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@ARTICLE{Brambilla:878465,
      author       = {Brambilla, Nora and Eidelman, Simon and Hanhart, Christoph
                      and Nefediev, Alexey and Shen, Cheng-Ping and Thomas,
                      Christopher E. and Vairo, Antonio and Yuan, Chang-Zheng},
      title        = {{T}he {X} {Y} {Z} states: {E}xperimental and theoretical
                      status and perspectives},
      journal      = {Physics reports},
      volume       = {873},
      issn         = {0370-1573},
      address      = {Amsterdam [u.a.]},
      publisher    = {Elsevier Science, North-Holland},
      reportid     = {FZJ-2020-02870},
      pages        = {1 - 154},
      year         = {2020},
      abstract     = {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
                      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 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 hadrons over the past few
                      years both in experiments and theory. It concludes with a
                      summary on future prospects and challenges.},
      cin          = {IAS-4 / IKP-3},
      ddc          = {530},
      cid          = {I:(DE-Juel1)IAS-4-20090406 / I:(DE-Juel1)IKP-3-20111104},
      pnm          = {511 - Computational Science and Mathematical Methods
                      (POF3-511) / DFG project 196253076 - TRR 110: Symmetrien und
                      Strukturbildung in der Quantenchromodynamik (196253076)},
      pid          = {G:(DE-HGF)POF3-511 / G:(GEPRIS)196253076},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000572119400001},
      doi          = {10.1016/j.physrep.2020.05.001},
      url          = {https://juser.fz-juelich.de/record/878465},
}