Search

We measured the ratios of electroproduction cross sections from a proton target for three exclusive meson-baryon final states: $\mathrm{\Lambda }{K}^{+}$, $p{\pi }^0$, and $n{\pi }^{+}$, with the CLAS detector at Jefferson Lab. Using a simple model of quark hadronization, we extract $q\overline{q}$ creation probabilities for the first time in exclusive two-body production, in which only a single $q\overline{q}$ pair is created. We observe a sizable suppression of strange quark-antiquark pairs compared to nonstrange pairs, similar to that seen in high-energy production.

The $(W,Q^2)$ dependence of the ratio of inclusive electron scattering cross sections for ${}^{15}\mathrm{N}$/${}^{12}\mathrm{C}$ was determined in the kinematic ranges $0.8<W<2$ GeV and $0.2<Q^2<1$ GeV${}^2$ using 2.285 GeV electrons and the CLAS detector at Jefferson Lab. The ratios exhibit only slight resonance structure, in agreement with a simple phenomenological model and an extrapolation of deep-inelastic scattering ratios to low $Q^2$. Ratios of ${}^4\mathrm{He}$/${}^{12}\mathrm{C}$ using 1.6 to 2.5 GeV electrons were measured with very high statistical precision and were used to correct for He in the N and C targets. The $(W,Q^2)$ dependence of the ${}^4\mathrm{He}$/${}^{12}\mathrm{C}$ ratios is in good agreement with that of the phenomenological model and exhibit significant resonance structure centered at $W=0.94,1.23$, and 1.5 GeV.

This paper reports the measurement of polarized and unpolarized cross sections for the $ep\to e^{\prime }{p}^{\prime }\gamma$ 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, $0.10<x_B<0.58, 1.0<Q^2<4.8{\text{GeV}}^2$, and $0.09<-t<2.00{\text{GeV}}^2$. 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.

We report the first beam-target double-polarization asymmetries in the $\gamma +n(p)\to {\pi }^{-}+p(p)$ reaction spanning the nucleon resonance region from invariant mass $W=1500$ to 2300 MeV. Circularly polarized photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been extracted using three very different analyses 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 significant revisions for several $\gamma n{N}^{*}$ resonance photocouplings.

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 $ep\to e^{\prime }{p}^{\prime }\gamma$ events, which arise from the interference of the deeply virtual Compton scattering and the Bethe-Heitler processes, were extracted over the widest kinematics in $Q^2$, $x_B$, $t$, and $\varphi$, 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.

Background: The discrepancy between proton electromagnetic form factors extracted using unpolarized and polarized scattering data is believed to be a consequence of two-photon exchange (TPE) effects. However, the calculations of TPE corrections have significant model dependence, and there is limited direct experimental evidence for such corrections.

Purpose: The TPE contributions depend on the sign of the lepton charge in $e^{\pm }p$ scattering, but the luminosities of secondary positron beams limited past measurement at large scattering angles, where the TPE effects are believe to be most significant. We present the results of a new experimental technique for making direct $e^{\pm }p$ comparisons, which has the potential to make precise measurements over a broad range in $Q^2$ and scattering angles.

Methods: We use the Jefferson Laboratory electron beam and the Hall B photon tagger to generate a clean but untagged photon beam. The photon beam impinges on a converter foil to generate a mixed beam of electrons, positrons, and photons. A chicane is used to separate and recombine the electron and positron beams while the photon beam is stopped by a photon blocker. This provides a combined electron and positron beam, with energies from 0.5 to 3.2 GeV, which impinges on a liquid hydrogen target. The large acceptance CLAS detector is used to identify and reconstruct elastic scattering events, determining both the initial lepton energy and the sign of the scattered lepton.

Results: The data were collected in two days with a primary electron beam energy of only 3.3 GeV, limiting the data from this run to smaller values of $Q^2$ and scattering angle. Nonetheless, this measurement yields a data sample for $e^{\pm }p$ with statistics comparable to those of the best previous measurements. We have shown that we can cleanly identify elastic scattering events and correct for the difference in acceptance for electron and positron scattering. Because we ran with only one polarity for the chicane, we are unable to study the difference between the incoming electron and positron beams. This systematic effect leads to the largest uncertainty in the final ratio of positron to electron scattering: $R=1.027\pm 0.005\pm 0.05$ for $\langle Q^2\rangle =0.206$ GeV${}^2$ and $0.830⩽\epsilon ⩽0.943$.

