Heavy quarkonium: progress, puzzles, and opportunities

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. The plethora of newly-found quarkonium-like 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 c\bar{c}, b\bar{b}, and b\bar{c} bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. 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.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K. Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D. Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A. Petrov, P. Robbe, A. Vair

Incomplete fusion and cluster production in heavy-ion collisions at 30 MeV/nucleon

none21The total energy dissipated in central collisions has been measured for the system S-32 + Ni-58 at about 1 GeV incident energy. An event-by-event reconstruction of the atomic charge of the reaction products was performed by means of a 4-pi charged-particle detector. Two distinct classes of events were thus separated: one consistent with a "conventional" incomplete fusion-evaporation process, a second where three or more heavy fragments are produced. A subtraction of the evaporative component from the particle spectra at all angles allowed extraction of the excitation energy removed from the system by pre-equilibrium emission. The average excitation energies corresponding to the two different classes of events were determined. Comparisons with statistical model calculations as well as a multifragmentation model are presented.noneG. NEBBIA;J. A. RUIZ;D. FABRIS;G. VIESTI;R. H. BURCH;F. GRAMEGNA;G. PRETE;A. GIORNI;A. LLERES;J. B. VIANO;B. CHAMBON;B. CHEYNIS;A. DEMEYER;D. DRAIN;D. GUINET;X. C. HU;M. GONIN;K. HAGEL;J. B. NATOWITZ;R. WADA;P. L. GONTHIERG., Nebbia; J. A., Ruiz; D., Fabris; Viesti, Giuseppe; R. H., Burch; F., Gramegna; G., Prete; A., Giorni; A., Lleres; J. B., Viano; B., Chambon; B., Cheynis; A., Demeyer; D., Drain; D., Guinet; X. C., Hu; M., Gonin; K., Hagel; J. B., Natowitz; R., Wada; P. L., Gonthie