Beam Spin Asymmetries in Deeply Virtual Compton Scattering (DVCS) with CLAS at 4.8 GeV

We report measurements of the beam spin asymmetry in deeply virtual Compton scattering (DVCS) at an electron beam energy of 4.8 GeV using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The DVCS beam spin asymmetry has been measured in a wide range of kinematics, 1.0 \u3c Q2 \u3c 2.8 (GeV/c)2, 0.12 \u3c xB \u3c 0.48, and 0.1 \u3c −t \u3c 0.8 (GeV/c)2, using the reaction ep → e\u27 p X. The number of H(e, e\u27 γ p) and H(e, e\u27π0p) events are separated in each (Q2, xB, t) bin by a fit to the line shape of the H(e, e\u27 p)X M2x distribution. The validity of the method was studied in detail using experimental and simulated data. It was shown that with the achieved missing mass squared resolution and the available statistics, the separation of DVCS–Bethe-Heitler and π0 events can reliably be done with less than 5% uncertainty. Also, the Q2 and t dependences of the sin φ moments of the asymmetry are extracted and compared with theoretical calculations

Electroexcitation of Nucleon Resonances from CLAS Data on Single Pion Electroproduction

We present results on the electroexcitation of the low mass resonances ∆(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 in a wide range of Q2. The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the ∆(1232)P33 show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q2 ≤ 6 GeV2. For the Roper resonance, N(1440)P11, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S11 allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ*p → N(1520)D13 and N(1535)S11 longitudinal helicity amplitudes with good precision. For the N(1535)S11, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D13 clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q2 \u3e 1 GeV2, confirming a long-standing prediction of the constituent quark model

Electroproduction of \u3ci\u3epπ\u3c/i\u3e+\u3ci\u3e​π\u3c/i\u3e- Off Protons at 0.2 \u3c \u3ci\u3eQ\u3c/i\u3e\u3csup\u3e2\u3c/sup\u3e \u3c 0.6 GeV\u3csup\u3e2\u3c/sup\u3e and 1.3 \u3c \u3ci\u3eW\u3c/i\u3e \u3c 1.57 GeV with the CLAS Detector

This paper reports on the most comprehensive data set obtained on differential and fully integrated cross sections for the process ep → e\u27pπ+π −. The data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the as yet unexplored kinematic region of photon virtuality 0.2 \u3c Q2 \u3c 0.6 GeV2 and invariant mass of the final hadron system W from 1.3 to 1.57 GeV. For the first time, nine independent one-fold differential cross sections were determined in each bin of W and Q2 covered by the measurements. A phenomenological analysis of the data allowed us to establish the most significant mechanisms contributing to the reaction. The nonresonant mechanisms account for a major part of cross sections. However, we find sensitivity to s-channel excitations of low-mass nucleon resonances, especially to the N(1440)P11 and N(1520)D13 states in kinematic dependencies of the one-fold differential cross sections

Measurements of the Gamma(Upsilon)p -\u3e p ’pi(+)Pi(- )Cross Section with the CLAS Detector for 0.4 GeV2 \u3c Q(2) \u3c 1.0 GeV2 and 1.3 GeV \u3c W \u3c 1.825 GeV

New results on the single-differential and fully integrated cross sections for the process γvp -\u3e p\u27π+π- are presented. The experimental data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the kinematic region of the reaction invariant mass W from 1.3 to 1.825 GeV and the photon virtuality Q2 from 0.4 to 1.0 GeV2. The cross sections were obtained in narrow Q2 bins (0.05 GeV2) with the smallest statistical uncertainties achieved in double-pion electroproduction experiments to date. The results were found to be in agreement with previously available data where they overlap. A preliminary interpretation of the extracted cross sections, which was based on a phenomenological meson-baryon reaction model, revealed substantial relative contributions from nucleon resonances. The data offer promising prospects to improve knowledge on the Q2 evolution of the electrocouplings of most resonances with masses up to similar to ~ 1.8 GeV

Exclusive Photoproduction of Pi Degrees up to Large Values of Mandelstam Variables s, t, and u with CLAS

Exclusive photoproduction cross sections have been measured for the process γp -\u3e pπ0 [e+ e- (γ)] with the Dalitz decay final state using tagged photon energies in the range of Eγ = 1.275 – 5.425 GeV. The complete angular distribution of the final state π0, 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 dσ / dt, at mid to large angles, decreases with energy as s-6.89 +/- 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 π0 photoproduction are more consistent with experimental data

Comment on the narrow structure reported by Amaryan et al

The CLAS Collaboration provides a comment on the physics interpretation of the results presented in a paper published by M. Amaryan et al. regarding the possible observation of a narrow structure in the mass spectrum of a photoproduction experiment.Comment: to be published in Physical Review

Target and beam-target spin asymmetries in exclusive pion electroproduction for Q2>1GeV2 . I. ep→eπ+n

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive π + electroproduction reaction γ ∗ p → n π + . 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 < 6 GeV 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. 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

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