Cascade Baryons: Spectrum and
production in photon-nucleon reactions
Yongseok Oh
(Kyungpook National University, Korea)
Workshop on “Extractions and interpretations of hadron resonances and
multi-meson production reactions with 12 GeV upgrade”, May 27-28, 2010
Overview
1.
2.
Introduction
Strangeness −2 and −3 baryons
1) In experiments
2) In theory
3. Photoproduction of X(1318)
4. Outlook
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1. Introduction
 What do we know about X baryons?
 Strangeness −2 baryons: 𝑞𝑠𝑠 (𝑞: light u/d quark)
 Baryon number = 1, isospin = ½
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Baryons in SU(3)
Baryons: made of three quarks (𝑞𝑞𝑞)
flavor : 3  3  3  1  8  8  10
1 1 1 1 3
spin :    , ⨁ 𝐿
2 2 2 2 2
Baryon octet
Baryon decuplet
𝑱𝑷 = 𝟏/𝟐+
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𝑱𝑷 = 𝟑/𝟐+
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1. Introduction
 What do we know about X baryons?
 Strangeness −2 baryons: 𝑞𝑠𝑠 (𝑞: light u/d quark)
 Baryon number = 1, isospin = ½
 If flavor SU(3) symmetry is exact for the classification of all particles,
then we have N(X*) = N(N*) + N(D*)
 Currently, only a dozen of X baryons have been identified so far.
(cf. more than 20 N*s & more than 20 D*s)
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X in PDG
• What do we know about X baryons?
Particle Data Group (2008): 11 X’s
1/2+
3/2+
P is not directly measured
Cf. Spin of Ω−
(= 3/2)
was confirmed
only recently
by BaBar
3/2−
PRL 97 (2006)
States whose 𝐽𝑃 is known
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1. Introduction
 What do we know about X baryons?
 Strangeness −2 baryons: 𝑞𝑠𝑠 (𝑞: light u/d quark)
 Baryon number = 1, isospin = ½
 If flavor SU(3) symmetry is exact for the classification of all particles,
then we have N(X*) = N(N*) + N(D*)
 Currently, only a dozen of X baryons have been identified so far.
(cf. more than 20 N*s & more than 20 D*s)
 Only Ξ(1318) and Ξ(1530) are in the four star status
 Only three states with known spin-parity  the quantum numbers of
other states should be identified
 Advantages & difficulties
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Advantages
•
•
•
•
Small decay widths
Identifiable in missing mass plots
Isospin is 1/2.
(↔ nonstrange sector: 𝑖 = 1/2 and 3/2)
No flavor singlet state (unlike Λ
hyperons)
Difficulties
•
•
In most cases, 𝑆 = −1 initial state has
been used  no hadron beams for X
physics
With 𝑆 = 0 initial state,
 3-body final states at least
 cross section is very small ~ 𝑂(nb)
 other technical difficulties
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PDG 2008
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1. Introduction
 What do we know about X baryons?
 Strangeness −2 baryons: 𝑞𝑠𝑠 (𝑞: light u/d quark)
 Baryon number = 1, isospin = ½
 If flavor SU(3) symmetry is exact for the classification of all particles,
then we have N(X*) = N(N*) + N(D*)
 Currently, only a dozen of X baryons have been identified so far.
(cf. more than 20 N*s & more than 20 D*s)
 Only Ξ(1318) and Ξ(1530) are in the four star status
 Only three states with known spin-parity  the quantum numbers of
other states should be identified
 Advantages & difficulties
 No meaningful information for the X resonances
⇛ it can open a new window for studying hadron structure
• Baryon structure from X spectroscopy
• Properties of 𝑆 = −1 hyperons (in production mechanisms)
• New particles
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2.1 Strangeness −2 and −3 baryons (Expt.)
Experiments
WA89 (CERN-SPS)
EPJC, 11 (1999), hep-ex/0406077
1690
Σ − -nucleus collisions
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CLAS@JLab
PRC 71 (2005)
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PRC 76 (2007)
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Questions
PDG 2008
Ξ(1530)
1620 ?
The 3rd lowest state
1690 ?
1. Does Ξ(1620) really exist?
2. Ξ(1620) or Ξ(1690)?
Most recent report on Ξ(1620): NPB 189 (1981)
3. What are their spin-parity quantum numbers?
↔ comparison with theoretical predictions
CLAS: PRC 76 (2007)
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2.2 Strangeness −2 and −3 baryons (Theory)
Direct extension of the classification in the quark model
•
Classify the states as members of octet or decuplet
•
Use spin-parity (if known) and Gell-Mann—Okubo mass relation
•
Works before 1975: reviewed by
Samlos, Goldberg, Meadows RMP 46 (1974)
•
Recent work along this line
Guzey & Polyakov, hep-ph/0512355 (2005)
•
No dynamics
Hadron models for X baryons
•
Most parameters of models are fixed by the 𝑆 = 0 and 𝑆 = −1 sector
 in principle, no free parameter for the 𝑆 = −2, −3
•
Most models give (almost) correct masses for 𝛯(1318) and 𝛯(1530)
 Requirement to survive
 SU(3) group structure
•
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But they give very different spectrum for the excited 𝛯 states!
