Eur. Phys. J. C (2017) 77: 378
https://doi.org/10.1140/epjc/s10052-017-4899-1

Regular Article - Experimental Physics

Ratios of helicity amplitudes for exclusive electroproduction on transversely polarized protons

The HERMES Collaboration

¹ Physics Division, Argonne National Laboratory, Argonne, IL, 60439-4843, USA
² Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70124, Bari, Italy
³ Department of Theoretical Physics, University of the Basque Country UPV/EHU, 48080, Bilbao, Spain
⁴ IKERBASQUE, Basque Foundation for Science, 48013, Bilbao, Spain
⁵ Nuclear Physics Laboratory, University of Colorado, Boulder, CO, 80309-0390, USA
⁶ DESY, 22603, Hamburg, Germany
⁷ DESY, 15738, Zeuthen, Germany
⁸ Joint Institute for Nuclear Research, 141980, Dubna, Russia
⁹ Physikalisches Institut, Universität Erlangen-Nürnberg, 91058, Erlangen, Germany
¹⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Ferrara, 44122, Ferrara, Italy
¹¹ Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, 44122, Ferrara, Italy
¹² Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, 00044, Frascati, Italy
¹³ Department of Physics and Astronomy, Ghent University, 9000, Gent, Belgium
¹⁴ II. Physikalisches Institut, Justus-Liebig Universität Gießen, 35392, Gießen, Germany
¹⁵ SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
¹⁶ Department of Physics, University of Illinois, Urbana, IL, 61801-3080, USA
¹⁷ Randall Laboratory of Physics, University of Michigan, Ann Arbor, MI, 48109-1040, USA
¹⁸ Lebedev Physical Institute, 117924, Moscow, Russia
¹⁹ National Institute for Subatomic Physics (Nikhef), 1009 DB, Amsterdam, The Netherlands
²⁰ B.P. Konstantinov Petersburg Nuclear Physics Institute, Gatchina, 188300, Leningrad Region, Russia
²¹ Institute for High Energy Physics, Protvino, 142281, Moscow Region, Russia
²² Institut für Theoretische Physik, Universität Regensburg, 93040, Regensburg, Germany
²³ Gruppo Collegato Sanità, Istituto Nazionale di Fisica Nucleare, Sezione di Roma, 00161, Rome, Italy
²⁴ Istituto Superiore di Sanità, 00161, Rome, Italy
²⁵ TRIUMF, Vancouver, BC, V6T 2A3, Canada
²⁶ Department of Physics, Tokyo Institute of Technology, Tokyo, 152, Japan
²⁷ Department of Physics and Astronomy, VU University, 1081 HV, Amsterdam, The Netherlands
²⁸ National Centre for Nuclear Research, 00-689, Warsaw, Poland
²⁹ Yerevan Physics Institute, 375036, Yerevan, Armenia

^* e-mail: gunar.schnell@desy.de

Received: 2 February 2017
Accepted: 8 May 2017
Published online: 8 June 2017

Abstract

Exclusive -meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV 7.0 GeV, 3.0 GeV 6.3 GeV, and 0.4 GeV. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing -meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude , which describes the production of a longitudinal -meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to . Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.