Photoproduction of Cascade baryons

Yongseok Oh (UGA)H. Haberzettl (GWU)K. Nakayama (UGA)

nucl-th/0605169

The University of GeorgiaWhat do we know about What do we know about ??

PDG

– If all the particles can be classified as SU(3) flavor octet or decuplet, N() = N(N*) + N(*)

– So far, only a dozen or so of have been identified.– Only (1318) and (1530) have four-star status.– Even the quantum numbers of most of the resonances are unknown. – So, very little is known about the resonances. But this may offer a

good opportunity to find many interesting physics.

possibility of being in part a pentaquark (1520)S11 (B.-S. Zou, this meeting).

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Cascade (S=-2) baryons:Cascade (S=-2) baryons:

GS: (1318)P11 1st ES:(1530)P13

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Theory of Theory of baryons (spectrum and decays):baryons (spectrum and decays):

Quark Models: ● SU(3), NR, EME decay model (Chao, Isgur, Karl, PRD23, ‘81). ● SU(3), NR, OPE model (Glozman, Riska, PR268, ‘96). ● SU(3), semi-rel., OBE model (Glozman et al., PRD58, ‘98). ● SU(3), OBE+OGE model (Valcarce, Garcilazo, Vijande, PRC72, ‘05).

● 1/Nc expansion of QCD (Schat, Goity, Scoccola, PRL88, ‘02).

Other works in progress: ● SU(3) quark model, relativistic (S. Capstick & collaborators). ● (quenched) lattice QCD (N. Mathur, D. Richards).

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baryon spectrum (predictions and expt):baryon spectrum (predictions and expt):

Extracted from S. Capstick, Cascades@Jlab, July 29 2006

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An interesting feature of Cascades:An interesting feature of Cascades:

* decays are suppressed with respect to N. For example: (1232) p ~ 120±5 MeV ~ 9-10 MeV- Other channels involve K, which cuts down the available phase space.

- Leads to the possibility of narrow excited states. - Why are they narrow? Some of this is phase space: decay momentum for (P-wave) is 227 MeV; *(1530) (P-wave) is 152 MeV.

The University of Georgia decay widths:decay widths:

Extracted from S. Capstick, Cascades@Jlab, July 29 2006

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baryons should be investigatedbaryons should be investigated

Cascade baryons should be studied as an integral part of the baryon spectroscopy program:

● being an S=-2 baryons they are produced only indirectly and have relatively low production rates (~ nb). ● it has received attention recently in connection with the search for pentaquark baryons (NA49 collab., PRL92, ’04). ● the CLAS collaboration at JLab has initiated a cascade physics program recently: cascade spectroscopy through photoproduction off nucleons (J.Price et al., PRC71, ’05 and refs.

therein). ● only one early inclusive photoproduction of reported (TAPS collab., NPB282, ‘87, at T=105 GeV).

The University of Georgiapp→K→K++KK++

L. Guo & D. P. Weygand, for CLAS collab., L. Guo & D. P. Weygand, for CLAS collab., hep-ex/0601011, Proc. NSTAR05hep-ex/0601011, Proc. NSTAR05

preliminary CLAS datapreliminary CLAS data

The University of GeorgiaAim of the present work Aim of the present work ::

(Exploratory) theoretical investigation of the reaction N→KKwithin a relativistic meson-exchange model of hadronic interactions.As a first step toward building a reliable reaction model for analyzing the cascade spectroscopy data, one needs to understand in detail the production mechanism(s) of the well established cascades ((1318)P11, (1530)P13). To date, no cascade photoproduction calculation is available so far, except for the hadronic model calculation by Liu and Ko (PRC69, ’04) in connection with the pentaquark cascade production in →KK5

[includes only the hyperon (1193) in the intermediate state].

(1520)S11? (B.-S. Zou).

