Eur. Phys. J. C (2011) 71: 1534
https://doi.org/10.1140/epjc/s10052-010-1534-9

Review

Heavy quarkonium: progress, puzzles, and opportunities

¹ Physik-Department, Technische Universität München, James-Franck-Str. 1, 85748, Garching, Germany
² Budker Institute of Nuclear Physics, Novosibirsk, 630090, Russia
³ Novosibirsk State University, Novosibirsk, 630090, Russia
⁴ Cornell University, Ithaca, NY, 14853, USA
⁵ Physics Division, Lawrence Livermore National Laboratory, Livermore, CA, 94551, USA
⁶ Physics Department, University of California at Davis, Davis, CA, 95616, USA
⁷ High Energy Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL, 60439, USA
⁸ Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL, 60510, USA
⁹ Physics Department, Florida State University, Tallahassee, FL, 32306-4350, USA
¹⁰ Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany
¹¹ Indiana University, Bloomington, IN, 47405, USA
¹² Physics Department, Brookhaven National Laboratory, Upton, NY, 11973-5000, USA
¹³ Department of Physics and Astronomy, Wayne State University, Detroit, MI, 48201, USA
¹⁴ Laboratoire de l’Accélérateur Linéaire, IN2P3/CNRS and Université Paris-Sud 11, Centre Scientifique d’Orsay, BP 34, 91898, Orsay Cedex, France
¹⁵ GSI Helmholtzzentrum für Schwerionenforschung, 64291, Darmstadt, Germany
¹⁶ INFN Sezione di Torino, Via P. Giuria 1, 10125, Torino, Italy
¹⁷ Department of Physics, The Ohio State University, Columbus, OH, 43210, USA
¹⁸ Institut für Theoretische Physik, Universität Regensburg, 93040, Regensburg, Germany
¹⁹ INFN Sezione di Padova, Via Marzolo 8, 35131, Padova, Italy
²⁰ Università di Ferrara and INFN Sezione di Ferrara, Via del Paradiso 12, 44100, Ferrara, Italy
²¹ Institute of Nuclear Physics, Polish Academy of Sciences, Kraków, Poland
²² Università di Bari and INFN Sezione di Bari, Via Amendola 173, 70126, Bari, Italy
²³ Max Planck Institute for Physics, München, Germany
²⁴ Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK
²⁵ CCAST (World Laboratory), P.O. Box 8730, Beijing, 100190, China
²⁶ Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing, 100190, China
²⁷ Department of Physics, Peking University, Beijing, 100871, China
²⁸ Thomas Jefferson National Accelerator Facility, 12000 Jefferson Ave., Newport News, VA, 23606, USA
²⁹ Clermont Université, Université Blaise Pascal, CNRS-IN2P3, LPC, BP 10448, 63000, Clermont-Ferrand, France
³⁰ University of Cincinnati, Cincinnati, OH, 45221, USA
³¹ Laboratoire de Physique Théorique, Unité mixte de Recherche, CNRS, UMR 8627, Université de Paris-Sud, 91405, Orsay, France
³² LIP, Av. Elias Garcia 14, 1000-149, Lisbon, Portugal
³³ SLAC National Accelerator Laboratory, Stanford, CA, 94309, USA
³⁴ Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2G7,
³⁵ Institut für Kernphysik, Jülich Center for Hadron Physics, and Institute for Advanced Simulation, Forschungszentrum Jülich, 52425, Jülich, Germany
³⁶ Department of Physics and Astronomy, University of Hawaii, Honolulu, HI, 96822, USA
³⁷ Illinois Institute of Technology, Chicago, IL, 60616, USA
³⁸ Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA
³⁹ IPNO, Université Paris-Sud 11, CNRS/IN2P3, Orsay, France
⁴⁰ Centre de Physique Théorique, École Polytechnique, CNRS, 91128, Palaiseau, France
⁴¹ INFN Sezione di Milano, Via Celoria 16, 20133, Milano, Italy
⁴² CERN, 1211, Geneva 23, Switzerland
⁴³ Center for Cosmology, Particle Physics and Phenomenology, Université Catholique de Louvain, 1348, Louvain-la-Neuve, Belgium
⁴⁴ Department of Math and Science, Pratt Institute, 200 Willoughby Ave, ARC LL G-35, Brooklyn, NY, 11205, USA
⁴⁵ Instituto de Física, Universidade de São Paulo, C.P. 66318, 05315-970, São Paulo, SP, Brazil
⁴⁶ Department of Physics & Astronomy, Seoul National University, Seoul, Korea
⁴⁷ Institute for Theoretical and Experimental Physics, Moscow, 117218, Russia
⁴⁸ INFN Sezione di Roma, Piazzale Aldo Moro 2, 00185, Roma, Italy
⁴⁹ Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
⁵⁰ C.N. Yang Institute for Theoretical Physics, Stony Brook University, Stony Brook, NY, 11794-3840, USA
⁵¹ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100049, China
⁵² Instituto de Física Corpuscular (IFIC) and Departamento de Física Teórica, Centro Mixto Universitat de Valencia-CSIC, Doctor Moliner 50, 46100, Burjassot, Valencia, Spain
⁵³ Excellence Cluster ‘Universe’, Technische Universität München, Garching, Germany
⁵⁴ Dipartimento di Fisica, Università di Milano, 20133, Milano, Italy
⁵⁵ Department of Physics, Tohoku University, Sendai, 980-8578, Japan
⁵⁶ William I. Fine Theoretical Physics Institute, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN, 55455, USA

^* e-mail: bkh2@cornell.edu

Received: 28 October 2010
Published online: 8 February 2011

Abstract

A golden age for heavy-quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the B-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations at BESIII, the LHC, RHIC, FAIR, the Super Flavor and/or Tau–Charm factories, JLab, the ILC, and beyond. The list of newly found conventional states expanded to include h _c (1P), χ _c2(2P), , and η _b (1S). In addition, the unexpected and still-fascinating X(3872) has been joined by more than a dozen other charmonium- and bottomonium-like “XYZ” states that appear to lie outside the quark model. Many of these still need experimental confirmation. The plethora of new states unleashed a flood of theoretical investigations into new forms of matter such as quark–gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of , , and bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. Lattice QCD has grown from a tool with computational possibilities to an industrial-strength effort now dependent more on insight and innovation than pure computational power. New effective field theories for the description of quarkonium in different regimes have been developed and brought to a high degree of sophistication, thus enabling precise and solid theoretical predictions. Many expected decays and transitions have either been measured with precision or for the first time, but the confusing patterns of decays, both above and below open-flavor thresholds, endure and have deepened. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark–gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.