Transcript Document

Excited Hadrons
Volker D. Burkert
Jefferson Lab
Contributions: T. Barnes, A. Dzierba, C. Meyer, A. Szczepaniak
Excited baryons
• Classification
• NΔ transition form factors
• Low mass N* excitations, Roper
• A near-threshold resonance S11(1535)
• Search for new baryon states
• Coupled channels analysis
Exotic hybrid mesons
• What are they, what do we know?
• Near term plans
• Plans for JLab Upgrade
• Summary
Workshop on Physics of Nucleons and Nuclei, October 16-17, 2006
1
Why excited baryons are important


Baryons (nucleons) make up most of the mass of the visible
universe.
They represent the simplest system where the non-abelian
character of QCD is manifest.
gluon self coupling
LQCD calculation of gluon flux
distribution in a 3-quark system.

Study of the excitation spectrum is necessary to understand
the ground state and explore origin quark confinement.
2
SU(6)xO(3) Classification of Baryons
Quark orbital angular momentum
3-Quark Shell Model
HO-Model Principal Energy Levels
3
Electromagnetic Excitation of N*’s
The experimental N* Program has two major components:
1) Transition form factors of known resonances to study their
internal structure and confining potential
2) Spectroscopy of excited baryon states, search for new
states.
Both parts of the program are being pursuit in various decay
channels, e.g. Nπ, pη, pπ+π-, KΛ, KΣ, pω, pρ0 using cross sections
and polarization observables.
4
Reach of Current Accelerators
Spring-8
JLAB
5
Quark orbital angular momentum
SU(6)xO(3) Classification of Baryons
S11(1535)
P33(1232)
P11(1440)
6
Examples of Exclusive Processes in N* Studies
p(e,e’)X
Hadronic mass
7
Electromagnetic Excitation of N*’s
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
DOE Milestone 2012
Measure the electromagnetic excitations of low-lying
baryon states (<2 GeV) and their transition form factors
over the range Q2 = 0.1 – 7 GeV2 and measure the electroand photo-production of final states with one and two
pseudo-scalar mesons.
8
Resonance Analysis Tools
•
Nucleon resonances are broad and overlapping, careful analyses of
angular distributions for differential cross sections and
polarization observables are needed.
•
Amplitude & multipole analysis (GWU, SAID)
•
Phenomenological analysis procedures have been developed, e.g.
unitary isobar models (UIM), dispersion relations (DR), that
separate non-resonant and resonant amplitudes in single channels.
•
Dynamical coupled channel approaches for single and double pion
analysis are being developed within the Exited Baryon Analysis
Center (EBAC) effort. They are most important in the extraction
of transition form factors for higher mass baryon states.
•
Event-based partial wave analyses with maximum-likelihood fit,
developed in the search for new mesons states are now being
utilized for baryon resonance studies. They fully utilize
correlations in the final state (CMU). (Comments by Curtis Meyer).
9
The γNΔ(1232) Quadrupole Transition
SU(6): E1+=S1+=0
Shape at low Q2
pQCD
limit
pQCD
limit
10
NΔ Multipole Ratios REM, RSM
1.0
2.0
3.0
4.0
5.0
6.0
 REM remains small and negative at -2% to -4%
from 0 ≤ Q2 ≤ 6 GeV2.
 No trend towards sign change or asymptotic
behavior. REM → +100%, RSM→constant. Higher
energies needed.
 Dynamical models need to include pion
contributions to explain magnitude and Q2
dependence.
 Quenched LQCD agrees with E/M, deviates at
low Q2 in S/M. Effect of pion cloud?
11
The Roper resonance N1/2+(1440)P11
RQM: P11(1440) = [56,0+]r
P11(1440) = Q3G
P11(1440) = (Q3)r(QQ)
Photocoupling amplitudes
carry information on the
the internal structure of
the state.
The Roper resonance is not a
gluonic excitation Q3G.
 At large distances meson
couplings may be important.
First observation of a sign change for
any nucleon resonance.
 At short distances the Roper
is best described as a radial
excitation of the nucleon.
12
S11(1535): A near-threshold resonances
S11(1535)
S11(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 very slow falloff of the
A1/2(S11) form factor with Q2
suggests a Q3 system rather than
a meson-baryon (QQQ-QQ)
molecule (no form factor
calculations exist for the molecular
case).
13
Cross section for Nπ and η-cusp
γp->π0p, Θ*=180o,
mostly resonant
γp->π+n, Θ*=180o,
mostly non-resonant.
0.6
0.6
0.7
0.8
Eγ(GeV)
0.9
0.7
0.8
Eγ(GeV)
0.9
14
Quark orbital angular momentum
SU(6)xO(3) Classification of Baryons
Missing
states
15
Search for undiscovered baryon states
|Q3>
 Symmetric CQM |Q3> predicts many more states
than are observed in elastic πN → πN scattering analysis.
 The diquark-quark model |Q2Q> has frozen degrees of
freedom → fewer states. It accommodates all observed
**** states.
|Q2Q>
 Discovery of any new state could have significant
impact on our understanding of the relevant degrees
of freedom in baryonic matter.
Example: An additional P13 below 1900MeV would
effectively rule out the |Q2Q> model.
 Search for new states in different final states, e.g.
