arXiv subjects

CLAS Collaboration

Publications and source records attributed to CLAS Collaboration.

48 records · Page 3Linked to original sources

arXiv2004

Complete measurement of three-body photodisintegration of 3He for photon energies between 0.35 and 1.55 GeV

The three-body photodisintegration of 3He has been measured with the CLAS detector at Jefferson Lab, using tagged photons of energies between 0.35 GeV and 1.55 GeV. The large acceptance of the spectrometer allowed us for the first time to cover a wide momentum and angular range for the two outgoing protons. Three kinematic regions dominated by either two- or three-body contributions have been distinguished and analyzed. The measured cross sections have been compared with results of a theoretical model, which, in certain kinematic ranges, have been found to be in reasonable agreement with the data.

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arXiv2004

Measurement of the Polarized Structure Function $σ_{LT^\prime}$ for $p(\vec{e},e'π^+)n$ in the $Δ(1232)$ Resonance Region

The polarized longitudinal-transverse structure function $σ_{LT^\prime}$ has been measured using the $p(\vec e,e'π^+)n$ reaction in the $Δ(1232)$ resonance region at $Q^2=0.40$ and 0.65 GeV$^2$. No previous $σ_{LT^\prime}$ data exist for this reaction channel. The kinematically complete experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an energy of 1.515 GeV. A partial wave analysis of the data shows generally better agreement with recent phenomenological models of pion electroproduction compared to the previously measured $π^0 p$ channel. A fit to both $π^0 p$ and $π^+ n$ channels using a unitary isobar model suggests the unitarized Born terms provide a consistent description of the non-resonant background. The $t$-channel pion pole term is important in the $π^0 p$ channel through a rescattering correction, which could be model-dependent.

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arXiv2004

Proton source size measurements in the eA->e'ppX reaction

Two-proton correlations at small relative momentum q were studied in the eA(3He, 4He, C, Fe)-> e'ppX reaction at E_0 = 4.46 GeV using the CLAS detector at Jefferson Lab. The enhancement of the correlation function at small q was found to be in accordance with theoretical expectation. Emission region sizes were extracted and proved to be dependent on A and proton momentum. The size of the two-proton emission region on the lightest possible nucleus, He, was measured for the first time.

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arXiv2004

Measurement of Beam-Spin Asymmetries for Deep Inelastic $π^+$ Electroproduction

We report the first evidence for a non-zero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic region. Data have been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of the $\sinϕ$ modulation increases with the momentum of the pion relative to the virtual photon, $z$, with an average amplitude of $0.038 \pm 0.005 \pm 0.003$ for $0.5 < z < 0.8$ range.

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arXiv2003

Measurement of the Polarized Structure Function $σ_{LT^\prime}$ for $p(\vec{e},e'p)π^o$ in the $Δ(1232)$ Resonance Region

The polarized longitudinal-transverse structure function $σ_{LT^\prime}$ has been measured in the $Δ(1232)$ resonance region at $Q^2=0.40$ and 0.65 GeV$^2$. Data for the $p(\vec e,e'p)π^o$ reaction were taken at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an energy of 1.515 GeV. For the first time a complete angular distribution was measured, permitting the separation of different non-resonant amplitudes using a partial wave analysis. Comparison with previous beam asymmetry measurements at MAMI indicate a deviation from the predicted $Q^2$ dependence of $σ_{LT^{\prime}}$ using recent phenomenological models.

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arXiv2003

Two-Nucleon Momentum Distributions Measured in 3He(e,e'pp)n

We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic range. The kinetic energy distribution for `fast' nucleons (p > 250 MeV/c) peaks where two nucleons each have 20% or less, and the third nucleon has most of the transferred energy. These fast pp and pn pairs are back-to-back with little momentum along the three-momentum transfer, indicating that they are spectators. Experimental and theoretical evidence indicates that we have measured distorted two-nucleon momentum distributions by striking the third nucleon and detecting the spectator correlated pair.

