Observation of ${\eta }_c(2S)\to p\bar{p}$ and search for $X(3872)\to p\bar{p}$ decays

Abstract

The first observation of the decay ${\eta }_c(2S)\to p\bar{p}$ is reported using proton-proton collision data corresponding to an integrated luminosity of $3.0\mathrm{f}{\mathrm{b}}^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ${\eta }_c(2S)$ resonance is produced in the decay $B^{+}\to [c\bar{c}]K^{+}$. The product of branching fractions normalised to that for the $J/\psi$ intermediate state, ${\mathcal{R}}_{{\eta }_c(2S)}$, is measured to be \begin{align*} {\cal R}_{η_{c}(2S)}\equiv\frac{{\mathcal B}(B^{+} \to η_{c}(2S) K^{+}) \times {\mathcal B}(η_{c}(2S) \to p \bar p)}{{\mathcal B}(B^{+} \to J/ψK^{+}) \times {\mathcal B}(J/ψ\to p \bar p)} =~& (1.58 \pm 0.33 \pm 0.09)\times 10^{-2}, \end{align*} where the first uncertainty is statistical and the second systematic. No signals for the decays $B^{+}\to X(3872)(\to p\bar{p})K^{+}$ and $B^{+}\to \psi (3770)(\to p\bar{p})K^{+}$ are seen, and the 95\% confidence level upper limits on their relative branching ratios are % found to be ${\mathcal{R}}_{X(3872)}<0.25\times 10^{-2}$ and ${\mathcal{R}}_{\psi (3770))}<0.10$. In addition, the mass differences between the ${\eta }_c(1S)$ and the $J/\psi$ states, between the ${\eta }_c(2S)$ and the $\psi (2S)$ states, and the natural width of the ${\eta }_c(1S)$ are measured as \begin{align*} M_{J/ψ} - M_{η_{c}(1S)} =~& 110.2 \pm 0.5 \pm 0.9 \rm \, MeV, M_{ψ(2S)} -M_{η_{c}(2S)} =~ & 52.5 \pm 1.7 \pm 0.6 \rm \, MeV, Γ_{η_{c}(1S)} =~& 34.0 \pm 1.9 \pm 1.3 \rm \, MeV. \end{align*}

16 pages, 2 figures All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2016-016.html

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