Nonstandard heavy mesons and baryons: Experimental evidence

Abstract

Quantum Chromodynamics (QCD), the generally accepted theory for the strong interactions, describes the interactions between quarks and gluons. The strongly interacting particles that are seen in nature are hadrons, which are composites of quarks and gluons. Since QCD is a strongly coupled theory at distance scales that are characteristic of observable hadrons, there are no rigorous, first-principle methods to derive the spectrum and properties of the hadrons from the QCD Lagrangian, except for Lattice QCD simulations that are not yet able to cope with all aspects of complex and short-lived states. Instead, a variety of "QCD inspired" phenomenological models have been proposed. Common features of these models are predictions for the existence of hadrons with substructures that are more complex than the standard quark-antiquark mesons and the three quark baryons of the original quark model that provides a concise description of most of the low-mass hadrons. Recently, an assortment of candidates for non-standard multi-quark mesons, meson-gluon hybrids and pentaquark baryons that contain heavy (charm or bottom) quarks have been discovered. Here we review the experimental evidence for these states and make some general comparisons of their measured properties with standard quark-model expectations and predictions of various models for non-standard hadrons. We conclude that the spectroscopy of all but simplest hadrons is not yet understood.

Study of the X±(5568) state with semileptonic decays of the BsAbazov1, Abbott2, Acharya3 et al. 2018Phys. Rev. D321130View full textAdd to dashboardCite

“…However, for exotic mesons containing only the up, down and strange quarks it has been difficult to make a definitive experimental case for such exotic states, although some persuasive arguments have been made (for recent comprehensive discussions of exotic hadrons containing both light and heavy quarks, see Refs. [3][4][5][6]). Multiquark states that contain heavy quarks can be more recognizable owing to the distinctive decay structure of heavy quark hadrons.…”

Implications of heavy quark-diquark symmetry for excited doubly heavy baryons and tetraquarks

“…Since then numerous experiments have observed bottomonium and charmonium states, the so-called XYZ mesons, that do not fit into the conventional quark-antiquark potential model of quarkonium, for reviews see Refs. [18][19][20][21][22]. Some of these states such as Z + c (4430) [23,24] are charged and hence must exotic mesons.…”

A new look at the Y tetraquarks and $$\Omega _c$$ Ω c baryons in the diquark model

“…We have analysed the masses of the four lightest Y states, using two experimental scenarios proposed in [1,6] and the Table 4 and κ cq = 67 MeV. N 1 is the number of spin-1 "bad" diquarks, defined in Eq.…”

“…The exotic states, called X, Y, Z and P c , have been analysed in a number of theoretical models. They have been claimed to be hybrid charmonia, molecules, disguised charmonium a e-mail: ahmed.ali@desy.de b e-mail: Luciano.Maiani@cern.ch c e-mail: borisov@phys.msu.ru d e-mail: ishtiaqmusab@gmail.com e e-mail: muhammadjamil.aslam@gmail.com f e-mail: parkh@uniyar.ac.ru g e-mail: antonio.polosa@roma1.infn.it h e-mail: Abdur.Rehman@fuw.edu.pl states, or just coupled-channel or threshold effects; see [1][2][3][4][5][6] for recent reviews and the references therein. We concentrate on the alternative diquark-antidiquark interpretation, tetraquark in brief, introduced in [7,8] following the light pentaquark picture discussed in [9], which has the potential to include all exotic hadrons seen thus far in a single scheme.…”