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Cascade Physics with CLAS

Cascade Physics with CLAS. Aleksandr Starostin Ben Nefkens UCLA. Outline. Motivation Measurements with CLAS6 Cascades with CLAS12 Summary. Cascades: Physics Motivation. Review of Cascade physics: Workshop “Cascade Physics: A New Window on Baryon Spectroscopy”, December 1-3, 2005, JLab

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Cascade Physics with CLAS

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  1. Cascade Physics with CLAS Aleksandr Starostin Ben Nefkens UCLA

  2. Outline • Motivation • Measurements with CLAS6 • Cascades with CLAS12 • Summary A.Starostin, Ξ@CLAS12

  3. Cascades: Physics Motivation Review of Cascade physics: Workshop “Cascade Physics: A New Window on Baryon Spectroscopy”, December 1-3, 2005, JLab Presentations available at http://conferences.jlab.org/cascade/ A.Starostin, Ξ@CLAS12

  4. What is known about cascades? • Only 11 Ξ* known; 6 “well-established” • Those we know aren't known well: only have JP for three states and a guess of a fourth • SU(3) symmetry requires one Ξ I=1/2 per octet and per decuplet: n(Ξ*)=n(N*) +n(Δ*) Ξ0 = uss, Ξ-= dss Adamovich et al., EPJ C5, 621 Hemingway et al., PLB 68, 1972 A.Starostin, Ξ@CLAS12

  5. Width of the Cascades N* and Δ* resonances overlap, because of that the spectroscopy of N* and Δ* requires complicated (and ambiguous) phase-shift analysis If cascades are as narrow as expected, the parameters of the cascade excited states can be extracted directly from the mass spectra A.Starostin, Ξ@CLAS12

  6. Cascades in quark models 45 cascades states with masses below 2.5 GeV S.Capstick and N. Isgur PRD 34 2809 (1986) A.Starostin, Ξ@CLAS12

  7. Cascades in algebraic model 33 cascades states with masses below 2.5 GeV A.R. Bijker, F.Iachello, and A. Leviatan Ann. Phys. 284 89 (2000) A.Starostin, Ξ@CLAS12

  8. Cascades in the Constituent Quark Model S.Capstick and N.Isgur • Reasons for Ξ* to be narrow: • Phase-space factor for the decay Ξ*->Ξπ compared to N*->Nπ and Δ*->Δπ • Flavor overlap factor Riska relation: Γ(N*,Δ*) : Γ(Λ*,Σ*) : Γ(Ξ*) ≈ 32: 22: 12 A.Starostin, Ξ@CLAS12

  9. Physics with Cascades • Spectroscopy of cascade exited states (complementary to N* and Δ*) • d-u quark mass difference (m(Ξ-)-m(Ξ0) ~ 6.5 MeV, requires good energy resolution), s-d quark mass difference, test of octet-decuplet mass relations • Search for exotic states: Ξ+ and Ξ--;ΞΞbound state A.Starostin, Ξ@CLAS12

  10. Search for parity doublets A.Starostin, Ξ@CLAS12

  11. Search for parity doublets The high-mass light baryon spectrum exhibits occurrence of parity doublets. Glozman suggested to explain the mass degeneracy by assuming that chiral symmetry is restored in highly excited hadronic states. L.Y. Glozman, PL B 475, 329 (2000) T.D. Cohen and L.Y. Glozman PR D 65, 016006 (2001) R.L. Jaffe et al. Phys. Rep. 435, 157 (2006) M. Shifman and A. Vainshtein PR D 77, 034002 (2008) “Evidence for a Parity Doublet Δ(1920)P33 and Δ(1940)D33 from γp->pπ0η” I.Horn et al., PRL 101, 202002 (2008) A.Starostin, Ξ@CLAS12

  12. Photoproduction of cascades • Cascades in CLAS g11: • Number of events: ground state – 7678, first exited state – 658 • maximum energy: 3.8 GeV for most of the run, some data at 4.8 GeV (production threshold ~2.4 GeV) • σtotal(γp -> 2K+Ξ-) ~ 10nb, flat between 3.5 GeV and 4.8 GeV (estimated by Denis Weygand). • New data – run g12: • The data were collected in spring 2007 • Cooking process is in progress J.W. Price et al. PRC 71, 058201 (2005), L. Guo et al. PRC 76, 025208 (2008) A.Starostin, Ξ@CLAS12

