Extraction of Beam-Spin Asymmetries from the Hard Exclusive π⁺ Channel Off Protons in a Wide Range of Kinematics

We have measured beam-spin asymmetries to extract the sinϕ moment ALUsinϕ from the hard exclusive e→p → e\u27nπ+ reaction above the resonance region, for the first time with nearly full coverage from forward to backward angles in the center of mass. The ALUsinϕ moment has been measured up to 6.6  GeV2 in -t, covering the kinematic regimes of generalized parton distributions (GPD) and baryon-to-meson transition distribution amplitudes (TDA) at the same time. The experimental results in very forward kinematics demonstrate the sensitivity to chiral-odd and chiral-even GPDs. In very backward kinematics where the TDA framework is applicable, we found ALUsinϕ to be negative, while a sign change was observed near 90° in the center of mass. The unique results presented in this Letter will provide critical constraints to establish reaction mechanisms that can help to further develop the GPD and TDA frameworks

Probing High-Momentum Protons and Neutrons in Neutron-Rich Nuclei

The atomic nucleus is one of the densest and most complex quantum-mechanical systems in nature. Nuclei account for nearly all the mass of the visible Universe. The properties of individual nucleons (protons and neutrons) in nuclei can be probed by scattering a high-energy particle from the nucleus and detecting this particle after it scatters, often also detecting an additional knocked-out proton. Analysis of electron- and proton-scattering experiments suggests that some nucleons in nuclei form close-proximity neutron–proton pairs with high nucleon momentum, greater than the nuclear Fermi momentum. However, how excess neutrons in neutron-rich nuclei form such close-proximity pairs remains unclear. In this study we measure protons and, for the first time, neutrons knocked out of medium-to-heavy nuclei by high-energy electrons and show that the fraction of high-momentum protons increases markedly with the neutron excess in the nucleus, whereas the fraction of high-momentum neutrons decreases slightly. This effect is surprising because in the classical nuclear shell model, protons and neutrons obey Fermi statistics, have little correlation and mostly fill independent energy shells. These high-momentum nucleons in neutron-rich nuclei are important for understanding nuclear parton distribution functions (the partial momentum distribution of the constituents of the nucleon) and changes in the quark distributions of nucleons bound in nuclei (the EMC effect). They are also relevant for the interpretation of neutrino-oscillation measurements and understanding of neutron-rich systems such as neutron stars

Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering

We present new precision measurements of the elastic electron-proton scattering cross section for momentum transfer (Q2) up to 15.75  (GeV/c)2. Combined with existing data, these provide an improved extraction of the proton magnetic form factor at high Q2 and double the range over which a longitudinal or transverse separation of the cross section can be performed. The difference between our results and polarization data agrees with that observed at lower Q2 and attributed to hard two-photon exchange (TPE) effects, extending to 8 (GeV/c)2 the range of Q2 for which a discrepancy is established at \u3e95% confidence. We use the discrepancy to quantify the size of TPE contributions needed to explain the cross section at high Q2

Polarized Structure Function σ\u3csub\u3eLT\u27\u3c/sub\u3e from ⁰p Electroproduction Data in the Resonance Region at 0.2 GeV² \u3c Q² \u3c 1.0 GeV²

The first results on the σLT′ structure function in exclusive π0p electroproduction at invariant masses of the final state of 1.5GeV \u3c W \u3c 1.8 GeV and in the range of photon virtualities 0.4 GeV2 \u3c Q2 \u3c 1.0 GeV2 were obtained from data on beam spin asymmetries and differential cross sections measured with the CLAS detector at Jefferson Lab. The Legendre moments determined from the σLT′ structure function have demonstrated sensitivity to the contributions from the nucleon resonances in the second and third resonance regions. These new data on the beam spin asymmetries in π0p electroproduction extend the opportunities for the extraction of the nucleon resonance electro-excitation amplitudes in the mass range above 1.6 GeV

Comparing Proton Momentum Distributions in A = 2 and 3 Nuclei Via \u3csup\u3e2\u3c/sup\u3eH \u3csup\u3e3\u3c/sup\u3eH and \u3csup\u3e3\u3c/sup\u3eHe (e,e′p) Measurements

