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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 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 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$\%$).

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.

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 $x_B$.

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.

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.

The inclusive polarized structure functions of the proton and deuteron, $g_1^p$ and $g_1^d$, were measured with high statistical precision using polarized 6 GeV electrons incident on a polarized ammonia target in Hall B at Jefferson Laboratory. Electrons scattered at laboratory angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual deep inelastic region kinematics, $Q^2>1$ ${\mathrm{GeV}}^2$ and the final-state invariant mass $W>2$ GeV, the ratio of polarized to unpolarized structure functions $g_1/F_1$ is found to be nearly independent of $Q^2$ at fixed $x$. Significant resonant structure is apparent at values of $W$ up to 2.3 GeV. In the framework of perturbative quantum chromodynamics, the high-$W$ results can be used to better constrain the polarization of quarks and gluons in the nucleon, as well as high-twist contributions.

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 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.

New cross sections for the reaction $\mathit{ep}\to e^{\text{'}}\eta p$ are reported for total center-of-mass energy $W=1.5\text{−}2.3$ GeV and invariant squared momentum transfer $Q^2=0.13\text{−}3.3$ GeV${}^2$. This large kinematic range allows the extraction of new information about response functions, photocouplings, and $\eta N$ coupling strengths of baryon resonances. A sharp structure is seen at $W~1.7$ GeV. The shape of the differential cross section is indicative of the presence of a $P$-wave resonance that persists to high $Q^2$. Improved values are derived for the photocoupling amplitude for the $S_{11}$(1535) resonance. The new data greatly expand the $Q^2$ range covered, and an interpretation of all data with a consistent parametrization is provided.

We have measured the moment $A_{LU}^{\sin \varphi }$ corresponding to the polarized electron beam-spin asymmetry in semi-inclusive deep inelastic scattering. $A_{LU}^{\sin \varphi }$ is a twist-3 quantity providing information about quark-gluon correlations. Data were taken with the CLAS Spectrometer at Jefferson Lab using a 5.498 GeV longitudinally polarized electron beam and an unpolarized liquid hydrogen target. All three pion channels (${\pi }^{+}$, ${\pi }^0$ and ${\pi }^{-}$) were measured simultaneously over a large range of kinematics within the virtuality range $Q^2\approx 1.0–4.5{\mathrm{GeV}}^2$. The observable was measured with better than 1% statistical precision over a large range of $z$, $P_T$, $x_B$, and $Q^2$, which permits comparison with several reaction models. The discussed measurements provide an upgrade in statistics over previous measurements, and serve as the first evidence for the negative sign of the ${\pi }^{-}$ $\sin \varphi$ moment.

New measurements of the spin structure functions of the proton and deuteron $g_1^p(x,Q^2)$ and $g_1^d(x,Q^2)$ in the nucleon resonance region are compared with extrapolations of target-mass-corrected next-to-leading-order (NLO) QCD fits to higher energy data. Averaged over the entire resonance region ($W<2$ GeV), the data and QCD fits are in good agreement in both magnitude and $Q^2$ dependence for $Q^2>1.7$ GeV${}^2/c^2$. This “global” duality appears to result from cancellations among the prominent “local” resonance regions: in particular strong ${\sigma }_{3/2}$ contributions in the $\Delta (1232)$ region appear to be compensated by strong ${\sigma }_{1/2}$ contributions in the resonance region centered on 1.5 GeV. These results are encouraging for the extension of NLO QCD fits to lower $W$ and $Q^2$ than have been used previously.