The University of Georgia KKKK (model):(model):

K-exchange N/N’

K*-exchange

contact current

’

+ ( K1(q1)↔K2(q2) )

Y= Y’ resonance currentY≠Y’ radiative decay

The University of GeorgiaNN→→KKKK(model):(model):

require an exotic meson (S=+2) exchange; therefore,they are not considered in thepresent model

t-channel Drell-type processes:

The University of Georgia KKKK (baryon resonances included):(baryon resonances included):

(1116), (1405), (1520)(1193), (1385)(1530)(1232) ← negligible

all the model parameters fixed from the relevant decay rates(PDG)and/or quark models and SU(3) symmetry considerations.

no enough information to fix the parameters ofthe model.

The University of Georgia KKKK (model parameters):(model parameters):

The University of GeorgiaNN→KK→KK (free parameters of the model) :(free parameters of the model) :

ps-pv mixing parameter: BYK vertex (spin-1/2 baryons B and Y):

= ps-pv mixing parameter) = 0 , ps-coupling = 1 , pv-coupling

gBK = ± 0.91 , ), B=N,

g′= ± 1.26 , (1116), ′(1520)g′= ± 2.22 , (1193), ′(1520)

signs of :

← radiative transition vertex

← BK vertex

€

ˆ Γ + = γ 5

€

ˆ Γ − =1

The University of Georgia KKKK (hadronic form factors):(hadronic form factors):

p p′

q

FB & n: free parameters but the same for all B

K = 1.3 GeV

K* = 1.0 GeV

[n→∞: fB(p2) → Gaussian with width B]

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NN→KK→KK (preliminary CLAS data, L. Guo & D. P. Weygand, for CLAS collab., (preliminary CLAS data, L. Guo & D. P. Weygand, for CLAS collab.,

hep-ex/0601011, Proc. NSTAR05) hep-ex/0601011, Proc. NSTAR05)

BYK (ps-coupling) (B, n)=(1.25GeV, 2)

BYK (pv-coupling) (B, n)=(1.38GeV, ∞)

phasespace

PRELIMINARY CLAS DATA

The University of GeorgiaNN→KK→KK (dynamical content : spin-3/2 hyperon contributions) :(dynamical content : spin-3/2 hyperon contributions) :

+

Y≠Y′ (rad. decay)

Y=Y′ (res)

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NN→KK→KK (preliminary CLAS data, L. Guo & D.P.Weygand , private (preliminary CLAS data, L. Guo & D.P.Weygand , private

communication) : communication) :

(x 15)

Y≠Y′ (rad. decay)

p→K+K+-

PRELIMINARY CLAS DATA

The University of GeorgiaNN→KK→KK (higher mass resonances)(higher mass resonances)

Consider spin-1/2 and -3/2 resonances: ● |gNYK| can be estimated from the partial decay widths.● unless gYK is unrealistically large :

JP=1/2+ and 3/2- are negligibly small !

on-shell:

))((

))((

2/3

2/1

YYN

YYN

mmmmM

mmmmM

±±∝∝

±

± mm

( ) ( )1 4 1 2 1 0 1K N K

K N K

g g

g g f f fΞΛ Λ

ΞΛ Λ

Λ =

Λ = − + < < <

For singlet ,

For octet , for

The University of GeorgiaNN→KK→KK ( addition of higher mass resonances) :( addition of higher mass resonances) :

(2000)3/2+ (gNKgK~2.5) (1850)1/2- (gNKgK~2.0)(1950)3/2+ (gNKgK~2.0)

(B,n) = (1.23GeV,∞)BYK (pv-coupling)

(B,n) = (1.25GeV,∞)BYK (pv-coupling)

The University of GeorgiaNN→KK→KK ( adding ( adding (1850)1/2(1850)1/2-- & & (1950)3/2(1950)3/2++ ) : ) :

PRELIMINARY CLAS DATA

The University of GeorgiaNN→KK→KK ( adding ( adding (1850)1/2(1850)1/2-- & & (1950)3/2(1950)3/2++ ) : ) :

PRELIMINAY CLAS DATA(L.Guo & D.Weygand,private communication)

The University of GeorgiaNN→KK→KK ( adding ( adding (1800)1/2(1800)1/2-- , , (1890)3/2(1890)3/2++ & & (2050)3/2(2050)3/2++ )) : :

