Towards a resolution of the proton form factor problem: new electron and positron scattering data

There is a significant discrepancy between the values of the proton electric form factor, $G_E^p$, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of $G_E^p$ from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization ($\varepsilon$) and momentum transfer ($Q^2$) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing $\varepsilon$ at $Q^2 = 1.45 \text{ GeV}^2$. This measurement is consistent with the size of the form factor discrepancy at $Q^2\approx 1.75$ GeV$^2$ and with hadronic calculations including nucleon and $\Delta$ intermediate states, which have been shown to resolve the discrepancy up to $2-3$ GeV$^2$.Comment: 6 pages, 4 figures, submitted to PR

Measurement of the neutron F2 structure function via spectator tagging with CLAS

We report on the first measurement of the F2 structure function of the neutron from semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to < 100 MeV and their angles to < 100 degrees relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F2n data collected cover the nucleon resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65 < Q2 < 4.52 GeV2, with uncertainties from nuclear corrections estimated to be less than a few percent. These measurements provide the first determination of the neutron to proton structure function ratio F2n/F2p at 0.2 < x < 0.8 with little uncertainty due to nuclear effects.Comment: 6 pages, 3 page

A comparison of forward and backward pp pair knockout in 3He(e,e'pp)n

Measuring nucleon-nucleon Short Range Correlations (SRC) has been a goal of the nuclear physics community for many years. They are an important part of the nuclear wavefunction, accounting for almost all of the high-momentum strength. They are closely related to the EMC effect. While their overall probability has been measured, measuring their momentum distributions is more difficult. In order to determine the best configuration for studying SRC momentum distributions, we measured the $^3$He$(e,e'pp)n$ reaction, looking at events with high momentum protons ($p_p > 0.35$ GeV/c) and a low momentum neutron ($p_n< 0.2$ GeV/c). We examined two angular configurations: either both protons emitted forward or one proton emitted forward and one backward (with respect to the momentum transfer, $\vec q$). The measured relative momentum distribution of the events with one forward and one backward proton was much closer to the calculated initial-state $pp$ relative momentum distribution, indicating that this is the preferred configuration for measuring SRC.Comment: 8 pages, 9 figures, submitted to Phys Rev C. Version 2 incorporates minor corrections in response to referee comment

Light Vector Mesons in the Nuclear Medium

The light vector mesons ($\rho$, $\omega$, and $\phi$) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the $\rho$ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The $\rho$ meson mass spectrum was extracted after the $\omega$ and $\phi$ signals were removed in a nearly model-independent way. Comparisons were made between the $\rho$ mass spectra from the heavy targets ($A > 2$) with the mass spectrum extracted from the deuterium target. With respect to the $\rho$-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.Comment: 15 pages, 18 figures, 3 table

Search for Θ+(1540)\Theta^+(1540) pentaquark in high statistics measurement of γp→Kˉ0K+n\gamma p \to \bar K^0 K^+ n at CLAS

The exclusive reaction $\gamma p \to \bar K^0 K^+ n$ was studied in the photon energy range between 1.6-3.8 GeV searching for evidence of the exotic baryon $\Theta^+(1540)\to nK^+$. The decay to $nK^+$ 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 $pb^{-1}$. 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 $nK^+$ mass. The 95% CL upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.Comment: Submitted to Physical Review Letter

Differential cross sections and spin density matrix elements for the reaction gamma p -> p omega

High-statistics differential cross sections and spin density matrix elements for the reaction gamma p -> p omega have been measured using the CLAS at Jefferson Lab for center-of-mass (CM) energies from threshold up to 2.84 GeV. Results are reported in 112 10-MeV wide CM energy bins, each subdivided into cos(theta_CM) bins of width 0.1. These are the most precise and extensive omega photoproduction measurements to date. A number of prominent structures are clearly present in the data. Many of these have not previously been observed due to limited statistics in earlier measurements

Photoproduction of phi(1020) mesons on the proton at large momentum transfer

The cross section for $\phi$ meson photoproduction on the proton has been measured for the first time up to a four-momentum transfer -t = 4 GeV^2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low four-momentum transfer, the differential cross section is well described by Pomeron exchange. At large four-momentum transfer, above -t = 1.8 GeV^2, the data support a model where the Pomeron is resolved into its simplest component, two gluons, which may couple to any quark in the proton and in the $\phi$.Comment: 5 pages; 7 figure

Q^2 Dependence of the S_{11}(1535) Photocoupling and Evidence for a P-wave resonance in eta electroproduction

New cross sections for the reaction $ep \to e'\eta p$ are reported for total center of mass energy $W$=1.5--2.3 GeV and invariant squared momentum transfer $Q^2$=0.13--3.3 GeV$^2$. This large kinematic range allows extraction of new information about response functions, photocouplings, and $\eta N$ coupling strengths of baryon resonances. A sharp structure is seen at $W\sim$ 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 photon coupling amplitude for the $S_{11}$(1535) resonance. The new data greatly expands the $Q^2$ range covered and an interpretation of all data with a consistent parameterization is provided.Comment: 31 pages, 9 figure

Precision measurements of g1g_1 of the proton and the deuteron with 6 GeV electrons

The inclusive polarized structure functions of the proton and deuteron, g1p and g1d, 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 lab angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual DIS kinematics, Q^2>1 GeV^2 and the final-state invariant mass W>2 GeV, the ratio of polarized to unpolarized structure functions g1/F1 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 QCD, 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