Beam-Target Helicity Asymmetry for γ[over →]n[over →]→π^{-}p in the N^{*} Resonance Region

We report the first beam-target double-polarization asymmetries in the γ+n(p)→π^{-}+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 γnN^{*} resonance photocouplings.

Study of η and η' Photoproduction at MAMI

The reactions γp→ηp and γp→η^{'}p are measured from their thresholds up to the center-of-mass energy W=1.96  GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for η photoproduction at the energies in the vicinity of the η^{'} threshold, W=1896  MeV (E_{γ}=1447  MeV). Within the framework of a revised ηMAID isobar model, the cusp, in connection with a steep rise of the η^{'} total cross section from its threshold, can only be explained by a strong coupling of the poorly known N(1895)1/2^{-} state to both ηp and η^{'}p. Including the new high-accuracy results in the ηMAID fit to available η and η^{'} photoproduction data allows the determination of the N(1895)1/2^{-} properties.

Insight into the Narrow Structure in η Photoproduction on the Neutron from Helicity-Dependent Cross Sections

The double polarization observable E and the helicity dependent cross sections σ_{1/2} and σ_{3/2} were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π^{0}. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ_{1/2} and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in η production are only N 1/2^{-} (S_{11}) and N 1/2^{+} (P_{11}). Furthermore, the extracted Legendre coefficients of the angular distributions for σ_{1/2} are in good agreement with recent reaction model predictions assuming a narrow resonance in the P_{11} wave as the origin of this structure.

Cross Sections for the Exclusive Photon Electroproduction on the Proton and Generalized Parton Distributions

Unpolarized and beam-polarized fourfold cross sections (d^{4}σ/dQ^{2}dx_{B}dtdϕ) for the ep→e^{'}p^{'}γ 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}.

Precision Measurement of the p(e,e^{'}p)π^{0} Reaction at Threshold

New results are reported from a measurement of π^{0} electroproduction near threshold using the p(e,e^{'}p)π^{0} reaction. The experiment was designed to determine precisely the energy dependence of s- and p-wave electromagnetic multipoles as a stringent test of the predictions of chiral perturbation theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the ϕ_{π}^{*} and θ_{π}^{*} angles in the pπ^{0} center of mass was obtained for invariant energies above threshold from 0.5 up to 15 MeV. The 4-momentum transfer Q^{2} coverage ranges from 0.05 to 0.155 (GeV/c)^{2} in fine steps. A simple phenomenological analysis of our data shows strong disagreement with p-wave predictions from ChPT for Q^{2}>0.07 (GeV/c)^{2}, while the s-wave predictions are in reasonable agreement.

Measurements of double-polarized compton scattering asymmetries and extraction of the proton spin polarizabilities

The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to extract the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the Δ(1232) region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields γ(E1E1)=-3.5±1.2, γ(M1M1)=3.16±0.85, γ(E1M2)=-0.7±1.2, and γ(M1E2)=1.99±0.29, in units of 10(-4)  fm(4).

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 (ϵ) and momentum transfer (Q(2)) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ϵ at Q(2)=1.45  GeV(2). This measurement is consistent with the size of the form factor discrepancy at Q(2)≈1.75  GeV(2) and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3  GeV(2).

Longitudinal target-spin asymmetries for deeply virtual compton scattering

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→e^{'}p^{'}γ 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 ϕ, 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.

Strangeness suppression of qq creation observed in exclusive reactions

We measured the ratios of electroproduction cross sections from a proton target for three exclusive meson-baryon final states: ΛK(+), pπ(0), and nπ(+), with the CLAS detector at Jefferson Lab. Using a simple model of quark hadronization, we extract qq creation probabilities for the first time in exclusive two-body production, in which only a single qq 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.

Measurement of the transverse target and beam-target asymmetries in η meson photoproduction at MAMI

We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the γ[over →]p[over →]→ηp reaction up to a center-of-mass energy of W=1.9  GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of η meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.

Neutron skin of (208)pb from coherent pion photoproduction

Information on the size and shape of the neutron skin on (208)Pb is extracted from coherent pion photoproduction cross sections measured using the Crystal Ball detector together with the Glasgow tagger at the MAMI electron beam facility. On exploitation of an interpolated fit of a theoretical model to the measured cross sections, the half-height radius and diffuseness of the neutron distribution are found to be c(n)=6.70±0.03(stat.)  fm and a(n)=0.55±0.01(stat.)(-0.03)(+0.02)(sys.)  fm, respectively, corresponding to a neutron skin thickness Δr(np)=0.15±0.03(stat.)(-0.03)(+0.01)(sys.)  fm. The results give the first successful extraction of a neutron skin thickness with an electromagnetic probe and indicate that the skin of (208)Pb has a halo character. The measurement provides valuable new constraints on both the structure of nuclei and the equation of state for neutron-rich matter.

Photoproduction of π0 mesons off neutrons in the nucleon resonance region

Precise angular distributions have been measured for the first time for the photoproduction of π0 mesons off neutrons bound in the deuteron. The effects from nuclear Fermi motion have been eliminated by a complete kinematic reconstruction of the final state. The influence of final-state-interaction effects has been estimated by a comparison of the reaction cross section for quasifree protons bound in the deuteron to the results for free protons and then applied as a correction to the quasifree neutron data. The experiment was performed at the tagged photon facility of the Mainz Microtron MAMI with the Crystal Ball and TAPS detector setup for incident photon energies between 0.45 and 1.4 GeV. The results are compared to the predictions from reaction models and partial-wave analyses based on data from other isospin channels. The model predictions show large discrepancies among each other and the present data will provide much tighter constraints. This is demonstrated by the results of a new analysis in the framework of the Bonn-Gatchina coupled-channel analysis which included the present data.

