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Measurements of $ep \to e'\pi^+n$ at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS

K. Park I.G. Aznauryan V.D. Burkert K.P. Adhikari M.J. Amaryan S. Anefalos Pereira H. Avakian M. Battaglieri R. Badui I. Bedlinskiy A.S. Biselli J. Bono W.J. Briscoe W.K. Brooks D.S. Carman A. Celentano S. Chandavar G. Charles L. Colaneri P.L. Cole M. Contalbrigo O. Cortes V. Crede A. d'Angelo N. Dashyan R. De Vita E. De Sanctis A. Deur C. Djalali D. Doughty R. Dupre H. Egiyan A. El Alaoui L. Elouadrhiri L. El Fassi P. Eugenio G. Fedotov S. Fegan R. Fersch A. Filippi J.A. Fleming B. Garillon M. Garçon 1 N. Gevorgyan G.P. Gilfoyle K.L. Giovanetti F.X. Girod H.S. Joo J.T. Goetz E. Golovatch R.W. Gothe K.A. Griffioen B. Guegan 2 M. Guidal 2 L. Guo H. Hakobyan C. Hanretty M. Hattawy 2 K. Hicks M. Holtrop S.M. Hughes C. E. Hyde Y. Ilieva D.G. Ireland B.S. Ishkhanov E.L. Isupov D. Jenkins H. Jiang H.S. Jo 2 K. Joo S. Joosten D. Keller M. Khandaker A. Kim W. Kim A. Klein F.J. Klein V. Kubarovsky S.E. Kuhn S.V. Kuleshov P. Lenisa K. Livingston H.Y. Lu I .J .D. Macgregor N. Markov David Martinez B. Mckinnon V. Mokeev 2 R.A. Montgomery H. Moutarde 1 C. Muñoz Camacho P. Nadel-Turonski S. Niccolai 2 G. Niculescu I. Niculescu M. Osipenko A.I. Ostrovidov M. Paolone E. Pasyuk P. Peng W. Phelps J.J. Phillips S. Pisano O. Pogorelko J.W. Price S. Procureur 1 Y. Prok D. Protopopescu A.J.R. Puckett B.A. Raue M. Ripani A. Rizzo G. Rosner P. Rossi Pascal Roy F. Sabatié 1 C. Salgado D. Schott R.A. Schumacher E. Seder Y.G. Sharabian A. Simonyan Iu. Skorodumina E.S. Smith G.D. Smith N. Sparveris P. Stoler I.I. Strakovsky S. Strauch V. Sytnik M. Taiuti W. Tang C.E. Taylor Ye Tian A. Trivedi M. Ungaro H. Voskanyan E. Voutier 3 N.K. Walford D.P. Watts X. Wei L.B. Weinstein M.H. Wood N. Zachariou L. Zana J. Zhang Z.W. Zhao I. Zonta
Abstract : Differential cross sections of the exclusive process $e p \to e^\prime \pi^+ n$ were measured with good precision in the range of the photon virtuality $Q^2 = 1.8 - 4.5$ GeV$^2$, and the invariant mass range of the $\pi^+ n$ final state $W = 1.6 - 2.0$ GeV using the CEBAF Large Acceptance Spectrometer. Data were collected with nearly complete coverage in the azimuthal and polar angles of the $n\pi^+$ center-of-mass system. More than 37,000 cross section points were measured. The contributions of the isospin $I = {1\over 2}$ resonances $N(1675){5\over 2}^-$, $N(1680){5\over 2}^+$ and $N(1710){1\over 2}^+$ were extracted at different values of $Q^2$ using a single-channel, energy-dependent resonance amplitude analysis. Two different approaches, the unitary isobar model and the fixed-$t$ dispersion relations, were employed in the analysis. We observe significant strength of the $N(1675){5\over 2}^-$ in the $A_{1/2}$ amplitude, which is in strong disagreement with quark models that predict both transverse amplitudes to be strongly suppressed. For the $N(1680){5\over 2}^+$ we observe a slow changeover from the dominance of the $A_{3/2}$ amplitude at the real photon point ($Q^2=0$) to a $Q^2$ where $A_{1/2}$ begins to dominate. The scalar amplitude $S_{1/2}$ drops rapidly with $Q^2$ consistent with quark model prediction. For the $N(1710){1\over2}^+$ resonance our analysis shows significant strength for the $A_{1/2}$ amplitude at $Q^2 < 2.5$ GeV$^2$.
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Submitted on : Wednesday, December 3, 2014 - 8:39:11 AM
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K. Park, I.G. Aznauryan, V.D. Burkert, K.P. Adhikari, M.J. Amaryan, et al.. Measurements of $ep \to e'\pi^+n$ at 1.6 < W < 2.0 GeV and extraction of nucleon resonance electrocouplings at CLAS. Physical Review C, American Physical Society, 2015, 91, pp.045203. ⟨10.1103/PhysRevC.91.045203⟩. ⟨in2p3-01090110⟩

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