Transcript ppt

Excited Baryon Program
- in part based on N* Workshop, Nov. 6-7, 2006, JLab -
Volker D. Burkert
Jefferson Lab
Town Meeting on QCD and Hadrons, Rutgers University, January 12-14, 2007
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Why excited baryons are important

Baryons (nucleons) make up most of the mass of the visible universe.

The 3-quark system are at the foundation of the development of the quark model.

Understanding the existence of the lowest excited Δ++ baryon required introduction
of a new quantum number (later called ‘color’) by O. Greenberg.

Baryon represent the simplest system where the non-abelian character of QCD is
manifest.
gluon self coupling
Lattice QCD calculation of
gluon flux distribution in a
system of 3 heavy quarks.

Study of the excited baryon states is necessary to fully understand the ground state
nucleon and to explore quark confinement.
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Quark orbital angular momentum
SU(6)SF x O(3) Classification of Baryons
F15(1680)
S11(1535)
D13(1520)
P33(1232)
Predicted
states
P11(1440)
Harmonic Oscillator-Potential - Principal Energy Levels
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Why study hadron structure with e.m. probes?
resolution
of probe
π
low
• γN may excite states not seen in πN.
• What are the appropriate degrees-of-freedom
describing hadron structure at varying distance?
N
LQCD - P.O. Bowman
DSE - C. Roberts
q
dressed quark (glue, qq)
bare quark
high
e.m. probe
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Electromagnetic Excitation of N*’s
The experimental N* Program has two major components:
1) Transition form factors of known states to probe their
internal structure and confining mechanism
e’
γv
e
p, h, pp,..
lgp=1/2
N*,△
N’
N
A3/2, A1/2, S1/2
Ml+/-, El+/-, Sl+/-
gv
N
lgp=3/2
2) Search for undiscovered states.
Both parts of the program are being pursued in various decay
channels with CLAS, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using
cross sections and polarization observables.
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Inclusive Electron Scattering
CLAS
ep→eX
Need to measure exclusive processes in full phase space to separate
resonances from each other and from non-resonant contributions.
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The γ*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.
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γ*NΔ Multipole Ratios REM, RSM before JLab
Sign @ Q2 > 0 ?
Q2 dependence?
γ*NΔ Multipole Ratios REM, RSM with JLab
 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
g
e
*
p0
Pion cloud
contribution
e
e
g*
T.-S. H. Lee
N. Sato
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.
Connection with elastic form factors and GPDs => Paul Stoler, Friday session
Lattice QCD results for P11(1440), S11(1535)
F. Lee, N*2004
Both states are considered
as possible nucleon-meson
molecular states: P11(1440) =
|Nσ >, S11(1535) = |YK>.
Masses of both states are
well reproduced in quenched
LQCD with valence quarks.
For a (Q3Q2) system one
expects a faster drop of the
transition form factors with
Q2.
Mπ2 (GeV2)
CLAS
Legendre Moments
Q2=3GeV2
~const.
Δ
with
Roper
σT +εσL for γ*p→π+n
~cosΘ
S11(1535)
D13(1520)
~ (a + bcos2Θ)
with
Roper
no
Roper
no Roper
Δ(1232)
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>
nr|Q3>
|Q3G>
zero
crossing
preliminary
r|Q3>LC
preliminary
|Q3G>
LC Models: S. Capstick & B. Keister; S. Simula; I. Aznauryan
 Exclude gluonic excitation Q3G.
 At short distances consistent with Q3- radial excitation.
 At large distances meson couplings may be important.
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CLAS Photocoupling amplitudes N(1535)S11
What is the nature of the N(1535) ?
preliminary
Nπ
pη
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.
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CLAS Photocoupling amplitudes N(1520)D13
preliminary
preliminary
Q2(GeV2)
Q2(GeV2)
A1/2 is dominant amplitude at high Q2 as expected
from asymptotic helicity conservation.
A1/2 amplitudes P11, S11, D13, (F15) appear to behave similarly at high Q2.
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CLAS
Test helicity conservation
