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

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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.

Figures and captions

Invariant-mass distribution of selected $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \phi K ^+ $ candidates with the fit overlaid.

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Dalitz plots for $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \phi K ^+ $ candidates in a region $\pm$15 $\text{ Me V}$ around the $B^+$ mass peak.

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Distributions of $\phi K ^+ $ (left), $ { J \mskip -3mu/\mskip -2mu\psi } \phi$ (middle) and $ { J \mskip -3mu/\mskip -2mu\psi } K ^+ $ (right) invariant masses for the $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \phi K ^+ $ candidates (black data points) compared with the fit results (red solid lines) of the default model (top row) and the Run 1 model (bottom row).

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Projections of the fits with the default model, performed in the full phase-space, onto $m_{ { J \mskip -3mu/\mskip -2mu\psi } K ^+ }$ in two slices of $m_{ { J \mskip -3mu/\mskip -2mu\psi } \phi}$ with and without the $1^+$ $ Z_{cs}^+ $ states. The narrow $ Z_{cs}^+ $ state at 4 $\text{ Ge V}$ is evident.

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Fitted values of the $Z_{cs}(4000)^+$ amplitude in eight $m_{ { J \mskip -3mu/\mskip -2mu\psi } K ^+ }$ intervals, shown on an Argand diagram (black points). The red curve represents the expected Breit-Wigner behaviour between $-1.4\Gamma_0$ to $1.4\Gamma_0$ around the $Z_{cs}(4000)^+$ mass.

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Animated gif made out of all figures.

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Tables and captions

Fit results from the default amplitude model. The significances are evaluated accounting for total (statistical) uncertainties. The listed masses and widths without uncertainties are taken from PDG [14] and are fixed in the fit. The listed world averages of the two $K_2$ and $K^*(1680)$ resonances do not contain the contributions from the previous LHCb Run 1 results.

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Created on 19 April 2024.