Eur. Phys. J. C (2020) 80: 1189
https://doi.org/10.1140/epjc/s10052-020-08587-3

Regular Article – Experimental Physics

Measurement of exclusive ${\mathit{\pi }}^{\mathbf{+}}{\mathit{\pi }}^{\mathbf{-}}$ and ${\mathit{\rho }}^{\mathbf{0}}$ meson photoproduction at HERA

H1 Collaboration

¹ I. Physikalisches Institut der RWTH, Aachen, Germany
² School of Physics and Astronomy, University of Birmingham, Birmingham, UK
³ Inter-University Institute for High Energies ULB-VUB, Brussels and Universiteit Antwerpen, Antwerp, Belgium
⁴ Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Bucharest, Romania
⁵ STFC, Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK
⁶ Institute of Nuclear Physics Polish Academy of Sciences, 31342, Kraków, Poland
⁷ Institut für Physik, TU Dortmund, Dortmund, Germany
⁸ Joint Institute for Nuclear Research, Dubna, Russia
⁹ Irfu/SPP, CE Saclay, Gif-sur-Yvette, France
¹⁰ DESY, Hamburg, Germany
¹¹ Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany
¹² Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany
¹³ Department of Physics, University of Lancaster, Lancaster, UK
¹⁴ Department of Physics, University of Liverpool, Liverpool, UK
¹⁵ School of Physics and Astronomy, Queen Mary University of London, London, UK
¹⁶ Aix Marseille Université, CNRS/IN2P3, CPPM UMR 7346, 13288, Marseille, France
¹⁷ Departamento de Fisica Aplicada, CINVESTAV Mérida, Mérida, Yucatán, Mexico
¹⁸ Institute for Theoretical and Experimental Physics, Moscow, Russia
¹⁹ Lebedev Physical Institute, Moscow, Russia
²⁰ Max-Planck-Institut für Physik, Munich, Germany
²¹ LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France
²² LLR, Ecole Polytechnique, CNRS/IN2P3, Palaiseau, France
²³ Faculty of Science, University of Montenegro, Podgorica, Montenegro
²⁴ Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
²⁵ Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
²⁶ Dipartimento di Fisica, Università di Roma Tre and INFN Roma 3, Rome, Italy
²⁷ Institute of Physics and Technology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
²⁸ Paul Scherrer Institut, Villigen, Switzerland
²⁹ Fachbereich C, Universität Wuppertal, Wuppertal, Germany
³⁰ Yerevan Physics Institute, Yerevan, Armenia
³¹ DESY, Zeuthen, Germany
³² Institut für Teilchenphysik, ETH Zürich, Zurich, Switzerland
³³ Physik-Institut der Universität Zürich, Zurich, Switzerland
³⁴ Université Claude Bernard Lyon 1, CNRS/IN2P3, Villeurbanne, France
³⁵ Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia
³⁶ Joint Laboratory of Optics, Palackỳ University, Olomouc, Czech Republic
³⁷ CERN, Geneva, Switzerland
³⁸ Ulaanbaatar University, Ulaanbaatar, Mongolia
³⁹ LAPP, Université de Savoie, CNRS/IN2P3, Annecy-le-Vieux, France
⁴⁰ II. Physikalisches Institut, Universität Göttingen, Göttingen, Germany
⁴¹ Institute for Information Transmission Problems RAS, Moscow, Russia
⁴² Brookhaven National Laboratory, 11973, Upton, NY, USA
⁴³ Department of Physics and Astronomy, Purdue University, 525 Northwestern Ave, 47907, West Lafayette, IN, USA
⁴⁴ Department of Physics, Oxford University, Oxford, UK
⁴⁵ Rice University, Houston, USA
⁴⁶ Shandong University, Shandong, People’s Republic of China
⁴⁷ Stony Brook University, 11794, Stony Brook, NY, USA

^ds sschmitt@mail.desy.de

Received: 2 June 2020
Accepted: 20 October 2020
Published online: 23 December 2020

Abstract

Exclusive photoproduction of ${\rho }^0(770)$ mesons is studied using the H1 detector at the ep collider HERA. A sample of about 900,000 events is used to measure single- and double-differential cross sections for the reaction $\gamma p\to {\pi }^{+}{\pi }^{-}Y$. Reactions where the proton stays intact ($m_Y=m_p$) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($m_p<m_Y<10\text{GeV}$). The double-differential cross sections are measured as a function of the invariant mass $m_{\pi \pi }$ of the decay pions and the squared 4-momentum transfer t at the proton vertex. The measurements are presented in various bins of the photon–proton collision energy $W_{\gamma p}$. The phase space restrictions are $0.5\le m_{\pi \pi }\le 2.2\text{GeV}$, $|t|\le 1.5{\text{GeV}}^2$, and $20\le W_{\gamma p}\le 80\text{GeV}$. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $m_{\pi \pi }$ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the ${\rho }^0(770)$ meson mass and width at $m_{\rho }=770.8{}_{-2.7}^{+2.6}(\text{tot.})\text{MeV}$ and ${\mathrm{\Gamma }}_{\rho }=151.3{}_{-3.6}^{+2.7}(\text{tot.})\text{MeV}$, respectively. The model is used to extract the ${\rho }^0(770)$ contribution to the ${\pi }^{+}{\pi }^{-}$ cross sections and measure it as a function of t and $W_{\gamma p}$. In a Regge asymptotic limit in which one Regge trajectory $\alpha (t)$ dominates, the intercept $\alpha (t=0)=1.0654{}_{-0.0067}^{+0.0098}(\text{tot.})$ and the slope ${\alpha }^{\prime }(t=0)=0.233{}_{-0.074}^{+0.067}(\text{tot.}){\text{GeV}}^{-2}$ of the t dependence are extracted for the case $m_Y=m_p$.