Workshop on “Extractions and interpretations of hadron resonances and multi-meson production  reactions with 12 GeV upgrade”, May 27-28, 2010

Cascade Baryons: Spectrum and production in photon-nucleon re-

actionsYongseok Oh

(Kyungpook National University, Korea)

2010-05-27 EBAC Workshop, JLab 2

1. Introduction2. Strangeness and baryons

1) In experiments2) In theory

3. Photoproduction of X4. Outlook

Overview

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1. Introduction What do we know about X baryons?

Strangeness baryons: (: light u/d quark) Baryon number = 1, isospin = ½

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Baryons in SU(3)

Baryon octet Baryon decuplet

Baryons: made of three quarks

10881333 :flavor 2321212121 ,:spin ⨁ 𝐿

𝑱 𝑷=𝟏/𝟐+¿¿ 𝑱 𝑷=𝟑/𝟐+¿¿

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1. Introduction What do we know about X baryons?

Strangeness baryons: (: 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

States whose is known

P is not directly measured

Cf. Spin of (

was confirmedonly recently

by BaBarPRL 97 (2006)

1/2+¿ ¿3 /2+¿¿

3 /2−

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1. Introduction What do we know about X baryons?

Strangeness baryons: (: 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 and are in the four star status Only three states with known spin-parity the quantum num-

bers of other states should be identified Advantages & difficulties

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Advantages

Difficulties

• Small decay widths• Identifiable in missing mass plots• Isospin is .

( nonstrange sector: and )• No flavor singlet state (unlike hyper-

ons)

• In most cases, initial state has been used no hadron beams for X physics

• With initial state, 3-body final states at least cross section is very small ~ other technical difficulties PDG 2008

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1. Introduction What do we know about X baryons?

Strangeness baryons: (: 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 and are in the four star status Only three states with known spin-parity the quantum num-

bers 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 hyperons (in production mechanisms)• New particles

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2.1 Strangeness and baryons (Expt.)Experiments

WA89 (CERN-SPS) EPJC, 11 (1999), hep-ex/0406077

-nucleus collisions

1690

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CLAS@JLab

PRC 71 (2005) PRC 76 (2007)

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Questions PDG 2008

The 3rd lowest state

1. Does really exist?2. or ?

Most recent report on : NPB 189 (1981)3. What are their spin-parity quantum num-

bers? comparison with theoretical predictions

CLAS: PRC 76 (2007)

1620?1690?

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2.2 Strangeness and baryons (Theory)

• Classify the states as members of octet or decuplet• Use spin-parity (if known) and Gell-Mann—Okubo mass rela-

tion• Works before 1975: reviewed by

Samlos, Goldberg, Meadows RMP 46 (1974)• Recent work along this line

Guzey & Polyakov, hep-ph/0512355 (2005)• No dynamics

Direct extension of the classification in the quark model

• Most parameters of models are fixed by the and sector in principle, no free parameter for the

• Most models give (almost) correct masses for and Requirement to survive SU(3) group structure

• But they give very different spectrum for the excited states!

Hadron models for X baryons

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Nonrelativistic quark modelChao, Isgur, Karl PRD 23 (1981)

from S. Capstick

• has ?• The first negative parity

state appears at MeV.• Decay widths are not fully

calculated by limiting the final state (but indicates narrow widths)The 3rd lowest state

at 1695 MeV?

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Relativistic quark modelCapstick, Isgur PRD 34 (1986)

from S. Capstick

Negative states have lower mass• The third lowest has

at MeV. • Where is ?

The 3rd lowest state ?

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One-boson exchange modelGlozman, Riska Phys. Rep 268 (1996)

from S. Capstick

Negative states have lower mass• Degeneracy pattern appears• No clear separation between

(+) and (–) parity states• Where is ?

The 3rd lowest state ?

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Large (constituent quark model)

• Based on quark model• Expand the mass operator by expansion• Mass formula (e.g. 70-plet)

• Fit the coefficients to the known masses and predict.

Large quark model

31

110 n

nnn

nn dc ˆˆ

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The 3rd lowest state ?

from J.L. Goity

• Where is ?

