Sensitivity of the η(′)→π0γγ and η′→ηγγ decays to a sub-GeV leptophobic U(1)B boson

Abstract

The sensitivity of the rare decays ${\ensuremath{\eta}}^{(\ensuremath{'})}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\eta}\ensuremath{\gamma}\ensuremath{\gamma}$ to signatures of a leptophobic $B$ boson in the MeV--GeV mass range is analyzed in this work. By adding an explicit $B$-boson resonance exchange, $\ensuremath{\eta}\ensuremath{\rightarrow}B\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$, to the Standard Model contributions from vector and scalar meson exchanges, and employing experimental data for the associated branching ratios, it allows us to improve the current constraints on the $B$-boson mass ${m}_{B}$ and coupling to Standard Model particles ${\ensuremath{\alpha}}_{B}$. From these constraints and the analysis of the available experimental $\ensuremath{\gamma}\ensuremath{\gamma}$ invariant mass distribution, we show that a $B$-boson signature in the resonant mass range ${m}_{{\ensuremath{\pi}}^{0}}\ensuremath{\lesssim}{m}_{B}\ensuremath{\lesssim}{m}_{\ensuremath{\eta}}$ is strongly suppressed and would be very difficult to experimentally identify, assuming that the leptophobic $B$ boson only decays to Standard Model particles. In contrast, the limits outside this mass window are less stringent and the corresponding $t$- and $u$-channel signatures may still be observable in the data, as it occurs with the nonresonant Standard Model $\ensuremath{\rho}$, $\ensuremath{\omega}$ and $\ensuremath{\phi}$ meson exchanges. In addition, we make use of experimental data from the ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\gamma}\ensuremath{\gamma}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\eta}\ensuremath{\gamma}\ensuremath{\gamma}$ decays to explore larger $B$-boson masses. Our results are relevant for the $B$-boson search programs at existing and forthcoming light-meson facilities, such as KLOE(-II) and Jefferson Lab Eta Factory experiments.