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Observation of e+e- -> eta Upsilon(2S) and search for e+e- -> eta Upsilon(1S), gamma X_b at sqrt(s) near 10.75 GeV

Physical Review Letters. 2026. Vol. D. No. 113. Article L051102.
Uglov T., Pakhlov P., Bodrov D., Aushev Т., Adachi I.

We present an analysis of the processes e+ e- -> eta Upsilon(1S), eta Upsilon(2S), and gamma X_b with X_b -> pi+ pi- chi_bJ, chi_bJ -> gamma Upsilon(1S) (J = 1, 2) reconstructed from gamma gamma pi+ pi- l+ l- (l = e, mu) final states in 19.6 fb^-1 of Belle II data collected at four energy points near the peak of the Upsilon(10753) resonance. Here, X_b is a hypothetical bottomonium-sector partner of the X(3872). In the process e+ e- -> eta Upsilon(2S), we observe a signal with a significance that exceeds 6.0 sigma when all energy points are combined. A signal with a significance of 4.2 sigma is observed at a center-of-mass energy of 10.653 GeV, close to the B* B*bar threshold. At this energy, we measure sigma(eta Upsilon(2S)) / sigma(pi+ pi- Upsilon(2S)) > 23 at the 90% confidence level, indicating a strong violation of heavy-quark spin symmetry. No significant signal is observed for e+ e- -> eta Upsilon(1S) or gamma X_b. Upper limits on the Born cross sections for the processes e+ e- -> eta Upsilon(1S) and e+ e- -> gamma X_b with X_b -> pi+ pi- chi_bJ are determined.

Research target: Physics
Language: English
Full text
DOI
Keywords: gamma Upsilon
Publication based on the results of:
Search for New physics in the experiments on e+e- colliders (2025)
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