We report the first measurement of the (e, e\u27 p) reaction cross-section ratios for Helium-3 (3He), Tritium (3H), and Deuterium (d). The measurement covered a missing momentum range of 40 ≤ pmiss ≤ 550 MeV/c, at large momentum transfer ({Q2} ≈ 1.9 (GeV/c)2) and xB \u3e 1, which minimized contributions from non quasi-elastic (QE) reaction mechanisms. The data is compared with planewave impulse approximation (PWIA) calculations using realistic spectral functions and momentum distributions. The measured and PWIA-calculated cross-section ratios for 3He/d and 3H/d extend to just above the typical nucleon Fermi-momentum (kF ≈ 250 MeV/c) and differ from each other by ∼ 20%, while for 3He/3H they agree within the measurement accuracy of about 3%. At momenta above kF , the measured 3He/3H ratios differ from the calculation by 20% −50%. Final state interaction (FSI) calculations using the generalized Eikonal Approximation indicate that FSI should change the 3He/3H cross-section ratio for this measurement by less than 5%. If these calculations are correct, then the differences at large missing momenta between the 3He/3H experimental and calculated ratios could be due to the underlying NN interaction, and thus could provide new constraints on the previously loosely-constrained short-distance parts of the NN interaction

Revealing the short-range structure of the "mirror nuclei" 3^3H and 3^3He

When protons and neutrons (nucleons) are bound into atomic nuclei, they are close enough together to feel significant attraction, or repulsion, from the strong, short-distance part of the nucleon-nucleon interaction. These strong interactions lead to hard collisions between nucleons, generating pairs of highly-energetic nucleons referred to as short-range correlations (SRCs). SRCs are an important but relatively poorly understood part of nuclear structure and mapping out the strength and isospin structure (neutron-proton vs proton-proton pairs) of these virtual excitations is thus critical input for modeling a range of nuclear, particle, and astrophysics measurements. Hitherto measurements used two-nucleon knockout or ``triple-coincidence'' reactions to measure the relative contribution of np- and pp-SRCs by knocking out a proton from the SRC and detecting its partner nucleon (proton or neutron). These measurementsshow that SRCs are almost exclusively np pairs, but had limited statistics and required large model-dependent final-state interaction (FSI) corrections. We report on the first measurement using inclusive scattering from the mirror nuclei $^3$H and $^3$He to extract the np/pp ratio of SRCs in the A=3 system. We obtain a measure of the np/pp SRC ratio that is an order of magnitude more precise than previous experiments, and find a dramatic deviation from the near-total np dominance observed in heavy nuclei. This result implies an unexpected structure in the high-momentum wavefunction for $^3$He and $^3$H. Understanding these results will improve our understanding of the short-range part of the N-N interaction

Observation of azimuth-dependent suppression of hadron pairs in electron scattering off nuclei

We present the first measurement of di-hadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the GiBUU model, which suggests that hadrons form near the nuclear surface and undergo multiple-scattering in nuclei. These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nucleiComment: 6 pages, 4 figure

First Measurement of Hard Exclusive π- Δ++ Electroproduction Beam-Spin Asymmetries off the Proton

The polarized cross-section ratio σLT′/σ0 from hard exclusive π-Δ++ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2 GeV/10.6 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 GeV2 up to 7 GeV2. The reaction provides a novel access to the d-quark content of the nucleon and to p→Δ++ transition generalized parton distributions. A comparison to existing results for hard exclusive π+n and π0p electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry

Beam–target helicity asymmetry E in K+Σ− photoproduction on the neutron

We report a measurement of a beam–target double-polarisation observable (E) for the γ→n→(p)→K+Σ−(p) reaction. The data were obtained impinging the circularly-polarised energy-tagged photon beam of Hall B at Jefferson Lab on a longitudinally-polarised frozen-spin hydrogen deuteride (HD) nuclear target. The E observable for an effective neutron target was determined for centre-of-mass energies 1.70≤W≤2.30 GeV, with reaction products detected over a wide angular acceptance by the CLAS spectrometer. These new double-polarisation data give unique constraints on the strange decays of excited neutron states. Inclusion of the new data within the Bonn-Gatchina theoretical model results in significant changes for the extracted photocouplings of a number of established nucleon resonances. Possible improvements in the PWA description of the experimental data with additional “missing” resonance states, including the N(2120)3/2− resonance, are also quantified