Understanding the enhanced $\varphi {\eta }^{\prime }$ decay mode of the $\varphi (2170)$ through strange-meson loops

Abstract

The nature of the strangeonium-like $\varphi (2170)$ remains controversial. Recent measurements of $e^{+}{e}^{-}\to \varphi {\eta }^{(\prime )}$ reveal a striking puzzle: a broad $\varphi (2170)$-like structure appears in the $\varphi {\eta }^{\prime }$ channel but not in $\varphi \eta$, despite the strong phase-space suppression of $\varphi {\eta }^{\prime }$. We investigate this unexpectedly large $\varphi {\eta }^{\prime }$ decay fraction within the excited-strangeonium assignment. A combined analysis of the $e^{+}{e}^{-}\to \varphi \eta$ and $e^{+}{e}^{-}\to \varphi {\eta }^{\prime }$ cross sections is performed by including short-distance amplitudes from vacuum quark-pair creation and long-distance transitions mediated by strange-meson loops. The short-distance mechanism predicts too small a $\varphi {\eta }^{\prime }/\varphi \eta$ strength ratio to explain the data. In contrast, strange-meson loops naturally enhance this ratio because the SU(3)-flavor factor at the $K^{(\ast )}{K}^{(\ast )}{\eta }^{(\prime )}$ vertex suppresses the transition to $\varphi \eta$ relative to $\varphi {\eta }^{\prime }$. The resulting overall description of both cross sections supports an important role for long-distance dynamics in hidden-strangeness decays of excited vector strangeonia. More intriguingly, although the excited strangeonium contributions have been included, the current high-precision $\varphi \eta$ data still favor the existence of an extra narrow vector state near $2.15\mathrm{GeV}$ with a width of about $25\mathrm{MeV}$. If confirmed, it would be a promising exotic-hadron candidate in the light-vector sector. Furthermore, we test $\Gamma (Y\to \varphi \eta )/\Gamma (Y\to \varphi {\eta }^{\prime })=0.25$, $1$, and $4$ as fit inputs and obtain similarly good descriptions of the cross sections in all three cases. The present data therefore do not allow this ratio to discriminate among different internal configurations of the narrow state.

Comments: 12 pages, 5 figures