My Ahmed , El Alaoui

Exclusive photoproduction cross sections have been measured for the process γp → pπ0[e+e−(γ )] with the Dalitz decay final state using tagged photon energies in the range of Eγ = 1.275–5.425 GeV. The complete angular distribution of the final state π0, for the entire photon energy range up to large values of t and u, has been measured for the first time. The data obtained show that the cross section dσ/dt, at mid to large angles, decreases with energy as s−6.89±0.26. This is in agreement with the perturbative QCD quark counting rule prediction of s−7. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD-based generalized parton distribution mechanism at highest available invariant energy s = 11 GeV2 . At the same time, the Regge-exchange-based models for π0 photoproduction are more consistent with experimental data.

Strange matter is believed to exist in the cores of neutron stars based on simple kinematics. If this is true, then hyperon-nucleon interactions will play a significant part in the neutron star equation of state. Yet, compared to other elastic scattering processes, there is very little data on Λ-N scattering. This experiment utilized the CEBAF Large Acceptance Spectrometer (CLAS) detector to study the Λp → Λp elastic scattering cross section in the incident Λ momentum range 0.9–2.0 GeV=c. These are the first data on this reaction since the 1970s. The new cross sections have significantly better accuracy and precision than the existing world data, and the techniques developed here can also be used in future experiments.

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 SK0 S channel using the CLAS detector. The reaction γp → fJp → K0 SK0 Sp, where J = 0,2, was measured with photon energies from 2.7–5.1 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 fourmomentum 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 a t-channel production mechanism.

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.

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π0 electroproduction reaction γ ∗p → pπ0, expanding an analysis of the γ ∗p → 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

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.

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 < xB < 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 𝑒⁢𝑝→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 𝑛⁢𝜋+ center-of-mass system. More than 37 000 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.

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 report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (𝑧) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high 𝑧 and an enhancement at low 𝑧. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high 𝑧. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.