Measurement of nuclear transparency ratios for protons and neutrons
Not Available
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Engineering topics
Kuleshov, S. V.
Publications and source records attributed to Kuleshov, S. V..
First Measurements of the Double-Polarization Observables F , P , and H in ω Photoproduction off Transversely Polarized Protons in the N * Resonance Region
First measurements of double-polarization observables in ${\omega}$ photoproduction off the proton are presented here using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry $F$ has been measured using circularly polarized, tagged photons in the energy range 1200–2700 MeV, and the beam-target asymmetries $H$ and $P$ have been measured using linearly polarized, tagged photons in the energy range 1200–2000 MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon (${N}^{*}$) resonances. In particular, contributions from new ${N}^{*}$ resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Measurement of the beam spin asymmetry of e → p → e ′ p ′ η in the deep-inelastic regime with CLAS
Not Available
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Measurement of unpolarized and polarized cross sections for deeply virtual Compton scattering on the proton at Jefferson Laboratory with CLAS
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Measurements of the γ v p → p ' π + π - cross section with the CLAS detector for 0.4 GeV 2 < Q 2 < 1.0 GeV 2 and 1.3 GeV < W < 1.825 GeV
New results on the single-differential and fully-integrated cross sections for the process $\gamma_{v} p \rightarrow p' \pi^{+} \pi^{-}$ 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 $Q^2$ from 0.4 to 1.0~GeV$^2$. The cross sections were obtained in narrow $Q^{2}$ bins (0.05 GeV$^{2}$) 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 $Q^{2}$ evolution of the electrocouplings of most resonances with masses up to $\sim$1.8~GeV.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Semi-inclusive π 0 target and beam-target asymmetries from 6 GeV electron scattering with CLAS
Here, 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 14 NH 3 target, and extracted double and single target spin asymmetries for ep → e'π 0 X in multidimensional bins in fourmomentum transfer (1.0 < Q 2 < 3.2 GeV 2 ), Bjorken-x (0.12 < x < 0.48), hadron energy fraction (0.4 < z < 0.7), transverse pion momentum (0 < P T < 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 P T , which provide access to transverse-momentum distributions of longitudinally polarized quarks. The double spin asymmetries depend weakly on P T . The sin 2Φ h moments are zero within uncertainties, which is consistent with the expected suppression of the Collins fragmentation function. The observed sin Φ h moments suggest that quark gluon correlations are significant at large x.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the beam asymmetry Σ and the target asymmetry T in the photoproduction of ω mesons off the proton using CLAS at Jefferson Laboratory
Here, the photoproduction of $\omega$ mesons off the proton has been studied in the reaction $\gamma p\to p\,\omega$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, $T$, has been measured in photoproduction from the decay $\omega\to\pi^+\pi^-\pi^0$, using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40% in the third-resonance region. New measurements for the photon-beam asymmetry, $\Sigma$, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional $\omega$-photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production were found in addition to $t$-channel exchange processes.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the differential and total cross sections of the γ d → K 0 Λ ( p ) reaction within the resonance region
Here, we report the first measurement of differential and total cross sections for the ${\gamma}d \to K^0{\Lambda}(p)$ reaction, using data from the CLAS detector at the Thomas Jefferson National Accelerator Facility. Data collected during two separate experimental runs were studied with photon-energy coverage 0.8 - 3.6 GeV and 0.5 - 2.6 GeV, respectively. The two measurements are consistent giving confidence in the method and determination of systematic uncertainties. The cross sections are compared with predictions from the KAON-MAID theoretical model (without kaon exchange), which deviate from the data at higher W and at forward kaon angles. These data, along with previously published cross sections for $K^+ {\Lambda}$ photoproduction, provide essential constraints on the nucleon resonance spectrum. A first partial wave analysis has been performed that describes the data without the introduction of new resonances.
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First Exclusive Measurement of Deeply Virtual Compton Scattering off He 4 : Toward the 3D Tomography of Nuclei
Here, we report on the first measurement of the beam-spin asymmetry in the exclusive process of coherent deeply virtual Compton scattering off a nucleus. The experiment used the 6 GeV electron beam from the CEBAF accelerator at Jefferson Lab incident on a pressurized $^4$He gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron was detected by CLAS and the photon by a dedicated electromagnetic calorimeter at forward angles. To ensure the exclusivity of the process, a specially designed radial time projection chamber was used to detect the recoiling $^4$He nuclei. We measured beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we were able to extract, in a model-independent way, the real and imaginary parts of the only $^4$He Compton form factor, $\cal H_A$. This first measurement of coherent deeply virtual Compton scattering on the $^4$He nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.
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Photon beam asymmetry Σ in the reaction γ → p → p ω for E= 1.152 to 1.876 GeV
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Differential cross section measurements for γ n → π - p above the first nucleon resonance region
The quasifree gamma d -> pi(-)p(p) differential cross section has been measured with CLAS at photon beam energies E-gamma from 0.445 to 2.510 GeV (corresponding to W from 1.311 to 2.366 GeV) for pion center-of-mass angles cos theta(c.m.)(pi) from -0.72 to 0.92. A correction for final state interactions has been applied to these data to extract the gamma n -> pi(-)p differential cross sections. These cross sections are quoted in 8428 (E-gamma, cos theta(c.m)(pi)) bins, a factor of nearly 3 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* -> gamma n at the resonance poles are determined for the first time and are reported here.
