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Register a new userElectroproduction of p pi^{+} pi^{-} off protons at 0.2<Q^{2}<0.6 GeV^{2} and 1.3<W<1.57 GeV with CLAS
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This paper reports on the most comprehensive data set obtained on differential and fully integrated cross sections for the process $e p to e p pi^{+} pi^{-} $. The data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the so-far unexplored kinematic region of photon virtuality 0.2 $<$ $Q^{2}$ $<$ 0.6 GeV$^{2}$ and invariant mass of the final hadron system $W$ from 1.3 to 1.57 GeV. For the first time, nine independent 1-fold differential cross sections were determined in each bin of $W$ and $Q^{2}$ covered by the measurements. A phenomenological analysis of the data allowed us to establish the most significant mechanisms contributing to the reaction. The non-resonant 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 $N(1440)P_{11}$ and $N(1520)D_{13}$ states in kinematical dependencies of the 1-fold differential cross-sections.
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Exclusive neutral-pion electroproduction ($epto e^prime p^prime pi^0$) was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4sigma/dtdQ^2dx_Bdphi_pi$ and structure functions $sigma_T+epsilonsigma_L, sigma_{TT}$ and $sigma_{LT}$ as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. The data are compared with Regge and handbag theoretical calculations. Analyses in both frameworks find that a large dominance of transverse processes is necessary to explain the experimental results. For the Regge analysis it is found that the inclusion of vector meson rescattering processes is necessary to bring the magnitude of the calculated and measured structure functions into rough agreement. In the handbag framework, there are two independent calculations, both of which appear to roughly explain the magnitude of the structure functions in terms of transversity generalized parton distributions.
New results on the single-differential and fully-integrated cross sections for the process $gamma_{v} p rightarrow p pi^{+} pi^{-}$ 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 $Q^2$ from 0.4 to 1.0 GeV$^2$. The cross sections were obtained in narrow $Q^{2}$ bins (0.05 GeV$^{2}$) 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 $Q^{2}$-evolution of the electrocouplings of most resonances with masses up to $sim$1.8 GeV.
The exclusive electroproduction process $ep rightarrow eppi^{0}$ was measured in the range of photon virtualities $Q^{2} = 0.4 - 1.0$~GeV$^{2}$ and the invariant mass range of the $ppi^{0}$ system of $W = 1.1 - 1.8$~GeV. These kinematics are covered in exclusive $pi^{0}$ electroproduction off the proton with nearly complete angular coverage in the $ppi^{0}$ center-of-mass system and with high statistical accuracy. Nearly 36000 cross section points were measured, and the structure functions $sigma_T+epsilonsigma_L$, $sigma_{LT}$, and $sigma_{TT}$, were extracted via fitting the $phi_{pi^{0}}$ dependence of the cross section. A Legendre polynomial expansion analysis demonstrates the sensitivity of our data to high-lying $N^*$ and $Delta^{*}$ resonances with $M~>~1.6$ GeV. As part of a broad effort to determine the electrocouplings of the $N^{*}$ and $Delta^{*}$ resonances using both single- and double-pion electroproduction, this dataset is crucial for the reliable extraction of the high-lying resonance electrocouplings from the combined isospin analysis of the $N pi$ and $pi^{+}pi^{-} p$ channels.
The differential cross sections sigma_0=sigma_T+epsilon sigma_L, sigma_{LT}, and sigma_{TT} of pi^0 electroproduction from the proton were measured from threshold up to an additional center of mass energy of 40 MeV, at a value of the photon four-momentum transfer of Q^2= 0.05 GeV^2/c^2 and a center of mass angle of theta=90^circ. By an additional out-of-plane measurement with polarized electrons sigma_{LT} was determined. This showed for the first time the cusp effect above the pi^+ threshold in the imaginary part of the s-wave. The predictions of Heavy Baryon Chiral Perturbation Theory are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.
The cross section of the exclusive $eta$ electroproduction reaction $epto e^prime p^prime eta$ was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections $d^4sigma/dtdQ^2dx_Bdphi_eta$ and structure functions $sigma_U = sigma_T+epsilonsigma_L, sigma_{TT}$ and $sigma_{LT}$, as functions of $t$ were obtained over a wide range of $Q^2$ and $x_B$. The $eta$ structure functions are compared with those previously measured for $pi^0$ at the same kinematics. At low $t$, both $pi^0$ and $eta$ are described reasonably well by generalized parton distributions (GPDs) in which chiral-odd transversity GPDs are dominant. The $pi^0$ and $eta$ data, when taken together, can facilitate the flavor decomposition of the transversity GPDs.
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