Negative parity pentaquarks in large Nc QCD and quark model

Recently, the 1/Nc expansion has been applied to the study of exotic baryons containing both quarks and antiquarks. We extend this approach to exotic states with mixed symmetric spin-flavor symmetry, which correspond in the quark model to negative parity pentaquarks, and discuss the large Nc predictions for their mass spectrum. The heavy exotics Qbar q^4 transform as 3, 6bar,15 and 15' under flavor SU(3), while the light states include the exotic multiplets 10bar, 27, 35. We give mass relations among these multiplets following from the 1/Nc expansion. In the quark model, the mass splittings between these states are given by color-spin interactions. Using the observation of an anticharmed exotic by the H1 collaboration, we give predictions for the masses of other expected heavy pentaquarks.Comment: 18 pages, one figure, uses youngtab.sty; v3: eq.(40) corrected, final version to appear in PR

Radiative Decays of the P-Wave Charmed Mesons

Minor (mainly numerical) corrections.Comment: 12 pages, LaTeX, MZ-TH/92-5

Improved variables for measuring the LambdabLambda_b polarization

We discuss a few possible strategies for measuring the polarization of the $\Lambda_{\mathrm{b}}$ baryons produced in $e^+e^-$-annihilation at the $\mathrm{Z}$ resonance through their inclusive semileptonic decays. After reviewing the existing methods, a new method is proposed, based on the ratio of the averages of the squared electron and neutrino energy, including both perturbative and nonperturbative corrections. This variable minimizes the statistical error on the $\Lambda_{\mathrm{b}}$ polarization, while keeping the systematic theoretical errors at the level of 1-2%. A number of other polarization-sensitive variables are also discussed, such as averages of ratios of the electron and neutrino energy and the distribution in the difference of the electron and neutrino rapidities.Comment: 23 pages, 4 uuencoded figures, REVTe

New Relations for Excited Baryons in Large N_c QCD

We show that excited baryons in large N_c QCD form multiplets, within which masses are first split at O(1/N_c). The dominant couplings of resonances to various mesons are highly constrained: The N(1535) decays at leading 1/N_c order exclusively to eta-N rather than pi-N, and vice versa for the N(1650). This multiplet structure is reproduced by a simple large N_c quark model, well studied in the literature, that describes resonances as single-quark excitations.Comment: 4 pages, no figures, ReVTeX 4. Includes new discussion of previous work on excited baryon tower

Decays of Non-strange Negative Parity Baryons in the 1/Nc Expansion

The decays of non-strange negative parity baryons via the emission of single $\pi$ and $\eta$ mesons are analyzed in the framework of the $1/N_c$ expansion. A basis of spin-flavor operators for the partial wave amplitudes is established to order $1/N_c$ and the unknown effective coefficients are determined by fitting to the S- and D-wave partial widths as provided by the Particle Data Group. A set of relations between widths that result at the leading order, i.e. order $N_c^0$, is given and tested with the available data. Up to a few exceptions, a good description of the partial decays widths is already obtained at that order. Because of the rather large errors in the empirical input data the next to leading order fit fails to pin down with satisfactory accuracy the subleading effective coefficients. The hierarchy expected from the $1/N_c$ expansion is reflected in the results.Comment: 24 pages, 8 table