First Measurement of Transferred Polarization in the Exclusive e⃗ p→e′K+Λ⃗Reaction

The first measurements of the transferred polarization for the exclusive e⃗ p→e′K+Λ⃗ reaction have been performed at Jefferson Laboratory using the CLAS spectrometer. A 2.567 GeV beam was used to measure the hyperon polarization over Q2 from 0.3 to 1.5   (GeV/c)2, W from 1.6 to 2.15 GeV, and over the full K+ center-of-mass angular range. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A nonrelativistic quark-model interpretation of our data suggests that the ss¯ quark pair is produced with spins predominantly antialigned. Implications for the validity of the most widely used quark-pair creation operator are discussed

Beam-recoil polarization transfer in the nucleon resonance region in the exclusive e→p→e′K+Λ→ and e→p→e′K+Σ→0 reactions at the CLAS spectrometer

Beam-recoil transferred polarizations for the exclusive e→p→e′K+Λ→,Σ→0 reactions have been measured using the Continuous Electron Beam Accelerator Facility\u27s large acceptance spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. New measurements have been completed at beam energies of 4.261 and 5.754 GeV that span a range of momentum transfer Q2 from 0.7 to 5.4 GeV2, invariant energy W from 1.6 to 2.6 GeV, and the full center-of-mass angular range of the K+ meson. These new data add to the existing CLAS K+Λ measurements at 2.567 GeV, and provide the first-ever data for the K+Σ0 channel in electroproduction. Comparisons of the data with several theoretical models are used to study the sensitivity to s-channel resonance contributions and the underlying reaction mechanism. Interpretations within two semiclassical partonic models are made to probe the underlying reaction mechanism and the ss¯ quark-pair creation dynamics

Data Analysis Techniques, Differential Cross Sections, and Spin Density Matrix Elements for the Reaction ᵞp → ϕp

High-statistics measurements of differential cross sections and spin density matrix elements for the reaction ᵞp → ϕp have been made using the CLAS detector at Jefferson Lab. We cover center-of-mass energies (√s) from 1.97 to 2.84 GeV, with an extensive coverage in the ϕ production angle. The high statistics of the data sample made it necessary to carefully account for the interplay between the ϕ natural lineshape and effects of the detector resolution, that are found to be comparable in magnitude. We study both the charged-(ϕ → K+ K-) and neutral( ϕ → (KOSKOL) K K- decay modes of the ϕ. Further, for the charged mode, we differentiate between the cases where the final K-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 ϕ mass resolution. Our final results are reported in 10- and mostly 30-MeV-wide √s bins for the charged-and the neutral-modes, respectively. Possible effects from K+Λ* channels with p K K- final states are discussed. These present results constitute the most precise and extensive ϕ photoproduction measurements to date and in conjunction with the ω photoproduction results recently published by CLAS, will greatly improve our understanding of low energy vector meson photoproduction

Induced Polarization of Λ1116 in Kaon Electroproduction

We have measured. the induced polarization of the Λ (1116) in the reaction ep →e′K+Λ , detecting the scattered e′ and K+ in the final state along with the proton from the decay Λ → pπ− . The present study used the CEBAF Large Acceptance Spectrometer (CLAS), which allowed for a large kinematic acceptance in invariant energy W (1.6≤ W ≤ 2.7 GeV) and covered the full range of the kaon production angle at an average momentum transfer Q2 = 1.90GeV2 . In this experiment a 5.50-GeV electron beam was incident upon an unpolarized liquid-hydrogen target. We have mapped out the W and kaon production angle dependencies of the induced polarization and found striking differences from photoproduction data over most of the kinematic range studied. However, we also found that the induced polarization is essentially Q2 independent in our kinematic domain, suggesting that somewhere below the Q2 covered here there must be a strong Q2 dependence. Along with previously published photo- and electroproduction cross sections and polarization observables, these data are needed for the development of models, such as effective field theories, and as input to coupled-channel analyses that can provide evidence of previously unobserved s -channel resonances

Beam Asymmetry Σ for π+ and π0 Photoproduction on the Proton for Photon Energies From 1.102 to 1.862 GeV

Beam asymmetries for the reactions ˠp -\u3e pπ0 and ˠp -\u3e nπ+have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged, linearly polarized photon beam with energies from 1.102-1.862 GeV. A Fourier moment technique for extracting beam asymmetries from experimental data is described. The results reported here possess greater precision and finer energy resolution than previous measurements. Our data for both pion reactions appear to favor the SAID and Bonn-Gatchina scattering analyses over the older Mainz MAID predictions. After incorporating the present set of beam asymmetries into the world database, exploratory fits made with the SAID analysis indicate that the largest changes from previous fits are for properties of the Δ(1700)3/2- and Δ(1905) 5/2+ states

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

Measurement of Target and Double-Spin Asymmetries for the ep → eπ+(n) Reaction in the Nucleon Resonance Region at Low Q²

We report measurements of target- and double-spin asymmetries for the exclusive channel →e→p → eπ+(n) in the nucleon resonance region at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). These asymmetries were extracted from data obtained using a longitudinally polarized NH3 target and a longitudinally polarized electron beam with energies 1.1, 1.3, 2.0, 2.3, and 3.0 GeV. The new results are consistent with previous CLAS publications but are extended to a low Q2 range from 0.0065 to 0.35(GeV/c)2. The Q2 access was made possible by a custom-built Cherenkov detector that allowed the detection of electrons for scattering angles as low as 6°. These results are compared with the unitary isobar models JANR and MAID, the partial-wave analysis prediction from SAID, and the dynamic model DMT. In many kinematic regions our results, in particular results on the target asymmetry, help to constrain the polarization-dependent components of these models

Single and Double Spin Asymmetries for Deeply Virtual Compton Scattering Measured with CLAS and a Longitudinally Polarized Proton Target

Single-beam, single-target, and double spin asymmetries for hard exclusive electroproduction of a photon on the proton →e→p~ → e\u27p\u27γ are presented. The data were taken at Jefferson Lab using the CEBAF large acceptance spectrometer and a longitudinally polarized 14NH3 target. The three asymmetries were measured in 165 four-dimensional kinematic bins, covering the widest kinematic range ever explored simultaneously for beam and target-polarization observables in the valence quark region. The kinematic dependences of the obtained asymmetries are discussed and compared to the predictions of models of generalized parton distributions. The measurement of three DVCS spin observables at the same kinematic points allows a quasi-model-independent extraction of the imaginary parts of the H and ~H Compton form factors, which give insight into the electric and axial charge distributions of valence quarks in the proton

First Results on Nucleon Resonance Photocouplings from the γp → π+π−p Reaction

We report the first experimental measurements of the nine 1-fold differential cross sections for the γ p → π+π−p reaction, obtained with the CLAS detector at Jefferson Laboratory. The measurements cover the invariant mass range of the final state hadrons from 1.6 GeV \u3c W \u3c 2.0 GeV. For the first time the photocouplings of all prominent nucleon resonances in this mass range have been extracted from this exclusive channel. Photoproduction of two charged pions is of particular importance for the evaluation of the photocouplings for the Δ (1620)1/2−, Δ (1700)3/2−, N(1720)3/2+, and Δ (1905)5/2+ resonances, which have dominant decays into the π π N final states rather than the more extensively studied single meson decay channels