Pentaquark and Tetraquark States

The past seventeen years have witnessed tremendous progress on the experimental and theoretical explorations of the multiquark states. The hidden-charm and hidden-bottom multiquark systems were reviewed extensively in Ref. [1]. In this article, we shall update the experimental and theoretical efforts on the hidden heavy flavor multiquark systems in the past three years. Especially the LHCb collaboration not only confirmed the existence of the hidden-charm pentaquarks but also provided strong evidence of the molecular picture. Besides the well-known XYZ and P_c states, we shall discuss more interesting tetraquark and pentaquark systems either with one, two, three or even four heavy quarks. Some very intriguing states include the fully heavy exotic tetraquark states QQ Q ¯ Q ¯ and doubly heavy tetraquark states QQ q ¯ q ¯ , where Q is a heavy quark. The QQ Q ¯ Q ¯ states may be produced at LHC while the QQ q ¯ q ¯ system may be searched for at BelleII and LHCb. Moreover, we shall pay special attention to various theoretical schemes such as the chromomagnetic interaction (CMI), constituent quark model, meson exchange model, heavy quark and heavy diquark symmetry, QCD sum rules, Faddeev equation for the three body systems, Skyrme model and the chiral quark-soliton model, and the lattice QCD simulations. We shall emphasize the model-independent predictions of various models which are truly/closely related to Quantum Chromodynamics (QCD). For example, the chromomagnetic interaction arises from the gluon exchange which is fundamental and universal in QCD and responsible for the mixing and mass splitting of the conventional mesons and baryons within the same multiplet. The same CMI mechanism shall also be responsible for the mixing of the different color configurations and mass splittings within the multiplets in the multiquark sector. There have also accumulated many lattice QCD simulations through multiple channel scattering on the lattice in recent years, which provide deep insights into the underlying structure/dynamics of the XYZ states. In terms of the recent P_c states, the lattice simulations of the charmed baryon and anti-charmed meson scattering are badly needed. We shall also discuss some important states which may be searched for at BESIII, BelleII and LHCb in the coming years.