$X(3872)$ and hidden charmed tetraquarks

Abstract

In a constituent quark model, a hidden charmed tetraquark is assumed consisting of a $cq$ diquark and an $\bar c\bar q$ antidiquark or vice versa. The Semay-Silvestre-Brac potentials are employed to calculate the masses of $cq$ (q=u, d) diquarks. The mass of the $cq$ diquark or $\bar c\bar q$ antidiquark with spin-$0$ is predicted with $\sim 2175$ MeV, and the spin-$1$ one is predicted with $\sim 2220$ MeV. The masses of hidden charmed tetraquarks from $1S$ to $2P$ excitations are systemically calculated in terms of the same potentials. It is found that the mass of hidden charmed tetraquark without radial excitation grows higher in $1^{+-},~1^{++},~1^{--},~0^{-+},~0^{--},~1^{-+},~\cdots$ sequence, and the tetraquarks with exotic $J^{PC}=0^{--},~1^{-+}$ have higher masses. The hidden charmed tetraquarks with radial excitations have masses larger than $4300$ MeV. The $1S-1P$ and $1S-2S$ mass splittings of the hidden charmed tetraquarks are about $390-400$ MeV and $550-570$ MeV, respectively, which are about $70$ MeV and $50$ MeV smaller than those of normal charmonium. The $1P-2P$ and $2S-2P$ mass splittings are similar to those for conventional $c\bar c$ charmonium mesons. Based on our predicted masses for hidden charmed tetraquarks, some XYZ exotics are analyzed and tentatively assigned. $X^*(3860)$ is possibly the $0^{++}$ tetraquark. $Z_c(3900)$ and $X(3940)$ are possibly the $1^{+-}$ tetraquarks, and $X(3872)$ is possibly a $1^{++}$ tetraquark. $X(4250)$ may be a $0^{-+}$, $0^{++}$ or $1^{-+}$ tetraquark, $X(4240)$ may be a $0^{--}$ tetraquark. With radial excitations, $X(4350)$ may be a $0^{++}$ tetraquark, $Z_c(4430)$ may be a $1^{+-}$ tetraquark, $X(4630)$ may be a $0^{-+}$ or $1^{-+}$ tetraquark, and $X(Y)(4660)$ may be the $1^{--}$ tetraquark. $Y(4008)$ or $Y(4390)$ seems impossibly the $1^{--}$ tetraquark.