https://doi.org/10.1140/epjc/s10052-020-08740-y
Regular Article – Experimental Physics
Spin density matrix elements in exclusive
meson muoproduction
1
Department of Physics, University of Aveiro, I3N, 3810-193, Aveiro, Portugal
2
Universität Bochum, Institut für Experimentalphysik, 44780, Bochum, Germany
3
Universität Bonn, Helmholtz-Institut für Strahlen- und Kernphysik, 53115, Bonn, Germany
4
Universität Bonn, Physikalisches Institut, 53115, Bonn, Germany
5
Institute of Scientific Instruments of the CAS, 61264, Brno, Czech Republic
6
Matrivani Institute of Experimental Research & Education, 700 030, Calcutta, India
7
Joint Institute for Nuclear Research, 141980, Dubna, Moscow region, Russia
8
Universität Freiburg, Physikalisches Institut, 79104, Freiburg, Germany
9
CERN, 1211, Geneva 23, Switzerland
10
Technical University in Liberec, 46117, Liberec, Czech Republic
11
LIP, 1649-003, Lisbon, Portugal
12
Universität Mainz, Institut für Kernphysik, 55099, Mainz, Germany
13
University of Miyazaki, 889-2192, Miyazaki, Japan
14
Lebedev Physical Institute, 119991, Moscow, Russia
15
Physik Department, Technische Universität München, 85748, Garching, Germany
16
Nagoya University, 464, Nagoya, Japan
17
Faculty of Mathematics and Physics, Charles University, 18000, Prague, Czech Republic
18
Czech Technical University in Prague, 16636, Prague, Czech Republic
19
State Scientific Center Institute for High Energy Physics of National Research Center ‘Kurchatov Institute’, 142281, Protvino, Russia
20
IRFU, CEA, Université Paris-Saclay, 91191, Gif-sur-Yvette, France
21
Academia Sinica, Institute of Physics, 11529, Taipei, Taiwan
22
Tel Aviv University, School of Physics and Astronomy, 69978, Tel Aviv, Israel
23
Tomsk Polytechnic University, 634050, Tomsk, Russia
24
Department of Physics, University of Trieste, 34127, Trieste, Italy
25
Trieste Section of INFN, 34127, Trieste, Italy
26
Department of Physics, University of Turin, 10125, Turin, Italy
27
Torino Section of INFN, 10125, Turin, Italy
28
Department of Physics, University of Illinois at Urbana-Champaign, IL 61801-3080, Urbana, USA
29
National Centre for Nuclear Research, 02-093, Warsaw, Poland
30
Faculty of Physics, University of Warsaw, 02-093, Warsaw, Poland
31
Warsaw University of Technology, Institute of Radioelectronics, 00-665, Warsaw, Poland
32
Yamagata University, 992-8510, Yamagata, Japan
an
Oleg.Denisov@cern.ch
fz
Andrzej.Sandacz@ncbj.gov.pl
gu
Fulvio.Tessarotto@cern.ch
Received:
22
September
2020
Accepted:
7
December
2020
Published online:
5
February
2021
We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive meson muoproduction on the proton at COMPASS using 160 GeV/c polarised
and
beams impinging on a liquid hydrogen target. The measurement covers the range 5.0 GeV/
17.0 GeV/
, with the average kinematics
2.1 (GeV/c)
,
GeV/
, and
(GeV/c)
. Here,
denotes the virtuality of the exchanged photon, W the mass of the final hadronic system and
the transverse momentum of the
meson with respect to the virtual-photon direction. The measured non-zero SDMEs for the transitions of transversely polarised virtual photons to longitudinally polarised vector mesons (
) indicate a violation of s-channel helicity conservation. Additionally, we observe a sizeable contribution of unnatural-parity-exchange (UPE) transitions that decreases with increasing W. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow to evaluate in a model-dependent way the contribution of UPE transitions and assess the role of parton helicity-flip GPDs in exclusive
production.
This article is dedicated to the memory of Bohdan Marianski.
© The Author(s) 2021
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