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The first observation of exotic states with a new quark content $c \bar{c} u\bar{s}$ decaying to the $J/\psi K^+$ final state is reported with highsignificance from an amplitude analysis of the $B^+ \to J/\psi \phi K^+$ decay.The analysis is carried out using proton-proton collision data corresponding toa total integrated luminosity of 9 fb$^{-1}$ collected by the LHCb experimentat centre-of-mass energies of 7, 8 and 13 TeV. The most significant state,$Z_{cs}(4000)^+$, has a mass of $4003\pm6\,^{+\,\phantom{0}4}_{-\,14}$ MeV, awidth of $131\pm15\pm26$ MeV, and spin-parity $J^P=1^+$, where the quoteduncertainties are statistical and systematic, respectively. A new $1^+$$X(4685)$ state decaying to the $J/\psi \phi$ final state is also observed withhigh significance. In addition, the four previously reported $J/\psi \phi$states are confirmed and two more exotic states, $Z_{cs}(4220)^+$ and$X(4630)$, are observed with significance exceeding five standard deviations.

Observation of new resonances decaying to $J/\psi K^+$ and $J/\psi \phi$