TY - JOUR

T1 - The XYZ states: Experimental and theoretical status and perspectives

AU - Brambilla, Nora

AU - Eidelman, Simon

AU - Hanhart, Christoph

AU - Nefediev, Alexey

AU - Shen, Cheng Ping

AU - Thomas, Christopher E.

AU - Vairo, Antonio

AU - Yuan, Chang Zheng

N1 - Funding Information: We would like to express our gratitude to Jaume Tarrús Castellà and Ulf-G. Meißner for useful comments on the manuscript. This work is supported in part by National Natural Science Foundation of China (NSFC) under contract Nos. 11575017 , 11521505 , 11661141008 , 11761141009 , 11835012 , and 11975076 ; the Ministry of Science and Technology of China under Contract Nos. 2015CB856701 and 2018YFA0403902 ; Key Research Program of Frontier Sciences, CAS , Grant No. QYZDJ-SSW-SLH011 ; the CAS Center for Excellence in Particle Physics (CCEPP) ; the Russian Science Foundation (A.N. was supported by Grant No. 18-12-00226 ) and the Ministry of Science and Education of Russian Federation (S.E. was supported by grant 14.W03.31.0026 ); the NSFC and Deutsche Forschungsgemeinschaft (DFG) through funds provided to the Sino-German Collaborative Research Center “Symmetries and the Emergence of Structure in QCD” (NSFC Grant No. 11621131001 , DFG Grant No. TRR110 ); the DFG cluster of excellence “Origins” ; the U.K. Science and Technology Facilities Council (STFC) [grant number ST/P000681/1] . Publisher Copyright: © 2020 Elsevier B.V. All rights reserved.

PY - 2020/8/20

Y1 - 2020/8/20

N2 - The quark model was formulated in 1964 to classify mesons as bound states made of a quark–antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as XYZ states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the XYZ states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic XYZ hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges.

AB - The quark model was formulated in 1964 to classify mesons as bound states made of a quark–antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as XYZ states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the XYZ states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic XYZ hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges.

KW - B factories

KW - BES

KW - Effective field theories

KW - Exotic hadrons

KW - Lattice QCD

KW - LHC experiments

KW - Potential models

KW - Quarkonium

UR - http://www.scopus.com/inward/record.url?scp=85086016839&partnerID=8YFLogxK

U2 - 10.1016/j.physrep.2020.05.001

DO - 10.1016/j.physrep.2020.05.001

M3 - Review article

AN - SCOPUS:85086016839

VL - 873

SP - 1

EP - 154

JO - Physics Reports

JF - Physics Reports

SN - 0370-1573

ER -