Eur. Phys. J. C (2023) 83: 924
https://doi.org/10.1140/epjc/s10052-023-11359-4

Regular Article - Experimental Physics

Spin density matrix elements in exclusive ${\rho }^0$ meson muoproduction

¹ A.I. Alikhanyan National Science Laboratory, 2 Alikhanyan Br. Street, 0036, Yerevan, Armenia
² Institute of Scientific Instruments of the CAS, 61264, Brno, Czech Republic
³ Technical University in Liberec, 46117, Liberec, Czech Republic
⁴ Czech Technical University in Prague, 16636, Prague, Czech Republic
⁵ Faculty of Mathematics and Physics, Charles University, 12116, Prague, Czech Republic
⁶ IRFU, CEA, Université Paris-Saclay, 91191, Gif-sur-Yvette, France
⁷ Universität Bochum, Institut für Experimentalphysik, 44780, Bochum, Germany
⁸ Universität Bonn, Helmholtz-Institut für Strahlen- und Kernphysik, 53115, Bonn, Germany
⁹ Universität Bonn, Physikalisches Institut, 53115, Bonn, Germany
¹⁰ Universität Freiburg, Physikalisches Institut, 79104, Freiburg, Germany
¹¹ Universität Mainz, Institut für Kernphysik, 55099, Mainz, Germany
¹² Technische Universität München, Physik Department, 85748, Garching, Germany
¹³ Ludwig-Maximilians-Universität, 80539, München, Germany
¹⁴ Matrivani Institute of Experimental Research & Education, 700 030, Calcutta, India
¹⁵ School of Physics and Astronomy, Tel Aviv University, 69978, Tel Aviv, Israel
¹⁶ Abdus Salam ICTP, 34151, Trieste, Italy
¹⁷ Trieste Section of INFN, 34127, Trieste, Italy
¹⁸ Dept. of Physics, University of Trieste, 34127, Trieste, Italy
¹⁹ Torino Section of INFN, 10125, Torino, Italy
²⁰ Dept. of Physics, University of Torino, 10125, Torino, Italy
²¹ University of Miyazaki, 889-2192, Miyazaki, Japan
²² Nagoya University, 464, Nagoya, Japan
²³ Yamagata University, 992-8510, Yamagata, Japan
²⁴ National Centre for Nuclear Research, 02-093, Warsaw, Poland
²⁵ Warsaw University of Technology, Institute of Radioelectronics, 00-665, Warsaw, Poland
²⁶ Faculty of Physics, University of Warsaw, 02-093, Warsaw, Poland
²⁷ Dept. of Physics, University of Aveiro, I3N, 3810-193, Aveiro, Portugal
²⁸ LIP, 1649-003, Lisbon, Portugal
²⁹ international laboratory covered by a cooperation agreement with CERN, Geneva, Switzerland
³⁰ institute covered by a cooperation agreement with CERN, Geneva, Switzerland
³¹ CERN, 1211 Geneva 23, Switzerland
³² Academia Sinica, Institute of Physics, 11529, Taipei, Taiwan
³³ University of Illinois at Urbana-Champaign, Dept. of Physics, 61801-3080, Urbana, IL, USA
³⁴ CERN, 1211, Geneva 23, Switzerland
³⁵ Retired from Ludwig-Maximilians-Universität, 80539, München, Germany

^a Bakur.Parsamyan@cern.ch, Fulvio.Tessarotto@cern.ch, Andrzej.Sandacz@ncbj.gov.pl

Received: 4 November 2022
Accepted: 24 February 2023
Published online: 13 October 2023

Abstract

We report on a measurement of Spin Density Matrix Elements (SDMEs) in hard exclusive ${\rho }^0$ meson muoproduction at COMPASS using 160 GeV/c polarised ${\mu }^{+}$ and ${\mu }^{-}$ beams impinging on a liquid hydrogen target. The measurement covers the kinematic range 5.0 GeV/$c^2$$<W<$ 17.0 GeV/$c^2$, 1.0 (GeV/c)${}^2$$<Q^2<$ 10.0 (GeV/c)${}^2$ and 0.01 (GeV/c)${}^2$$<p_{\text{T}}^2<$ 0.5 (GeV/c)${}^2$. Here, W denotes the mass of the final hadronic system, $Q^2$ the virtuality of the exchanged photon, and $p_{\text{T}}$ the transverse momentum of the ${\rho }^0$ 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 (${\gamma }_T^{\ast }\to V_L^{}$) indicate a violation of s-channel helicity conservation. Additionally, we observe a dominant contribution of natural-parity-exchange transitions and a very small contribution of unnatural-parity-exchange transitions, which is compatible with zero within experimental uncertainties. The results provide important input for modelling Generalised Parton Distributions (GPDs). In particular, they may allow one to evaluate in a model-dependent way the role of parton helicity-flip GPDs in exclusive ${\rho }^0$ production.