arXiv subjects

Z. E. Meziani

Publications and source records attributed to Z. E. Meziani.

45 records · Page 3Linked to original sources

arXiv2017

Differential Cross Section Measurements for $γn\toπ^-p$ Above the First Nucleon Resonance Region

The quasi-free $γd\toπ^{-}p(p)$ differential cross section has been measured with CLAS at photon beam energies $E_γ$ from 0.445 GeV to 2.510 GeV (corresponding to $W$ from 1.311 GeV to 2.366 GeV) for pion center-of-mass angles $\cosθ_π^{c.m.}$ from -0.72 to 0.92. A correction for final state interactions has been applied to this data to extract the $γn\toπ^-p$ differential cross sections. These cross sections are quoted in 8428 $(E_γ,\cosθ_π^{c.m.})$ bins, a factor of nearly three increase in the world statistics for this channel in this kinematic range. These new data help to constrain coupled-channel analysis fits used to disentangle the spectrum of $N^*$ resonances and extract their properties. Selected photon decay amplitudes $N^* \to γn$ at the resonance poles are determined for the first time and are reported here.

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arXiv2017

Photon beam asymmetry $Σ$ in the reaction $\vecγ p \to p ω$ for $E_γ$ = 1.152 to 1.876 GeV

Photon beam asymmetry $Σ$ measurements for $ω$ photoproduction in the reaction $\vecγ p \to ωp$ are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between $t$-channel meson exchange and $s$- and $u$-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the $Σ$ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the $J^P=3/2^+$ partial wave), as well as the resonant portions of the smaller partial waves with $J^P$= $1/2^-$, $3/2^-$, and $5/2^+$.

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arXiv2017

Exclusive $η$ electroproduction at $W>2$ GeV with CLAS and transversity generalized parton distributions

The cross section of the exclusive $η$ electroproduction reaction $ep\to e^\prime p^\prime η$ was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4σ/dtdQ^2dx_Bdϕ_η$ and structure functions $σ_U = σ_T+εσ_L, σ_{TT}$ and $σ_{LT}$, as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. The $η$ structure functions are compared with those previously measured for $π^0$ at the same kinematics. At low $t$, both $π^0$ and $η$ are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The $π^0$ and $η$ data, when taken together, can facilitate the flavor decomposition of the transversity GPDs.

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arXiv2016

Electroexcitation of the $Δ^{+}(1232)$ at low momentum transfer

We report on new p$(e,e^\prime p)π^\circ$ measurements at the $Δ^{+}(1232)$ resonance at the low momentum transfer region. The mesonic cloud dynamics is predicted to be dominant and rapidly changing in this kinematic region offering a test bed for chiral effective field theory calculations. The new data explore the low $Q^2$ dependence of the resonant quadrupole amplitudes while extending the measurements of the Coulomb quadrupole amplitude to the lowest momentum transfer ever reached. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.

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arXiv2014

Spectroscopy of Lambda-9Li by electroproduction

In the absence of accurate data on the free two-body hyperon-nucleon interaction, the spectra of hypernuclei can provide information on the details of the effective hyperon-nucleon interaction. Electroproduction of the hypernucleus Lambda-9Li has been studied for the first time with sub-MeV energy resolution in Hall A at Jefferson Lab on a 9Be target. In order to increase the counting rate and to provide unambiguous kaon identification, two superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH) were added to the Hall A standard equipment. The cross section to low-lying states of Lambda-9Li is concentrated within 3 MeV of the ground state and can be fitted with four peaks. The positions of the doublets agree with theory while a disagreement could exist with respect to the relative strengths of the peaks in the doublets. A Lambda separation energy of 8.36 +- 0.08 (stat.) +- 0.08 (syst.) MeV was measured, in agreement with an earlier experiment.

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arXiv2010

Measurements of the Electric Form Factor of the Neutron up to Q2=3.4 GeV2 using the Reaction He3(e,e'n)pp

The electric form factor of the neutron was determined from studies of the reaction He3(e,e'n)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2-range over which it is known, we find GEn = 0.0225 +/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142 +/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.

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arXiv2009

High Resolution Spectroscopy of 16N_Lambda by Electroproduction

An experimental study of the 16O(e,e'K^+)16N_Lambda reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e,e'K^+)Lambda,Sigma_0 exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76 +/- 0.16 MeV was obtained for 16N_Lambda with better precision than previous measurements on the mirror hypernucleus 16O_Lambda. Precise energies have been determined for peaks arising from a Lambda in s and p orbits coupled to the p_{1/2} and p_{3/2} hole states of the 15N core nucleus.

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arXiv2007

High Resolution Spectroscopy of 12B_Lambda by Electroproduction

An experiment measuring electroproduction of hypernuclei has been performed in Hall A at Jefferson Lab on a $^{12}$C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a Ring Imaging CHerenkov detector (RICH) were added to the Hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed \lam{12}{B} spectrum shows for the first time identifiable strength in the core-excited region between the ground-state {\it s}-wave $Λ$ peak and the 11 MeV {\it p}-wave $Λ$ peak.

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arXiv2001

Is the Coulomb sum rule violated in nuclei?

Guided by the experimental confirmation of the validity of the Effective Momentum Approximation (EMA) in quasi-elastic scattering off nuclei, we have re-examined the extraction of the longitudinal and transverse response functions in medium-weight and heavy nuclei. In the EMA we have performed a Rosenbluth separation of the available world data on $^{40}$Ca, $^{48}$Ca, $^{56}$Fe, $^{197}$Au, $^{208}$Pb and $^{238}$U. We find that the longitudinal response function for these nuclei is "quenched" and that the Coulomb sum is not saturated, at odds with claims in the literature.

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