arXiv subjects
Publications and source records attributed to R. L. Workman.
Given the growing evidence for an exotic S = +1 resonance, seen in kaon, photon and neutrino induced reactions, we reexamine the existing K+p and K+d database in order to understand how such a state could have been missed in previous studies. The lack of structure in this database implies a width of an MeV or less, assuming a state exists near 1540 MeV.
Experiments that study the hadronic and electromagnetic production of the pseudoscalar mesons -- pions, etas and kaons, contribute to our knowledge of the properties of baryon and hyperon resonances. Fixed-target programs at hadronic facilities such as BNL-AGS have been phased out. However, the availability of modern experimental facilities with pseudo-monochromatic or tagged medium-energy photon beams at GRAAL, SPring-8, Bonn, Mainz, and Jefferson Lab, together with LEGS, Max-Lab, and HIGS at lower energies, are beginning to produce high-quality results. These new data have smaller statistical uncertainties and better understood systematic uncertainties, than those obtained at the older bremsstrahlung facilities, for measurements of differential and integrated cross sections, as well as polarization and asymmetry. Experimental results are compared with the predictions of QCD-based approaches, such as the lattice-gauge calculations of baryon properties, and Chiral Perturbation Theory applied to threshold photoproduction, and are essential to the performance of Partial-Wave Analyses (PWA). These PWA studies are less model dependent than in the past, and are used in coupled-channels calculations that incorporate unitarity dynamically, and combine results from hadronic reaction channels with electromagnetic processes. This approach is necessary to extract resonance properties and may lead to the discovery of the "missing resonances" predicted by a number of different QCD-inspired calculations. We discuss recent experimental and phenomenological results for single and double pseudoscalar meson hadronic and photoproduction channels with emphasis on the JLab Hall B and the BNL/AGS Crystal Ball programs.
The K+ nucleon elastic scattering process has been reexamined in light of recent measurements which have found a narrow exotic S=+1 resonance in their KN invariant mass distributions. We have analyzed the existing database in order to consider the effect of a narrow state on fits to K+ nucleon observables.
The problem of nucleon resonances N' with masses below the Delta is considered. We derive bounds for the properties of such states. Some of these are new, while others improve upon existing limits. We discuss the nature of N' states, and their unitary partners, assuming their existence can be verified.
We investigate the influence of new Jefferson Lab polarization-transfer data, Cx and Cz, and recoil polarization, Py, measurements in multipole fits to the single-pion photoproduction database. Results are compared to those found with the addition of a double-polarization quantity associated with the Gerasimov- Drell-Hearn sum rule.
Experiments that study the photoproduction of pseudoscalar mesons; pions, etas and kaons, have the potential to increase our knowledge of baryon and hyperon resonance properties. Recent experiments at JLab, Mainz, GRAAL, and Bonn are beginning to produce results in the form of polarization and asymmetry measurements and determinations of the differential and integrated cross sections. These new data are essential to the performance of Partial-Wave Analyses that are less model dependent and coupled-channels calculations that incorporate unitarity dynamically,combining hadronic reaction channels together with electromagnetic processes. This approach is necessary to extract resonance properties and may lead to the identification of missing, but predicted, resonances. Some recent experimental and phenomenological results for single and double pseudoscalar meson photoproduction are discussed.
Model-dependence of the single-pion contribution to the GDH, Baldin, and forward spin-polarizability sum rules is explored, using the most recent SAID multipole analysis. Results from SAID and MAID are compared.
Neutral pion photo- and electroproduction at threshold is analyzed in the framework of dispersion relations. For this purpose, we evaluate the real threshold amplitudes in terms of Born contributions and dispersion integrals determined by the imaginary parts of the MAID and SAID multipoles. The results show considerable cancellations between Born terms and resonance contributions. Good agreement with the data is found for photoproduction. While our dispersion analysis suggests considerable discrepancies for electroproduction, the present state of the experimental multipole analysis at finite Q^2 does not permit drawing conclusions at this time.
A partial-wave analysis of single-pion photoproduction data has been completed. This study extends from threshold to 2 GeV in the laboratory photon energy, focusing mainly on the influence of new measurements and model-dependence in the choice of parameterization employed above the two-pion threshold. Results are used to evaluate sum rules and estimate resonance photo-decay amplitudes. These are compared to values obtained in the MAID analysis.
