GLUEBALLS, HYBRIDS, PENTAQUARKS: Introduction to Hadron Spectroscopy and Review of Selected Topics Eberhard Klempt Helmholtz–Institut f¨urStrahlen- und Kernphysik der Universit¨atBonn Nußallee 14 -16, 53115 Bonn, Germany E-mail: [email protected] Hadron spectroscopy has received revitalised interest due to the discovery of states with unexpected properties. The BABAR collaboration found a DsJ(2317) (likely scalar) me- son, accompanied by a second state, the DsJ(2463) with preferred spin J = 1, discovered at CLEO. Both are found at an unexpectedly low mass and are narrow. A further nar- row resonance, discovered at BELLE in its decay into ¼¼J/Ã, might be a DD¤ molecule or a cc¯ meson in which the color field, concentrated in a flux tube, is excited. A qq¯ state with excited gluon field is called hybrid, such excitations are expected from QCD. The mass of the state at the DD¤ threshold underlines the importance of meson–meson interactions or four–quark dynamics at the opening of new thresholds. BES reports a signal in radiative J/à decays into a proton and an antiproton which has the proper- ties as predicted for NN¯ quasi-nuclear bound states. And last not least the Θ+(1540), seen in several experiments, shows that the ’naive’ quark model needs to be extended. There is also considerable progress in understanding the dynamics of quarks in more conventional situations even though the view presented here is not uncontested. In me- son and in baryon spectroscopy, evidence is emerging that one–gluon–exchange does not provide the appropiate means to understand low–energy QCD; instanton–induced inter- actions yield much more insight. In particular the rich spectrum of baryon resonances is very well suited to test dynamical quark models using constituent quarks, a confine- ment potential plus some residual interactions. The baryon spectrum favors definitely instanton–induced interactions over long–range one–gluon exchange. A still controversial issue is the question if glueballs and hybrids exist. There is the possibility that two qq¯ scalar states and a glueball form three observed resonances by mixing. However, there is also rather conclusive evidence against this interpretation. Mesons with exotic quan- tum numbers have been reported but there is no reason why they should be hybrids: a four–quark interpretation is enforced for the ¼1(1400) and not ruled out in the other cases. This report is based on a lecture series which had the intent to indroduce young scien- tists into hadron spectroscopy. The attempt is made to transmit basic ideas standing behind some models, without any formulae. Of course, these models require (and de- serve) a much deeper study. However, it may be useful to explain in a simple language some of the ideas behind the formalisms. Often, a personal view is presented which is not shared by many experts working in the field. In the last section, the attempt is made to combine the findings into a picture of hadronic interactions and to show some of the con- sequences the picture entails and to suggest further experimental and theoretical work. 18th Annual Hampton University Graduate Studies Jefferson Lab, Newport News, Virginia June 2-20, 2003 1 Acknowledgments First of all I would like to thank the organizers for this nice workshop. I enjoyed the friendly atmosphere and the scientific environment provided by the Jefferson Labo- ratory, and I enjoyed giving lectures to an interested audiences of young scientists. It is a pleasure for me to acknowledge the numerous discussions with friends and colleagues from various laboratories and universities. In naming some of them I will undoubtedly forget some very important conversations; nevertheless I would like to mention fruitful discussions with R. Alkofer, A.V. Anisovich, V.V. Anisovich, C. Amsler, Chr. Batty, F. Bradamante, D. Bugg, S. Capstick, S.U. Chung, F.E. Close, D. Diakonov, W. Dunwoodie, St. Dytman, A. Dzierba, W. D¨unnweber, P. Eugenio, A. F¨assler,M. F¨assler,H. Fritzsch, U. Gastaldi, K. Goeke, D. Herzog, N. Isgur, K. K¨onigsmann,F. Klein, S. Krewald, H. Koch, G.D. Lafferty, R. Landua, D.B. Lichtenberg, M. Locher, R.S. Longacre, V.E. Markushin, A. Martin, U.