Exotic Baryons in Two-Dimensional QCD

Two-dimensional QCD has often been used as a laboratory for studying the full four-dimensional theory, providing, for example, an explicit realization of baryons as solitons. We review aspects of conventional baryons in two-dimensional QCD, including the classical and quantum contributions to their masses. We then discuss the spectrum of exotic baryons in two-dimensional QCD, commenting on the solitonic radius inferred from the excitation spectrum as well as the two-dimensional version of the Goldberger-Treiman relation relating meson couplings to current matrix elements. Two-dimensional QCD provides strong overall support to the chiral-soliton picture for the structure of normal and exotic baryons in four dimensions.Comment: 15 pages latex, no figure

Light hadron and diquark spectroscopy in quenched QCD with overlap quarks on a large lattice

A simulation of quenched QCD with the overlap Dirac operator has been completed using 100 Wilson gauge configurations at beta = 6 on an 18^3 x 64 lattice and at beta = 5.85 on a 14^3 x 48 lattice, both in Landau gauge. We present results for light meson and baryon masses, meson final state "wave functions," and other observables, as well as some details on the numerical techniques that were used. Our results indicate that scaling violations, if any, are small. We also present an analysis of diquark correlations using the quark propagators generated in our simulation.Comment: 28 LaTeX pages, 41 figures, v2: minor updates, version published in JHE

Spin 3/2 Pentaquarks

We investigate the possible existence of the spin 3/2 pentaquark states using interpolating currents with K-N color-octet structure in the framework of QCD finite energy sum rule (FESR). We pay special attention to the convergence of the operator product expansion

Experimentalni program uz SPring-8

A highly-polarized photon beam produced via the backward Compton scattering of polarized laser photons off the high-energy electrons is utilized for nuclear and particle physics experiments at SPring-8. The present status of the experiments at the Laser Electron Photon beamline at SPring-8 (LEPS) is reported. From the recent results, the evidence for a five-quark baryon resonance is presented. A sharp baryon resonance peak was observed at 1.54 ± 0.01 GeV/c2 with a width smaller than 25 MeV/c2 in the γn → K+K−n reaction on 12C. The resonance has the strangeness quantum number S = +1 and it can not be formed by three quarks.Za istraživanja u nuklearnoj i čestičnoj fizici sa SPring-8 rabimo visoko-polarizirane fotone koji se postižu raspršenjem unatrag polariziranih laserskih fotona na elektronima visoke energije. Opisujemo napredak eksperimenata sa laserskoelektronskim fotonskim snopom. U nedavnim mjerenjima utvrdili smo petokvarkovsku barionsku rezonanciju. Opaža se kao oštar barionski rezonantni vrh u reakciji γn → K+K−n u meti 12C, na 1.54 ± 0.01 GeV/c2 , širine manje od 25 MeV/c2 . Ima kvantni broj stranosti S = +1 i ne može biti sastavljena od samo tri kvarka

Pentaquarks: review of the experimental evidence

Pentaquarks, namely baryons made by 4 quarks and one antiquark have been predicted and searched for since several decades without success. Theoretical and experimental advances in the last 2 years led to the observation of a number of pentaquark candidates. We review the experimental evidence for pentaquarks as well as their non-observations by some experiments, and discuss to which extend these sometimes contradicting informations may lead to a consistent picture.Comment: Contribution to the International Conference on 'Strangeness in Quark Matter', 15-21 Sept. 2004, Cape Tawn, South Afric

First measurement of coherent ϕ\phi-meson photoproduction from helium-4 near threshold

The differential cross sections and decay angular distributions for coherent $\phi$-meson photoproduction from helium-4 have been measured for the first time at forward angles with linearly polarized photons in the energy range $E_{\gamma} = \text{1.685-2.385 GeV}$. Thanks to the target with spin-parity $J^{P} = 0^{+}$, unnatural-parity exchanges are prohibited, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the dominance ($> 94\%$) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section ($\theta = 0^\circ$) for the $\gamma p \rightarrow \phi p$ reaction near the threshold, the energy dependence of the forward cross section ($\theta = 0^\circ$) for the $\gamma {^{4}\text{He}} \rightarrow \phi {^{4}\text{He}}$ reaction was analyzed. The comparison to $\gamma p \rightarrow \phi p$ data suggests that enhancement of the forward cross section arising from natural-parity exchanges, and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the $\gamma p \rightarrow \phi p$ reaction near the threshold.Comment: 6 pages, 4 figure

Polarization in Hyperon Photo- and Electro- Production

Multiple polarization observables must be measured to access the amplitude structure of pseudoscalar meson photoproduction off the proton. The hyperon-producing reactions are especially attractive to study, since the weak decays allow straightforward measurement of the induced and recoil polarization observables. In this paper we emphasize $\gamma + p \to K^+ + \Lambda$, discussing recent measurements of $C_x$, $C_z$, and $P$ for this reaction. An empirical constraint on the helicity amplitudes is obtained. A simplified model involving spin-flip and spin non-flip amplitudes is presented. Finally, a semi-classical model of how the polarization may arise is presented.Comment: Invited paper at NStar2007, Accepted for EPJ A 1-21-08, 6 pages, 6 figure

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)Sigma0 reactions at Egamma=1.5-2.4 GeV

Beam polarization asymmetries for the p(gamma,K+)Lambda and p(gamma,K+)sigma0 reactions are measured for the first time for Egamma=1.5-2.4 GeV and 0.6<cos(theta_cm(K+))<1.0 by using linearly polarized photons at the Laser-Electron-Photon facility at SPring-8 (LEPS). The observed asymmetries are positive and gradually increase with rising photon energy. The data are not consistent with theoretical predictions based on tree-level effective Lagrangian approaches. Including the new results in the development of the models is, therefore, crucial for understanding the reaction mechanism and to test the presence of baryon resonances which are predicted in quark models but are sofar undiscovered