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

TL;DR

This LHCb study reports the first observation of ηc(2S) → p pbar in B+ decays and performs a search for X(3872) → p pbar and ψ(3770) → p pbar. Using 3 fb−1 of data, they measure the relative branching fraction of B+ → ηc(2S) K+ to B+ → J/ψ K+ and J/ψ → p pbar, normalised to the J/ψ channel, and extract ηc(2S) signal yields from a p pbar mass spectrum analysis that accounts for interference with a non-resonant component. They set upper limits on X(3872) and ψ(3770) decays to p pbar, and provide precise mass-difference and width measurements for ηc states, along with a visible branching fraction for the ηc(2S) channel. The results contribute to charmonium spectroscopy by refining the ηc(2S) properties and constraining exotic-state decays to p pbar, with systematic uncertainties carefully catalogued and mitigated through data-driven calibrations and ratio techniques.

Abstract

The first observation of the decay $η_{c}(2S) \to p \bar p$ is reported using proton-proton collision data corresponding to an integrated luminosity of $3.0\rm \, fb^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The $η_{c}(2S)$ resonance is produced in the decay $B^{+} \to [c\bar c] K^{+}$. The product of branching fractions normalised to that for the $J/ψ$ intermediate state, ${\cal R}_{η_{c}(2S)}$, 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 $B^{+} \to ψ(3770) (\to p \bar p) K^{+}$ are seen, and the 95\% confidence level upper limits on their relative branching ratios are % found to be ${\cal R}_{X(3872)}<0.25\times10^{-2}$ and ${\cal R}_{ψ(3770))}<0.10$. In addition, the mass differences between the $η_{c}(1S)$ and the $J/ψ$ states, between the $η_{c}(2S)$ and the $ψ(2S)$ states, and the natural width of the $η_{c}(1S)$ 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*}

Figures (2)

• Figure 1: Invariant mass spectrum of the $p \bar{p} K^+$ candidates. The total fit curve and individual fit components are superimposed on the data.
• Figure 2: Invariant mass spectrum of the $p \bar{p}$ candidates. Background in the $B^{+} \rightarrow\xspace p \bar{p} K^{+}$ distribution is subtracted using the sPlot technique as described in the text. The total fit curve is superimposed.