
- single-spin asymmetry **TL’** for  e p    e’p (******o**)  and   e p    e’******+** (n)
- polarization transfer in  e p    e’p (******o**)
- differential cross sections for  e p    e’******+** n  (**** less important)

- CLAS

- Hall A

- CLAS

## Polarized Beam Observables

- LT’  response

- e p    e p 

- CLAS

- function for

- LT’ = 0  if only a single diagram contributes   (sensitive to the interference between  and background)

**Notes:**
The Mami beam asymmetry data at large cms angles show a strong sensitivity to model descriptions is. The dotted line is a rescaled MAID model. 
The CLAS data on the sigma_LT' response function cover the full angle range and show similar sensitivity to model descriptions. 
Since the resonance contributions are nearly the same in either model, the discrepancies are largely due to the different description of non-resonant terms. 

## Polarization Measurement in  e p    e’ p (o)  

**Polarization Measurement in****  e p    e’ p (********o****)**

- Hall A

- Q2 = 1 GeV2
- W = 1.232 GeV
- Results sensitive to non-resonant contributions

- Parametrisations of available data

- SAID MAID

****b/sr

## + Electroproduction 

- CLAS

**Notes:**
allows ectraction of resonant amplitudes in the 2nd resonance region.  
(next slide) 
the next graph shows the preliminary results for one Q2 point on the Roper resonance....   

## Electromagnetic Probe

- helicity amplitudes are sensitive to the difference in wave functions of N and N*
- can separate electric and magnetic parts of the transition amplitude
- is linearly polarized

- varying Q2 allows to change the spatial resolution and enhances different multipoles

- sensitive to missing resonance states

## Quark Model Classification of N*

**Quark Model Classification of N***

**+ q3g**
**+ q3qq**
**+ N-Meson**
**+ ...**

**Notes:**
Lambda(1405) N-K molecule, Roper N-gluon excitation

## Possible solutions: 

-  fewer degrees-of-freedom
-  open question: mechanism for q2  formation?

- Problem: symmetric CQM predicts many more states than observed (in N scattering)

- Possible solutions:

- 1. di-quark model

- 2. not all states have been found

- possible reason: decouple from N-channel
- model calculations: missing states couple to
- N, N,  N,  KY

-  coupling not suppressed      electromagnetic excitation is ideal

- 3. chiral symmetry approach

- all baryonic and mesonic excitations beyond the groundstate octets and decuplet are generated by coupled channel dynamics (not only (1405), (1520), S11(1535) or f0(980))

- old but always young

- new

**Notes:**
Lambda(1405) KbarN, Lambda(1520) KstarN, S11(1535) KbarSigma or f0(980) KKbar Matthias Lutz Darmstadt, SU(3) chiral symmetry, Weinberg-Tomazawa meson baryon interaction, 
Coupled channel Bethe-Salpeter equation
In recent years a number of alternatives to the "standard" constituent quark model have been proposed, in large measures   to fix some of the real or perceived shortcomings of that model, especially to explain the absence of many states predicted in that model. Several talks at this conference are dedicated to these new approaches. One of the oldest alternative models is the quark-diquark model, where two of the quarks form an elementary object. This model has the advantage that due to the fewer excitation degrees of freedom a much smaller number of states are predicted which corresponds more to what we see in the data...
The various alternatives correspond to quite different underlying quark symmetries and interactions, search for at least some of the states predicted in the standard model, but not in the alternative models is imperative. 
A series of P13 states with coupling to the omega channel are predicted in the standard model but not in the quark-diquark model..... . 
Electromagnetic production of omegas suffers from strong diffractive or t-channel contributions, leading to strong forward peaking which complicates the analysis. However, diffractive production is reduced with increasing Q2.  

While the diffractive or t-channel contribution gives the forward peaking, s-channel resonance production produces a more symmetric angular dependence with significant large angle production. 
(next) The data exhibit ..

## preliminary

**Resonances in Hyperon Production?**

-     *p        K+Y

- backward hemisphere

- forward hemisphere

- N* ?

- CLAS

##  Photoproduction off the Proton

- Dominant resonances
- S11(1650)
- P11(1710)
- P13(1720)

- Carnegie Mellon

- CLAS

## Resonances in  *p      p+-

**Resonances in**  *p      p+-

- extra strength

- CLAS

- Analysis performed by Genova-Moscow collaboration
- Step #1:

- use the best information presently available
- N:  PDG
- N :   expt. Data or
-             SQTM

- Q2
- 0.65 GeV2

- 1.30 GeV2

- 0.95 GeV2

**Notes:**
These are the first high statistics electroproduction data in the 2-pi channel. Victor Mokeev: phenomenological isobar model, SQTM: single quark transition model. Shaded area systematic error.
The data can be best fitted by a new P13 state with properties that are very different from the properties of the known 
P13 located in the same mass range. 

## Attempts to fit observed extra strength

- CLAS

- Step #2:

- vary parameters of the photocouplings
- vary parameters of known P13, P11, D13
- introduce new P13

- New P13(1720)  consistent with predictied missing P13 , but at lower mass.

