2293560
Kaon photoproduction on the neutron using deuterium.
The three processes \ensuremath{\gamma}+d\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Lambda}}}^{0}$(${\mathrm{\ensuremath{\Sigma}}}^{0}$)+${\mathit{K}}^{0}$+p and \ensuremath{\gamma}+d\ensuremath{\rightarrow}${\mathrm{\ensuremath{\Sigma}}}^{\mathrm{\ensuremath{-}}}$+${\mathit{K}}^{+}$+p are investigated in the framework of the impulse approximation. The exclusive reactions d(\ensuremath{\gamma},Kp)Y where the proton is detected in coincidence with the ${\mathit{K}}^{0}$ or ${\mathit{K}}^{+}$ should provide unique signatures for each of the processes. In most kinematic situations the final-state Yp interaction is insignificant and the elementary process cross section for \ensuremath{\gamma}+n\ensuremath{\rightarrow}Y+K can be reliably extracted. Cross-section estimates are given for several models of the basic production operator.
53400647
Study of pentaquarks on the lattice with overlap fermions
We present a quenched lattice QCD calculation of spin-$1/2$ five-quark states with $uudd\overline{s}$ quark content for both positive and negative parities. We do not observe any bound pentaquark state in these channels for either $I=0$ or $I=1$. The states we found are consistent with $KN$ scattering states which are checked to exhibit the expected volume dependence of the spectral weight. The results are based on overlap-fermion propagators on two lattices, ${12}^{3}\ifmmode\times\else\texttimes\fi{}28$ and ${16}^{3}\ifmmode\times\else\texttimes\fi{}28$, with the same lattice spacing of 0.2 fm, and pion mass as low as $\ensuremath{\sim}180\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}$.
37147201
Low-dimensional long-range topological structure in the QCD vacuum☆
Abstract Lattice topological charge associated with Ginsparg-Wilson fermions exhibits generic topological stability over quantum ensemble of configurations contributing to the QCD path integral. Moreover, the underlying chiral symmetry leads to the suppression of ultraviolet noise in the associated topological charge densities (“ chiral smoothing ”). This provides a solid foundation for the direct study of the role of topological charge fluctuations in the physics of QCD vacuum. Using these tools it was recently demonstrated that: (a) there is a well-defined space-time structure (order) in topological charge density (defined through overlap fermions) for typical configurations contributing to QCD path integral; (b) this fundamental structure is low-dimensional, exhibiting sign-coherent behavior on subsets of dimension less than four and not less than one; (c) the structure has a long-range global character (spreading over maximal space-time distances) and is built around the locally one-dimensional network of strong fields (skeleton). In this talk we elaborate on certain aspects and implications of these results.
17008691
Excited Baryons and Chiral Symmetry Breaking of QCD
N* masses in the spin-1/2 and spin-3/2 sectors are computed using two non-perturbative methods: lattice QCD and QCD sum rules. States with both positive and negative parity are isolated via parity projection methods. The basic pattern of the mass splittings is consistent with experiments. The mass splitting within the same parity pair is directly linked to the chiral symmetry breaking QCD.
8673005
Hadron masses and quark condensate from overlap fermions
Abstract We present results on hadron masses and quark condensate from Neuberger's overlap fermion. The scaling and chiral properties and finite volume effects from this new Dirac operator are studied. We find that the generalized Gell-Mann-Oakes-Renner relation is well satisfied down to the physical u and d quark mass range. We find that in the range of the lattice spacing we consider, the π and ϱ masses at a fixed m π / m ϱ ratio have weak O ( a 2 ) dependence.
119511266
Excited decuplet baryons from QCD sum rules
Abstract A calculation of the mass spectrum in the baryon decuplet sector is presented using the method of QCD sum rules. Sum rules are derived for states of spin-parity 3 / 2 ± and 1 / 2 + using both the conventional method and a parity-projection method. The predictive ability of the sum rules is explored by a Monte Carlo based analysis procedure in which the three phenomenological parameters (mass, coupling, threshold) are treated as free parameters and fitted simultaneously. Taken together, the results give an improved determination of the mass spectrum in this sector from the perspective of non-perturbative QCD.
28317308
Coplanar and noncoplanar nucleon-nucleon bremsstrahlung calculation: A study of pseudoscalar and pseudovector pi N couplings.
The {ital pp}{gamma} and {ital np}{gamma} processes have been used to study pseudoscalar (ps) and pseudovector (pv) {pi}{ital N} couplings. We calculate both coplanar and noncoplanar cross sections using a realistic one-boson exchange model in the energy region between 100 and 300 MeV. We find that the difference in cross sections calculated using the two couplings increases as the nucleon scattering angles decrease. Furthermore, this difference becomes greater with increasing incident energy. We show for the {ital pp}{gamma} case that this difference will be significantly increased upon the inclusion of electromagnetic form factors. For {ital pp}{gamma} with large proton scattering angles ({ge}30{degree}), cross sections (both ps and pv) calculated using an on-shell {ital p}{gamma}{ital p} vertex are in very good agreement with the data, and they are also extremely close to a recent soft-photon calculation at 157 MeV using the two-{ital u}-two-{ital t} special amplitude. The ps and pv calculations are both in good agreement with the TRIUMF data at 200 and 280 MeV. More accurate data are required to differentiate between these two couplings. Agreement with sparsely available {ital np}{gamma} data is good for most cases, but discrepancies do exist for some data points. For large nucleon scattering angles,more » noncoplanar cross sections for both {ital np}{gamma} and {ital pp}{gamma} change rapidly with the noncoplanar angle. However, the respective shapes of the noncoplanar {ital np}{gamma} and {ital pp}{gamma} curves are very different.« less
20647092
Anomalous but consistent gravity in two dimensions.
Two-dimensional gravity coupled to chiral fermions possesses a Lorentz anomaly, but it can be made consistent by allowing the gauge degree of freedom to be physical at the quantum level. The theory has massless excitations or a ghost particle depending on the sign of a counterterm. The possibility of adding the Wess-Zumino term in order to cancel the anomaly is also discussed.
17940028
Perturbative QCD analysis of B meson decays.
The resummation of large QCD radiative corrections, including leading and next-to-leading logarithms, in the pion electromagnetic form factor is reviewed. A similar formalism is applied to exclusive processes involving heavy mesons, and leads to Sudakov suppression for the semileptonic decay {ital B}{r_arrow}{pi}{ital l}{nu}. It is found that, with the inclusion of Sudakov effects, a perturbative QCD analysis of this decay is reliable for the energy fraction of the pion above 0.4. Combining predictions from soft pion theorems we estimate that the upper limit of the matrix element {parallel}{ital V}{sub {ital ub}}{parallel} is 2.8{endash}4.8{times}10{sup {minus}3} from different models of the {ital B} meson wave function. {copyright} {ital 1996 The American Physical Society.}
38608728
Comment on "Role of g2 in relating the Schwinger and Gerasimov-Drell-Hearn sum rules"
10326293
New light on time machines: Against the chronology protection conjecture.
I construct a spacetime with closed timelike curves and without closed causal geodesics. There is a noncompactly generated Cauchy horizon separating the region with closed timelike curves from that without closed causal curves. This spacetime might be stable against the vacuum fluctuation of matter fields. Whether such a spacetime is stable against the vacuum fluctuation of a gravitational field is an open question.