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Nonrelativistic quark model
Chao, Isgur, Karl
PRD 23 (1981)
The 3rd lowest state
at 1695 MeV?
•
𝛯(1690)∗∗∗ has 𝐽𝑃 = 1/2+ ?
•
The first negative parity state
appears at ~1800 MeV.
•
Decay widths are not fully
calculated by limiting the final
state (but indicates narrow
widths)
from S. Capstick
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Relativistic quark model
Capstick, Isgur
PRD 34 (1986)
Negative states have lower mass
• The third lowest has 𝐽𝑃 = 1/2−
at ~1750 MeV.
• Where is 𝛯(1690)?
The 3rd lowest state ?
from S. Capstick
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One-boson exchange model
Glozman, Riska
Phys. Rep 268 (1996)
Negative states have lower mass
• Degeneracy pattern appears
• No clear separation between (+)
and (–) parity states
• Where is 𝛯(1690)?
The 3rd lowest state ?
from S. Capstick
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Large 𝑁𝑐 (constituent quark model)
Large 𝑁𝑐 quark model
•
Based on 𝑂(3) × 𝑆𝑈(6) quark model
•
Expand the mass operator by 1/𝑁𝑐 expansion
•
Mass formula (e.g. ℓ = 1 70-plet)
11
3
n 0
n 1

   cnˆ n   d n ˆ n
•
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Fit the coefficients to the known masses and predict.
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from J.L. Goity
• Where is 𝛯(1690)?
The 3rd lowest
state ?
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Summary
PRC 75
QM (Pervin, Roberts)
1530
(expt.)
1325
1891
2014
1520
1934
2020
1725
1811
1820
(expt.)
1759
1826
1320
(expt.)
Expt.: Ξ(1620)∗ , Ξ(1690)∗∗∗
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: the 3rd lowest state
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Summary
Highly model-dependent !
•
The predicted masses for the third lowest state are higher than 1690
MeV (except NRQM)
•
•
How to describe 𝛯(1690)?
The presence of 𝛯(1620) is puzzling, if it exits.
Cf. similar problem in QM: Λ(1405)
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Skyrme model
Bound state approach
(Callan, Klebanov)
bound kaon
SU(3) is badly broken
Treat light flavors and strangeness
on the different footing
L = LSU(2) + LK/K*
Anomaly terms
(i) Push up the 𝑆 = +1 state
to the continuum
} no bound state
(ii) Pull down the 𝑆 = −1 state
below the threshold
} bound state
} give hyperons
Soliton provides background potential
which traps K/K* (or heavy) meson
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Bound state model
•
Renders two bound states with negative strangeness
 p-wave: lowest state
 s-wave: excited state
•
270 MeV energy difference
After quantization
 p-wave: positive parity hyperons
Λ(1116)
 s-wave: negative parity hyperons
Λ(1405)
Mass formula
•
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Includes parameters
•
They should be computed with a given Lagrangian (dynamics).
•
Or fix them to known masses and then predict.
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Hyperon spectrum (expt.)
parity undetermined
negative parity
290 MeV
positive parity
285 MeV
289 MeV
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Hyperon spectrum (Skyrme model)
Recently confirmed by COSY
PRL 96 (2006)
BaBar : 𝐽𝑃 of Ξ(1690) is 1/2−
PRD 78 (2008)
NRQM predicts1/2+
High precision experiments are required!
Unique prediction of this model.
The Ξ(1620) should be there.
still one-star resonance
YO, PRD 75 (2007)
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W’s would be discovered in
future.
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More comments
Two 𝜩 states
Kaons: one in p-wave and one in s-wave
•
 𝐽 = 𝐽𝑠𝑜𝑙 + 𝐽𝑚 (𝐽𝑚 = 𝐽1 + 𝐽2 )
𝐽𝑠𝑜𝑙 : soliton spin (= 1/2),
𝐽1 (𝐽2 ): spin of the p(s)-wave kaon (= 1/2)
𝐽𝑚 = 0 and 1: both of them can lead to 𝐽𝑃 = 1/2− 𝛯 states
Therefore, two 𝐽 = 1/2− states and one 𝐽 = 3/2− state
 ∴ In this model, it is natural to have two 1/2− states and their masses
are 1616 MeV & 1658 MeV!