Nππ, KΛ, KΣ, pω, pη’. Analyses are more complex and
channel couplings become important.
16
New N* states in KS production?
K+S
17
New N* states in KL/KS production?
• PWA of data on gp → K+L, K+S, K0S+
K+L
CLAS
K+S0
Partial wave analyses yield tantalizing hints of several new states.
However solutions are not unique due to insufficient polarization data.
18
New N* candidate at 1710 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
19
CLAS
Search for Exited Baryon States
Experiment
reactions
beam pol.
target pol. recoil status
===================================================================================
G1/G10
γp→Nπ, pη, pππ, KΛ/Σ
Λ,Σ complete
G8
γp→p(ρ,φ,ω)
linear
-
-
complete
----------------------------------------------------------------------------------------------------G9-FROST
γp→Nπ, pη, pππ, KΛ
lin./circ.
long./trans. Λ,Σ
2007
G13
γD→KΛ, KΣ
circ./lin.
unpol.
Λ,Σ
2006/2008
G14-HD
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 (more than 7 independent polarization
measurements at each kinematics), and nearly complete information on
the Nπ final states.
20
γp→KY (K+Λ, K+Σ0, K0Σ+)
21
γp→KY (K+Λ, K+Σ0, K0Σ+)
22
γn→K0Λ
DE=100MeV
23
Coupled Channel Analysis (EBAC)
24
Coupled Channel Analysis (EBAC)
 Pion-nucleon and 2pion-nucleon
contributions to the
non-resonant T
matrix.
25
Hybrid mesons
Flux Tube Model
Comments by Ted Barnes, Alex Dzierba, Adam Szczepaniak
– Provides a framework to understand gluonic excitations.
– The quarks in mesons are sources of color electric flux
which is trapped in a flux tube connecting the quarks.
– Conventional mesons have the flux tube in the ground state.
When the flux tube is excited hybrid mesons emerge. For
static quarks the excitation level above the ground state is
~1 GeV.
– The excitation of the flux tube, when combined with the
quarks, can lead to spin-parity quantum numbers that cannot
be obtained in the quark model
(JPC - exotics).
qqG
qq
1GeV
JPC = 0+-, 1-+, 2+– The decay of hybrid mesons leads to complex final states.
26
LQCD supports the idea of flux tubes.
Flux distribution between
static quarks.
Flux tubes lead to a linear confining
potential.
27
Exotic Hybrid Mesons Masses
With 3 light quarks the
conventional and hybrid
mesons form flavor
nonets for each JPC.
28
Evidence for Exotic Meson with JPC = 1-+
E852 - Evidence for the π1(1600) in π-p → η′π-p
2.0
1.5
2.5
M(η’π-) (GeV)
45
M = 1597 10 10 MeV / c
2
 = 340  40 50
MeV
/
c
50
2
29
Evidence for Exotic Mesons
JPC = 1-+
E852 BNL
There is controversy about some
of these channels.
(A. Dzierba et al., Phys.Rev.D73:072001,2006)
• experimental issues.
• truncation of partial wave included
in analysis.
• interpretation of line shape and
phase motion.
30
Photons may be more suited to excite exotics
• In the flux tube model, using photon beams, the production rate of hybrid
mesons is not suppressed compared to conventional mesons.
N. Isgur, PRD (1999); A. Afanasev & A. Szczepaniak, PRD (2000); F. Close & J. Dudek (2004)
31
A first search for exotic meson with photons
102 Events/ 20 MeV
 Clarify evidence for exotic meson states,
e.g. at 1600 MeV with high statistics.
 Prepare for full study with GlueX.
Experiment planned to
run in 2007/2008.
Events from previous CLAS experiment.
a2
45
35
p2
a1
25
Gluonic Meson?
p1(1600)
15
5
0.8
1.2
1.6
2.0
Expect 1-2 million 3-pion events,
3 orders more than any previously
published meson photoproduction
results, allowing a partial wave analysis.
32
GlueX – Exotic meson program at 12GeV
To meet these goals GlueX will:
33
GlueX – Mass reach
Other facilities:
PANDA (GSI/Darmstadt).
cc and cc-hybrid production in
pp annihilation.
34
Summary
•
Transition form factors of the NΔ(1232) measured in large Q2 range.
– no sign of approaching asymptotic QCD limit, needs 12 GeV upgrade
– pion dressing of vertex needed to describe form factors
•
Roper P11 transition form factor determined for the first time.
– zero-crossing of magnetic form factor
– behaves like a Q3 radial excitation at short distances
•
Tantalizing hints of new baryon states in KY and Nππ channels
– require polarization data to resolve ambiguities in analysis
•
Measurement of multiple polarization observables in Nπ, pη, and KY production
needed to resolve ambiguities in baryon resonance analysis. EBAC essential to
support the baryon resonance program with coupled channel calculations.
•
Experiment to clarify status of some exotic meson candidates with photon
beams in preparation.
•
Full program with GlueX at the JLab 12GeV Upgrade will map out exotic hybrid
meson mass spectrum with high precision.
35
JLAB-RIA Workshop
Wash. DC
16-17 Oct 2006
Ted Barnes
Physics Div. ORNL
Dept. of Physics and
Astronomy, U.Tenn.
Structure of Exotic Mesons
…in 2-3 slides, 5 mins, concentrate on future developments.
1. Definition(s) of exotic mesons
2. Current theoretical expectations (< 2.5 GeV)
3. Future developments
1. Exotic meson defn. (Wikipedia):
“…quantum numbers not possible for mesons in the quark model.” (hence .ne. qq )

