Browsing by Author "Niccolai, S."

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.

The f0(1500) meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we present the first photoproduction data for the f0(1500) via decay into the K0 SK 0 S channel using the CLAS detector. The reaction γp → fJp → K0 SK0 Sp, where J = 0,2, was measured with photon energies from 2.7 to 5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured 4-momentum transfer to the proton target is restricted to be less than 1.0 GeV2. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low t, which is consistent with at-channel production mechanism.

In the past two decades, deeply virtual Compton scattering of electrons has been successfully used to advance our knowledge of the partonic structure of the free proton and investigate correlations between the transverse position and the longitudinal momentum of quarks inside the nucleon. Meanwhile, the structure of bound nucleons in nuclei has been studied in inclusive deep-inelastic lepton scattering experiments off nuclear targets, showing a significant difference in longitudinal momentum distribution of quarks inside the bound nucleon, known as the EMC effect. In this Letter, we report the first beam spin asymmetry (BSA) measurement of exclusive deeply virtual Compton scattering off a proton bound in 4 He . The data used here were accumulated using a 6 GeV longitudinally polarized electron beam incident on a pressurized 4 He gaseous target placed within the CLAS spectrometer in Hall-B at the Thomas Jefferson National Accelerator Facility. The azimuthal angle (𝜙) dependence of the BSA was studied in a wide range of virtual photon and scattered proton kinematics. The 𝑄2, 𝑥𝐵, and t dependencies of the BSA on the bound proton are compared with those on the free proton. In the whole kinematical region of our measurements, the BSA on the bound proton is smaller by 20% to 40%, indicating possible medium modification of its partonic structure.

A measurement of the electroproduction of photons off protons in the deeply inelastic regime was performed at Jefferson Lab using a nearly 6 GeV electron beam, a longitudinally polarized proton target, and the CEBAF Large Acceptance Spectrometer. Target-spin asymmetries for ep → e0 p0 γ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in Q2, xB, t, and ϕ, for 166 four-dimensional bins. In the framework of generalized parton distributions, at leading twist the t dependence of these asymmetries provides insight into the spatial distribution of the axial charge of the proton, which appears to be concentrated in its center. These results also bring important and necessary constraints for the existing parametrizations of chiral-even generalized parton distributions.

Exclusive π⁰ electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in Q², xB, t, and φ, in the Q² range from 1.0 to 4.6 GeV², t up to 2 GeV², and xB from 0.1 to 0.58. Structure functions σT+L, σTT, and σLT were extracted as functions of t for each of 17 combinations of Q² and xB. The data were compared directly with two handbag-based calculations including both longitudinal and transversity generalized parton distributions (GPDs). Inclusion of only longitudinal GPDs very strongly underestimates σT+L and fails to account for σTT and σLT, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity-flip and helicity nonflip processes. The results confirm that exclusive π⁰ electroproduction offers direct experimental access to the transversity GPDs.

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.

This paper reports the measurement of polarized and unpolarized cross sections for the ep → e'p'γ reaction, which is composed of deeply virtual Compton scattering (DVCS) and Bethe-Heitler (BH) processes, at an electron beam energy of 5.88 GeV at the Thomas Jefferson National Accelerator Facility using the Large Acceptance Spectrometer CLAS. The unpolarized cross sections and polarized cross section differences have been measured over broad kinematics, 0.10 < x_B < 0.58, 1.0 < Q² < 4.8 GeV², and 0.09 < -t < 2.00 GeV². The results are found to be consistent with previous CLAS data, and these new data are discussed in the framework of the generalized parton distribution approach. Calculations with two widely used phenomenological models are approximately compatible with the experimental results over a large portion of the kinematic range of the data.

Differential cross sections of the exclusive process ep → e'π⁺n were measured with good precision in the range of the photon virtuality Q² = 1.8-4.5 GeV² and the invariant mass range of the π⁺n final state W = 1.6-2.0 GeV using the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the nπ⁺ center-of-mass system. More than 37000 cross-section points were measured. The contributions of the isospin I = 1/2 resonances N(1675)5/2⁻, N(1680)5/2⁺, and N(1710)1/2⁺ were extracted at different values of Q² using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-t dispersion relations, were employed in the analysis. We observe significant strength of the N(1675)5/2⁻ in the A₁/₂ amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the N(1680)5/2⁺ we observe a slow changeover from the dominance of the A₃/₂ amplitude at the real photon point (Q² = 0) to a Q² where A₁/₂ begins to dominate. The scalar amplitude S₁/₂ drops rapidly with Q² consistent with quark model prediction. For the N(1710)1/2⁺ resonance our analysis shows significant strength for the A₁/₂ amplitude at Q² < 2.5 GeV².

New results on the single-differential and fully integrated cross sections for the process γvp → p π+π− are presented. The experimental data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the kinematic region of the reaction invariant mass W from 1.3 to 1.825 GeV and the photon virtuality Q2 from 0.4 to 1.0 GeV2 . The cross sections were obtained in narrow Q2 bins (0.05 GeV2 ) with the smallest statistical uncertainties achieved in double-pion electroproduction experiments to date. The results were found to be in agreement with previously available data where they overlap. A preliminary interpretation of the extracted cross sections, which was based on a phenomenological meson-baryon reaction model, revealed substantial relative contributions from nucleon resonances. The data offer promising prospects to improve knowledge on the Q2 evolution of the electrocouplings of most resonances with masses up to ∼1.8 GeV.

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.