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arXiv2003

Measurement of the Proton Spin Structure Function $g_1(x,Q^2)$ for $Q^2$ from 0.15 to 1.6 GeV$^2$ with CLAS

Double-polarization asymmetries for inclusive $ep$ scattering were measured at Jefferson Lab using 2.6 and 4.3 GeV longitudinally polarized electrons incident on a longitudinally polarized NH$_3$ target in the CLAS detector. The polarized structure function $g_1(x,Q^2)$ was extracted throughout the nucleon resonance region and into the deep inelastic regime, for $Q^2 = 0.15 -1.64 $GeV$^2$. The contributions to the first moment $Γ_1(Q^2) = \int g_1(x,Q^2)dx$ were determined up to $Q^2=1.2$ GeV$^2$. Using a parametrization for $g_1$ in the unmeasured low $x$ regions, the complete first moment was estimated over this $Q^2$ region. A rapid change in $Γ_1$ is observed for $Q^2 < 1 $GeV$^2$, with a sign change near $Q^2 = 0.3 $GeV$^2$, indicating dominant contributions from the resonance region. At $Q^2=1.2$ GeV$^2$ our data are below the pQCD evolved scaling value.

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arXiv2002

First Measurement of Transferred Polarization in the Exclusive e p --> e' K+ Lambda Reaction

The first measurements of the transferred polarization for the exclusive ep --> e'K+ Lambda reaction have been performed in Hall B at the Thomas Jefferson National Accelerator Facility using the CLAS spectrometer. A 2.567 GeV electron beam was used to measure the hyperon polarization over a range of Q2 from 0.3 to 1.5 (GeV/c)2, W from 1.6 to 2.15 GeV, and over the full center-of-mass angular range of the K+ meson. Comparison with predictions of hadrodynamic models indicates strong sensitivity to the underlying resonance contributions. A non-relativistic quark model interpretation of our data suggests that the s-sbar quark pair is produced with spins predominantly anti-aligned. Implications for the validity of the widely used 3P0 quark-pair creation operator are discussed.

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arXiv2002

$Q^2$ Dependence of Quadrupole Strength in the $γ^*p\toΔ^+(1232)\to p π^0$ Transition

Models of baryon structure predict a small quadrupole deformation of the nucleon due to residual tensor forces between quarks or distortions from the pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through the $Q^2$ dependence of the magnetic ($M_{1+}$), electric ($E_{1+}$), and scalar ($S_{1+}$) multipoles in the $γ^* p \to Δ^+ \to p π^0$ transition. We report new experimental values for the ratios $E_{1+}/M_{1+}$ and $S_{1+}/M_{1+}$ over the range $Q^2$= 0.4-1.8 GeV$^2$, extracted from precision $p(e,e 'p)π^{\circ}$ data using a truncated multipole expansion. Results are best described by recent unitary models in which the pion cloud plays a dominant role.

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arXiv2002

First Measurement of the Double Spin Asymmetry in $\vec{e}\vec{p}\to e' π^+ n$ in the Resonance Region

The double spin asymmetry in the $\vec{e}\vec{p}\to e' π^+ n$ reaction has been measured for the first time in the resonance region for four-momentum transfer $Q^2=0.35-1.5$ GeV$^2$. Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH$_3$ target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity-1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with $Q^2$ of the helicity asymmetry $A_1$ for the $F_{15}(1680)$ resonance than expected from the analysis of the unpolarized data.

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arXiv2001

Observation of exclusive DVCS in polarized electron beam asymmetry measurements

We report the first results of the beam spin asymmetry measured in the reaction e + p -> e + p + gamma at a beam energy of 4.25 GeV. A large asymmetry with a sin(phi) modulation is observed, as predicted for the interference term of Deeply Virtual Compton Scattering and the Bethe-Heitler process. The amplitude of this modulation is alpha = 0.202 +/- 0.028. In leading-order and leading-twist pQCD, the alpha is directly proportional to the imaginary part of the DVCS amplitude.

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arXiv2001

Exclusive electroproduction of phi mesons at 4.2 GeV

We studied the exclusive reaction e p --> e' p' phi using the phi --> K^+ K^- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CLAS detector at Jefferson Lab. Our experiment covers the range in Q^2 from 0.7 to 2.2 GeV^2, and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of phi production on the proton. Our measurement shows the expected decrease of the t-slope with the vector meson formation time c Delta tau below 2 fm. At = 0.6 fm, we measure b_phi = 2.27 +- 0.42 GeV^-2. The cross section dependence on W as W^{0.2+-0.1} at Q^2 = 1.3 GeV^2 was determined by comparison with phi production at HERA after correcting for threshold effects. This is the same dependence as observed in photoproduction.

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