Browsing by Author "Adikaram, D."

The CLAS detector was used to obtain the first ever measurement of the electromagnetic decay of the Σ*⁺(1385) from the reaction γp → K⁰Σ*⁺(1385). A real photon beam with a maximum energy of 3.8 GeV was incident on a liquid-hydrogen target, resulting in the photoproduction of the kaon and Σ* hyperon. Kinematic fitting was used to separate the reaction channel from the background processes. The fitting algorithm exploited a new method to kinematically fit neutrons in the CLAS detector, leading to the measured decay widths ratio: Γ(Σ⁺(1385) → Σ⁺γ) / Γ(Σ⁺(1385) → Σ⁺π⁰) = 11.95 ± 2.21 (stat) ⁺⁰.⁵³₋₁.₂₁ (sys) % and a deduced partial width of: Γ(Σ⁺(1385) → Σ⁺γ) = 250.0 ± 56.9 (stat) ⁺³⁴.³₋₄₁.² (sys) keV A U-spin symmetry test using the SU(3) flavor-multiplet representation yields predictions for the Σ*⁺(1385) → Σ⁺γ and Σ*⁰(1385) → Λγ partial widths that agree with the experimental measurements.

Unpolarized and beam-polarized fourfold cross sections ðd4σ=dQ2dxBdtdϕÞ for the ep → e0 p0 γ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 (Q2; xB; t) bins over the widest phase space ever explored in the valencequark region. Several models of generalized parton distributions (GPDs) describe the data well at most of our kinematics. This increases our confidence that we understand the GPD H, expected to be the dominant contributor to these observables. Through a leading-twist extraction of Compton form factors, these results support the model predictions of a larger nucleon size at lower quark-momentum fraction xB.

The beam-spin asymmetry, , for the reaction γ d → pn has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins, between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, θc.m., between 25◦ and 160◦. These are the first measurements of beam-spin asymmetries at θc.m. = 90◦ for photon-beam energies above 1.6 GeV, and the first measurements for angles other than θc.m. = 90◦. The angular and energy dependence of is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.

We have measured the nuclear transparency of the incoherent diffractive A(e, e ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced ρ0’s on a nucleus relative to deuterium, which is sensitive to ρ A interaction, was studied as function of the coherence length (lc ), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q 2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q 2 dependence was measured, which is consistent with calculations that included the color transparency effects.

The reaction 𝛾+𝑝→𝐾++𝛴+𝜋 was used to determine the invariant mass distributions or “line shapes” of the 𝛴+⁢𝜋−, 𝛴−⁢𝜋+, and 𝛴0⁢𝜋0 final states, from threshold at 1328 MeV/𝑐2 through the mass range of the 𝛬⁡(1405) and the 𝛬⁡(1520). The measurements were made with the CLAS system at Jefferson Lab using tagged real photons, for center-of-mass energies 1.95<𝑊<2.85 GeV. The three mass distributions differ strongly in the vicinity of the 𝐼=0 𝛬⁡(1405), indicating the presence of substantial 𝐼=1 strength in the reaction. Background contributions to the data from the 𝛴0⁡(1385) and from 𝐾*⁢𝛴 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 Math output error threshold. A best fit to all the data was obtained after including a phenomenological 𝐼=1, 𝐽𝑃=1/2− amplitude with a centroid at 1394±20 MeV/𝑐2 and a second 𝐼=1 amplitude at 1413±10 MeV/𝑐2. The centroid of the 𝐼=0 𝛬⁡(1405) strength was found at the 𝛴⁢𝜋 threshold, with the observed shape determined largely by channel coupling, leading to an apparent overall peak near 1405 MeV/𝑐2.

The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four-momentum transfer (Q²). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. We produced a mixed simultaneous electron-positron beam in Jefferson Lab’s Hall B by passing the 5.6-GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a converter to produce electron-positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm-long liquid hydrogen (LH₂) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons, we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. We present previously unpublished results for the quantity R₂γ, the TPE correction to the elastic-scattering cross section, at Q² ≈ 0.85 and 1.45 GeV² over a large range of virtual photon polarization ε. Our results, along with recently published results from VEPP-3, demonstrate a nonzero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q² ≈ 2 GeV². These data are consistent with an increase in R₂γ with decreasing ε at Q² ≈ 0.85 and 1.45 GeV². There are indications of a slight increase in R₂γ with Q².

We report the measurement of near-threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range Q2 from 2 to 4.5 GeV2 and W from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles E0+ and S0+ in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors Gπ0p 1 (Q2) and Gπ0p 2 (Q2). The data are compared to these generalized form factors and the results for Gπ0p 1 (Q2) are found to be in good agreement with the LCSR predictions, but the level of agreement with Gπ0p 2 (Q2) is poor.

The inclusive polarized structure functions of the proton and deuteron, g1p and g1d, were measured with high statistical precision using polarized 6 GeV electrons incident on a polarized ammonia target in Hall B at Jefferson Laboratory. Electrons scattered at lab angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual DIS kinematics, Q^2>1 GeV^2 and the final-state invariant mass W>2 GeV, the ratio of polarized to unpolarized structure functions g1/F1 is found to be nearly independent of Q^2 at fixed x. Significant resonant structure is apparent at values of W up to 2.3 GeV. In the framework of perturbative QCD, the high-W results can be used to better constrain the polarization of quarks and gluons in the nucleon, as well as high-twist contributions.

We present a search for ten baryon-number violating decay modes of Λ hyperons using the CLAS detector at Jefferson Laboratory. Nine of these decay modes result in a single meson and single lepton in the final state (Λ → m`) and conserve either the sum or the difference of baryon and lepton number (B ±L). The tenth decay mode (Λ → pπ¯ +) represents a difference in baryon number of two units and no difference in lepton number. We observe no significant signal and set upper limits on the branching fractions of these reactions in the range (4 − 200) × 10−7 at the 90% confidence level.

We present precision measurements of the target and beam-target spin asymmetries from neutral pion electroproduction in deep-inelastic scattering (DIS) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. We scattered 6-GeV, longitudinally polarized electrons off longitudinally polarized protons in a cryogenic 14NH3 target, and extracted double and single target spin asymmetries for ep → e π0 X in multidimensional bins in four-momentum transfer (1.0 < Q 2 < 3.2 GeV2), Bjorken-x (0.12 < x < 0.48), hadron energy fraction (0.4 < z < 0.7), transverse pion momentum (0 < PT < 1.0 GeV), and azimuthal angle φh between the lepton scattering and hadron production planes. We extracted asymmetries as a function of both x and PT , which provide access to transverse-momentum distributions of longitudinally polarized quarks. The double spin asymmetries depend weakly on PT . The sin 2φh moments are zero within uncertainties, which is consistent with the expected suppression of the Collins fragmentation function. The observed sinφh moments suggest that quark gluon correlations are significant at large x.

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π⁰ electroproduction reaction γp → pπ⁰, expanding an analysis of the γp → nπ⁺ reaction from the same experiment. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic ranges covered are 1.1 < W < 3 GeV and 1 < Q² < 6 GeV². Results were obtained for about 5700 bins in W, Q², cos(θ*), and φ*. The beam-target asymmetries were found to generally be greater than zero, with relatively modest φ* dependence. The target asymmetries exhibit very strong φ* dependence, with a change in sign occurring between results at low W and high W, in contrast to π⁺ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for W < 1.6 GeV, but significant differences are seen at higher W. When combined with cross-sectional measurements, as well as π⁺ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q², for resonances with masses as high as 2.4 GeV.