Nucleon Excited States

1
Nucleon Excited States

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

2
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

3
CLAS Coverage for ep eX at 4 GeV
5.0
4.0
3.0
2.0
1.0
CLAS
0
2.0
1.0
1.5
2.5
4
CLAS Coverage for ep epX at 4 GeV
2.0
missing states
1.5
1.0
CLAS
1.5
0.
0.5
1.0
5
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

6
N D(1232) Transition Form Factors
SU(6) E1S10
7
Cross Section Decomposition
example
e p e p po
8
gp ppo Response Functions
CLAS
9
Multipole Analysis for gp ppo
CLAS
Q2 0.9 GeV2
M12
Re(E1M1)
M12
Re(S1M1)
10
Multipole Ratios REM, RSM before 1999

• Data could not determine sign or Q2 dependence

11
Multipole Ratios REM, RSM in 2002
Sign?
lt 0 !
Q2 dependence ! Slope lt 0

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

12
N D(1232) Transition Form Factors

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

13
N D(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.

14
Preliminary Multipole Ratios REM, RSM

• Dynamical models and full LQCD calculations
  indicate the importance of the pion cloud at low
  Q2 consistent with the trend of data.
• Full LQCD results indicate a small oblate
  deformation of the D(1232).
• Data at high Q2 needed to study the transition
  to pQCD.

preliminary
15
Very Preliminary Multipole Ratios REM , RSM
preliminary

• New trend towards pQCD behavior may or may not
  show up.
• Experiment E-01-002 in Hall C will reach Q²8
  GeV² and after the energy upgrade REM and RSM
  can be measured up to Q²12 GeV².

16
N D Transition and Background Terms

• systematic uncertainties in extraction of E1/M1
  from
• ep ep po around 0.5
• differences in treatment of background terms
  (models not constrained)
• will become more severe for higher Q2 (D
  dropping faster)
• more experimental information on hand
  (polarization and isospin)
• single-spin asymmetry sTL for e p ep (po)
  and e p ep (n)
• polarization transfer in e p ep (po)
• differential cross sections for e p ep n
  (D less important)

CLAS
Hall A
CLAS
17
Polarized Beam Observables
CLAS
sLT response
function for
e p e p po
sLT 0 if only a single diagram contributes
(sensitive to the interference between D and
background)
18
Polarization Measurement in e p e p (po)
Hall A
mb/sr
Q2 1 GeV2 W 1.232 GeV Results sensitive to
non-resonant contributions
SAID MAID
Parametrisations of available data
19
p Electroproduction
CLAS
20
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

21
Quark Model Classification of N
q³g q³qq N-Meson
22
Missing Resonances?
Problem symmetric CQM predicts many more states
than observed (in pN scattering)
Possible solutions
1. di-quark model
old but always young
fewer degrees-of-freedom open question
mechanism for q2 formation?
2. not all states have been found

• possible reason decouple from pN-channel
• model calculations missing states couple to
• Npp (Dp, Nr), Nw, KY

3. chiral symmetry approach
new
all baryonic and mesonic excitations beyond the
groundstate octets and decuplet are generated by
coupled channel dynamics (not only L(1405),
L(1520), S11(1535) or f0(980))

• g coupling not suppressed electromagnetic
  excitation is ideal

23
Resonances in Hyperon Production?
CLAS
gp KY
backward hemisphere
forward hemisphere
preliminary
N ?
24
L Photoproduction off the Proton
CLAS
Dominant resonances S11(1650) P11(1710) P13(1720)
Carnegie Mellon
25
Resonances in gp ppp-
CLAS
Analysis performed by Genova-Moscow
collaboration Step 1
Q² 0.65 GeV²
0.95 GeV²
use the best information presently
available GNpp PDG GNg expt. Data or
SQTM
1.30 GeV²
extra strength
26
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.
27
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)
28
PWA in gp ppp-
CLAS
W2240 MeV
29
Resonances in gp ppp-
CLAS
30
Resonances in gp ppp-
CLAS
F15(1680)
P13(1720)
preliminary
31
JLab Q- Exclusive Process I
CLAS
gd K-Kp(n)
Q

