Hunting for the heavy quark spin symmetry partner of $Z_{cs}$

Abstract

The discovery of a charged strange hidden-charm state $Z_{cs}$(3985) implies another higher $Z^*_{cs}$ state coupling to $\bar{D}_s^{*-}D^{*0} + c.c$ under the heavy quark spin symmetry. In this paper we discuss a possible hunt for it with data taken at existing facilities. We point out a hint for $Z^*_{cs}$ in the data of $\bar{B}_s^0 \to J/\psi K^- K^+$ at LHCb, though weak, in line with the production mechanism of pentaquark $P_c$. We also study the triangular singularity which would possibly enhance the production of $Z^*_{cs}$ in electron-positron collision. Surprisingly, the production rate of $Z^*_{cs}$ is expected to be maximum at the $e^+ e^-$ center of mass energy of 4.648 GeV, which is lower than 4.681 GeV for $Z_{cs}$ due to the inverted coupling hierarchy of $D_{s1} \bar D K$ and $D_{s2} \bar D^* K$ in the triangle diagrams. Their bottom analogue under heavy quark flavor symmetry is also discussed. Our theoretical analysis would confront with future experiment of LHCb, BESIII, and Bell II.