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Summary

QM (Pervin, Roberts)132518912014

PRC 75

15201934202017251811

17591826

1820 (expt.)

Expt.: ,

1320 (expt.)

1530 (expt.)

: the 3rd lowest state

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Summary

• The predicted masses for the third lowest state are higher than 1690 MeV (except NRQM)

• How to describe ? • The presence of is puzzling, if it exits.

Highly model-dependent !

Cf. similar problem in QM:

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Skyrme model

bound kaon

SU(3) is badly broken

Treat light flavors and strangenesson the different footing

L = LSU(2) + LK/K*

Soliton provides background potentialwhich traps K/K* (or heavy) meson

Bound state approach(Callan, Klebanov)

Anomaly terms(i) Push up the state

to the continuum} no bound state

(ii) Pull down the statebelow the threshold} bound state} give hyperons

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Bound state model

• Renders two bound states with negative strangeness p-wave: lowest state s-wave: excited state

• After quantization p-wave: positive parity hyperons s-wave: negative parity hyperons

270 MeV energy differ-ence

• Includes parameters• They should be computed with a given Lagrangian (dynam-

ics).• Or fix them to known masses and then predict.

Mass formula

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Hyperon spectrum (expt.)

289 MeV

290 MeV

285 MeV positive parity

negative parity

parity undetermined

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Hyperon spectrum (Skyrme model)

YO, PRD 75 (2007)

spin-parity

Recently confirmed by COSY

PRL 96 (2006)

BaBar : of is PRD 78 (2008)NRQM predicts

Unique prediction of this model.

The should be there.still one-star resonance

High precision experiments are re-quired!

W’s would be discovered in future.

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More comments

• Kaons: one in p-wave and one in s-wave ()

: soliton spin (), : spin of the p(s)-wave kaon (): both of them can lead to statesTherefore, two states and one state

In this model, it is natural to have two states and their masses are 1616 MeV & 1658 MeV!

• Clearly, different from quark models

Two states

Unitary extension of chiral perturbation theoryRamos, Oset, Bennhold PRL 89 (2002)

state at MeVGarcia-Recio, Lutz, Nieves, PLB 582 (2004)

Claim that the and are states

Other ap-proaches

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• Earlier work– A few experiments on inclusive photoproduction– Tagged Photon Spectrometer Collab. NPB 282 (1987)

• photoproduction by CLAS@JLab PRC 76 (2007)– The reaction of – Total cross sections– Differential cross sections for X and 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

3. Photoproduction of

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• In kaon—anti-kaon production, , meson production processes, es-pecially meson production, are important.

• In photoproduction, – such processes are suppressed since the produced meson should be

exotic having strangeness in order to decay into two kaons.– by the same reason, -channel meson-exchange for is also suppressed

as the exchange meson should have .

Forbidden or suppressed mechanisms

E: exotic meson with

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• Consider and exchange only.

– Axial-vector mesons: lack of information & heavy mass

– Scalar or mesons: not al-lowed since coupling is for-bidden by angular momen-tum and parity conservation.

• Consider– and – low-lying and hyperons– and

Considered diagrams

+ exchanged diagrams q1 n q2

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• Problems – There are many hyperon resonances of , 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 Nijme-gen 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:

– Their couplings are rather well-known.• Higher mass hyperons:

– Expect important role of higher mass hyperon resonances GeV– Photoproduction amplitude at the intermediate hyperon on-shell point

)(), )(), – Consider and resonances only

Methods

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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 sectionCLAS: PRC 76

(2007)

𝛾𝑝→𝐾 +¿𝐾+¿ Ξ−¿ ¿

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Differential cross sections

𝑑𝜎 /𝑑 cos𝜃Ξ 𝑑𝜎 /𝑑 cos𝜃𝐾

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invariant mass distribution

No structure

Absence of exotic meson

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invariant mass distribution

NOT from a resonance

hyperon resonancein the mass ~ 2

GeV ?

Needs higher-mass reso-nances

More works are needed!

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• 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 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

4. Outlook

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Preliminary