Measurement of the 1H(γ, p)π0 reaction using a novel nucleon spin polarimeter

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of 1H(γ, p)π0 in the range 0.4<Eγ<1.4  GeV validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

Narrow structure in the excitation function of η photoproduction off the neutron

The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)  MeV and Γ=(30±15)  MeV. For the first time precise results for the angular dependence of this structure have been extracted.

Accurate test of chiral dynamics in the γp→π0p reaction

A precision measurement of the differential cross sections dσ/dΩ and the linearly polarized photon asymmetry Σ≡(dσ⊥-dσ∥)/(dσ⊥+dσ∥) for the γp→π0p reaction in the near-threshold region has been performed with a tagged photon beam and almost 4π detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multiphoton detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the S- and all three P-wave amplitudes for the first time and provide the most stringent test to date of the predictions of chiral perturbation theory and its energy region of agreement with experiment.

Hard two-body photodisintegration of 3He

We have measured cross sections for the γ(3)He → pd reaction at photon energies of 0.4-1.4 GeV and a center-of-mass angle of 90°. We observe dimensional scaling above 0.7 GeV at this center-of-mass angle. This is the first observation of dimensional scaling in the photodisintegration of a nucleus heavier than the deuteron.

Measurement of exclusive π(0) electroproduction structure functions and their relationship to transverse generalized parton distributions

Exclusive π(0) electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in Q(2), x(B), t, and ϕ(π), in the Q(2) range from 1.0 to 4.6  GeV(2), -t up to 2  GeV(2), and x(B) from 0.1 to 0.58. Structure functions σ(T)+ϵσ(L), σ(TT), and σ(LT) were extracted as functions of t for each of 17 combinations of Q(2) and x(B). The data were compared directly with two handbag-based calculations including both longitudinal and transversity generalized parton distributions (GPDs). Inclusion of only longitudinal GPDs very strongly underestimates σ(T)+ϵσ(L) and fails to account for σ(TT) and σ(LT), while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity-flip and helicity nonflip processes. The results confirm that exclusive π(0) electroproduction offers direct experimental access to the transversity GPDs.

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

We report on the first measurement of the F(2) structure function of the neutron from the 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/c and their angles to ≳100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The F(2)(n) data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65<Q(2)<4.52 GeV(2), 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 F(2)(n)/F(2)(p) at 0.2≲x≲0.8 with little uncertainty due to nuclear effects.

Measurement of single- and double-spin asymmetries in deep inelastic pion electroproduction with a longitudinally polarized target

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.

Tensor correlations measured in 3He(e,e' pp)n

We have measured the 3He(e,e' pp)n reaction at an incident energy of 4.7 GeV over a wide kinematic range. We identified spectator correlated pp and pn nucleon pairs by using kinematic cuts and measured their relative and total momentum distributions. This is the first measurement of the ratio of pp to pn pairs as a function of pair total momentum p(tot). For pair relative momenta between 0.3 and 0.5  GeV/c, the ratio is very small at low p(tot) and rises to approximately 0.5 at large p(tot). This shows the dominance of tensor over central correlations at this relative momentum.

Absorption of the ω and ϕ mesons in nuclei

Because of their long lifetimes, the ω and ϕ mesons are the ideal candidates for the study of possible modifications of the in-medium meson-nucleon interaction through their absorption inside the nucleus. During the E01-112 experiment at the Thomas Jefferson National Accelerator Facility, the mesons were photoproduced from 2H, C, Ti, Fe, and Pb targets. This Letter reports the first measurement of the ratio of nuclear transparencies for the e+e- channel. The ratios indicate larger in-medium widths compared with what have been reported in other reaction channels. The absorption of the ω meson is stronger than that reported by the CBELSA-TAPS experiment and cannot be explained by recent theoretical models.

Beam-helicity asymmetries in double-pion photoproduction off the proton

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels gamma[over -->]p-->pi(+)pi(0)n, gamma[over -->]p-->pi(0)pi(0)p, and gamma[over -->]p-->pi(+)pi(-)p. The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of pi(0) mesons were detected in a 4pi electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.

Measurement of the differential cross section for the reaction gamman-->pi- p from deuterium

We report a measurement of the differential cross section for the gamman-->pi- p process from the CLAS detector at Jefferson Laboratory in Hall B for photon energies between 1.0 and 3.5 GeV and pion center-of-mass (c.m.) angles (thetac.m.) between 50 degrees and 115 degrees. We confirm a previous indication of a broad enhancement around a c.m. energy ([sqrt]s) of 2.1 GeV at thetac.m.=90 degrees in the scaled differential cross section s7dsigma/dt and a rapid falloff in a center-of-mass energy region of about 400 MeV following the enhancement. Our data show an angular dependence of this enhancement as the suggested scaling region is approached for thetac.m. from 70 degrees to 105 degrees.