**Notes:**
D13(1700), P11(1710), P13(1720) ...

## Attempts to fit observed extra strength

- CLAS

- Step #2:

- vary parameters of the photocouplings
- vary parameters of known P13, P11, D13
- introduce new P13

- W(GeV)

**Notes:**
D13(1700), P11(1710), P13(1720) ...

## PWA in  p      p+-

**PWA in**  p      p+-

- CLAS

- W=2240 MeV

**Notes:**
Matt Bellis, shaded CLAS data, red accepted MC data unweighted, blue accepted MC data weighted by energy-independent fit, S.134+135, 775,553 exclusive events

## Resonances in  p      p+-

**Resonances in**  p      p+-

- CLAS

**Notes:**
These are the first high statistics electroproduction data in the 2-pi channel. SQTM: single quark transition model.
The data can be best fitted by a new P13 state with properties that are very different from the properties of the known 
P13 located in the same mass range. 

## preliminary

**Resonances in**  p      p+-

- CLAS

- F15(1680)

- P13(1720)

**Notes:**
PDG F15(1680), P13(1720) Bellis F15(1650), P13(1770) S.166

## JLab: +- Exclusive Process I 

**JLab:** **+****- ****Exclusive Process I**** **

******+**

- Mass** **1.542**±**0.005 GeV**/**c2
**<** 21 MeV**/**c2
- Significance 5.2**±**0.6 

- CLAS

- d     K**-**K**+**p(n)

**Notes:**
Distribution of Lambda*(1520) events, Gauss+Gauss+P0, GEANT four body phase space background pK+K-n, tagged range 2.474 to 3.115 GeV

## JLab: +- Exclusive Process II

- CLAS

- p     **+**K**-**K**+**(n)

**Events**

**K***

**M(K****-********+****)**

********

**Notes:**
cos *(+, K-) > 0.5, background shape taken from spectrum without small-t  cut, estimate 4.8  significance, 1537

## JLab: +- Exclusive Process III

- CLAS

- p     **+**K**-**K**+**(n)

- Mass** **1.555**±**0.010 GeV**/**c2
**<** 26 MeV**/**c2
- Significance 7.8**±**1.0 

********

- combined analysis II & III

- cut

**Notes:**
cos *(+)> 0.8, cos *(+)< 0.6, GEANT four body phase space background pi+K+K-n, tagged range 3.2-3.95 + 3.0-5.25 + 4.8-5.47 GeV

## JLab: +- Exclusive Process III

- CLAS

- p     **+**K**-**K**+**(n)

- Mass** ~ **2.440** **GeV**/**c2

- N* ?

**Notes:**
Why not seen in piN elastic ? Need 2.6 GeV/c pion beam. PDG shows gap from 2.3 – 2.7 GeV/c with no elastic data!

## Outlook: Observation of Exotic −−

- (1530)

**CERN SPS hep-ex/0310014**

- M=1.862± 0.002 GeV/c2
- **<** 18 MeV**/**c2

- 0

- combined analyis NA49

**Notes:**
Diakonov 2070 (1997), NA49 CERN SPS pp at 158 GeV/c.

## Outlook: Observation of Exotic −−

- (1530)

**CERN SPS hep-ex/0310014**

- M=1.862± 0.002 GeV/c2

- 0

**Diakonov et al.     Hep-ph/9703373**
**Diakonov, Petrov hep-ph/0310212**
**Jaffee, Wilczek     hep-ph/0307341**
**Jaffee, Wilczek     hep-ph/0312369**
**R.A. Arndt et al.   nucl-th/0312126**

- PAC 25 at JLab

**Notes:**
Diakonov 2070 (1997), NA49 CERN SPS pp at 158 GeV/c.

## Hadron multiplets

- Baryons qqq

- Mesons qq

- Baryons built from meson-baryon, or qqqqq

**...**

## New Tools: Exploit Weak Decays in Direct Reconstruction 

**New Tools: Exploit Weak Decays in Direct Reconstruction**** **

- Electron
-   beam

- Negatives bend outwards

******−**** ****c******** = 4.9 cm**
**  ****c******** = 7.9 cm**

******−−**** ****→ ********− ********−**

******−**** ****→ ********− ******

****** ****→ ********− ****p**

******n****→K****+****K****+************−−**

****** ****~ 1.5**

## New Tools: Frozen Spin Target for CLAS

**Solution:**
-  - frozen spin target
-  - temperature 50mK
-  - magnetic field 5kG

**Status:**
-  - design in progress at JLab
-  - procurement started for
-    polarizing magnet

**Technical problem:**
- build polarized target for tagged photon beam
-   - minimum obstruction of CLAS solid angle
-  - low distortion of particle trajectories in
-    magnetic field

- 5 Tesla polarizing magnet

- CLAS

- Bonn target at Mainz GDH experiment

- JLab design

**Notes:**
80% p and n polarization factor of two more than previously achieved, GDH at n in Mainz in one week better data than in 4weeks at p
Missing resonances and background

