Observation of $e^+e^-\toηΥ(2S)$ and search for $e^+e^-\toηΥ(1S),~γX_b$ at $\sqrt{s}$ near 10.75 GeV

TL;DR

This Belle II study searches for hadronic transitions and exotic bottomonium-like states in $e^+e^-$ collisions near 10.65–10.81 GeV. It reports a robust observation of $e^+e^- o ηΥ(2S)$ with significance >$6σ$ and shows that a simple resonance-only interpretation involving $Υ(5S)$ or $Υ(10753)$ is disfavored, hinting at a possible new state near the $B^{*} ar{B}^{*}$ threshold or substantial continuum production; a three-resonance fit to the Born cross sections (including a near-threshold state) describes the data better than two-resonance hypotheses. No apparent signals are found for $e^+e^- o ηΥ(1S)$ or $e^+e^- o γX_b$, and upper limits are placed on their Born cross sections. The results, including a notable production-ratio behavior near the $B^{*} ar{B}^{*}$ threshold that challenges heavy-quark-spin symmetry, contribute to the understanding of bottomonium-like structures and motivate further energy scans in this region.

Abstract

We present an analysis of the processes $e^{+}e^{-}\toηΥ(1S)$, $ηΥ(2S)$, and $γX_b$ with $X_b\toπ^+π^-χ_{bJ},~χ_{bJ}\toγΥ(1S)$ $(J=1,~2)$ reconstructed from $γγπ^+π^-\ell^+\ell^-~(\ell=e,~μ)$ final states in $19.6\rm~\rm fb^{-1}$ of Belle II data collected at four energy points near the peak of the $Υ(10753)$ resonance. Here, $X_b$ is a hypothetical bottomonium-sector partner of the $X(3872)$. In the process $e^{+}e^{-} \to ηΥ(2S)$, we observe a signal with a significance that exceeds $6.0σ$. Based on an analysis of the Born cross-section, the hypothesis that these events are associated solely with the production of the $Υ(5S)$ or $Υ(10753)$ resonances is rejected at the level of 3.6$σ$. This suggests that the observed events are more likely to arise from a new state near the $B^{*}\bar{B}^{*}$ threshold or from a substantial continuum production. No significant signal is observed for $e^{+}e^{-}\toηΥ(1S)$ or $γX_b$. Upper limits on the Born cross sections for the processes $e^{+}e^{-}\toηΥ(1S)$ and $e^{+}e^{-}\toγX_b$ with $X_b\toπ^+π^-χ_{bJ}$ are determined.

Figures (7)

• Figure 1: Two dimensional distribution of $M(\ell^+\ell^-\xspace)$ versus $M(\pi^+\pi^-\pi^0\xspace)$ (top) and $M(\pi^+\pi^-\xspace 焇(1S))$ versus $M'(\gamma\gamma)$ (bottom) from data. Horizontal lines represents the $焇(2S)$ signal (red) and sideband boundaries (blue).
• Figure 2: Simultaneous fit to $M(\pi^+\pi^-\pi^0\xspace)$ (top) and $M'(\gamma\gamma)$ (bottom) from each energy point. Plots in the same column correspond to the same scan energy. Dots with error bars are experimental data, the red solid lines represent the fit result, and the green histograms represent events from the $焇(2S)$ sideband.
• Figure 3: Born cross-sections for $e^+e^-\xspace\rightarrow\xspace\eta 焇(2S)$ with fit results overlaid. Points with error bars show measured cross sections, the solid curve is the nominal fit result with $焇(5S)$, $焇(10753)$ and a state near $B^*\bar{B}^*$ threshold, the green dashed curve is the fit result with $焇(5S)$ and $焇(10753)$, and the blue dashed curve is the fit result with only $焇(5S)$. The new state near the $B^*\bar{B}^*$ threshold is decoupled from the other two states, with only its amplitude contributing to the fit.
• Figure 4: Simultaneous fit to $M(\gamma\gamma)$ and $M(\pi^+\pi^-\pi^0)$ in the data sample where all four energy points are combined; the green histogram shows events from the $焇(2S)$ sidebands.
• Figure 5: $\Delta(-2\ln\mathcal{L})$ as a function of the Born cross-section in the fit to the $e^+e^-\rightarrow\xspace\eta 焇(2S)$ signal yield.