https://doi.org/10.1140/epjc/s10052-014-3110-1
Regular Article - Experimental Physics
Spin density matrix elements in exclusive 
electroproduction on 
H and 
H targets at 27.5 GeV beam energy
1
Physics Division, Argonne National Laboratory, Argonne, IL, 60439-4843, USA
2
Sezione di Bari, Istituto Nazionale di Fisica Nucleare, 70124, Bari, Italy
3
School of Physics, Peking University, Beijing, 100871, China
4
Department of Theoretical Physics, University of the Basque Country UPV/EHU, 48080, 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
Sezione di Ferrara, Istituto Nazionale di Fisica Nucleare, 44122, Ferrara, Italy
11
Laboratori Nazionali di Frascati, Istituto Nazionale di Fisica Nucleare, 00044, Frascati, Italy
12
Department of Physics and Astronomy, Ghent University, 9000, Gent, Belgium
13
II. Physikalisches Institut, Justus-Liebig Universität Gießen, 35392, Gießen, Germany
14
SUPA, School 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
B.P. Konstantinov Petersburg Nuclear Physics Institute, Gatchina, 188300, Leningrad Region, Russia
20
Institute for High Energy Physics, Protvino, 142281, Moscow Region, Russia
21
Institut für Theoretische Physik, Universität Regensburg, 93040, Regensburg, Germany
22
Sezione di Roma, Gruppo Collegato Sanità, Istituto Nazionale di Fisica Nucleare, 00161, Rome, Italy
23
TRIUMF, Vancouver, BC, V6T 2A3, Canada
24
Department of Physics, Tokyo Institute of Technology, Tokyo, 152, Japan
25
Department of Physics and Astronomy, VU University, 1081 HV, Amsterdam, The Netherlands
26
National Centre for Nuclear Research, 00-689, Warsaw, Poland
27
Yerevan Physics Institute, 375036, Yerevan, Armenia
28
IKERBASQUE, Basque Foundation for Science, 48013, Bilbao, Spain
29
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, 44122, Ferrara, Italy
30
Istituto Superiore di Sanità, 00161, Roma, Italy
* e-mail: gunar.schnell@desy.de
Received:
8
July
2014
Accepted:
30
September
2014
Published online:
15
November
2014
Exclusive electroproduction of mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of 
GeV, 3.0 GeV 
6.3 GeV, and 
GeV. Results on the angular distribution of the meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996–2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of 
or 
. Violation of 
-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse production by longitudinal and transverse virtual photons, 
and 
, is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the 
transition dominates over the two natural-parity-exchange amplitudes describing the 
and transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.
© The Author(s), 2014