  13. Detector requirements • Very good K/π separation – RICH detector can be great help • Two detached vertices – high resolution tracking device is essential • Hall-B: tagged photons with energies below 7.5 GeV, 12 GeV electron beam • CLAS12: very good momentum resolution, good particle ID in forward direction, central tracker A.Starostin, Ξ@CLAS12

  14. Photoproduction vs. Electroproduction • First stage: photoproduction at energies below 7.5 GeV. The energy range covers cascades with masses below 2.85 GeV • Second stage: electroproduction at energies above 7.5 GeV. Will cover masses from 2.5 to 3.8 GeV. LowQ2 spectrometer is required. A.Starostin, Ξ@CLAS12

  15. Acceptance Estimates • Kinematics: γp->Ξ-K+K+, followed by Ξ-->π-Λ and Λ->π-p+ • Resolutions: FD >1%, CD>5% • PID: both kaons in FD • Momentum cutoffs: 300 MeV/c and 500 MeV/c A.Starostin, Ξ@CLAS12

  16. γp->Ξ0K+K0, followed by Ξ0->π0Λ and K0->π-π+, Λ->π-p+ Acceptance Estimates for γp->Ξ0K+K0 A.Starostin, Ξ@CLAS12

  17. Decay modes of excited cascades • Ξ* will be “tagged” using KK in the forward detector • The background can be suppressed by detecting products of the decays • States with similar masses but different JP can be separated using different decay modes A.Starostin, Ξ@CLAS12

  18. Reactions to detect – charged cascades γp->Ξ*K+K+: • γp->(Ξ-π0)K+K+, or γp->(Ξ0π-)K+K+ • γp->(ΛK-)K+K+ • γp->(Σ0K-)K+K+, orγp->(Σ-K0)K+K+ Assuming decaysΞ-->Λπ-(100%), Ξ0->Λπ0(100%), Λ->pπ-(64%), Σ-->nπ-(100%), Σ0->Λγ(100%) and K0s->π+π-all these decay modes can be detected, some with one missing particles A.Starostin, Ξ@CLAS12

  19. Reactions to detect – neutral cascades γp->Ξ*K0K+: • γp->(Ξ0π0)K0K+, orγp->(Ξ-π+)K0K+ • γp->(ΛK0)K0K+ • γp->(Σ0K0)K0K+, orγp->(Σ+K-)K+K+, orγp->(Σ-K+)K+K+ Assuming decaysΞ-->Λπ-(100%), Ξ0->Λπ0(100%), Λ->pπ-(64%), Σ-->nπ-(100%), Σ0->Λγ(100%), Σ+->pπ0(52%), and K0s->π+π-all these decay modes except forγp->(Ξ0π0)K0K+, can be detected, some with one missing particles A.Starostin, Ξ@CLAS12

  20. Summary • CLAS12 will provide almost 4π geometrical acceptance, very good momentum resolution and PID capabilities combined with 7.5GeV tagged photon beam. All this makes cascade program with CLAS12 feasible. Both, charged and neutral cascades can be detected using missing mass and invariant mass techniques. • The objectives of the cascade program will be to establish spectrum of the excited states and their spin-parity. The data can also be used to investigate existence of the cascade parity doublets, calculate d-u quark mass difference and search for the exotic cascade states. A.Starostin, Ξ@CLAS12

  21. A.Starostin, Ξ@CLAS12

  22. Cascades in 1/Nc expansion • 70-plet: • 7 masses in range 1750-2000 MeV • Two pairs of same J states nearly degenerate • L=2 56-plet: • 6 masses in range 2000-2250 MeV • Same J states separated by >140 MeV • Larger errors in predictions than in 70-plet • Parameter free mass relations rather well tested. Presented by J L Goity A.Starostin, Ξ@CLAS12

  23. Search for “Cascaderium” NN scattering length a =-17.3 fm. If ΞΞ 1S0 potential is same as for NN: predicted a BOUND STATE: dibaryon S=-4 decays weakly to ΞΛπ->NN4π ΞΞ binding predicted by many potential models γd -> 2K+ 2K0 [Ξ0 Ξ-] γα -> 2K+ 2K0 [Ξ0 Ξ-] n p Neutral cascades are preferred to avoid repulsion (J.Miller, Cascade workshop) Figure by R.Jones A.Starostin, Ξ@CLAS12

  24. Search for exotic Ξ+ and Ξ-- • Ξ+ and Ξ–- are unique signatures for a five-quark state, member of the antidecupled • Evidences for existance of Ξ–- were found by CERN NA49 (PRL, 92, 042003 (2004)) • Search was also conducted by CLAS (run g12?) A.Starostin, Ξ@CLAS12

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