Conclusions: We have demonstrated that the tertiary $e^{\pm }$ beam generated using this technique provides the opportunity for dramatically improved comparisons of $e^{\pm }p$ scattering, covering a significant range in both $Q^2$ and scattering angle. Combining data with different chicane polarities will allow for detailed studies of the difference between the incoming $e^{+}$ and $e^{-}$ beams.

We report the first observation of the line shape of the $\Lambda \left(1405\right)$ from electroproduction, and show that it is not a simple Breit-Wigner resonance. Electroproduction of $K^{+}\Lambda \left(1405\right)$ off the proton was studied by using data from CLAS at Jefferson Lab in the range $1.0<Q^2<3.0$ (GeV/$c$)${}^2$. The analysis utilized the decay channels ${\Sigma }^{+}{\pi }^{-}$ of the $\Lambda \left(1405\right)$ and $p{\pi }^0$ of the ${\Sigma }^{+}$. Neither the standard Particle Data Group resonance parameters, nor free parameters fitting to a single Breit-Wigner resonance represent the line shape. In our fits, the line shape corresponds approximately to predictions of a two-pole meson-baryon picture of the $\Lambda \left(1405\right)$, with a lower mass pole near 1368 MeV/$c^2$ and a higher mass pole near 1423 MeV/$c^2$. Furthermore, with increasing photon virtuality the mass distribution shifts toward the higher mass pole.

The transition helicity amplitudes from the proton ground state to the $P_{11}\left(1440\right)$ and $D_{13}\left(1520\right)$ excited states (${\gamma }_{v}p{N}^{*}$ electrocouplings) were determined from the analysis of nine independent one-fold differential ${\pi }^{+}{\pi }^{-}p$ electroproduction cross sections off a proton target, taken with CLAS at photon virtualities of $0.25<Q^2<0.60$ GeV${}^2$. The phenomenological reaction model was employed for separation of the resonant and nonresonant contributions to the final state. The $P_{11}\left(1440\right)$ and $D_{13}\left(1520\right)$ electrocouplings were obtained from the resonant amplitudes parametrized within the framework of a unitarized Breit-Wigner ansatz. They are consistent with results obtained in the previous CLAS analyses of the ${\pi }^{+}n$ and ${\pi }^{0}p$ channels. The successful description of a large body of data in dominant meson-electroproduction channels off protons with the same ${\gamma }_{v}p{N}^{*}$ electrocouplings offers clear evidence for the reliable extraction of these fundamental quantities from meson-electroproduction data. This analysis also led to the determination of the long-awaited hadronic branching ratios for the $D_{13}\left(1520\right)$ decay into $\Delta \pi$ (24$\%$–32$\%$) and $N\rho$ (8$\%$–17$\%$).

The exclusive electroproduction process $\overrightarrow{e}p\to e^{\text{'}}n{\pi }^{+}$ was measured in the range of the photon virtuality $Q^2=1.7\text{−}4.5\hspace{1em}{\mathrm{GeV}}^2$, and the invariant mass range for the $n{\pi }^{+}$ system of $W=1.15\text{−}1.7\hspace{1em}\mathrm{GeV}$ using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive ${\pi }^{+}$ production from protons with nearly full coverage in the azimuthal and polar angles of the $n{\pi }^{+}$ center-of-mass system. The $n{\pi }^{+}$ channel has particular sensitivity to the isospin ½ excited nucleon states, and together with the $p{\pi }^0$ final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the ${\sigma }_{{\mathit{LT}}^{\text{'}}}$ structure function. In this experiment, 31,295 cross section and 4,184 asymmetry data points were measured. Because of the large volume of data, only a reduced set of structure functions and Legendre polynomial moments can be presented that are obtained in model-independent fits to the differential cross sections.

