${\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525)$; $f_2^{\prime }(1525)\to K_{\mathrm{S}}^0{K}_{\mathrm{S}}^0$ decays

We report on a study of exclusive radiative decays of the ${\rm Y}(1\mathrm{S})$ resonance into a final state consisting of a photon and two $K_{\mathrm{S}}^0$ candidates. We find evidence for a signal for ${\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525)$; $f_2^{\prime }(1525)\to \gamma K_{\mathrm{S}}^0{K}_{\mathrm{S}}^0$, at a rate $\mathcal{B}({\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525))=(4.0\pm 1.3\pm 0.6)\times {10}^{-5}$, consistent with previous observations of ${\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525)$; $f_2^{\prime }(1525)\to K^{+}{K}^{-}$, and isospin. Combining this branching fraction with existing branching fraction measurements of ${\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525)$ and $J/\psi \to \gamma f_2^{\prime }(1525)$, we obtain the ratio of branching fractions: $\mathcal{B}({\rm Y}(1\mathrm{S})\to \gamma f_2^{\prime }(1525))/\mathcal{B}(J/\psi \to \gamma f_2^{\prime }(1525))=0.09\pm 0.02$, approximately consistent with expectations based on soft-collinear effective theory.

Figure 1

$K_{\mathrm{S}}^0{K}_{\mathrm{S}}^0$ invariant mass vs (Total visible energy—center-of-mass energy) for events satisfying overall momentum conservation. Acceptance region is bounded by vertical lines.

Figure 2

Invariant mass of $K_{\mathrm{S}}^0{K}_{\mathrm{S}}^0$ candidates for events satisfying all energy, momentum, and photon selection requirements, showing signal as well as background estimators from the continuum and also $K_{\mathrm{S}}^0$ sidebands. Also overlaid is the fit to the relativistic, spin-2 Breit-Wigner signal shape. Sideband and continuum contributions have not been explicitly subtracted, and are implicitly included in our background parametrization.