Search for a Dark Leptophilic Scalar in $e^{+}{e}^{-}$ Collisions

Many scenarios of physics beyond the standard model predict the existence of new gauge singlets, which might be substantially lighter than the weak scale. The experimental constraints on additional scalars with masses in the MeV to GeV range could be significantly weakened if they interact predominantly with leptons rather than quarks. At an $e^{+}{e}^{-}$ collider, such a leptophilic scalar (${\varphi }_L$) would be produced predominantly through radiation from a $\tau$ lepton. We report herein a search for $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}{\varphi }_L$, ${\varphi }_L\to {\ell }^{+}{\ell }^{-}$ ($\ell =e$, $\mu$) using data collected by the BABAR experiment at SLAC. No significant signal is observed, and we set limits on the ${\varphi }_L$ coupling to leptons in the range $0.04<m_{{\varphi }_L}<7.0\mathrm{GeV}$. These bounds significantly improve upon the current constraints, excluding almost entirely the parameter space favored by the observed discrepancy in the muon anomalous magnetic moment below 4 GeV at 90% confidence level.

Figure 1

The distribution of (top) the dielectron invariant mass and (bottom) the dimuon invariant mass for prompt decays, together with simulated predictions for the indicated processes normalized to the integrated luminosity of the data (stacked histograms).

Figure 2

The distribution of the dielectron invariant mass for the (top) $c{\tau }_{\varphi L}=1$, (middle) $c{\tau }_{\varphi L}=10$, and (bottom) $c{\tau }_{\varphi L}=100\mathrm{mm}$ samples, together with simulated predictions for the indicated processes normalized to the integrated luminosity of the data (stacked histograms).

Figure 3

The 90% C.L. upper limits on the $\sigma (e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}{\varphi }_L)$ cross section at the $\mathrm{\Upsilon }(4S)$ resonance derived from (top) the dielectron and (bottom) dimuon final states. The gray bands indicate the regions excluded from the search around the nominal $J/\psi$ and $\psi (2S)$ masses.

Figure 4

The 90% C.L. limits on the coupling $\xi$ as a function of the ${\varphi }_L$ mass (green shaded area), together with existing constraints [9, 10, 12, 13, 14] (gray shaded areas) and the parameter space preferred by the muon anomalous magnetic moment [3, 10] (red band).