1 / 39

New Cascade Physics Program

New Cascade Physics Program. Yongseok Oh (Univ. of Georgia) With K. Nakayama (UGA) & H. Haberzettl (GWU) Cascade Physics Working group: B. Nefkens et al. Contents. Motivation Experiments Theories Photoproduction process Outlook. 1. Motivation. Characters of the X hyperons

kuper
Télécharger la présentation

New Cascade Physics Program

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. New Cascade Physics Program Yongseok Oh (Univ. of Georgia) With K. Nakayama (UGA) & H. Haberzettl (GWU) Cascade Physics Working group: B. Nefkens et al.

  2. Contents • Motivation • Experiments • Theories • Photoproduction process • Outlook Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  3. 1. Motivation • Characters of the X hyperons • strangeness = -2, baryon number = 1, and isospin = 1/2 • Narrow widths: G(X*)/G(N* or D*) ~ 1/10 for pionic decays • G is proportioanl to (# of light quarks)2Riska, EPJA 17 (2003) • Insignificant sea quark contributions to hyperons expected to have larger effects for excited states Decuplet g octet + p from J. Price Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  4. Why X ? • What do we know about X baryons? • 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 have been identified so far. (cf. more than 20 N*s & more than 20 D*s) • Only X(1318) and X(1530) have four-star status. (cf. the rating is based on the clearness of the peak.) • Even the quantum numbers of most X resonances are still to be identified: practically, no meaningful information for the X resonances. Particle Data Group (2006) Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  5. Advantages • Easy identification • Small decay widths • Identifiable in a missing mass plot, e.g.,missing mass m(K+K+) in g + p g K+ + K+ + X,invariant mass of decay products such as Xgp L • Background is less complicated. (g + p g K+ + K+ + X*g K+ + K+ + p + Xgs) • Isospin ½ (cf. nucleonic resonances have N* & D*; I=1/2 and 3/2) • No flavor singlet state (unlike L hyperons) • What can we learn from it? • Baryon structure from X spectroscopy • Properties of S=-1 resonances • Exotic particles (penta-quarks & tetra-quarks) • New particles (perhaps S=-4 dibaryon?) Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  6. Exotic X(1860) or F(1860) • Isospin-3/2 state: therefore, penta-quark exotic • Report from NA49 in pp collision PRL 92 (2004) • but never be confirmed by other experiments with higher statistics, e.g. WA89 in S--nucleus collisions, PRC 70 (2004)(no signal of X(1860) with the 40-year accumulation of Xp spectra) WA89 NA49 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  7. 2. Experiments • Difficulties in searching for X* • Mostly processes through K-p reactions or the S-hyperon induced reactions were used. (initial state has S=-1) • No current activity in X physics with hadron beams • They can only produced via indirect processes from the nucleon. (initial state has S=0) • In the case of photon-nucleon reaction, we have at least three-body final state. • The current CLAS data indicate that the production cross section is less than 20 nb at low energies. (cf. KL or KS photoproduction have cross sections of order of a few mb). • Other technical difficulties • Questions • What is the third lowest state following X(1320) and X(1530)? • Can we confirm the existence of X(1620)? Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  8. Earlier experiments • WA89 results with S- beam (hep-ex/0406077) • Comments by PDG (2006) 1530 1690 1860(?) Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  9. Recent activity • CLAS at JLab: initiated new Cascade physics programphotoproduction processes: g p g K K X • More data with higher statistics are under analyses. g6a g6b PRC 71 (2005) CLAS preprint (2006) Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  10. 3. Theories • Review on the works before 1975Samlos, Goldberg, and Meadows, Rev. Mod. Phys. 46 (1974) 49 • What is the first excited state following X(1320) and X(1530)? What is X(1690)? • Even the parity of the ground state X was not directly measured. Model predictions for the X spectrum are needed. • Most model builders have not considered X spectrum or the structure of X resonances seriously, except the lowest X’s of octet and decuplet. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  11. Quark model (One-gluon-exchange model) • Non-relativistic quark model Chao, Isgur, Karl, PRD 23 (1981) • First order perturbation calculation in anharmonic terms (linear, Coulomb) and in hyperfine interactions. • X(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) • Relativistic quark model ? from S. Capstick Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  12. One-boson-exchange model Glozman, Riska, Phys. Rep. 268 (1996) • Exchange of octet pseudoscalar mesons. • First order perturbation calculation around harmonic oscillator spectrum. • Negative parity state seems to have lower mass: but no clear separation between +ve and –ve parity states • Strong decay widths are not calculated. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  13. Comparison of OGE and OBE • The two models show very different X hyperon spectrum. • The predictions on the candidate for X(1690) are different. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  14. Improved OBE model • Semi-relativistic OBE model Glozman et al., PRD 58 (1998) • OBE + OGE Valcarce, Garcilazo, Vijande, PRC 72 (2005) Glozman et al. Valcarce et al. MN Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  15. 1/Nc (constituent quark model) Schat, Scoccola, Goity, PRL 88 (2002) and other groups Expand the mass operator by 1/Nc expansion • Basically O(3) X SU(6) quark model • Mass formula (e.g. 70-plet: L=1, p=-1) • Fit the coefficients to the known particle masses and then predict. Where is X(1690)? from J.L. Goity Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  16. QCD sum rules • Mass splitting between 1/2+ and1/2- baryons. • Jido & Oka, hep-ph/9611322 (unpublished) • Interpolating field (with a parameter t) • X(1/2+) = 1320 MeV and X(1/2-) = 1630 MeV. • So, X(1690) would be X(1/2-). • Sum rules for 1/2+, 1/2-, and 3/2-. • F.X. Lee & X. Liu, PRD 66 (2002) • Three-parameter calculation (similar interpolating field) • X(1/2+) = 1320 MeV, X(1/2-) = 1550 MeV, X(3/2-) = 1840 MeV (exp. 1820 MeV) • X(1820) is well reproduced, but where is X(1690)? Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  17. Other hadron models • No rigorous calculation for X spectrum was done in other hadron models in the market. • NJL model, Skyrme model, bag models(?), … • This is a good place to test and improve hadron models. • Various model calculations are highly desirable. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  18. Lattice calculationF.X. Lee et al., Nucl. Phys. B (PS) 119 (2003) • Quenched approx. with Bayesian statistics • Level cross-over in the physical region? • Results for 1/2+ and 1/2- states • Higher-spin states? Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  19. Lattice calculation Bern-Graz-Regensburg Coll., PRD 74 (2006) • Quenched approx. (variational method) • The first excited state seems to have negative parity. • Higher-spin states? X octet with J = 1/2 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  20. S=-4 dibaryon? • A new dibaryon (possibly at JLab & J-PARC) • Feasibility of an 1S0 di-Cascade bound state? • A simple estimate G.A. Miller, nucl-th/0607006 • Both NN and XX are in the same 27-plet representation of SU(3). • N and X iso-doublets occupy analogous positions. • Use 4-point interactions (meson-exchange is ignored) • Invariant under NN 1XX • Maybe good for 1S0 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  21. di-Cascade • NR Schroedinger equation with a potential whose parameters are fixed by n-n/p-p or n-p system. • Square well potential, non-local separable potential, delta-shell potential. • Obtained results for XX system • Scattering length: 8~11 fm • Binding energy: 0.5 ~ 7.5 MeV • Deuteron binding energy ~ 2.2 MeV • With Nijmegen potential (6 versions of it) • Binding energy: 0.1 ~ 16 MeV • Suggests the existence of a XX bound state. • Needs other model predictions. • Possible reactions: gD (JLab) or KD (J-PARC) Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  22. 4. Photoproduction Nakayama, Oh, Haberzettl PRC 74 (2006) 035205 • CLAS at JLab succeeded to produce X by photon-induced reactions. • So far, only a few inclusive X photoproduction were reported. Tagged Photon Spectrometer Collab., NPB 282 (1987) • No theoretical work on X photoproduction • Except one for pentaquark X photoproduction Liu, Ko, PRC 69 (2004) • Our strategy • Investigate the production mechanism using the currently available information only. • Then consider other possible (and important) mechanisms. • Final-state interactions & coupled channel? • Ideal, but practically impossible at this stage. • Use the tree-level approximation as the first attempt to understand the production mechanism. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  23. Forbidden or suppressed mechanisms • In kaon—anti-kaon production , meson production processes, especially f meson production, are important. • In X 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 g KX is also suppressed as the exchange meson should have S=+2. E: exotic meson with S=+2 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  24. Considered diagrams • Consider K and K* exchange only. • Axial-vector K1 mesons: lack of information & heavy mass • Scalar k or K0 mesons: not allowed since kgKg coupling is forbidden by angular momentum and parity conservation. • Consider • N’ = N and D • Y, Y’ = low-lying L and S hyperons • X’ = X(1320) and X(1530) + exchanged diagrams q1n q2 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  25. Strategy • 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. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  26. Intermediate hyperons Particle Data Group Decay widths and couplings are in a very wide range. No information for the other couplings. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  27. Strategy • 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. • The details are in nucl-th/0605169. • Preliminary CLAS data (of Weygand and Guo) • The total cross sections data (hep-ex/0601011) is used to determine the cutoff parameter of the form factors. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  28. Model (A) • First, consider only the low mass hyperons:L(1116), L(1405), L(1520), S(1190), S(1385) • Their couplings are rather well-known. • The cross sections for the two non-identical kaon productions are larger than those for two identical kaon productions: isospin factors • The dominant contribution to gp g K+K+X- comes from the spin-1/2 hyperon resonances. spin-1/2 baryons spin-3/2 baryons Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  29. Invariant mass distributions • Invariant mass distributions of K+X- and K+K+. • No structure for K+K+ distribution as expected: absence of S=+2 exotic mesons in this calculation. • No structure for K+X-distribution since we are considering the low-lying hyperons only whose masses are below 1.6 GeV, while the minimum value for m(K+X-) is > 1.8 GeV pv coupling ps coupling Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  30. Higher-mass resonances • As the K+X-mass distribution covers the energy larger than 1.8 GeV, it is natural to expect important role from the higher-mass hyperon resonances around 1.8 GeV and above. • The properties of higher-mass hyperons are poorly known. • We first consider the hyperons of spin-1/2 and 3/2 only. • What we know are • The broad range of the NYK couplings: from G(YgNK) of PDG • The photoproduction amplitudes at the hyperon on-shell point have • So 1/2- and 3/2+ hyperon resonances around M = 1.8 GeV are expected to be important. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  31. Intermediate hyperons Particle Data Group Decay widths and couplings are in a very wide range. No information for the other couplings. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  32. Model (B) • Assumptions. • In order to reduce the number of unknown couplings, we consider two hyperon resonances only, L(1800)1/2- and L(1890)3/2+ in addition to the low mass hyperons. • Neglect their magnetic moments and radiative transitions. • Then the only unknown is the product of the coupling constants, gNLKgXLK. • We take gNLKgXLK = 2for simplicity. • Form factors are readjusted to fit the total cross section data. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  33. Results (I) • Total cross sections Nearly the same results as before. • total cross section alone cannot distinguish the contributions from the low-mass and the high-mass resonances. • Other quantities should be measured. Spin -3/2 Spin -1/2 Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  34. Results (II) • Invariant mass distributions of K+X- and K+K+. • No structure for K+K+ distribution as before. • Two bump structure for K+X-distribution is seen. • L(1800) bump cannot be seen.: below threshold • The first bump at lower mass is due to L(1890). • The second bump is not from a resonance at higher mass. • The position depends on the energy. • So-called kinematic reflections of the three-body final states. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  35. Model (C) • What happens if we have (unknown) hyperon resonance at a mass around 2 GeV which couples strongly enough to the nucleon and X? • In fact, the preliminary CLAS data do not show a sharp peak in K+X- channel. • Some well-established L and S resonances of spin-5/2 and 7/2 at around 2 GeV. • Consider a fictitious spin-3/2+ hyperon at around 2 GeV, so we consider three high-mass resonances in addition to the low-lying resonances.L(1800)1/2-, L(1890)3/2+, and L(2050)3/2+(fictitious particle) • Adjust the parameters so that we have similar total cross sections. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  36. Results (IV) • But we have very different K+X- invariant mass distribution. • The bump structure disappears. • The valley between the two peaks is now filled up by the additional resonance. • This shows that the higher-mass resonance at around 2 GeV should be examined. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  37. 5. Outlook • CLAS at JLab initiated Cascade Physics Program. • Opens the door to many avenues of research for X hyperons. • More data are coming! • Does X(1620) exist? • Should confirm other X resonances in PDG. • Role of L and S resonances in X photoproduction. • Offers a chance to study those hyperons. • Higher mass and high spin resonances: under progress • Theoretical models for X spectrum • Only a few model gives the X spectrum. • Where is the low-lying X resonances? • Possible di-baryon? • Etc … Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

  38. Cascade Physics Working Group • Members B. Nefkens, D.S. Carman, S. Capstick, J.L. Goity, L. Guo, H. Haberzettl, N. Marthur, K. Nakayama, Y. Oh, J. Price, D.G. Richards, S. Stepanyan, D.P. Weygand, and more. Quarks, Nuclei and Universe, Nov. 2006Yongseok Oh

More Related