2 2
4
FLAVOR EXOTICS: e.g. isospin=2, m : requires higher Fock states, e.g. q q . Controversial! (recall q q)
SPIN-PARITY EXOTICS: J
PC

forbidden to qq, e.g. 1 . Hybrid mesons “qqg”. Not controversial!
2. Current theoretical expectations for exotic mesons: (< 2.5 GeV)
SPIN-PARITY (J
All J
PC
PC
) exotics in this mass range are expected only from hybrids “qqg”.
can be formed from qqg.
____________________________________________________________________________________________________________________________________________________________
The lightest hybrid multiplet is predicted to contain
0  , 1  , 2  , 1 
all this and 0  , 1  , 2  , 1 
JPC =
(bag model)
(flux tube model)
9 flavor states (qq > flavor nonets). Hence 36 new states (bag) or 72 new states (f.t.)
____________________________________________________________________________________________________________________________________________________________
At what mass?
MH = 1.9 GeV (famous flux-tube estimate, Isgur and Paton),
ca. 1½ GeV (bag model), 2.0 GeV (LGT).
____________________________________________________________________________________________________________________________________________________________
Decaying to what?
Famous flux-tube prediction: H > S+P modes, e.g. f p, b p.
This may be wrong. Simple S+S modes (hp, rp, h’p) should also be studied.
The best experimental exotic candidate, p (1600), is seen in h’p !
1
1
3. Future developments:
_____________________________________________________________
Experiment JLAB:
High statistics investigation of meson spectroscopy using photoproduction
(Gluex/HallD/Jlab).
Other facilities:
PANDA (GSI/Darmstadt). cc and cc-hybrid production in pp annihilation.
“Super-LEAR” Start date ca. 2012. 350 collaborators.
____________________________________________________________
Theory Improved LGT studies of hybrid spectroscopy. LGT H strong decays.
Understanding photoproduction of meson resonances (CEX vs diffractive).
Adam Szczepaniak, University of Indiana
p p  hp p
50
M = 1370 16 30 MeV / c
2
 = 385  4065
105 MeV / c