-G. Meißner, C.A. Meyer, V. Metag, B. Metsch, L. Montanet, W. Ochs, M. Ostrick, Ph. Page, M. Pennington, K. Peters, H. Petry, M.V. Polyakov, J.M. Richard, A. Sarantsev, B. Schoch, E.S. Swanson, W. Schwille, A.P. Szczepaniak, J. Speth, L. Tiator, P. Tru¨ol,U. Thoma, Chr. Weinheimer, W. Weise, U. Wiedner, H. Willutzki, A. Zaitsev, and C. Zupancic. The work described in this report is partly based on experiments I had to pleasure to participate in. Over the time I have had the privilege to work with a large number of PhD students. The results were certainly not achieved without their unflagged enthusiasm for physics. I would like to mention O. Bartholomy, J. Brose, V. Crede, K.D. Duch, A. Ehmanns, I. Fabry, M. Fuchs, G. G¨artner,J. Junkersfeld, J. Haas, R. Hackmann, M. Heel, Chr. Heimann, M. Herz, G. Hilkert, I. Horn, B. Kalteyer, F. Kayser, R. Landua, J. Link, J. Lotz, M. Matveev, K. Neubecker, H. v.Pee, K. Peters, B. Pick, W. Povkov, J. Reifenr¨other,G. Reifenr¨other,J. Reinnarth, St. Resag, E. Sch¨afer,C. Schmidt, R. Schulze, R. Schneider, O. Schreiber, S. Spanier, Chr. Straßburger, J.S. Suh, T. Sczcepanek, U. Thoma, F. Walter, K. Wittmack, H. Wolf, R.W. Wodrich, M. Ziegler. Very special thanks go to my colleague Hartmut Kalinowsky with whom I have worked with jointly for more than 30 years. Anyone familiar with one of the ex- periments I was working on knows his contributions are invaluable to our common effort. To him my very personal thanks. 2 Contents 1 Getting started 7 1.1 Historical remarks . 7 Nuclear interactions . 7 Resonances in strong interactions . 7 Clebsch Gordan coefficients . 8 The particle zoo . 9 Color . 10 Units . 10 1.2 Mesons and their quantum numbers . 10 Mixing angles . 11 Mixing angles, examples . 12 The Gell-Mann-Okubo mass formula . 13 Naming scheme . 13 Regge trajectories . 13 1.3 Charmonium and bottonium . 15 Discovery of the J/à ........................... 15 Width of the J/à ............................. 16 The OZI rule and flavor tagging . 17 J/à decays to e+e¡ ........................... 18 Charmonium states in radiative decays . 19 Charmonium states inpp ¯ annihilation . 21 New players at Bejing and Cornell . 22 1.4 D and B mesons . 23 1.5 The new states . 24 The BABAR resonance . 24 The CLEO resonance . 25 The BELLE resonance . 25 The BES resonance . 26 Discussion . 27 1.6 Baryons . 28 The spatial wave function . 28 SU(3) and SU(6) . 29 Regge trajectories . 30 2 Particle decays and partial wave analysis 31 2.1 Particle decays . 31 The Argand circle . 33 The K–matrix . 33 Three-body decays . 35 The Dalitz plot . 36 2.2 Angular distributions . 37 Zemach formalism . 37 Helicity formalism . 37 The helicity formalism in photo–production processes . 39 3 2.3 Flavor structure of mesons . 42 Isoscalar coefficients for meson decays . 42 Fits . 44 3 Particles and their interaction 45 3.1 The particles: quark and leptons . 45 Leptons . 45 Quarks and their quantum numbers . 45 3.2 Quarks and leptons and their interactions . 46 The Standard Model and QCD . 46 From large energies to large distances . 48 Basic questions in strong QCD . 49 Modeling strong QCD . 50 Gluon exchange and the flux tube model . 50 Chiral symmetry and instanton–induced interactions . 50 The chiral soliton model . 52 Confinement . 53 3.3 Quark models for mesons . 53 The Godfrey-Isgur model . 54 Meson exchange between quarks ? . 55 The Bonn model . 55 3.4 Quark models for baryons . 56 The spatial wave function . 56 Quark model predictions . 59 3.5 Conclusions . 59 4 The quest for glueballs 61 4.1 Glueballs in “gluon-rich” processes . 61 4.2 E/¶ saga . 62 Short history of the ´(1440) . 62 The ´(1295) and the ´(1440) in γγ at LEP . 64 The ´(1295) and ´(1440) in pp¯ annihilation . 64 3 E=¶ decays in the P0 model . 65 Conclusions . 66 4.3 Glueball masses from the lattice . 67 4.4 The enigmatic scalar mesons . 67 Scalar mesons below 1 GeV . 69 Scalar mesons above 1 GeV . 69 4.5 Scalar mesons: interpretation . 70 The ‘narrow’ glueball . 70 The ‘narrow’ glueball scrutinized . 71 Evidence for a very wide glueball . 72 The ‘wide’ glueball scrutinized . 73 Is there no glueball ? . 75 4.6 Hybrids . 75 The ¼1(1370) . 76 4 The ¼1(1625) . 78 Higher-mass exotics . 78 The Fock-space expansion . 79 SU(3) relations . 79 Four-quark states in SU(3) . 80 4.7 Exotic(s) summary . 80 5 Baryon spectroscopy 89 5.1 N¤ and ∆¤ resonances . 89 Spin–orbit forces and the multiplet structure . 89 Regge trajectories . 89 Radial excitations . ..