We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, $γp\to π^+ n$, $γn\to π^- p$, $γp\to π^0 p$, and $γn\to π^0 n$. Leaving the $πNΔ$ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the first resonance region. Within this effective Lagrangian approach, strongly constrainted by the quark model, we consider the issue of double-counting which may occur if additional {\it t}-channel contributions are included.
A new result for the pion nucleon Sigma term from a George Washington University/TRIUMF group analysis of pion nucleon data is presented. The value Sigma=79$\pm$7 MeV was obtained, compared to the canonical value 64$\pm$8 MeV found by Koch. The difference is explained simply by the PSI pionic hydrogen value for a(pi -p), the latest results for the $πNN$ coupling onstant, and a narrower Delta resonance. Many systematic effects have been investigated, including Coulomb corrections, and database changes, and our results are found to be robust. In the standard interpretation, our value of Sigma implies a nucleon strangeness fraction y/2~0.23. The implausibility of such a large strange component suggests that the relationship between Sigma and nucleon strangeness ought to be re-examined.
A fit to the existing pion electroproduction data is presented. This work builds upon our previous analyses of pion photoproduction and elastic pion-nucleon scattering over the Delta resonance region. We comment on the extraction of E_1+ ^3/2 / M_1+ ^3/2 (E2/M1) and S_1+ ^3/2 / M_1+ ^3/2 (S2/M1) ratios, and note that the E2/M1 ratio approaches, and possibly crosses, zero below a Q2 of 5 (GeV/c}^2.
We have fitted low- and medium-energy benchmark datasets employing methods used in the MAID/SAID and dynamical model analyses. Independent fits from the Mainz, RPI, Yerevan, and Kharkov groups have also been performed over the low-energy region. Results for the multipole amplitudes are compared in order to gauge the model-dependence of such fits, given identical data and a single method for error handling.
The cross section asymmetry Sigma has been measured for the photoproduction of pi0-mesons off protons, using polarized photons in the energy range Eg = 0.5 - 1.1 GeV. The CM angular coverage is Theta = 85 - 125 deg with energy and angle steps of 25 MeV and 5 deg, respectively. The obtained Sigma data, which cover the second and third resonance regions, are compared with existing experimental data and recent phenomenological analyses. The influence of these measurements on such analyses is also considered.
A new result for the nucleon sigma term from an updated pion-nucleon partial-wave and dispersion relation analysis of the Virginia Polytechnic Institute (now George Washington University) group is discussed. Using a method similar to that of Gasser, Leutwyler, Locher, and Sainio, we obtain Sigma=90 +/- 8 MeV (preliminary), in disagreement with the canonical result 64 +/- 8 MeV, but consistent with expectations based on new information on the $π$NN coupling constant, pionic atoms, and the Delta resonance width.
We compare results for the D13(1520) photon-decay amplitudes determined in analyses of eta- and pion-photoproduction data. The ratio of helicity amplitudes (A_3/2 / A_1/2), determined from eta-photoproduction data, is quite different from that determined in previous analyses of pion-photoproduction data. We consider how strongly the existing pion-photoproduction data constrain both this ratio and the individual photon-decay amplitudes.
Our extraction of the pion-nucleon coupling constant from πp elastic scattering data is outlined. A partial wave analysis (T_π < 2100 MeV) is performed simultaneously with a fixed-t dispersion relation analysis (T_π < 800 MeV). The πNN coupling constant g^2/4πis searched to find the best fit. The result 13.73 \pm 0.01 \pm 0.07 (first error statistical, second systematic) is found to be insensitive to database changes and Coulomb barrier corrections. This value satisfies important elements of low energy QCD like the Goldberger-Treiman discrepancy, the Dashen-Weinstein sum rule, and chiral perturbation theory predictions of threshold pion photoproduction.
We summarize results obtained in our studies of the pion-nucleon coupling constant. Several different techniques have been applied to pi-N and NN elastic scattering data, and the existing database for single-pion photoproduction. The most reliable determination comes from pi-N elastic scattering. The sensitivity in this reaction was found to be greater, by at least a factor of 3, when compared with analyses of NN elastic scattering or single-pion photoproduction.