An updated review of the new hadron states

Abstract

The past decades witnessed the golden era of hadron physics. Many excited open heavy flavor mesons and baryons have been observed since 2017. We shall provide an updated review of the recent experimental and theoretical progresses in this active field. Besides the conventional heavy hadrons, we shall also review the recently observed open heavy flavor tetraquark states X(2900) and T + cc (3875) as well as the hidden heavy flavor multiquark states X(6900), P cs (4459) 0 , Z cs (3985) − , Z cs (4000) + , and Z cs (4220) + . We will also cover the recent progresses on the glueballs and light hybrid mesons, which are the direct manifestations of the non-Abelian SU (3) gauge interaction of the Quantum Chromodynamics in the low-energy region.

Glueball molecules

“…A possible (if small) mixture of a vector 1 −− glueball state G V with a vector cc state [17] could provide a glueball molecular component πG V of the X(3872) state containing no charmed quarks at all. This follows if the mass of the vector glueball is m GV ≈ 3.8 GeV, as suggested by some lattice QCD studies [3]. Such molecular states would also be different from the hybrid meson states, which can also be described in the framework of effective field theory [18].…”

“…Since the bulk of theoretical predictions points to the glueball mass values of m G ∼ 1.6 − 1.7 GeV [1][2][3], it is natural to assume that the mass of the molecular state should be around m P ≈ 1.8 GeV. Interestingly, the state π(1800) has the correct quantum numbers and could be seen as a candidate for the glueball molecular state.…”

“…Despite various predictions for their properties, no candidates for the pure glueball states have been experimentally observed, even in such glue-rich environments as nuclear collisions. Comprehensive reviews of the current state of theoretical calculations of glueball properties and their experimental searches can be found in [1][2][3].…”

$Σ_cΣ_c$ interactions in chiral effective field theory

“…However, since the discoveries of the D * s0 (2317) [3] and X(3872) [4] in 2003, a large number of states that cannot be classified into conventional hadrons are continuously reported [5]. Different pictures have been proposed to understand the nature of these exotic states, such as the compact multiquark states, loosely bound hadronic molecules, and kinetic effects, etc [6][7][8][9][10][11][12][13][14][15]. Many exotic states are near the thresholds of a pair of heavy hadrons from several to several tens MeVs, which makes the hadronic molecular picture the popular one.…”

Newly observed $a_0(1817)$ as the scaling point of constructing the scalar meson spectroscopy

“…In the hadron zoo, there exist different hadron configurations including the conventional meson and baryon, multiquark state, hybrid and glueball [1][2][3]. Due to the observations of the charmoniumlike XYZ states and P c states, big progress in the search of the multiquark states has been made in the past two decades [4][5][6][7]. Early this year, the reported η 1 (1855) in the J/ψ → γηη decay from the BESIII Collaboration aroused theorists' growing interest in the hybrid meson [8,9].…”