→ Expect approach to flat behavior for Q3A1/2, Q5A3/2 at high Q2
Q3A1/2
S11
Q5A3/2
D13
P11
F15
F15
D13
Helicity conserving amplitude appears
to approach scaling, but needs to be
confirmed at higher Q2.
No scaling seen for helicity
non-conserving amplitude A3/2
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Quark orbital angular momentum
SU(6)xO(3) Classification of Baryons
Predicted
states
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Summary of recent N* and Δ* findings
R. Arndt, W. Briscoe, I. Strakovsky, R. Workman
Analysis of elastic πN→πN (2006)
 Does not support several N* and D* reported by PDG2006:
*** D(1600)P33, N(1700)D13, N(1710)P11, D(1920)P33
** N(1900)P13, D(1900)S31, N(1990)F17, D(2000)F35,
N(2080)D13, N(2200)D15, D(2300)H39, D(2750)I313
*
D(1750)P31, D(1940)D33, N(2090)S11, N(2100)P11,
D(2150)S31, D(2200)G37, D(2350)D35, D(2390)F37
Discover new baryon states
|Q3>
 SU(6) symmetric quark model |Q3> predicts many
states that have not been seen in elastic πN scattering
analysis.
 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.
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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
O(3)
Partial wave
L2J,2I
Mass
(MeV)
Decays
[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
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New N* states in KY production?
K+S
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New N* states in KΛ/KΣ production?
• PWA of data on gp→
K+L,
K+S,
K0S+
A. Sarantsev et al.,
C. Bennhold, et al.,
J. McNabb et al, PRC69 (2004)
K+Λ
K+Σ0
• Analyses find needs for various new candidate states.
• Solutions based on unpolarized cross sections alone have ambiguities;
demonstrates the need for polarization measurements.
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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
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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
2006/2009
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.
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Instrumentation for Excited Baryon Search
LInearly polarized
photon beam
CLAS
Photon Tagger
Frozen Spin Target
60
P(H
)
FROST
40
(%)
20
BNL Fall’06
P(D
)
Considered
to be used
at CLAS.
Polarized
HD - Target
0
days
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→→
γp
→
+
→K Λ
Projected Accuracy of Data (4 of over 100 bins)
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→→
γn
→
0
→K Λ
Projected Accuracy of Data (4 of over 100 bins)
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Need for Theory Support
• For small resonance cross sections, channel
couplings due to unitary constraints can lead to
strong distortions of amplitudes.
• Requires coupled-channel computation that
includes all major channels.
• The Excited Baryon Analysis Center (EBAC) was
established in 2006 at JLab to provide theoretical
support for the excited baryons experimental
program.
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CLAS12
JLab Upgrade to 12 GeV
Forward Tracker,
Calorimeter,
Particle ID
Luminosity > 1035cm-2s-1
• General Parton Distributions
• Transverse parton distributions
• 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?
+100 ??
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CLAS12 Projections for A1/2 @ 12 GeV
Full transition to quark core behavior ?
CLAS published
CLAS preliminary
CLAS12 projected
DVCS - a new tool in N* physics
ep
egN*
CLAS (preliminary)
e
g*
e
x+x
x-x
GPDs
p
g
hard process
Bjorken regime
ep→egp+n
N*
t, ξ dependence of N* transition
- map out Transition-GPDs

Decouple γ virtuality from
momentum transfer to the nucleon
D
N*’s


Nucleon dynamics at the parton level
Mnp (GeV)
Strangeness = -2 Ξ Baryons
Advantage - Narrow widths, easier to separate from background.
Disadvantage – No s-channel production, low cross sections.
Flavor SU(3) predicts same number of Ξ’s as N*’s and Δ*’s. Only 3
Ξ’s have established JP.
γp ->
Ξ(1320)
K+K+X-
γp -> K+K+Ξ0π-
Ξ(1530)
Needs higher energy for spectroscopy -> 2007/2008.
JLab @ 12 GeV is a good place for cascade spectroscopy.
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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.
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