No Arabic abstract
Results of a partial wave analysis of new high-statistics data on $gamma pto peta$ from MAMI are presented. A fit using known broad resonances and only standard background amplitudes can not describe the relatively narrow peaking structure in the cross section in the mass region of 1660-1750 MeV which follows a minimum. An improved description of the data can be reached by either assuming the existence of a narrow resonance at a mass of about 1700 MeV with small photo-coupling or by a threshold effect. In the latter case the observed structure is explained by a strong (resonant or non-resonant) $gamma ptoomega p$ coupling in the $S_{11}$ partial wave. When the beam asymmetry data, published by part of the GRAAL collaboration, are included in the fit, the solution with a narrow $P_{11}$ state is slightly preferred. In that fit, mass and width of the hypothetical resonance are determined to $Msim$1694 MeV and $Gammasim 40$ MeV, respectively, and the photo-coupling to $sqrt{{rm Br}_{eta N}} A_{1/2}^p sim 2.6cdot 10^{-3}$ GeV$^{-1/2}$. High precision measurements of the target asymmetry and $F$-observable are mandatory to establish the possible existence of such a narrow state and to provide the necessary information to define which partial wave is responsible for the structure observed in the data.
Revised analysis of $Sigma$ beam asymmetry for $eta$ photoproduction on the free proton reveals a resonant structure at $Wsim 1.69$ GeV. Comparison of experimental data with multipole decomposition based on the E429 solution of the SAID partial wave analysis and including narrow states, suggests a narrow ($Gamma leq 15$ MeV) resonance. Possible candidates are $P_{11}$, $P_{13}$, or $D_{13}$ resonances. The result is considered in conjunction with the recent evidence for a bump-like structure at $Wsim 1.67 - 1.68$ GeV in quasi-free $eta$ photoproduction on the neutron.
The claim that the light quark mass ratio (m_d - m_u)/m_s can be extracted from the decay width ratio Gamma(eta -> pi^0 pi^+ pi^-)/Gamma(eta -> eta pi^+ pi^-) is critically investigated within a U(3) chiral unitary framework. The influence of the recent VES data on the eta -> eta pi^+ pi^- decay is also discussed.
We investigate $S=-2$ production from the $Lambda pto K^+X$ reactions within the effective Lagrangian approach. The $Lambda pto K^+LambdaLambda$ and $Lambda pto K^+Xi^-p$ reactions are considered to find the lightest $S=-2$ system, which is $H$-dibaryon. We assume that the $H(2250)toLambdaLambda$, and $H(2270)toXi^-p$ decays with the intrinsic decay width of 1 MeV. According to our calculations, the total cross-sections for $Lambda pto K^+LambdaLambda$ and $Lambda pto K^+Xi^-p$ reactions were found to be of the order of a few $mu$b in the $Lambda$ beam momentum range of up to 5 GeV$/c$. Furthermore, the direct access of information regarding the interference patterns between the $H$-dibaryon and non-resonant contributions was demonstrated.
We investigate the reaction mechanism of the $phi$-meson photoproduction off the proton target, i.e., $gamma ptophi p$, up to $sqrt{s}=2.8$ GeV. For this purpose, we employ an effective Lagrangian approach in the tree-level Born approximation, and we employ various experimental and theoretical inputs. As a theoretical setup, the vectorlike Pomeron ($C=+1$) is taken into account as a parameterized two-gluon exchange contribution. We also consider $f_1(1285)$ axial-vector-meson, ($pi,eta$) pseudoscalar-meson, and ($a_0,f_0$) scalar-meson exchanges in the $t$ channel, in addition to the experimentally confirmed nucleon resonances, such as $N^*(2000,5/2^+)$ and $N^*(2300,1/2^+)$, for the direct $phi$-meson radiations in the $s$ and $u$ channels. We provide numerical results for the total and differential cross sections as well as the spin-density matrices in the Gottfried-Jackson, Adair, and helicity frames. We observe that, together with the universally accepted pomeron contribution, the considered meson and nucleon-resonance contributions play significant roles in reproducing the experimental data for the forward and backward $phi$-meson scattering-angle regions, respectively, indicating the nontrivial interferences between mesonic and baryonic contributions.
A resonance-like structure as narrow as 10 MeV is observed in the $K^-p$ invariant mass distributions in $Lambda_c^+to p K^- pi^+$ at Belle. Based on the large data sample of about 1.5 million events and the small bin width of just 1 MeV for the $K^-p$ invariant mass spectrum, the narrow peak is found precisely lying at the $Lambdaeta$ threshold. While lacking evidence for a quark model state with such a narrow width at this mass region, we find that this narrow structure can be naturally identified as a threshold cusp but enhanced by the nearby triangle singularity via the $Lambda$-$a_0(980)^+$ or $eta$-$Sigma(1660)^+$ rescatterings.