Unpolarized and beam-polarized fourfold cross sections $(d^4\sigma /d{Q}^{2}d{x}_{B}dtd\varphi )$ for the $ep\to e^{\prime }{p}^{\prime }\gamma$ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 ($Q^2,x_B,t$) bins over the widest phase space ever explored in the valence-quark 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 $x_B$.

Exclusive photoproduction cross sections have been measured for the process $\gamma p\to p{\pi }^0\left[e^{+}{e}^{-}\left(\gamma \right)\right]$ with the Dalitz decay final state using tagged photon energies in the range of $E_{\gamma }=1.275–5.425$ GeV. The complete angular distribution of the final state ${\pi }^0$, 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 $d\sigma /dt$, at mid to large angles, decreases with energy as $s^{-6.89\pm 0.26}$. 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 $s=11{\mathrm{GeV}}^2$. At the same time, the Regge-exchange-based models for ${\pi }^0$ photoproduction are more consistent with experimental data.

The cross section of the exclusive $\eta$ electroproduction reaction $ep\to e^{\prime }{p}^{\prime }\eta$ was measured at Jefferson Laboratory with a 5.75 GeV electron beam and the CLAS detector. Differential cross sections $d^4\sigma /dtd{Q}^{2}d{x}_{B}d{\varphi }_{\eta }$ and structure functions ${\sigma }_U={\sigma }_T+\epsilon {\sigma }_L,{\sigma }_{TT}$, and ${\sigma }_{LT}$, as functions of $t$, were obtained over a wide range of $Q^2$ and $x_B$. The $\eta$ structure functions are compared with those previously measured for ${\pi }^0$ at the same kinematics. At low $t$, both ${\pi }^0$ and $\eta$ are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The ${\pi }^0$ and $\eta$ data, when taken together, can facilitate the flavor decomposition of the transversity GPDs.

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 uses the 6 GeV electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) accelerator at Jefferson Lab incident on a pressurized ${}^4\mathrm{He}$ gaseous target placed in front of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron is 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 is used to detect the recoiling ${}^4\mathrm{He}$ nuclei. We measure beam-spin asymmetries larger than those observed on the free proton in the same kinematic domain. From these, we are able to extract, in a model-independent way, the real and imaginary parts of the only ${}^4\mathrm{He}$ Compton form factor, ${\mathcal{H}}_A$. This first measurement of coherent deeply virtual Compton scattering on the ${}^4\mathrm{He}$ nucleus, with a fully exclusive final state via nuclear recoil tagging, leads the way toward 3D imaging of the partonic structure of nuclei.

The exclusive reaction $\gamma p\to {\overline{K}}^0{K}^{+}n$ was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon ${\Theta }^{+}(1540)\to n{K}^{+}$. The decay to $n{K}^{+}$ requires the assignment of strangeness $S=+1$ to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of $70{\mathrm{pb}}^{-1}$. No evidence for the ${\Theta }^{+}$ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and $n{K}^{+}$ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.

The polarized longitudinal-transverse structure function ${\sigma }_{{\mathit{LT}}^{\text{'}}}$ measures the interference between real and imaginary amplitudes in pion electroproduction and can be used to probe the coupling between resonant and nonresonant processes. We report new measurements of ${\sigma }_{{\mathit{LT}}^{\text{'}}}$ in the $N(1440){\frac{1}{2}}^{+}$ (Roper) resonance region at $Q^2=0.40$ and $0.65{\text{GeV}}^2$ for both the ${\pi }^{0}p$ and ${\pi }^{+}n$ channels. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at a beam energy of 1.515 GeV. Complete angular distributions were obtained and are compared to recent phenomenological models. The ${\sigma }_{{\mathit{LT}}^{\text{'}}}({\pi }^{+}n)$ channel shows a large sensitivity to the Roper-resonance multipoles $M_{1-}$ and $S_{1-}$ and provides new constraints on models of resonance formation.

