Relativistic effects in radiative transitions of charmonia

In the framework of a relativistic quark model constructed in an infinite momentum frame, corrections of the order of $v^{2}/c^{2}$ are obtained in the formulas which express amplitudes of the transitions $\psi(\psi') \to \eta_{_{c}}(\eta'_{_{c}})\gamma$, $\chi_{_{J}} \to \psi\gamma$, $\psi' \to \chi_{_{J}}\gamma$, $h_{_{c}} \to \eta_{_{c}}\gamma$, $\eta'_{_{c}} \to h_{_{c}}\gamma$ through overlap integrals $\int \phi^{*}_{f}(q) \phi_{i}(q) d{\bf q}$, $\int \phi^{*}_{f}(r)r \phi_{i}(r) d{\bf r}$. These corrections lead to a considerable suppression of $\Gamma(\psi\to\eta_{_{c}}\gamma)$, however, they are insufficient to remove the existing disagreement of quark model predictions with experiment. Taking into account the relativistic corrections, the ratios of overlap integrals for $\chi_{_{0}} \to \psi\gamma, \chi_{_{2}} \to \psi\gamma$ and $\psi' \to \chi_{_{0}}\gamma, \psi' \to \chi_{_{2}}\gamma$ transitions are extracted from experiment; the obtained results are compared with quark model predictions.Comment: 9 pages, LaTeX file, no fig

Electroexcitation of the Roper resonance in the relativistic quark models

The amplitudes of the transition gamma* N -> P11(1440) are calculated within light-front relativistic quark model assuming that the P11(1440) is the first radial excitation of the 3q ground state. The results are presented along with the predictions obtained in other relativistic quark models. In comparison with the previous calculations, we have extended the range of Q^2 up to 4.5 GeV^2 to cover the kinematic interval of the forthcoming experimental data. Using approach based on PCAC, we have checked the relative sign between quark model predictions for the N and P11(1440) contributions to the pion electroproduction found in previous investigations.Comment: 5 pages, 1 figure, accepted by PR

Electroexcitation of the Delta(1232)3/2+ and Delta(1600)3/2+ in a light-front relativistic quark model

The magnetic-dipole form factor and the ratios R_EM and R_SM for the gamma* N -> Delta(1232)3/2}+ transition are predicted within light-front relativistic quark model up to photon virtuality Q2=12 GeV2. We also predict the helicity amplitudes of the gamma* N -> Delta(1600)3/2+ transition assuming the Delta(1600)3/2+ is the first radial excitation of the ground state Delta(1232)3/2+.Comment: 7 pages, 3 figure

Electroexcitation of the Roper resonance from CLAS data

The helicity amplitudes of the electroexcitation of the Roper resonance on proton are extracted at 1.7 < Q2 < 4.2 GeV2 from recent high precision JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for pi+ electroproduction on protons. The analysis is made using two approaches, dispersion relations and unitary isobar model, which give consistent results. It is found that the transverse helicity amplitude for the gamma* p --> P11(1440) transition, which is large and negative at Q2=0, becomes large and positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. Longitudinal helicity amplitude, that was previously found from CLAS data as large and positive at Q2=0.4,0.65 GeV2, drops with Q2. These results rule out the presentation of P11(1440) as a 3qG hybrid state, and provide strong evidence in favor of this resonance as a first radial excitation of the 3q ground state.Comment: 3 pages, 2 figures, Talk on the Workshop on "The Physics of Excited Nucleons", Bonn, Germany, October 200

Single photo and electroproduction of pions at EBAC@JLAB

Within the Excited Baryon Analysis Center we have performed a dynamical coupled-channels analysis of the available $p(e,e' \pi)N$ data in the region of $W \leq$ 1.6 GeV and $Q^2 \leq$ 1.45 (GeV/c)$^2$. The channels included are $\gamma^* N$, $\pi N$, $\eta N$, and $\pi\pi N$ which has $\pi\Delta$, $\rho N$, and $\sigma N$ components. With the hadronic parameters of the model determined in our previous investigations of $\pi N\to \pi N$ reaction, we have found that the available data in the considered $W \leq$ 1.6 GeV region can be fitted well by only adjusting the bare $\gamma^* N \to N^*$ helicity amplitudes for the lowest $N^*$ states in $P_{33}$, $P_{11}$, $S_{11}$ and $D_{13}$ partial waves. The meson cloud effect, as required by the unitarity conditions, on the $\gamma^* N \to N^*$ form factors are examined.Comment: 6 pages, invited talk at NSTAR 2009, Beijing (China), 2009, to appear in the proceeding