Excited Baryon Decay Widths in Large N_c QCD

We study excited baryon decay widths in large N_c QCD. It was suggested previously that some spin-flavor mixed-symmetric baryon states have strong couplings of O(N_c^{-1/2}) to nucleons [implying narrow widths of O(1/N_c)], as opposed to the generic expectation based on Witten's counting rules of an O(N_c^0) coupling. The calculation obtaining these narrow widths was performed in the context of a simple quark-shell model. This paper addresses the question of whether the existence of such narrow states is a general property of large N_c QCD. We show that a general large N_c QCD analysis does not predict such narrow states; rather they are a consequence of the extreme simplicity of the quark model.Comment: 9 page

Excited Baryons in Large N_c QCD Revisited: The Resonance Picture Versus Single-Quark Excitations

We analyze excited baryon properties via a 1/N_c expansion from two perspectives: as resonances in meson-nucleon scattering, and as single-quark excitations in the context of a simple quark model. For both types of analysis one can derive novel patterns of degeneracy that emerge as N_c --> \infty, and that are shown to be compatible with one another. This helps justify the single-quark excitation picture and may give some insight into its successes. We also find that in the large N_c limit one of the S_{11} baryons does not couple to the pi-N channel but couples to the eta-N channel. This is empirically observed in the N(1535), which couples very weakly to the pi-N channel and quite strongly to the eta-N channel. The comparatively strong coupling of the N(1650) to the pi-N channel and weak coupling to eta-N channel is also predicted. In the context of the simple quark model picture we reproduce expressions for mixing angles that are accurate up to O(1/N_c) corrections and are in good agreement with mixing angles extracted phenomenologically.Comment: 13 pages, ReVTeX

Large N_c, Constituent Quarks, and N, Delta Charge Radii

We show how one may define baryon constituent quarks in a rigorous manner, given physical assumptions that hold in the large-N_c limit of QCD. This constituent picture gives rise to an operator expansion that has been used to study large-N_c baryon observables; here we apply it to the case of charge radii of the N and Delta states, using minimal dynamical assumptions. For example, one finds the relation r_p^2 - r_{Delta^+}^2 = r_n^2 - r_{Delta^0}^2 to be broken only by three-body, O(1/N_c^2) effects for any N_c.Comment: 15 pages, 1 eps figure. Version to appear in Phys. Rev.

Some (further) Comments on the Theta(1540) Pentaquark

Additional broader I=0 states in the KN channel near $\Theta^+$(1540) are expected in many models, making the absence of any signature in the K$^+$-deuteron scattering data even more puzzling. In an ideal "three-body" picture the $\Theta$ is viewed as two compact ud(1)ud(2) $\bar{3}$ color diquarks and an $\bar{s}$ quark. A "QCD-type" inequality involving $m(\Theta^+), m(\Lambda)$, the mass of the $\Lambda(1/2^-)$ L=1 excitation and that of a new I=0 tetraquark vector meson then follows. The inequality suggests a very light new vector meson, and is violated. We note that "associated production" of the pentaquark with another quadriquark or anti-pentaquark may be favored. This along with some estimates of the actual production cross sections suggest that the $\Theta$ can be found in BaBar or Belle e$^+$-e$^-$ colliders.Comment: 6 page

Possible Effects of Noncommutative Geometry on Weak CP Violation and Unitarity Triangles

Possible effects of noncommutative geometry on weak CP violation and unitarity triangles are discussed by taking account of a simple version of the momentum-dependent quark mixing matrix in the noncommutative standard model. In particular, we calculate nine rephasing invariants of CP violation and illustrate the noncommutative CP-violating effect in a couple of charged D-meson decays. We also show how inner angles of the deformed unitarity triangles are related to CP-violating asymmetries in some typical B_d and B_s transitions into CP eigenstates. B-meson factories are expected to help probe or constrain noncommutative geometry at low energies in the near future.Comment: RexTev 16 pages. Modifications made. References added. Accepted for publication in Phys. Rev.

A Naturally Narrow Positive Parity Theta^+

We present a consistent color-flavor-spin-orbital wave function for a positive parity Theta^+ that naturally explains the observed narrowness of the state. The wave function is totally symmetric in its flavor-spin part and totally antisymmetric in its color-orbital part. If flavor-spin interactions dominate, this wave function renders the positive parity Theta^+ lighter than its negative parity counterpart. We consider decays of the Theta^+ and compute the overlap of this state with the kinematically allowed final states. Our results are numerically small. We note that dynamical correlations between quarks are not necessary to obtain narrow pentaquark widths.Comment: 10 pages, 1 figure, Revtex4, two-column format, version to be published in Phys. Rev. D, includes numerical estimates of decay width

1/N Expansion for Exotic Baryons

The 1/N expansion for exotic baryons is developed, and applied to the masses, meson couplings and decay widths. Masses and widths of the 27 and 35 pentaquark states in the same tower as the Theta+ are related by spin-flavor symmetry. The 27 and 35 states can decay within the pentaquark tower, as well as to normal baryons, and so have larger decay widths than the lightest pentaquark Theta. The 1/N expansion also is applied to baryon exotics containing a single heavy antiquark. The decay widths of heavy pentaquarks via pion emission, and to normal baryons plus heavy D^(*),B^(*) mesons are studied, and relations following from large-N spin-flavor symmetry and from heavy quark symmetry are derived.Comment: Major additions: plots of widths and branching ratios, discussion of strong decays of heavy pentaquarks, including consequences of heavy quark symmetr

Towards an Axiomatization of Simple Analog Algorithms

International audienceWe propose a formalization of analog algorithms, extending the framework of abstract state machines to continuous-time models of computation