Eur. Phys. J. C (2007) 49: 545-558
https://doi.org/10.1140/epjc/s10052-006-0139-9

Regular Article - Experimental Physics

A Measurement of the ψ^′ to J/ψ production ratio in 920 GeV proton-nucleus interactions

¹ NIKHEF, 1009 DB, Amsterdam, Netherlands
² Department ECM, Faculty of Physics, University of Barcelona, 08028, Barcelona, Spain
³ Institute for High Energy Physics, Beijing, 100039, P.R. China
⁴ Institute of Engineering Physics, Tsinghua University, Beijing, 100084, P.R. China
⁵ Institut für Physik, Humboldt-Universität zu Berlin, 12489, Berlin, Germany
⁶ Dipartimento di Fisica, Università di Bologna and INFN Sezione di Bologna, 40126, Bologna, Italy
⁷ Department of Physics, University of Cincinnati, Cincinnati, OH, 45221, USA
⁸ LIP Coimbra, 3004-516, Coimbra, Portugal
⁹ Niels Bohr Institutet, 2100, Copenhagen, Denmark
¹⁰ Institut für Physik, Universität Dortmund, 44221, Dortmund, Germany
¹¹ Joint Institute for Nuclear Research Dubna, 141980, Dubna, Moscow region, Russia
¹² DESY, 22603, Hamburg, Germany
¹³ Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany
¹⁴ Physikalisches Institut, Universität Heidelberg, 69120, Heidelberg, Germany
¹⁵ Department of Physics, University of Houston, Houston, TX, 77204, USA
¹⁶ Institute for Nuclear Research, Ukrainian Academy of Science, 03680, Kiev, Ukraine
¹⁷ J. Stefan Institute, 1001, Ljubljana, Slovenia
¹⁸ University of Ljubljana, 1001, Ljubljana, Slovenia
¹⁹ University of California, Los Angeles, CA, 90024, USA
²⁰ Lehrstuhl für Informatik V, Universität Mannheim, 68131, Mannheim, Germany
²¹ University of Maribor, 2000, Maribor, Slovenia
²² Institute of Theoretical and Experimental Physics, 117259, Moscow, Russia
²³ Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, 80805, München, Germany
²⁴ Dept. of Physics, University of Oslo, 0316, Oslo, Norway
²⁵ Fachbereich Physik, Universität Rostock, 18051, Rostock, Germany
²⁶ Fachbereich Physik, Universität Siegen, 57068, Siegen, Germany
²⁷ Institute for Nuclear Research, INRNE-BAS, Sofia, Bulgaria
²⁸ Universiteit Utrecht/NIKHEF, 3584 CB, Utrecht, The Netherlands
²⁹ DESY, 15738, Zeuthen, Germany
³⁰ Physik-Institut, Universität Zürich, 8057, Zürich, Switzerland
³¹ Dipartimento di Energetica, Università di Firenze and INFN Sezione di Bologna, Bologna, Italy
³² P.N. Lebedev Physical Institute, 117924, Moscow B-333, Russia
³³ Moscow Physical Engineering Institute, 115409, Moscow, Russia
³⁴ Moscow State University, 119899, Moscow, Russia
³⁵ Institute for High Energy Physics, Protvino, Russia
³⁶ High Energy Physics Institute, 380086, Tbilisi, Georgia

^* e-mail: Alexander.Spiridonov@desy.de

Received: 27 July 2006
Published online: 18 November 2006

Abstract

Ratios of the ψ^′ over the J/ψ production cross sections in the dilepton channel for C, Ti and W targets have been measured in 920 GeV proton-nucleus interactions with the HERA-B detector at the HERA storage ring. The ψ^′ and J/ψ states were reconstructed in both the μ⁺μ^- and the e⁺e^- decay modes. The measurements covered the kinematic range -0.35≤x_F≤0.1 with transverse momentum p_T≤4.5 GeV/c. The angular dependence of the ratio has been used to measure the difference of the ψ^′ and J/ψ polarization. All results for the muon and electron decay channels are in good agreement: their ratio, averaged over all events, is R_ψ′(μ)/R_ψ′(e)=1.00±0.08±0.04. This result constitutes a new, direct experimental constraint on the double ratio of branching fractions, (B^′(μ)B(e))/(B(μ)B^′(e)), of ψ^′ and J/ψ in the two channels. The ψ^′ to J/ψ production ratio is almost constant in the covered x_F range and shows a slow increase with p_T.