J/ψ azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 GeV per nucleon

F. Prino; B. Alessandro; C. Alexa; R. Arnaldi; M. Atayan; S. Beolè; V. Boldea; P. Bordalo; G. Borges; C. Castanier; J. Castor; B. Chaurand; B. Cheynis; E. Chiavassa; C. Cicalò; M. P. Comets; S. Constantinescu; P. Cortese; A. De Falco; N. De Marco; G. Dellacasa; A. Devaux; S. Dita; J. Fargeix; P. Force; M. Gallio; C. Gerschel; P. Giubellino; M. B. Golubeva; A. A. Grigoryan; S. Grigoryan; F. F. Guber; A. Guichard; H. Gulkanyan; M. Idzik; D. Jouan; T. L. Karavicheva; L. Kluberg; A. B. Kurepin; Y. Le Bornec; C. Lourenço; M. Mac Cormick; A. Marzari-Chiesa; M. Masera; A. Masoni; M. Monteno; A. Musso; P. Petiau; A. Piccotti; J. R. Pizzi; G. Puddu; C. Quintans; L. Ramello; S. Ramos; L. Riccati; H. Santos; P. Saturnini; E. Scomparin; S. Serci; R. Shahoyan; F. Sigaudo; M. Sitta; P. Sonderegger; X. Tarrago; N. S. Topilskaya; G. L. Usai; E. Vercellin; L. Villatte; N. Willis; T. Wu

doi:10.1140/epjc/s10052-009-0907-4

2022 Impact factor 4.4

Particles and Fields

Open Access

Eur. Phys. J. C (2009) 61: 853-858
https://doi.org/10.1140/epjc/s10052-009-0907-4

Regular Article - Experimental Physics

J/ψ azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 GeV per nucleon

F. Prino¹⁰^*, B. Alessandro¹⁰, C. Alexa³, R. Arnaldi¹⁰, M. Atayan¹², S. Beolè¹⁰, V. Boldea³, P. Bordalo⁶, G. Borges⁶, C. Castanier², J. Castor², B. Chaurand⁹, B. Cheynis¹¹, E. Chiavassa¹⁰, C. Cicalò⁴, M. P. Comets⁸, S. Constantinescu³, P. Cortese¹, A. De Falco⁴, N. De Marco¹⁰, G. Dellacasa¹, A. Devaux², S. Dita³, J. Fargeix², P. Force², M. Gallio¹⁰, C. Gerschel⁸, P. Giubellino¹⁰, M. B. Golubeva⁷, A. A. Grigoryan¹², S. Grigoryan¹², F. F. Guber⁷, A. Guichard¹¹, H. Gulkanyan¹², M. Idzik¹³, D. Jouan⁸, T. L. Karavicheva⁷, L. Kluberg⁵^,9, A. B. Kurepin⁷, Y. Le Bornec⁸, C. Lourenço⁵, M. Mac Cormick⁸, A. Marzari-Chiesa¹⁰, M. Masera¹⁰, A. Masoni⁴, M. Monteno¹⁰, A. Musso¹⁰, P. Petiau⁹, A. Piccotti¹⁰, J. R. Pizzi¹¹, G. Puddu⁴, C. Quintans⁶, L. Ramello¹, S. Ramos⁶, L. Riccati¹⁰, H. Santos⁶, P. Saturnini², E. Scomparin¹⁰, S. Serci⁴, R. Shahoyan⁶, F. Sigaudo¹⁰, M. Sitta¹, P. Sonderegger⁵, X. Tarrago⁸, N. S. Topilskaya⁷, G. L. Usai⁴, E. Vercellin¹⁰, L. Villatte⁸, N. Willis⁸ and T. Wu⁸

¹ Univ. del Piemonte Orientale, Alessandria and INFN-Torino, Italy
² LPC, Univ. Blaise Pascal and CNRS-IN2P3, Aubière, France
³ IFA, Bucharest, Romania
⁴ Univ. di Cagliari/INFN, Cagliari, Italy
⁵ CERN, Geneva, Switzerland
⁶ LIP, Lisbon, Portugal
⁷ INR, Moscow, Russia
⁸ IPN, Univ. de Paris-Sud and CNRS-IN2P3, Orsay, France
⁹ Laboratoire Leprince-Ringuet, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France
¹⁰ Univ. di Torino/INFN, Torino, Italy
¹¹ IPN, Univ. Claude Bernard Lyon-I and CNRS-IN2P3, Villeurbanne, France
¹² YerPhI, Yerevan, Armenia
¹³ Faculty of Physics and Applied Computer Science, AGH Univ., Cracow, Poland

^* e-mail: prino@to.infn.it

Received: 13 September 2008
Revised: 5 December 2008
Published online: 12 February 2009

Abstract

The J/ψ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/ψ mesons at SPS energies. Hence, the measurement of J/ψ elliptic anisotropy, quantified by the Fourier coefficient v ₂ of the J/ψ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/ψ suppression observed in Pb-Pb collisions. We present the measured J/ψ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v ₂ as a function of the collision centrality and of the J/ψ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p _T sub-samples. The extracted v ₂ values are significantly larger than zero for non-central collisions and are seen to increase with p _T.