Double and single pion photoproduction within a dynamical coupled-channels model

Within a dynamical coupled-channels model which has already been fixed from analyzing the data of the pi N -> pi N and gamma N -> pi N reactions, we present the predicted double pion photoproduction cross sections up to the second resonance region, W< 1.7 GeV. The roles played by the different mechanisms within our model in determining both the single and double pion photoproduction reactions are analyzed, focusing on the effects due to the direct gamma N -> pi pi N mechanism, the interplay between the resonant and non-resonant amplitudes, and the coupled-channels effects. The model parameters which can be determined most effectively in the combined studies of both the single and double pion photoproduction data are identified for future studies.Comment: Version to appear in PRC. 16 pages, 13 figure

Effects of Pre-Lift Intake Valve Strategies on the Performance of a DISI VVA Turbocharged Engine at Part and Full Load Operation☆

Abstract In the paper, the potentialities offered by an advanced valve lift design are numerically analyzed. In particular, the study is carried out by a 1D approach and regards the characterization of a VVA strategy named " pre-lift" applied to a downsized turbocharged four-cylinder engine. The pre-lift consists of a small, almost constant lift of the intake valve during the exhaust stroke, so to increase the valves overlapping. The results show a benefit on the fuel economy and on the gas-dynamic noise at part load and a substantial increase in the delivered torque at full load, while preserving the fuel consumption

A numerical model of the human cornea accounting for the fiber-distributed collagen microstructure

We present a fiber-distributed model of the reinforcing collagen of the human cornea. The model describes the basic connections between the components of the tissue by defining an elementary block (cell) and upscaling it to the physical size of the cornea. The cell is defined by two sets of collagen fibrils running in approximately orthogonal directions, characterized by a random distribution of the spatial orientation and connected by chemical bonds of two kinds. The bonds of the first kind describe the lamellar crosslinks, forming the ribbon-like lamellae; while the bonds of the second kind describe the stacking crosslinks, piling up the lamellae to form the structure of the stroma. The spatial replication of the cell produces a truss structure with a considerable number of degrees of freedom. The statistical characterization of the collagen fibrils leads to a mechanical model that reacts to the action of the deterministic intraocular pressure with a stochastic distribution of the displacements, here characterized by their mean value and variance. The strategy to address the solution of the heavy resulting numerical problem is to use the so-called stochastic finite element improved perturbation method combined with a fully explicit solver. Results demonstrate that the variability of the mechanical properties affects in a non-negligible manner the expected response of the structure to the physiological action

Measurement of the Sigma pi photoproduction line shapes near the Lambda(1405)

The reaction gamma + p -\u3e K+ + Sigma + pi was used to determine the invariant mass distributions or line shapes of the Sigma(+) pi-, Sigma - pi(+), and Sigma(0)pi(0) final states, from threshold at 1328 MeV/c(2) through the mass range of the Lambda(1405) and the Lambda(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95 \u3c W \u3c 2.85 GeV. The three mass distributions differ strongly in the vicinity of the I = 0 Lambda(1405), indicating the presence of substantial I = 1 strength in the reaction. Background contributions to the data from the Sigma(0)(1385) and from K*Sigma production were studied and shown to have negligible influence. To separate the isospin amplitudes, Breit-Wigner model fits were made that included channel-coupling distortions due to the N (K) over bar threshold. A best fit to all the data was obtained after including a phenomenological I = 1, J(P) = 1/2(-) amplitude with a centroid at 1394 +/- 20 MeV/c(2) and a second I = 1 amplitude at 1413 +/- 10 MeV/c(2). The centroid of the I = 0 Lambda(1405) strength was found at the Sigma pi threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/c(2)

Spin and parity measurement of the Lambda(1405) baryon

A determination of the spin and parity of the Lambda(1405) is presented using photoproduction data from the CLAS detector at Jefferson Lab. The reaction gamma + p -\u3e K+ + Lambda(1405) is analyzed in the decay channel Lambda(1405) -\u3e Sigma(+) + pi(-), where the decay distribution to Sigma(+)pi(-) the variation of the Sigma(+) polarization with respect to the Lambda(1405) polarization direction determines the parity. The Lambda(1405) is produced, in the energy range 2.55 \u3c W \u3c 2.85 GeV and for 0.6 \u3c cos theta(c.m.)(K+) \u3c 0.9, with polarization P = 0.45 +/- 0.02(stat) +/- 0.07(syst). The analysis shows that the decays are in S wave, with the Sigma(+) polarized such that the Lambda(1405) has spin-parity J(P) = 1(-)/2, as expected by most theories

Multivariate side-band subtraction using probabilistic event weights

A common situation in experimental physics is to have a signal which can not be separated from a non-interfering background through the use of any cut. In this paper, we describe a procedure for determining, on an event-by-event basis, a quality factor ($Q$-factor) that a given event originated from the signal distribution. This procedure generalizes the "side-band" subtraction method to higher dimensions without requiring the data to be divided into bins. The $Q$-factors can then be used as event weights in subsequent analysis procedures, allowing one to more directly access the true spectrum of the signal.Comment: 17 pages, 9 figure

Differential photoproduction cross sections of the Sigma(0)(1385), Lambda(1405), and Lambda(1520)

We report the exclusive photoproduction cross sections for the Sigma(0)(1385), Lambda(1405), and Lambda(1520) in the reactions gamma + p -\u3e K+ + Y* using the CLAS detector for energies from near the respective production thresholds up to a center-of-mass energy W of 2.85 GeV. The differential cross sections are integrated to give the total exclusive cross sections for each hyperon. Comparisons are made to current theoretical models based on the effective-Lagrangian approach and fit to previous data. The accuracy of these models is seen to vary widely. The cross sections for the Lambda(1405) region are strikingly different for the Sigma(+)pi(-), Sigma(0)pi(0), and Sigma(-)pi(+) decay channels, indicating the effect of isospin interference, especially at W values close to the threshold

Data analysis techniques, differential cross sections, and spin density matrix elements for the reaction gamma p -\u3e phi p

High-statistics measurements of differential cross sections and spin density matrix elements for the reaction gamma p -\u3e phi p have been made using the CLAS detector at Jefferson Lab. We cover center-of-mass energies (v s) from 1.97 to 2.84 GeV, with an extensive coverage in the phi production angle. The high statistics of the data sample made it necessary to carefully account for the interplay between the f natural lineshape and effects of the detector resolution, that are found to be comparable in magnitude. We study both the charged-(phi -\u3e K+ K-) and neutral( phi -\u3e(KsKL0)-K-0) K (K) over bar decay modes of the phi Further, for the charged mode, we differentiate between the cases where the final K-track is directly detected or its momentum reconstructed as the total missing momentum in the event. The two charged-mode topologies and the neutral-mode have different resolutions and are calibrated against each other. Extensive usage is made of kinematic fitting to improve the reconstructed f mass resolution. Our final results are reported in 10- and mostly 30-MeV-wide root s bins for the charged-and the neutral-modes, respectively. Possible effects from K+Lambda* channels with pK (K) over bar final states are discussed. These present results constitute the most precise and extensive phi photoproduction measurements to date and in conjunction with the omega photoproduction results recently published by CLAS, will greatly improve our understanding of low energy vector meson photoproduction