Eur. Phys. J. C (2009) 60: 517-524
https://doi.org/10.1140/epjc/s10052-009-0957-7

Regular Article - Experimental Physics

Angular distributions of leptons from J/ψ’s produced in 920 GeV fixed-target proton-nucleus collisions

¹ NIKHEF, 1009 DB, Amsterdam, The Netherlands
² Department ECM, Faculty of Physics, University of Barcelona, 08028, Barcelona, Spain
³ Institute for High Energy Physics, Beijing, 100039, China
⁴ Institute of Engineering Physics, Tsinghua University, Beijing, 100084, China
⁵ Institut für Physik, Humboldt-Universität zu Berlin, 12489, Berlin, Germany
⁶ Dipartimento di Fisica dell’ 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, 117218, Moscow, Russia
²³ Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, 80805, München, Germany
²⁴ Department 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

^* e-mail: pietro.faccioli@cern.ch

Received: 9 January 2009
Revised: 11 February 2009
Published online: 26 February 2009

Abstract

A study of the angular distributions of leptons from decays of J/ψ’s produced in p-C and p-W collisions at has been performed in the J/ψ Feynman-x region −0.34<x _F <0.14 and for J/ψ transverse momenta up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3×10⁵ J/ψ’s reconstructed in both the e ⁺ e ⁻ and μ ⁺ μ ⁻ decay channels, indicate that J/ψ’s are produced polarized. The magnitude of the effect is maximal at low p _T . For p _T >1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises.