[en] We report on the measurement of the beam asymmetry $\mathrm{\Sigma <!--\; ??\; -->}$ for the reactions $\stackrel{\to <!--\; ???\; -->}{\mathrm{\gamma <!--\; ??\; -->}}p\to <!--\; ???\; -->p\mathrm{\eta <!--\; ??\; -->}$ and $\stackrel{\to <!--\; ???\; -->}{\mathrm{\gamma <!--\; ??\; -->}}p\to <!--\; ???\; -->p{\mathrm{\eta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ from the GlueX experiment, using an 8.2--8.8 GeV linearly polarized tagged photon beam incident on a liquid hydrogen target in Hall D at Jefferson Lab. These measurements are made as a function of momentum transfer $-<!--\; ???\; -->t$, with significantly higher statistical precision than our earlier $\mathrm{\eta <!--\; ??\; -->}$ measurements, and are the first measurements of ${\mathrm{\eta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ in this energy range. We compare the results to theoretical predictions based on $t$--channel quasi-particle exchange. We also compare the ratio of ${\mathrm{\Sigma <!--\; ??\; -->}}_{\mathrm{\eta <!--\; ??\; -->}}$ to ${\mathrm{\Sigma <!--\; ??\; -->}}_{{\mathrm{\eta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}}$ to these models, as this ratio is predicted to be sensitive to the amount of $s\stackrel{¯<!--\; ??\; -->}{s}$ exchange in the production. We find that photoproduction of both $\mathrm{\eta <!--\; ??\; -->}$ and ${\mathrm{\eta <!--\; ??\; -->}}^{\mathrm{\prime <!--\; ???\; -->}}$ is dominated by natural parity exchange with little dependence on $-<!--\; ???\; -->t$.

[en] We report on the measurement of the $\mathrm{\gamma <!--\; ??\; -->}p\to <!--\; ???\; -->J/\mathrm{\psi <!--\; ??\; -->}p$ cross section from $E_{\mathrm{\gamma <!--\; ??\; -->}}=11.8$ GeV down to the threshold at $8.2$ GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section $d\mathrm{\sigma <!--\; ??\; -->}/dt$ has an exponential slope of $1.67\pm <!--\; ??\; -->0.39$ GeV${}^{-<!--\; ???\; -->2}$ at $10.7$ GeV average energy. The LHCb pentaquark candidates $P_c^{+}$ can be produced in the $s$-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions $\mathcal{B}(P_c^{+}\to <!--\; ???\; -->J/\mathrm{\psi <!--\; ??\; -->}p)$ and cross sections $\mathrm{\sigma <!--\; ??\; -->}(\mathrm{\gamma <!--\; ??\; -->}p\to <!--\; ???\; -->P_c^{+})\times <!--\; ??\; -->\mathcal{B}(P_c^{+}\to <!--\; ???\; -->J/\mathrm{\psi <!--\; ??\; -->}p)$.

[en] Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π"+ electroproduction reaction γ*p→nπ"+. The results were obtained from scattering of 6-GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Laboratory. The kinematic range covered is 1.1 < W < 3 GeV and 1 < Q"2 < 6GeV"2. Results were obtained for about 6000 bins in W, Q"2, cos(θ*), and Φ*. Except at forward angles, very large target-spin asymmetries are observed over the entire W region. Reasonable agreement is found with phenomenological fits to previous data for W < 1.6 GeV, but very large differences are seen at higher values of W. A generalized parton distributions (GPD)-based model is in poor agreement with the data. As a result, when combined with cross-sectional measurements, the present results provide powerful constraints on nucleon resonance amplitudes at moderate and large values of Q"2, for resonances with masses as high as 2.4 GeV.

[en] Exclusive photoproduction cross sections have been measured for the process $\mathrm{\gamma <!--\; ??\; -->}p\to <!--\; ???\; -->p{\mathrm{\pi <!--\; ??\; -->}}^0(e^{+}{e}^{-<!--\; ???\; -->}(\mathrm{\gamma <!--\; ??\; -->}))$ with the Dalitz decay final state using tagged photon energies in the range of $E_{\mathrm{\gamma <!--\; ??\; -->}}=1.275-<!--\; ???\; -->5.425$ GeV. The complete angular distribution of the final state ${\mathrm{\pi <!--\; ??\; -->}}^0$, 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 $d\mathrm{\sigma <!--\; ??\; -->}/dt$, 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 GeV². At the same time, the Regge-exchange-based models for ${\mathrm{\pi <!--\; ??\; -->}}^0$ photoproduction are more consistent with experimental data.

