Observation of e⁺e^- → ηΥ(2S) and search for e⁺e^- → ηΥ(1S), γX_bat √s near 10.75 GeV

We present an analysis of the processes e⁺e^- → ηΥ(1S), ηΥ(2S), and γX_b with X_b → π⁺ π^- χ_bJ, χ_bJ → γΥ(1S) (J = 1, 2) reconstructed from γγπ⁺π^- ℓ⁺ℓ^- (ℓ = e, μ) final states in 19.6 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^- → ηΥ(2S), we observe a signal with a significance that exceeds 6.0σ when all energy points are combined. A signal with a significance of 4.2σ is observed at a center-of-mass energy of 10.653 GeV, close to the B*B* threshold. At this energy, we measure σ(ηΥ(2S))/σ(π⁺π^- Υ(2S)) > 23 at the 90% confidence level, indicating a strong violation of heavy-quark spin symmetry. No significant signal is observed for e⁺e^- → ηΥ(1S) or γX_b. Upper limits on the Born cross sections for the processes e⁺e^- → ηΥ(1S) and e⁺e^- → γX_b with X_b → π⁺ π^- χ_bJ are determined.