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

1 DPHN - Département de Physique Nucléaire (ex SPhN)
2 IPNO - Institut de Physique Nucléaire d'Orsay
3 LPSC - Laboratoire de Physique Subatomique et de Cosmologie

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$^2$.

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

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, 2015, 91, pp.045203. ⟨10.1103/PhysRevC.91.045203⟩. ⟨in2p3-01090110⟩