0
p p  hp n


p p  h p p
45
M = 1597 10 10 MeV / c
2
 = 340  40 50
50 MeV / c
2
BNL (E852)
confirmed by Crystal
Barrel
2
BNL (E852)
What is the QCD nature of exotic signals? For reactions involving ground state
pseudo-scalars there exists the possibility of effective theory formulation (chiral GB’s
have derivative couplings and the U(1) GB couples via (heavy) glueball). What
follows is a coupled channel analysis. For example to order p4 (schematically)
(other approaches use
dispersion relations - Roy
eqs. - yield similar
amplitudes)
Meson spectrum up to   1.2 GeV
(Gasser, et al., Pelaez et al., Oset et al., Lesniak
et al.)
Includes the ρ,K*; σ,κ, f0, a0 ;
f2
Effective Lagrangian projected onto the exotic
channel (Marco,Bass) reproduces the E852
ηπ and η’π signals (AS et al.)
P-wave
P -wave  ,  
2 coupled channels
_
S, D -wave ,  ,  
3 coupled channels
...but can we say something about the what is the quark content
...
Yes, study the large-Nc dependence of S-matrix poles
resonance pole moves
towards the real axis
resonance pole moves
deep into the complex
plane
like a quark model
state
complicated !
The Nc dependence of the E852 exotic is
currently under study (J.R.Pelaez, AS)
Partial Wave Analysis at CMU
Curtis A. Meyer, Carnegie Mellon University
Photoproduction of hadrons
gp  XN
Now:
In the CLAS experiment at Jefferson Lab, there significant photoproduction
data that can be used to provide information on baryon resonances.
Future:
The GlueX experiment will study photoproduction of mesons to search for
exotic-quantum-number hybrids. States that involve the confining gluonic
field in their quantum numbers.
A partial wave analysis combines information from the initial state
with angular and energy distributions of the final state particles
to reconstruct the quantum numbers of intermediate states.
gp  N *  p(  p p p 0 )
s-channel process in CLAS
t-channel process in GlueX
gp  p(b2  p ( r  pp ))
Missing Baryons?
Baryons are made from three quarks:
proton = uud neutron = ddu
Predict a spectrum of baryons of spin (J) and parity (P)
p,n Positive Parity States
(Observed)
Negative Parity States
(Observed)
Positive Parity States
(About 50% Missing)
What are the effective degrees of freedom in a baryon?
If “quarks”, then we are missing states.
If “quarks” and “small di-quarks”, then we might have all states.
Analysis Procedure
For two-body final states, the analysis can be simplified. For more
than two bodies, the correlations between particles provides
significant. This information is most effectively retained by keeping
the events themselves, and not trying to bin the data.
Using 11TB of CLAS data from a recent run period,
simultaneously analyzing reactions:
gp!ph
g p ! p h0
gp!p
g p ! L K+
g p ! pp+p-
700,000 Events
250,000 Events
8,000,000 Events
1,200,000 Events
¼1
Events
These channels have not been extensively studied
and are supposed to couple to some missing
baryons.
The Machinery
Very CPU intense process.
Can impose constraints at the event level between different final states
E.g. h and h0 have to couple to the same states
Get total and differential cross sections as a free by-product. They
are projected out.
Get partial wave intensities and phases as a function of energy,
which can then be fit to a resonance picture.
We have the ability to put some models directly into the fit and
se how well they describe the data.
Preliminary Results
h and h0 showing similar
coupling to the same
partial wave.
hp
p
Exotic Mesons
49