Front. Phys. 10, 101401 (2015) DOI 10.1007/s11467-014-0449-6 REVIEW ARTICLE A new hadron spectroscopy Stephen Lars Olsen Center for Underground Physics, Institute of Basic Science, Daejeon 305-811, Korea E-mail:
[email protected] Received August 13, 2014; accepted September 16, 2014 QCD-motivated models for hadrons predict an assortment of “exotic” hadrons that have structures that are more complex than the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H-dibaryon, and tetraquark, hybrid and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have been identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed- anticharmed quark pairs or bottom-antibottom quark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD- inspired exotics. Here I briefly review results from a recent search for the H-dibaryon, and discuss some properties of the newly discovered states –the proton-antiproton state and the so-called XYZ mesons– and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states. Keywords quarks, charmonium, pentaquarks, di-baryons, baryonium, tetraquarks PACS numb ers 14.40.Gx, 12.39.Mk, 13.20.He Contents gluons are related to mesons and baryons by the long- distance regime of Quantum Chromodynamics (QCD), 1 Introduction 1 which remains the least understood aspect of the the- 2 Pentaquarks and H-dibaryons 2 ory.