Exotic hadrons with heavy flavors: X, Y, Z, and related states

In the past decade, exotic hadrons with charm and bottom flavors have been extensively studied both in experiments and in theories. In this review, we provide topical discussions by selecting $X,Y,Z$ particles, to which Belle has made important contributions. These are $X(3872)$, $Y(4260)$, $Z_c(4430)^+$, $Z_c(3900)^+$, $Z_{b}(10610)^+$, and $Z_{b}(10650)^+$. Based on the current experimental observations, we discuss these states with emphasis on the hadronic molecule whose dynamics is governed by chiral symmetry and heavy-quark symmetry of QCD. We also mention briefly various interpretations and some theoretical predictions for the as yet undiscovered exotic hadrons.