For the first time, the reaction $\gamma d\to \Lambda n{K}^{+}$ has been analyzed in order to search for the exotic pentaquark baryon ${\Theta }^{+}(1540)$. The data were taken at Jefferson Laboratory, using the Hall-$B$ tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the $n{K}^{+}$ invariant-mass distribution. The upper limit on the $\gamma d\to \Lambda {\Theta }^{+}$ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported.

The quark structure of the $f_2(1270)$ meson has, for many years, been assumed to be a pure quark-antiquark ($q\overline{q}$) resonance with quantum numbers $J^{\mathrm{PC}}=2^{++}$. Recently, it was proposed that the $f_2(1270)$ is a molecular state made from the attractive interaction of two $\rho$ mesons. Such a state would be expected to decay strongly to final states with charged pions due to the dominant decay $\rho \to {\pi }^{+}{\pi }^{-}$, whereas decay to two neutral pions would likely be suppressed. Here, we measure for the first time the reaction $\gamma p\to {\pi }^0{\pi }^{0}p$, using the CEBAF Large Acceptance Spectrometer detector at Jefferson Lab for incident beam energies between 3.6 and 5.4 GeV. Differential cross sections, $d\sigma /dt$, for $f_2(1270)$ photoproduction are extracted with good precision due to low backgrounds and are compared to theoretical calculations.

We report the first measurements of deep inelastic scattering spin-dependent azimuthal asymmetries in back-to-back dihadron electroproduction in the deep inelastic scattering process. In this reaction, two hadrons are produced in opposite hemispheres along the $z$ axis in the virtual photon-target nucleon center-of-mass frame, with the first hadron produced in the current-fragmentation region and the second in the target-fragmentation region. The data were taken with longitudinally polarized electron beams of 10.2 and 10.6 GeV incident on an unpolarized liquid-hydrogen target using the CLAS12 spectrometer at Jefferson Lab. Observed nonzero $\sin \mathrm{\Delta }\varphi$ modulations in $ep\to e^{\prime }p{\pi }^{+}X$ events, where $\mathrm{\Delta }\varphi$ is the difference of the azimuthal angles of the proton and pion in the virtual photon and target nucleon center-of-mass frame, indicate that correlations between the spin and transverse momenta of hadrons produced in the target- and current-fragmentation regions may be significant. The measured beam-spin asymmetries provide a first access in dihadron production to a previously unexplored leading-twist spin- and transverse-momentum-dependent fracture function. The fracture functions describe the hadronization of the target remnant after the hard scattering of a virtual photon off a quark in the target particle and provide a new avenue for studying nucleonic structure and hadronization.

The polarized cross-section ratio ${\sigma }_{L{T}^{\prime }}/{\sigma }_0$ from hard exclusive ${\pi }^{-}{\mathrm{\Delta }}^{++}$ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a $10.2\mathrm{GeV}/10.6\mathrm{GeV}$ incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from $1.5{\mathrm{GeV}}^2$ up to $7{\mathrm{GeV}}^2$. The reaction provides a novel access to the $d$-quark content of the nucleon and to $p\to {\mathrm{\Delta }}^{++}$ transition generalized parton distributions. A comparison to existing results for hard exclusive ${\pi }^{+}n$ and ${\pi }^{0}p$ electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry.

We present the first threefold differential measurement for neutral-pion multiplicity ratios produced in semi-inclusive deep-inelastic electron scattering on carbon, iron, and lead nuclei normalized to deuterium from CLAS at Jefferson Lab. We found that the neutral-pion multiplicity ratio is maximally suppressed for the leading hadrons (energy fraction $z\to$ 1), suppression varying from 25% in carbon up to 75% in lead. An enhancement of the multiplicity ratio at low $z$ and high $p_T^2$ is observed, suggesting an interconnection between these two variables. This behavior is qualitatively similar to the previous twofold differential measurement of charged pions by the HERMES Collaboration and, recently, by CLAS Collaboration. The largest enhancement was observed at high $p_T^2$ for heavier nuclei, namely, iron and lead, while the smallest enhancement was observed for the lightest nucleus, carbon. This behavior suggests a competition between partonic multiple scattering, which causes enhancement, and hadronic inelastic scattering, which causes suppression.