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Photoproduction of Cascade baryons

Yongseok Oh (UGA) H. Haberzettl (GWU) K. Nakayama (UGA) nucl-th/0605169. Photoproduction of Cascade baryons. If all the particles can be classified as SU(3) flavor octet or decuplet, N( X ) = N(N*) + N( D *) So far, only a dozen or so of X have been identified.

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Photoproduction of Cascade baryons

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  1. Yongseok Oh (UGA) H. Haberzettl (GWU)K. Nakayama (UGA) nucl-th/0605169 Photoproduction of Cascade baryons

  2. If all the particles can be classified as SU(3) flavor octet or decuplet, N(X) = N(N*) + N(D*) So far, only a dozen or so of X have been identified. Only X(1318) and X(1530) have four-star status. Even the quantum numbers of most of the X resonances are unknown. So, very little is known about the  resonances. But this may offer a good opportunity to find many interesting physics. What do we know about ? PDG possibility of being in part a pentaquark (1520)S11 (B.-S. Zou, this meeting).

  3. Cascade (S=-2) baryons: GS: (1318)P11 1st ES:(1530)P13

  4. Theory of 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).

  5. baryon spectrum (predictions and expt): Extracted from S. Capstick, Cascades@Jlab, July 29 2006

  6. 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.

  7.  decay widths: Extracted from S. Capstick, Cascades@Jlab, July 29 2006

  8.  baryons 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 X • 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).

  9. gp→K+K+ L. Guo & D. P. Weygand, for CLAS collab., hep-ex/0601011, Proc. NSTAR05 preliminary CLAS data

  10. Aim of the present work : (Exploratory) theoretical investigation of the reaction gN→KKXwithin 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 (X(1318)P11, X(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 g→KKX5 [includes only the hyperon S(1193) in the intermediate state]. (1520)S11?(B.-S. Zou).

  11. N  KKX(model): K-exchange N/N’ X/X’ contact current Y= Y’ resonance current Y≠Y’ radiative decay + ( K1(q1)↔K2(q2) ) K*-exchange

  12. gN→KKX (model): t-channel Drell-type processes: require an exotic meson (S=+2) exchange; therefore, they are not considered in the present model

  13. N  KKX(baryon resonances included): L(1116), L(1405), L(1520) S(1193), S(1385) X(1530) D(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 of the model.

  14. N  KKX(model parameters):

  15. gN→KKX (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 signs of : gBLK = ± 0.91 , L(1405), B=N,X gLL′g = ± 1.26 , L(1116), L′(1520) gSL′g = ± 2.22 , S(1193), L′(1520) ← BLK vertex ← radiative transition vertex

  16. N  KKX(hadronic form factors): q p p′ F LB & n: free parameters but the same for all B [n→∞: fB(p2) → Gaussian with width LB] LK = 1.3 GeV LK* = 1.0 GeV

  17. gN→KK(preliminary CLAS data, L. Guo & D. P. Weygand, for CLAS collab., hep-ex/0601011, Proc. NSTAR05) BYK (ps-coupling) (LB, n)=(1.25GeV, 2) BYK (pv-coupling) (LB, n)=(1.38GeV, ∞) phase space PRELIMINARY CLAS DATA

  18. gN→KK(dynamical content : spin-3/2 hyperon contributions) : Y≠Y′ (rad. decay) Y=Y′ (res) +

  19. gN→KK(preliminary CLAS data, L. Guo & D.P.Weygand , private communication) : gp→K+K+X- PRELIMINARY CLAS DATA (x 15) Y≠Y′ (rad. decay)

  20. gN→KK(higher mass resonances) Consider spin-1/2 and -3/2 resonances: ● |gNYK| can be estimated from the partial decay widths. ● unless gYK is unrealistically large : JP=1/2+ and 3/2- are negligibly small ! on-shell:

  21. gN→KK( addition of higher mass resonances) : L(2000)3/2+ (gNLKgXLK~2.5) L(1850)1/2- (gNLKgXLK~2.0) L(1950)3/2+ (gNLKgXLK~2.0) (LB,n) = (1.23GeV,∞) BYK (pv-coupling) (LB,n) = (1.25GeV,∞) BYK (pv-coupling)

  22. gN→KK( adding L(1850)1/2- & L(1950)3/2+ ) : PRELIMINARY CLAS DATA

  23. gN→KK( adding L(1850)1/2- & L(1950)3/2+ ) : PRELIMINAY CLAS DATA (L.Guo & D.Weygand, private communication)

  24. gN→KK( adding L(1800)1/2- , L(1890)3/2+ & L(2050)3/2+) : L(1800)1/2- (gNLKgXLK~2.0) L(1890)3/2+ (gNLKgXLK~1.2) L(2050)3/2+ (gNLKgXLK~1.4)

  25. gN→KK( adding L(1800)1/2- , L(1890)3/2+ & L(2050)3/2+ ) : PRELIMINARY CLAS DATA (L.Guo & D. Weygand, private communication)

  26. gN→KK(higher spin resonances in the 2.0-2.1 GeV region) ●work in progress to include them ! ● unidentified L(2050)3/2+: simulating these high spin states as far as the invariant mass distribution is concerned .

  27. Spin 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?

  28. Beam Asymmetry B 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.

  29. Target Asymmetry T with higher-mass hyperons ps coupling pv coupling • Target symmetry has different sign depending on the coupling scheme.

  30. Summary of our findings : • The dominant - production mechanism in p→K+K+- is thet-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.

  31. 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.

  32. 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.

  33. The End

  34. Resonance widths , , , R→Np : qiR =qi (W=mR ) R→Npp :

  35. N  KKX(phenomenological contact current): q1 q2 p p′ B bare NBKg contact vertex GmC= G1 = NBK vertex ei-eB-e1=0

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