Eur. Phys. J. C (2009) 62: 659-695
https://doi.org/10.1140/epjc/s10052-009-1082-3

Regular Article - Experimental Physics

Spin density matrix elements in exclusive ρ ⁰ electroproduction on ¹H and ²H targets at 27.5 GeV beam energy

¹ Physics Division, Argonne National Laboratory, Argonne, IL, 60439-4843, USA
² Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70124, Bari, Italy
³ School of Physics, Peking University, Beijing, 100871, China
⁴ Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui, 230026, China
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 and Dipartimento di Fisica, Università di Ferrara, 44100, Ferrara, Italy
¹¹ Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, 00044, Frascati, Italy
¹² Department of Subatomic and Radiation Physics, University of Gent, 9000, Gent, Belgium
¹³ Physikalisches Institut, Universität Gießen, 35392, Gießen, Germany
¹⁴ Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
¹⁵ Department of Physics, University of Illinois, Urbana, IL, 61801-3080, USA
¹⁶ Randall Laboratory of Physics, University of Michigan, Ann Arbor, MI, 48109-1040, USA
¹⁷ Lebedev Physical Institute, 117924, Moscow, Russia
¹⁸ National Institute for Subatomic Physics (Nikhef), 1009 DB, Amsterdam, The Netherlands
¹⁹ Petersburg Nuclear Physics Institute, Gatchina, Leningrad Region, 188300, Russia
²⁰ Institute for High Energy Physics, Protvino, Moscow Region, 142281, Russia
²¹ Institut für Theoretische Physik, Universität Regensburg, 93040, Regensburg, Germany
²² Istituto Nazionale di Fisica Nucleare, Sezione Roma 1, Gruppo Sanità and Physics Laboratory, Istituto Superiore di Sanità, 00161, Roma, Italy
²³ TRIUMF, Vancouver, British Columbia, V6T 2A3, Canada
²⁴ Department of Physics, Tokyo Institute of Technology, Tokyo, 152, Japan
²⁵ Department of Physics, Vrije Universiteit, 1081 HV, Amsterdam, The Netherlands
²⁶ Andrzej Soltan Institute for Nuclear Studies, 00-689, Warsaw, Poland
²⁷ Yerevan Physics Institute, 375036, Yerevan, Armenia
²⁸ 36 Mizzen Circle, Hampton, VA, 23664, USA

^* e-mail: klaus.rith@desy.de

Received: 19 January 2009
Revised: 6 June 2009
Published online: 17 July 2009

Abstract

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ ⁰ electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1<Q ²<7 GeV², 3.0<W<6.3 GeV, and −t<0.4 GeV². Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q ² and t are presented, as well as the longitudinal-to-transverse ρ ⁰ electroproduction cross-section ratio as a function of Q ². A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.