CLAS Collaboration↗
Measurements of e p → e ' π + π – p ' cross sections with CLAS at 1.40 GeV < W < 2.0 GeV and 2.0 GeV 2 < Q 2 < 5.0 GeV 2
This study reports new exclusive cross sections for ep→e'π + π – p' using the CLAS detector at Jefferson Laboratory. These results are presented for the first time at photon virtualities 2.0 GeV 2 < Q 2 < 5.0 GeV 2 in the center-of-mass energy range 1.4 GeV < W < 2.0 GeV, which covers a large part of the nucleon resonance region. Using a model developed for the phenomenological analysis of electroproduction data, we see strong indications that the relative contributions from the resonant cross sections at W < 1.74 GeV increase with Q 2 . These data considerably extend the kinematic reach of previous measurements. Exclusive ep→e'π + π – p' cross section measurements are of particular importance for the extraction of resonance electrocouplings in the mass range above 1.6 GeV.
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Beam-target helicity asymmetry for γ → n → → π - p in the N * resonance region
In this paper, we report the first beam-target double-polarization asymmetries in the $\gamma + n(p) \rightarrow \pi^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $HD$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analysis techniques that show excellent agreement, and these have been used to deduce the $E$ polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to revised values for several $\gamma nN^*$ resonance photo-couplings.
43 PARTICLE ACCELERATORS↗
Measurement of two-photon exchange effect by comparing elastic e ± p cross sections
Here, 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 ($Q^{2}$). 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 convertor 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 (LH$_2$) 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.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Target and double spin asymmetries of deeply virtual π 0 production with a longitudinally polarized proton target and CLAS
The target and double spin asymmetries of the exclusive pseudoscalar channel e→p→→epπ0 were measured for the first time in the deep-inelastic regime using a longitudinally polarized 5.9 GeV electron beam and a longitudinally polarized proton target at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS). The data were collected over a large kinematic phase space and divided into 110 four-dimensional bins of Q2, xB, -t and Φ. Large values of asymmetry moments clearly indicate a substantial contribution to the polarized structure functions from transverse virtual photon amplitudes. The interpretation of experimental data in terms of generalized parton distributions (GPDs) provides the first insight on the chiral-odd GPDs H˜T and ET, and complement previous measurements of unpolarized structure functions sensitive to the GPDs HT and E¯T. These data provide a crucial input for parametrizations of essentially unknown chiral-odd GPDs and will strongly influence existing theoretical calculations based on the handbag formalism.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Exclusive η electroproduction at W > 2 GeV with CLAS and transversity generalized parton distributions
The cross section of the exclusive η electroproduction reaction ep → e'p'η was measured at Jefferson Laboratory with a 5.75 GeV electron beam and the CLAS detector. Differential cross sections d 4 σ/dtdQ 2 dx B dΦ 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. As a result, the π 0 and η data, when taken together, can facilitate the flavor decomposition of the transversity GPDs.
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Target and beam-target spin asymmetries in exclusive π + and π - electroproduction with 1.6- to 5.7-GeV electrons
Beam-target double-spin asymmetries and target single-spin asymmetries in exclusive pi(+) and quasiexclusive pi(-) electroproduction were obtained from scattering of 1.6- to 5.7-GeV longitudinally polarized electrons from longitudinally polarized protons (for pi(+)) and deuterons (for pi(-)) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The kinematic range covered is 1.1 < W < 2.6 GeV and 0.05 < Q(2) < 5 GeV2, with good angular coverage in the forward hemisphere. The asymmetry results were divided into approximately 40 000 kinematic bins for pi(+) from free protons and 15 000 bins for pi(-) production from bound nucleons in the deuteron. The present results are found to be in reasonable agreement with fits to previous world data for W < 1.7 GeV and Q(2) < 0.5 GeV2, with discrepancies increasing at higher values of Q(2), especially for W > 1.5 GeV. Very large target-spin asymmetries are observed for W > 1.6 GeV. When combined with cross-section measurements, the present results can provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q(2), for resonances with masses as high as 2.3 GeV.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Photoproduction of the f 1 ( 1285 ) meson
The f 1 (1285) meson with mass 1281.0 ± 0.8 MeV/c 2 and width 18.4 ± 1.4 MeV (full width at half maximum) was measured for the first time in photoproduction from a proton target using CLAS at Jefferson Lab. Differential cross sections were obtained via the ηπ + π - , K + K 0 π - , and K - K 0 π + decay channels from threshold up to a center-of-mass energy of 2.8 GeV. The mass, width, and an amplitude analysis of the ηπ + π - final-state Dalitz distribution are consistent with the axial-vector J P = 1 + f 1 (1285) identity, rather than the pseudoscalar 0-η (1295). The production mechanism is more consistent with s -channel decay of a high-mass N * state and not with t -channel meson exchange. Decays to η π π go dominantly via the intermediate a$^±_0$ (980) π ∓ states, with the branching ratio Γ [a 0 π (no $\overline{K}K$) ] / Γ [ηππ (all) ] = 0.74 ± 0.09 . The branching ratios Γ ($\overline{K}K$ π )/ Γ (ηππ) = 0.216 ± 0.033 and Γ (γρ 0 ) / Γ (ηππ) = 0.047 ± 0.018 were also obtained. The first is in agreement with previous data for the f 1 (1285), while the latter is lower than the world average.
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