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

TL;DR

This paper reports the discovery of new exotic hadronic states with a unique quark composition, including the $Z_{cs}(4000)^+$ and $X(4685)$, through amplitude analysis of $B^+$ decays in proton-proton collision data from the LHCb experiment.

Contribution

It presents the first observation of exotic states with $c ar{c} u ar{s}$ quark content decaying to $J/ar{ ext{}}$psi $K^+$, including new states and confirmation of previously reported ones.

Findings

Observation of $Z_{cs}(4000)^+$ with mass 4003 MeV and width 131 MeV.

Detection of a new $1^+$ $X(4685)$ state.

Confirmation of four known $J/ar{ ext{}}$psi $ o$ $ ext{phi}$ states and discovery of $Z_{cs}(4220)^+$ and $X(4630)$.

Abstract

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 high significance from an amplitude analysis of the $B^+ \to J/\psi \phi K^+$ decay. The analysis is carried out using proton-proton collision data corresponding to a total integrated luminosity of 9 fb$^{-1}$ collected by the LHCb experiment at 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, a width of $131\pm15\pm26$ MeV, and spin-parity $J^P=1^+$, where the quoted uncertainties are statistical and systematic, respectively. A new $1^+$ $X(4685)$ state decaying to the $J/\psi \phi$ final state is also observed with high 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.