[en] Experimental energy and angular distributions are presented for inclusive protons emitted from the ¹²C nucleus bombarded by bremsstrahlung with maximum energy 4.5 GeV. The protons were detected in the range of angles 20--120⁰ and momenta 0.4--1.3 GeV/c. It is shown that the energy distributions of photoprotons in the case of cumulative production are satisfactorily described by an exponential, while for noncumulative protons the spectrum is flattened out in the high-energy region. The experimental data obtained are analyzed from the point of view of a number of theoretical models proposed for description of cumulative particle production

[en] The reaction ep→e'K⁺Λ(1520) with Λ(1520)→p'K^- was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cos θ_K#sup +#, φ_K#sup +#, Q², and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9< Q²<2.4 GeV² and 1.95< W<2.65 GeV. Also, the Q² dependence of the Λ(1520) decay angular distribution is presented for the first time. The cos θ_K#sup +# angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the m_z=±1/2 parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K⁺ exchange or longitudinal coupling to an exchanged K^*. The Q² dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo- and electroproduction

[en] Electromagnetic calorimeter modules are being constructed for the CEBAF Large Acceptance Spectrometer that will utilize plastic plate scintillators that can be over 4 meters in length. At shower maximum, it is expected that up to 100 Gray may be deposited over a ten year lifetime in the detector elements. Although it is believed that PVT-based scintillators are radiation-hard at that level, the large size of the plates motivated an aging test of a 2 meter long samples so as to gain a realistic estimate of the loss in intrinsic light output and especially in attenuation performance of the chosen scintillator (Bicron BC412). A test setup was designed so that the sample was irradiated (by a ⁶⁰Co line source) at a rate sufficiently low (0.5 Gray/hr) to allow complete diffusion of oxygen into the material during the irradiation. Additionally, in order to ensure complete diffusion and to gain a pessimistic estimate of damage, a pure oxygen atmosphere was circulated through the sample container during the irradiation, and the total dose was increased to a total of 160 Gray. Within the same setup, tests were also made of two additional materials: (1) a 1 meter long sample of an uniquely bright, fast green scintillator with superior attenuation properties, and (2) a customized acrylic scintillator (with a high doping level of fluors) read out by embedded (into smooth grooves cut into the surface of the scintillator) wavelength shifting fibers doped with the same green fluor as the aforementioned scintillator. The data indicates that the BC412 shows no loss in intrinsic light output, but does have an attenuation loss amounting to 10% at 1 meter and 16% at 2 meters. The fast green scintillator (and fibers) showed no damage. When read out by WLS fibers, the acrylic scintillator displayed a loss in intrinsic light output, but no change in attenuation

[en] Experimental data on quasielastic electron scattering by carbon nuclei in the reaction (e,e'p) are presented for incident-electron energies of 1.6 and 2.06 GeV, the electron scattering angles being 19.5 degree sign and 14.2 degree sign , respectively. Recoil protons are detected in the momentum range between 420 and 640 MeV/c. Owing to the chosen kinematical conditions, information about the interaction of electrons with a bound proton can be deduced from analysis based on the plane-wave impulse approximation. The results are compared with predictions of theoretical models

[en] The GlueX experiment at Jefferson Lab has been designed to study photoproduction reactions with a 9-GeV linearly polarized photon beam. The energy and arrival time of beam photons are tagged using a scintillator hodoscope and a scintillating fiber array. The photon flux is determined using a pair spectrometer, while the linear polarization of the photon beam is determined using a polarimeter based on triplet photoproduction. Charged-particle tracks from interactions in the central target are analyzed in a solenoidal field using a central straw-tube drift chamber and six packages of planar chambers with cathode strips and drift wires. Electromagnetic showers are reconstructed in a cylindrical scintillating fiber calorimeter inside the magnet and a lead-glass array downstream. Charged particle identification is achieved by measuring energy loss in the wire chambers and using the flight time of particles between the target and detectors outside the magnet. The signals from all detectors are recorded with flash ADCs and/or pipeline TDCs into memories allowing trigger decisions with a latency of 3.3 $\mathrm{\mu }\mathrm{s}$. The detector operates routinely at trigger rates of 40 kHz and data rates of 600 megabytes per second. We describe the photon beam, the GlueX detector components, electronics, data-acquisition and monitoring systems, and the performance of the experiment during the first three years of operation.

[en] We report the first measurement of the transverse momentum dependence of double-spin asymmetries in semi-inclusive production of pions in deep-inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Jefferson Lab (JLab). Modulations of single spin asymmetries over the azimuthal angle between lepton scattering and hadron production planes φ have been measured over a wide kinematic range in Bjorken x and virtual photon squared four-momentum Q². A significant nonzero sin2φ single spin asymmetry was observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton.