nLab XYZ particle

Contents

matter field fermions (spinors, Dirac fields)

flavors of fundamental fermions in the standard model of particle physics:
generation of fermions	1st generation	2nd generation	3d generation
quarks ( $q$ )
up-type	up quark ( $u$ )	charm quark ( $c$ )	top quark ( $t$ )
down-type	down quark ( $d$ )	strange quark ( $s$ )	bottom quark ( $b$ )
leptons
charged	electron	muon	tauon
neutral	electron neutrino	muon neutrino	tau neutrino

bound states:
mesons	light mesons: pion ( $u d$ ) ρ-meson ( $u d$ ) ω-meson ( $u d$ ) f1-meson a1-meson	strange-mesons: ϕ-meson ( $s \bar s$ ), kaon, K-meson ( $u s$ , $d s$ ) eta-meson ( $u u + d d + s s$ ) charmed heavy mesons*: D-meson ( $u c$ , $d c$ , $s c$ ) J/ψ-meson ( $c \bar c$ )	bottom heavy mesons: B-meson ( $q b$ ) ϒ-meson ( $b \bar b$ )
baryons	nucleons: proton $(u u d)$ neutron $(u d d)$

hadrons (bound states of the above quarks)

bosinos:

dark matter candidates

Exotica

auxiliary fields

Idea

XYZ particles are charmonium-like exotic mesons which are increasingly being observed in experiment (typically in B-meson-experiments such as Belle, LHCb, , BaBar), but whch are hard to interpret as bound states in the traditional quark model (in theoretical physics) of quantum chromodynamics.

Review:

Stephen Godfrey and Stephen L. Olsen, The Exotic XYZ Charmonium-Like Mesons, Annual Review of Nuclear and Particle Science Vol. 58:51-73 (Volume publication date 23 November 2008) (arXiv:10.1146/annurev.nucl.58.110707.171145)
Xiang Liu, An overview of XYZ new particles, Chin. Sci. Bull. 59, 3815-3830 (2014) (arXiv:1312.7408)
Stephen Lars Olsen, The XYZ mesons: what they aren’t, EPJ Web of Conferences 202, 01003 (2019) (doi:10.1051/epjconf/201920201003)
Nora Brambilla, Simon Eidelman, Christoph Hanhart, Alexey Nefediev, Cheng-Ping Shen, Christopher E. Thomas, Antonio Vairo, Chang-Zheng Yuan, The XYZ states: experimental and theoretical status and perspectives, Physics Reports 2020 (arXiv:1907.07583)