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

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

First measurement of the helicity asymmetry E in eta photoproduction on the proton

Results are presented for the first measurement of the double-polarization helicity asymmetry E for the eta photoproduction reaction gamma p - \u3e eta p. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich-Bonn model to examine the case for the existence of a narrow N* resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the Eobservable from that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances. (C) 2016 Published by Elsevier B.V

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)

Cross sections for the gamma p -\u3e K*(+)Lambda and gamma p -\u3e K*(+)Sigma(0) reactions measured at CLAS

The first high-statistics cross sections for the reactions gamma p -\u3e K*(+)Lambda and gamma p -\u3e K*(+)Sigma(0) were measured using the CLAS detector at photon energies between threshold and 3.9 GeV at the Thomas Jefferson National Accelerator Facility. Differential cross sections are presented over the full range of the center-of-mass angles, and then fitted to Legendre polynomials to extract the total cross section. Results for the K*(+)Lambda final state are compared with two different calculations in an isobar and a Regge model, respectively. Theoretical calculations significantly underestimate the K*(+)Lambda total cross sections between 2.1 and 2.6 GeV, but are in better agreement with present data at higher photon energies

Exclusive pi(0) electroproduction at W \u3e 2 GeV with CLAS

Exclusive neutral-pion electroproduction (ep -\u3e e\u27p\u27pi(0)) was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections d(4)sigma/dtdQ(2)dx(B)d phi(pi) and structure functions sigma(T) + sigma(L), sigma(TT), and sigma(LT) as functions of t were obtained over a wide range of Q(2) and x(B). The data are compared with Regge and handbag theoretical calculations. Analyses in both frameworks find that a large dominance of transverse processes is necessary to explain the experimental results. For the Regge analysis it is found that the inclusion of vector meson rescattering processes is necessary to bring the magnitude of the calculated and measured structure functions into rough agreement. In the handbag framework, there are two independent calculations, both of which appear to roughly explain the magnitude of the structure functions in terms of transversity generalized parton distributions

The Heavy Photon Search test detector

The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment\u27s technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e(+)e(-) invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW0(4) crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top bottom. This arrangement provides sensitivity to low mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e(+)e(-) pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab. (C) 2014 The Authors. Published by Elsevier B.V

Photoproduction of Lambda and Sigma(0) hyperons using linearly polarized photons

Background: Measurements of polarization observables for the reactions (gamma) over right arrowp -\u3e K+ Lambda and (gamma) over right arrowp -\u3e K+ Sigma(0) have been performed. This is part of a program of measurements designed to study the spectrum of baryon resonances in particular, and nonperturbative QCD in general. Purpose: The accurate measurement of several polarization observables provides tight constraints for phenomenological fits, which allow the study of strangeness in nucleon and nuclear systems. Beam-recoil observables for the (gamma) over right arrowp -\u3e K+ Sigma(0) reaction have not been reported before now. Method: Themeasurements were carried out using linearly polarized photon beams incident on a liquid hydrogen target, and the CLAS detector at the Thomas Jefferson National Accelerator Facility. The energy range of the results is 1.71 \u3c W \u3c 2.19 GeV, with an angular range -0.75 \u3c cos theta(K)* \u3c +0.85. Results: The observables extracted for both reactions are beam asymmetry Sigma, target asymmetry T, and the beam-recoil double polarization observables O-x and O-z. Conclusions: Comparison with theoretical fits indicates that, in the regions where no previous data existed, the new data contain significant new information, and strengthen the evidence for the set of resonances used in the latest Bonn-Gatchina fit

Separated structure functions for exclusive K+Lambda and K+Sigma(0) electroproduction at 5.5 GeV measured with CLAS

We report measurements of the exclusive electroproduction of K+Lambda and K+Sigma(0) final states from an unpolarized proton target using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The separated structure functions sigma(U), sigma(LT), sigma(T) T, and sigma(LT\u27) were extracted from the Phi-dependent differential cross sections acquired with a longitudinally polarized 5.499 GeV electron beam. The data span a broad range of momentum transfers Q(2) from 1.4 to 3.9GeV(2), invariant energy W from threshold to 2.6GeV, and nearly the full center-of-mass angular range of the kaon. The separated structure functions provide an unprecedented data sample, which, in conjunction with other meson photo-and electroproduction data, will help to constrain the higher-level analyses being performed to search for missing baryon resonances. DOI: 10.1103/PhysRevC.87.02520

Photoproduction of the f(1)(1285) meson

The f(1)(1285) meson withmass 1281.0 +/- 0.8MeV/c(2) and width 18.4 +/- 1.4MeV (full width at half maximum) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the eta pi(+)pi(-), K+(K) over bar (0) pi(-), and (K-K0)pi(+) decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the eta pi(+)pi(-) final-state Dalitz distribution are consistent with the axial-vector J(P) = 1(+) f(1)(1285) identity, rather than the pseudoscalar 0(-) eta(1295). The production mechanism is more consistent with s-channel decay of a high-mass N* state and not with t-channel meson exchange. Decays to eta pi pi go dominantly via the intermediate a(0)(+/-) (980)pi(-/+) states, with the branching ratio Gamma [a(0)pi (no (K) over barK)]/Gamma[eta pi pi (all)] = 0.74 +/- 0.09. The branching ratios Gamma (K (K) over bar pi)/Gamma(eta pi pi) = 0.216 +/- 0.033 and Gamma (gamma rho(0))/Gamma(eta pi pi) = 0.047 +/- 0.018 were also obtained. The first is in agreement with previous data for the f(1)(1285), while the latter is lower than the world average