(1800)1/2- (gNKgK~2.0)(1890)3/2+ (gNKgK~1.2)2050)3/2+ (gNKgK~1.4)

The University of GeorgiaNN→KK→KK ( adding ( adding (1800)1/2(1800)1/2-- , , (1890)3/2(1890)3/2++ & & (2050)3/2(2050)3/2++ ) : ) :

PRELIMINARY CLAS DATA (L.Guo & D. Weygand, private communication)

The University of GeorgiaNN→KK→KK (higher spin resonances in the 2.0-2.1 GeV region)(higher spin resonances in the 2.0-2.1 GeV region)

● work in progress to include them !● unidentified (2050)3/2+: simulating these high spin states as far as the invariant mass distribution is concerned .

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Spin asymmetriesSpin asymmetries

Photon beam asymmetry & target asymmetry

Caution: Spin asymmetries may be sensitive to production mechanisms and need careful and detailed analyses.

What do we have in these simple models?

( 1) ( 1),

( 1) ( 1)

( 1) :

( 1) :

( 1) ( 1),

( 1) ( 1)

( 1) :

B

N NT

N N

N

σ σ σ σ

σ σ

σ σ σ σ

σ

=+ − =− ≡

=+ + =−=+=−

=+ − =− ≡

=+ + =−=±

linearly polarized photon along the y axis

linearly polarized photon along the x axis

polarized target ± nucleon along the y axis

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Beam Asymmetry Beam Asymmetry BB

Low-mass hyperons + higher-mass hyperons

pv coupling

ps coupling

• K-exchange = -1.

• pv and ps couplings give the similar beam asymmetry.

• beam asymmetry distinguishes the models with and without higher resonances.

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Target Asymmetry Target Asymmetry TT

pv coupling

ps coupling

• Target symmetry has different sign depending on the coupling scheme. 5 5 qμμ vs

with higher-mass hyperons

The University of GeorgiaSummary of our findings :Summary of our findings :

The dominant - production mechanism in p→K+K+- is the t-channel K-exchange process which is crucial in describing the observed backward peaked - and forward peaked K+ angular distributions. Also, the beam asymmetry can possibly provide an independent test of the t-channel K-exchange dominance.

Higher mass hyperons in the mass region of ~1.8-2.1 GeV (in particular, (1800)1/2- and (1890)3/2+) are needed to possibly provide the required t-channel K-exchange dominance. Low mass hyperons instead give raise to a dominant radiative hyperon-hyperon transition processes which lead to a forward peaked - and backward peaked K+ angular distributions (just opposite to what is observed in the preliminary CLAS data).

The target asymmetry can possibly impose a constraint on the ps-pv mixing parameter.

The University of GeorgiaSummary of our findings :Summary of our findings :

The K+- invariant mass distribution data indicate a need for additional resonance(s) in the ~2.0-2.1 GeV region. In fact, there are known spin-5/2 and -7/2 hyperons (with 3 and 4 stars status) precisely in this energy region. We are currently working to include these resonances into the model.

(the unknown (2050)3/2+ was introduced in the present calculation for illustration

purposes to make this point)

Measurements of other isospin channels would help disentangle the isoscalar and isovector hyperon resonance contributions.

The University of GeorgiaConclusion :Conclusion :

To our knowledge, this is the first quantitative calculation of the cascade photoproduction off nucleons.

The basic features of the p→K+K+-(1318) reaction could be understood. In particular, this reaction can be used to help extract information on higher mass hyperon resonances.

The findings of the present work should serve as a basis for building more complete models of cascade photoproduction to help analyze the forthcoming cascade data.

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The End

The University of GeorgiaResonance widthsResonance widths

,

,

,qiR =qi (W=mR )

R→N

R→N

The University of Georgia KKKK (phenomenological contact current):(phenomenological contact current):

q1

p p′

q2

B

bare NBK contact vertex

= NBK vertex

μC=

1

ei-eB-e1=0