Partial wave analysis of the reaction gamma p -> p omega$ and the search for nucleon resonances

An event-based partial wave analysis (PWA) of the reaction gamma p -> p omega has been performed on a high-statistics dataset obtained using the CLAS at Jefferson Lab for center-of-mass energies from threshold up to 2.4 GeV. This analysis benefits from access to the world's first high precision spin density matrix element measurements, available to the event-based PWA through the decay distribution of omega-> pi+ pi - pi0. The data confirm the dominance of the t-channel pi0 exchange amplitude in the forward direction. The dominant resonance contributions are consistent with the previously identified states F[15](1680) and D[13](1700) near threshold, as well as the G[17](2190) at higher energies. Suggestive evidence for the presence of a J(P)=5/2(+) state around 2 GeV, a "missing" state, has also been found. Evidence for other states is inconclusive

Wake-induced vibration interference between a fixed square cylinder and a 2-DOF downstream square cylinder at low Reynolds numbers

Wake-induced vibration (WIV) of a two-degree-of-freedom (2-DOF) downstream square cylinder behind a stationary equal-size upstream square cylinder is numerically investigated at low Reynolds numbers by using Characteristics-Based Split (CBS) finite element algorithm. Due to the interference between the cylinders, the flow pattern and dynamic characteristics of become more complex than an isolated cylinder case. The spacing ratio of the two equal-sized square cylinders in the tandem arrangement is fixed as L/D = 5.0. The Reynolds numbers of this two-cylinder system have five types, varying between 40 and 200 with a unit step of △Re = 40. The reduced mass of the square cylinder is Mr = 2.0, while its reduced velocity changes between Ur = 3.0–18.0. The numerical results show that the reduced velocity and Reynolds number can affect the characteristics of the flow patterns, oscillatory frequency, maximum amplitudes, and X-Y trajectories of the downstream square cylinder. The predominant vortex shedding patterns are 2S, 2S* 2P, P + S, 2T and steady mode. Additionally, the figure “8” and figure “dual-8” are observed in the X-Y vibrating trajectories of the downstream square cylinder. Finally, the interactions between cylinders underlying the vibration characteristics of the square cylinder behind a stationary square cylinder with different Reynolds numbers are revealed

First observation of the Λ(1405) line shape in electroproduction

We report the first observation of the line shape of the Λ(1405) from electroproduction, and show that it is not a simple Breit-Wigner resonance. Electroproduction of K+Λ(1405) off the proton was studied by using data from CLAS at Jefferson Lab in the range 1.0<Q2<3.0 (GeV/c)2. The analysis utilized the decay channels Σ+π− of the Λ(1405) and pπ0 of the Σ+. Neither the standard Particle Data Group resonance parameters, nor free parameters fitting to a single Breit-Wigner resonance represent the line shape. In our fits, the line shape corresponds approximately to predictions of a two-pole meson-baryon picture of the Λ(1405), with a lower mass pole near 1368 MeV/c2 and a higher mass pole near 1423 MeV/c2. Furthermore, with increasing photon virtuality the mass distribution shifts toward the higher mass pole

Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

We present results on the electroexcitation of the low mass resonances Δ(1232)P<sub>33</sub>, N(1440)P<sub>11</sub>, N(1520)D<sub>13</sub>, and N(1535)S<sub>11</sub> in a wide range of Q<sup>2</sup>. The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Δ(1232)P<sub>33</sub> show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q<sup>2</sup>⩽6 GeV<sup>2</sup>. For the Roper resonance, N(1440)P<sub>11</sub>, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S<sub>11</sub> allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ*p→N(1520)D<sub>13</sub> and N(1535)S<sub>11</sub> longitudinal helicity amplitudes with good precision. For the N(1535)S<sub>11</sub>, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D<sub>13</sub> clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q<sup>2</sup>>1 GeV<sup>2</sup>, confirming a long-standing prediction of the constituent quark model