• Mass 1.5420.005 GeV/c²
• lt 21 MeV/c²
• Significance 5.20.6 s

32
JLab Q- Exclusive Process II
CLAS
gp pK-K(n)
Events
Q
K
M(K-p)
33
JLab Q- Exclusive Process III
CLAS
gp pK-K(n)
Q
combined analysis II III

• Mass 1.5550.010 GeV/c²
• lt 26 MeV/c²
• Significance 7.81.0 s

cut
34
JLab Q- Exclusive Process III
CLAS
gp pK-K(n)
N ?
Mass 2.440 GeV/c²
35
Outlook Observation of Exotic X--
combined analyis NA49
X0
X(1530)
M1.862 0.002 GeV/c² Glt 18 MeV/c²
CERN SPS hep-ex/0310014
36
Outlook Observation of Exotic X--
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
X0
X(1530)
PAC 25 at JLab
M1.862 0.002 GeV/c²
CERN SPS hep-ex/0310014
37
Hadron multiplets
Mesons qq
Baryons qqq
Baryons built from meson-baryon, or qqqqq

38
New Tools Exploit Weak Decays in Direct
Reconstruction
X -- ? p- X-
X - ? p- L
L ? p- p
Electron beam
gn?KKX5--

• X- ct 4.9 cm
• ct 7.9 cm

Negatives bend outwards
ltgbgt 1.5
39
New Tools Frozen Spin Target for CLAS
Technical problem build polarized target for
tagged photon beam - minimum obstruction of
CLAS solid angle - low distortion of particle
trajectories in magnetic field
Solution - frozen spin target - temperature
50mK - magnetic field 5kG
Bonn target at Mainz GDH experiment
5 Tesla polarizing magnet
Status - design in progress at JLab -
procurement started for polarizing magnet
CLAS
JLab design
40
New Tools Bound Nucleon Structure (BoNuS)
Physics issue tag process off a neutron bound
in deuterium by detecting the spectator proton in
coincidence with the scattered e
CLAS coils

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

pair spectrometer

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

41
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

42
New Tools Gas Cherenkov Counter

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

PMTs Winston cones
multilayer m-metal iron shield
Mirrors
43
New Tools DVCS in N Physics
ep egN
e
g (p, h, w, )
e
g
CLAS (preliminary)
hard process
xx
x-x
Bjorken regime
GPDs
N
p
2-particle correlations function

• t, x dependence of N transition
• map out transition-GPDs

Ns
D

• decouple g virtuality from momentum transfer to
  the nucleon

• study nucleon dynamics at the parton level

Mnp (GeV)
44
New Tools Setup for DVCS
Physics Goal measure x, t, Q2 - dependence of ep
ep g 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

45
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46
Deeply Virtual Meson Production

47
DVCS Experiment
Superconducting solenoid needed for shielding
Moller electrons PbWO4 e.m. calorimeter needed
for photon detection

• Solenoid under
• construction at
• SACLAY

cryostat
coil windings
calorimeter
48
DVCS - 100 Crystal Prototype for Test Run
Connexion board APD / Preamplifier
Back frame
Support frame
Mother board preamplifier
Optical fiber system mounted on the front frame
Alignement system / - 5 mm
Connectors
Fixing plate on the CLAS support
Ph. Rosier, Orsay
49
DVCS Experiment

• 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

50
PrimEx Experiment
Electron beam test results
PbWO4 crystal channel
51
ep eX at 4 GeV
events
CLAS
52
A di-quark model for pentaquarks
JW hep-ph/0307341
JM hep-ph/0308286
SZ hep-ph/0310270
Decay Width
Mass Prediction for X-- is 1.75 instead of 2.07
GeV
53
Frozen Spin Target

• 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.

• 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.

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