Eur. Phys. J. C (2012) 72: 1869
https://doi.org/10.1140/epjc/s10052-012-1869-5

Regular Article - Experimental Physics

Exclusive electroproduction of two pions at HERA

¹ Argonne National Laboratory, Argonne, IL, 60439-4815, USA
² Andrews University, Berrien Springs, MI, 49104-0380, USA
³ INFN Bologna, Bologna, Italy
⁴ University and INFN Bologna, Bologna, Italy
⁵ Physikalisches Institut der Universität Bonn, Bonn, Germany
⁶ H.H. Wills Physics Laboratory, University of Bristol, Bristol, UK
⁷ Department of Physics, Panjab University, Chandigarh, India
⁸ Physics Department and INFN, Calabria University, Cosenza, Italy
⁹ Institute for Universe and Elementary Particles, Chonnam National University, Kwangju, South Korea
¹⁰ Jabatan Fizik, Universiti Malaya, 50603, Kuala Lumpur, Malaysia
¹¹ Nevis Laboratories, Columbia University, Irvington on Hudson, NY, 10027, USA
¹² The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland
¹³ Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology, Krakow, Poland
¹⁴ Department of Physics, Jagellonian University, Cracow, Poland
¹⁵ Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
¹⁶ Deutsches Elektronen-Synchrotron DESY, Zeuthen, Germany
¹⁷ INFN Florence, Florence, Italy
¹⁸ University and INFN Florence, Florence, Italy
¹⁹ Fakultät für Physik der Universität Freiburg i.Br., Freiburg i.Br., Germany
²⁰ School of Physics and Astronomy, University of Glasgow, Glasgow, UK
²¹ Department of Engineering in Management and Finance, Univ. of the Aegean, Chios, Greece
²² Institute of Experimental Physics, Hamburg University, Hamburg, Germany
²³ High Energy Nuclear Physics Group, Imperial College London, London, UK
²⁴ Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan
²⁵ Institute of Physics and Technology of Ministry of Education and Science of Kazakhstan, Almaty, Kazakhstan
²⁶ Institute for Nuclear Research, National Academy of Sciences, Kyiv, Ukraine
²⁷ Department of Nuclear Physics, National Taras Shevchenko University of Kyiv, Kyiv, Ukraine
²⁸ Center for High Energy Physics, Kyungpook National University, Daegu, South Korea
²⁹ Institut de Physique Nucléaire, Université Catholique de Louvain, Louvain-la-Neuve, Belgium
³⁰ Departamento de Física Teórica, Universidad Autónoma de Madrid, Madrid, Spain
³¹ Department of Physics, McGill University, Montréal, Québec, Canada H3A 2T8,
³² Faculty of General Education, Meiji Gakuin University, Yokohama, Japan
³³ Moscow Engineering Physics Institute, Moscow, Russia
³⁴ Institute of Nuclear Physics, Moscow State University, Moscow, Russia
³⁵ Max-Planck-Institut für Physik, München, Germany
³⁶ NIKHEF and University of Amsterdam, Amsterdam, Netherlands
³⁷ Physics Department, Ohio State University, Columbus, OH, 43210, USA
³⁸ Department of Physics, University of Oxford, Oxford, UK
³⁹ INFN Padova, Padova, Italy
⁴⁰ Dipartimento di Fisica dell’ Università and INFN, Padova, Italy
⁴¹ Department of Physics, Pennsylvania State University, University Park, PA, 16802, USA
⁴² Polytechnic University, Sagamihara, Japan
⁴³ Dipartimento di Fisica, Università ‘La Sapienza’ and INFN, Rome, Italy
⁴⁴ Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK
⁴⁵ Raymond and Beverly Sackler Faculty of Exact Sciences, School of Physics, Tel Aviv University, Tel Aviv, Israel
⁴⁶ Department of Physics, Tokyo Institute of Technology, Tokyo, Japan
⁴⁷ Department of Physics, University of Tokyo, Tokyo, Japan
⁴⁸ Department of Physics, Tokyo Metropolitan University, Tokyo, Japan
⁴⁹ Università di Torino and INFN, Torino, Italy
⁵⁰ Università del Piemonte Orientale, Novara, and INFN, Torino, Italy
⁵¹ Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7,
⁵² Physics and Astronomy Department, University College London, London, UK
⁵³ Faculty of Physics, University of Warsaw, Warsaw, Poland
⁵⁴ National Centre for Nuclear Research, Warsaw, Poland
⁵⁵ Department of Particle Physics and Astrophysics, Weizmann Institute, Rehovot, Israel
⁵⁶ Department of Physics, University of Wisconsin, Madison, WI, 53706, USA
⁵⁷ Department of Physics, York University, Toronto, Ontario, Canada M3J 1P3,

^* e-mail: levy@alzt.tau.ac.il

Received: 21 November 2011
Published online: 25 January 2012

Abstract

The exclusive electroproduction of two pions in the mass range 0.4<M _ππ <2.5 GeV has been studied with the ZEUS detector at HERA using an integrated luminosity of 82 pb⁻¹. The analysis was carried out in the kinematic range of 2<Q ²<80 GeV², 32<W<180 GeV and |t|<0.6 GeV², where Q ² is the photon virtuality, W is the photon-proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The two-pion invariant-mass distribution is interpreted in terms of the pion electromagnetic form factor, |F(M _ππ )|, assuming that the studied mass range includes the contributions of the ρ, ρ′ and ρ′′ vector-meson states. The masses and widths of the resonances were obtained and the Q ² dependence of the cross-section ratios σ(ρ′→ππ)/σ(ρ) and σ(ρ′′→ππ)/σ(ρ) was extracted. The pion form factor obtained in the present analysis is compared to that obtained in e ⁺ e ⁻→π ⁺ π ⁻.