## New Tools: Bound Nucleon Structure  (BoNuS)

- pair spectrometer

**Solution: **
-  - high pressure gas target
-  - surrounded by radial drift chamber
-  - GasElectronMultiplier gap

**Technical problem:**
- spectator protons have
-  - low momentum and low range
-  - isotropic angular distribution (no correlation)
-  - high rate

- CLAS coils

**Physics issue:**
- tag process off a neutron bound in deuterium by detecting the spectator proton in coincidence with the scattered e’

## New Tools: BoNuS Detector

- Radial Time Projection Chamber (RTPC)

- Goal: detect spectator protons with momenta as low as 70 MeV/c

-  Cylindrical prototype with GEM readout being developed by Howard Fenker
-  Flat GEM prototype has been built and is being tested

## New Tools: Gas Cherenkov Counter

**Mirrors**

**PMTs +**
**Winston**
**cones**

**New Tools: Gas Cherenkov Counter**

-   Needed for E-03-106 (GDH Integral at very low Q2), M. Ripani, et al.

**multilayer ********-metal **
**+ iron shield**

## ep     e’N*

- ep     e’N*

-    t,  dependence of N* transition
-   map out transition-GPDs

- GPDs

- p

- N*

- e

- e’

- 

- ...

- hard process

- Bjorken regime

- 2-particle correlations function

- x+

- x-

-   decouple  virtuality from momentum transfer to the nucleon

-   study nucleon dynamics at the parton level

- 

- N*’s

- Mn(GeV)

- CLAS (preliminary)

**Notes:**
xi = xB/(2-xB). Virtual Compton Scattering (as well as meson production) can provide a completely new way of resonance studies which may be called "Hard Nucleon Spectroscopy". In this case the reaction is in the deep virtual regime of Bjorken scaling where theory is under control. 
The N* is produced at small t where factorization between the upper vertices and the soft part is guaranteed. While the virtual photon has high virtuality to guarantee elementary quark coupling , the N-N* transition is probed at small momentum transfer t.  This process allows to map out GPDs for the N-N* transition as a function of xi and t, obtaining a more complete picture of nucleon dynamics, that includes quark, mesosn, and double meson contributions.

## New Tools: Setup for DVCS

**Physics Goal**
- measure** ******, t, Q2 - dependence of ep     e’p in a wide kinematics range to constrain GPD models.

- Technical Problem** **
- need to detect all final state particles to identify process
- double luminosity to 2x1034 cm-2 s-1

- Technical solution
- add forward calorimeter (436 lead PbWO4 crystals)
- readout via avalanche photodiodes (APD)
- super conducting 5 Tesla solenoid Moller shield

**Deeply Virtual Meson Production**

## DVCS Experiment

**DVCS Experiment**

- Superconducting solenoid: needed for shielding Moller electrons
- PbWO4 e.m. calorimeter needed for photon detection

- calorimeter

- coil windings

- cryostat

- Solenoid under
- construction at
- SACLAY

## DVCS - 100 Crystal Prototype for Test Run

**Fixing plate on the CLAS support**

**Optical fiber system **
**mounted on the front frame**

**Connectors**

**Alignement system**
**+ / - 5 mm**

**Back frame**

**Mother board **
**+ preamplifier**

**Connexion board **
**APD / Preamplifier**

**Support frame**

**DVCS - 100 Crystal Prototype for Test Run**

**Ph. Rosier, Orsay**

## PbWO4 crystal calorimeter 
440 tapered crystals, APDs, on site (ITEP, JLab)
Mechanical structure in final design stage (Orsay)
Preamps - designs being evaluated (ITEP, Orsay)
5 x 5 crystal prototype built and being tested 

- 440 tapered crystals, APDs, on site (ITEP, JLab)
- Mechanical structure in final design stage (Orsay)
- Preamps - designs being evaluated (ITEP, Orsay)
- 5 x 5 crystal prototype built and being tested

**DVCS Experiment**

## PrimEx Experiment

**PrimEx Experiment**

**Electron beam test results**

**PbWO****4**** crystal channel**

## ep    e’X  at  4 GeV

**ep    e’X****  ****at  4 GeV**

- CLAS

**events**

## A di-quark model for pentaquarks

**JW hep-ph/0307341**

**JM hep-ph/0308286**

**SZ hep-ph/0310270**

- Decay Width:

- Mass Prediction for ******−−**** **is 1.75 instead of 2.07 GeV

-  Needed for Search for missing N* in pion and kaon photoproduction,
- Experiment E-02-112, F. Klein et al.,
- E-03-105, S. Strauch et al.

-  Polarizing magnet ordered, estimated delivery: Feb. 2004
-  Longitudinal holding magnet; prototype constructed – being tested
-  Transverse holding magnet: prototype for “racetrack” design completed
-  Design for dilution refrigerator 50% completed, construction underway.

**Work by Target group (Chris Keith, et al.)**

**Frozen Spin Target**