The hidden-charm pentaquark and tetraquark states

Abstract

In the past decade many charmonium-like states were observed experimentally. Especially those charged charmoniumlike Z c states and bottomonium-like Z b states can not be accommodated within the naive quark model. These charged Z c states are good candidates of either the hidden-charm tetraquark states or molecules composed of a pair of charmed mesons. Recently, the LHCb Collaboration discovered two hidden-charm pentaquark states, which are also beyond the quark model. In this work, we review the current experimental progress and investigate various theoretical interpretations of these candidates of the multiquark states. We list the puzzles and theoretical challenges of these models when confronted with the experimental data. We also discuss possible future measurements which may distinguish the theoretical schemes on the underlying structures of the hidden-charm multiquark states.

Baryons as relativistic three-quark bound states

“…Such exotic states are heavily studied in the meson sector, where a number of candidates for qqqq states have been detected in recent experiments such as Belle, BABAR, BES and LHCb, see e.g. [282,283] for recent reviews. In the baryon sector, five-quark states are of considerable interest especially since the experimental signature of a potential pentaquark has been reported at LHCb [2].…”

Threshold and Kinematic Effects for the Exotic Resonance-Like Structures

“…This result immediately initiated a lot of theoretical studies and revived the interests in pentaquark baryons (see e.g. recent reviews of the relevant topics [40,41]). …”

The Mass and Current-Meson Coupling Constant of the Exotic X(3872) State from QCD Sum Rules

“…Later the X(3872) was observed also through direct production mode in pp annihilation [2,3], as well as in the e + e − annihilation and pp collisions [13]. The assigned quantum numbers to X state are 1 ++ , its average mass extracted from different experiments is m X = (3871.69 ± 0.17) MeV , and the decay width was estimated as Γ < 1.2 MeV [10,19,20].…”

“…The D0 Collaboration measured its mass, width of the dominant decay channel X(5568) → B s π, and studied its quantum numbers considering J PC = 0 ++ as preferable ones. The observed and studied exotic states are collected into the XY Z family of new particles (see, the reviews [6][7][8][9][10][11][12][13] and references therein).…”