Clearly, different from quark models
•
Other approaches
Unitary extension of chiral perturbation theory
Ramos, Oset, Bennhold PRL 89 (2002)
1/2− state at 1606 MeV
Garcia-Recio, Lutz, Nieves, PLB 582 (2004)
Claim that the Ξ(1620) and Ξ(1690) are 1/2− states
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3. Photoproduction of Ξ(1318)
•
Earlier work
– A few experiments on inclusive Ξ photoproduction
– Tagged Photon Spectrometer Collab. NPB 282 (1987)
•
Ξ photoproduction by CLAS@JLab
–
–
–
–
•
PRC 76 (2007)
The reaction of 𝛾𝑝 → 𝐾 + 𝐾 + 𝛯−
Total cross sections
Differential cross sections for X and 𝐾 + production angles
Invariant mass distributions in the KK and K X channels
Theoretical work
Nakayama, YO, Haberzettl, PRC 74 (2006)
– Strategy
• Investigate the production mechanism using the currently available
information only
• Then consider other possible (and important) mechanisms
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Forbidden or suppressed mechanisms
•
•
In kaon—anti-kaon production, 𝛾𝑁 → 𝐾𝐾𝑁, meson production processes,
especially 𝜙 meson production, are important.
In 𝛯 photoproduction,
– such processes are suppressed since the produced meson should be exotic
having strangeness 𝑆 = +2 in order to decay into two kaons.
– by the same reason, 𝑡-channel meson-exchange for 𝐾𝑁 → 𝐾𝑋 is also
suppressed as the exchange meson should have 𝑆 = +2.
E: exotic meson with 𝑆 = +2
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Considered diagrams
•
Consider 𝐾 and 𝐾 ∗ exchange
only.
– Axial-vector 𝐾1 mesons: lack of
information & heavy mass
– Scalar 𝜅 or 𝐾0 mesons: not
allowed since 𝜅 → 𝐾𝛾 coupling
is forbidden by angular
momentum and parity
conservation.
•
Consider
– 𝑁′ = 𝑁 and 𝛥
– 𝑌, 𝑌′ = low-lying Λ and Σ
hyperons
– Ξ′ = Ξ(1318) and Ξ(1530)
+ exchanged diagrams
q1 n q2
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Methods
• Problems
– There are many hyperon resonances of 𝑆 = −1, which can contribute to the
production process.
– We start with a very simple model for the production mechanism by choosing
only a few intermediate hyperon states.
• Lots of unknown coupling constants and ambiguities.
– We make use of the experimental (PDG) or empirical data (like Nijmegen
potential) if available.
– Or we use model predictions for the unknowns: SU(3) relations, quark model,
ChPT, Skyrme model, chiral quark model etc.
• Low mass hyperons:
Λ(1116), Λ(1405), Λ(1520), Σ(1190), Σ(1385)
– Their couplings are rather well-known.
• Higher mass hyperons:
– Expect important role of higher mass hyperon resonances ≳ 1.8 GeV
– Photoproduction amplitude at the intermediate hyperon on-shell point
𝑀1/2± ∝ (𝑚𝑌 ∓ 𝑚𝑁 )(𝑚𝑌 ∓ 𝑚Ξ ),
𝑀3/2± ∝ (𝑚𝑌 ± 𝑚𝑁 )(𝑚𝑌 ± 𝑚Ξ ),
– Consider 1/2− and 3/2+ resonances only
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Intermediate hyperons
Particle Data Group
Decay widths (and couplings) are in a very wide range. No information for the other couplings.
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Total cross section
CLAS: PRC 76 (2007)
𝛾𝑝 → 𝐾 + 𝐾 + Ξ−
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Differential cross sections
𝑑𝜎/𝑑 cos 𝜃𝐾
𝑑𝜎/𝑑 cos 𝜃Ξ
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𝐾𝐾 invariant mass distribution
No structure
Absence of 𝑆 = +2
exotic meson
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𝐾Ξ invariant mass distribution
Needs higher-mass resonances
More works are needed!
𝑆 = −1 hyperon resonance
in the mass ~ 2 GeV ?
NOT from a resonance
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4. Outlook
• Study on the spectrum of X hyperons
– Opens a new window for understanding baryon structure
• Theoretical models for X spectrum
– Different and even contradictory predictions
– What is the third lowest X resonance?
And the quantum numbers?
• Experimentally, more data are required!
– Does Ξ(1620) exist?
– Should confirm other poorly established X resonances in PDG
as well as their quantum numbers
– Almost no information on the W baryon resonances
• Role of L and S resonances in X photoproduction.
– Offers a chance to study those hyperons.
– Higher mass and high spin resonances
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Preliminary
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