Evidence for the fourth P11 resonance predicted by the constituent quark model

It is pointed out that the third of five low-lying P11 states predicted by a constituent quark model can be identified with the third of four states in a solution from a three-channel analysis by the Zagreb group. This is one of the so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb group to the pi N -> eta N data is the crucial element in finding this fourth resonance in the P11 partial wave.Comment: 8 pages, revtex; expanded acknowledgement

Extraction of the Ratio of the N^*(1535) Electromagnetic Helicity Amplitudes from Eta Photoproduction off Neutrons and Protons

Using the recent precise measurements of eta photopro- duction in proton and deuteron targets, we extract the ratio of the helicity amplitudes A^n_1/2 / A^p_1/2, for the excitation of N^*(1535), in the effective Lagrangian approach, It is fairly model-independent, free from the final-state interaction effects, and negative as predicted by the quark models. We stress the importance of polarization observables in further elucidation of the N^*(1520) photoexcitation amplitudes.Comment: 4 figures. It will be published in Phys. Lett.

Semileptonic Decays of Heavy Lambda Baryons in a Quark Model

The semileptonic decays of Lambda_c and Lambda_b are treated in the framework of a constituent quark model. Both nonrelativistic and semirelativistic Hamiltonians are used to obtain the baryon wave functions from a fit to the spectra, and the wave functions are expanded in both the harmonic oscillator and Sturmian bases. The latter basis leads to form factors in which the kinematic dependence on q^2 is in the form of multipoles, and the resulting form factors fall faster as a function of q^2 in the available kinematic ranges. As a result, decay rates obtained in the two models using the Sturmian basis are significantly smaller than those obtained using the harmonic oscillator basis. In the case of the Lambda_c, decay rates calculated using the Sturmian basis are closer to the experimentally reported rates. However, we find a semileptonic branching fraction for the Lambda_c to decay to excited Lambda* states of 11% to 19%, in contradiction with what is assumed in available experimental analyses. Our prediction for the Lambda_b semileptonic decays is that decays to the ground state Lambda_c provide a little less than 70% of the total semileptonic decay rate. For the decays Lambda_b to Lambda_c, the analytic form factors we obtain satisfy the relations expected from heavy-quark effective theory at the non-recoil point, at leading and next-to-leading orders in the heavy-quark expansion. In addition, some features of the heavy-quark limit are shown to naturally persist as the mass of the heavy quark in the daughter baryon is decreased.Comment: 51 pages, 8 figures, submitted to Physical Review

Optimizing an array of antennas for cellular coverage from a high altitude platform

In a wireless communications network served by a high altitude platform (HAP) the cochannel interference is a function of the antenna beamwidth, angular separation and. sidelobe level. At the millimeter wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells. We present a method for predicting cochannel interference based on curve-fit approximations for radiation patterns of elliptic beams which illuminate cell edges with optimum power, and a means of estimating optimum beamwidths for each cell of a regular hexagonal layout. The method is then applied to a 121 cell architecture. Where sidelobes are modeled As a flat floor at 40-dB below peak directivity, a cell cluster size of four yields carrier-to-interference ratios (CIRs), which vary from 15 dB at cell edges to 27 dB at cell centers. On adopting a cluster size of seven, these figures increase, respectively, to 19 and 30 dB. On reducing the sidelobe level, the. improvement in CIR can be quantified. The method also readily allows for regions of overlapping channel coverage to be shown

K+ photoproduction at SPring-8/LEPS

A series of experiments have been carried out by using a linearly polarized photon beam at the SPring-8/LEPS facility from December 2000 to June 2001. The photon beam asymmetries and differential cross sections of the p(gamma,K+)Lambda and p(gamma,K+)Sigma^0 reactions have been measured in the photon energy range from 1.5 GeV to 2.4 GeV at forward angles, 0 deg. < Theta_cm < 60 deg.. We report preliminary results of the photon beam asymmetries.Comment: 4 pages, 4 figures, to be published in the Proceedings (Nuclear Physics A) of the PANIC02, Osaka, Sept 30 - Oct 4, 200

New Baryons in the Delta eta and Delta omega Channels

The decays of excited nonstrange baryons into the final states Delta eta and Delta omega are examined in a relativized quark pair creation model. The wavefunctions and parameters of the model are fixed by previous calculations of N pi and N pi pi, etc., decays through various quasi-two body channels including N eta and N omega. Our results show that the combination of thresholds just below the region of interest and the isospin selectivity of these channels should allow the discovery of several new baryons in such experiments.Comment: 10 pages, RevTe

The role of the pion cloud in electroproduction of the Δ\Delta(1232)

We calculate the ratios $E2/M1$ and $C2/M1$ of the multipole amplitudes for electroproduction of the $\Delta$(1232) in the range of photon virtuality $0<-K^2<1$~GeV$^2$ in a chiral chromodielectric model and a linear $\sigma$-model. We find that relatively large experimental values can be explained in terms of the pion contribution alone; the contribution arising from d-state quark admixture remains below 10\%. We describe the pion cloud as a coherent state and use spin and isospin projection to obtain the physical nucleon and the $\Delta$. The $A_{1/2}$ and $A_{3/2}$ amplitudes are reasonably well reproduced in the $\sigma$-model; in the chromodielectric model, however, they are a factor of two too small.Comment: 10 pages LaTeX2e, 3 LaTeX figures within the text; Requires elsart.cls (included in the self-unpacking uuencoded gzipped file). (Accepted for publication in Phys. Lett. B

η\eta production off the proton in a Regge-plus-chiral quark approach

A chiral constituent quark model approach, embodying s- and u-channel exchanges,complemented with a Reggeized treatment for t-channel is presented. A model is obtained allowing data for $\pi^- p \to \eta n$ and $\gamma p \to \eta p$ to be describe satisfactorily. For the latter reaction, recently released data by CLAS and CBELSA/TAPS Collaborations in the system total energy range $1.6 \lesssim W \lesssim 2.8$ GeV are well reproduced due to the inclusion of Reggeized trajectories instead of simple $\rho$ and $\omega$ poles. Contribution from "missing" resonances is found to be negligible in the considered processes.Comment: 23 pages.4 figures,4 tables, to appear in Phys.Rev.

Strange Decays of Nonstrange Baryons

The strong decays of excited nonstrange baryons into the final states Lambda K, Sigma K, and for the first time into Lambda(1405) K, Lambda(1520) K, Sigma(1385) K, Lambda K*, and Sigma K*, are examined in a relativized quark pair creation model. The wave functions and parameters of the model are fixed by previous calculations of N pi and N pi pi, etc., decays. Our results show that it should be possible to discover several new negative parity excited baryons and confirm the discovery of several others by analyzing these final states in kaon production experiments. We also establish clear predictions for the relative strengths of certain states to decay to Lambda(1405) K and Lambda(1520) K, which can be tested to determine if a three-quark model of the Lambda(1405) K is valid. Our results compare favorably with the results of partial wave analyses of the limited existing data for the Lambda K and Sigma K channels. We do not find large Sigma K decay amplitudes for a substantial group of predicted and weakly established negative-parity states, in contrast to the only previous work to consider decays of these states into the strange final states Lambda K and Sigma K.Comment: 25 pages, 8 figures, RevTe