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1. Theoretical analysis of the doubly radiative decays ${\eta }^{(\prime )}\to {\pi }^0\gamma \gamma$ and ${\eta }^{\prime }\to \eta \gamma \gamma$

   https://doi.org/10.1103/PhysRevD.102.034026  

   Escribano, Rafel; Gonzàlez-Solís, Sergi; Jora, Renata; Royo, Emilio (August 2020, Physical Review D)

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2. Observation of $e^{+}{e}^{-}\to \gamma {\chi }_{c1}$ and search for $e^{+}{e}^{-}\to \gamma {\chi }_{c0}$ , $\gamma {\chi }_{c2}$ , and $\gamma {\eta }_c$ at $\sqrt{s}$ near 10.6 GeV at Belle

   https://doi.org/10.1103/PhysRevD.98.092015  

   Jia, S.; Wang, X. L.; Shen, C. P.; Yuan, C. Z.; Adachi, I.; Aihara, H.; Al Said, S.; Asner, D. M.; Atmacan, H.; Aulchenko, V.; et al (November 2018, Physical Review D)
3. Observation of $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(2S\right)$ and search for $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(1S\right)$ , $\gamma X_b$ at $\sqrt{s}$ near 10.75 GeV

   https://doi.org/10.1103/9f61-rmvx  

   Adachi, I; Aggarwal, L; Ahmed, H; Ahn, Y; Aihara, H; Akopov, N; Alghamdi, S; Alhakami, M; Aloisio, A; Althubiti, N; et al (March 2026, Physical Review D)

   We present an analysis of the processes $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(1S\right)$ , $\eta \mathrm{\Upsilon }\left(2S\right)$ , and $\gamma X_b$ with $X_b\to {\pi }^{+}{\pi }^{-}{\chi }_{bJ},{\chi }_{bJ}\to \gamma \mathrm{\Upsilon }\left(1S\right)$ ( $J=1$ , 2) reconstructed from $\gamma \gamma {\pi }^{+}{\pi }^{-}{\ell }^{+}{\ell }^{-}$ $(\ell =e,\mu )$ final states in $19.6{\mathrm{fb}}^{-1}$ of Belle II data collected at four energy points near the peak of the $\mathrm{\Upsilon }\left(10753\right)$ resonance. Here, $X_b$ is a hypothetical bottomonium-sector partner of the $X\left(3872\right)$ . In the process $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(2S\right)$ , 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 $B^{*}{\overline{B}}^{*}$ threshold. At this energy, we measure $\sigma \left(\eta \mathrm{\Upsilon }\right(2S\left)\right)/\sigma \left({\pi }^{+}{\pi }^{-}\mathrm{\Upsilon }\right(2S\left)\right)>23$ at the 90% confidence level, indicating a strong violation of heavy-quark spin symmetry. No significant signal is observed for $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(1S\right)$ or $\gamma X_b$ . Upper limits on the Born cross sections for the processes $e^{+}{e}^{-}\to \eta \mathrm{\Upsilon }\left(1S\right)$ and $e^{+}{e}^{-}\to \gamma X_b$ with $X_b\to {\pi }^{+}{\pi }^{-}{\chi }_{bJ}$ are determined. 

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4. Search for the ${\eta }_{c2}\left(1D\right)$ in $e^{+}{e}^{-}\to \gamma {\eta }_{c2}\left(1D\right)$ at $\sqrt{s}$ near 10.6 GeV at Belle

   https://doi.org/10.1103/PhysRevD.104.012012  

   Jia, S.; Shen, C. P.; Adachi, I.; Aihara, H.; Al Said, S.; Asner, D. M.; Aushev, T.; Ayad, R.; Babu, V.; Behera, P.; et al (July 2021, Physical Review D)
5. Observation of an alternative ${\chi }_{c0}\left(2P\right)$ candidate in $e^{+}{e}^{-}\to J/\psi D\overline{D}$

   https://doi.org/10.1103/PhysRevD.95.112003  

   Chilikin, K.; Adachi, I.; Aihara, H.; Al Said, S.; Asner, D. M.; Aulchenko, V.; Ayad, R.; Babu, V.; Badhrees, I.; Bakich, A. M.; et al (June 2017, Physical Review D)

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