The $XYZ$ states: experimental and theoretical status and perspectives

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 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.

Effective range expansion for narrow near-threshold resonances

“…The real parts of the loop functions provide range corrections in Eq. (7), which are taken into account by replacing in that equation…”

“…The experimental progress in the last two decades led to the discovery of many states in the charmonium and bottomonium mass range that are in conflict with the naive quarkmodel picture, for recent reviews see, for example, Refs. [1][2][3][4][5][6][7][8]. Among these exotic states, the first unconventional quarkonium-like one, the χ c1 (3872), also known as X(3872), discovered more than a decade ago by the Belle Collaboration [9] remains one of the most popular and controversial states in theoretical and experimental studies.…”

Double Exotic States Productions in Pion and Kaon Induced Reactions

“…The investigations of exotic states beyond the conventional hadrons are one of the most intriguing topics in hadron physics in the past two decades. Since the observations of X(3872) in 2003 [1], a series of new hadron states have been observed (see Refs [2][3][4][5][6][7][8][9][10][11] for recent reviews of experimental and theoretical status), which turns a new page of searching exotic states. Among these new hadron states, some of them are particular interesting, such as the series of Z c /Z cs [12][13][14][15][16][17][18] and P c /P cs states [19][20][21][22], which are good candidates of [9,23]and pentaquark states [24][25][26] respectively.…”

The lineshape of the compact fully heavy tetraquark

“…That challenges the conventional quark model and stimulates research enthusiasm on the so called exotic hadrons. Up to now, tens of exotic candidates have been observed by experimental collaborations (such as LHCb, BESIII, BelleII, JLab, CMS, ATLAS) and numerous studies [1][2][3][4][5][6][7][8][9][10] have been proposed for the understanding of their properties.…”