Search for Invisible Decays of Sub-GeV Dark Photons in Missing-Energy Events at the CERN SPS

We report on a direct search for sub-GeV dark photons ($A^{\prime }$), which might be produced in the reaction $e^{-}Z\to e^{-}Z{A}^{\prime }$ via kinetic mixing with photons by 100 GeV electrons incident on an active target in the NA64 experiment at the CERN SPS. The dark photons would decay invisibly into dark matter particles resulting in events with large missing energy. No evidence for such decays was found with $2.75\times 10^9$ electrons on target. We set new limits on the $\gamma -A^{\prime }$ mixing strength and exclude the invisible $A^{\prime }$ with a mass $\lesssim 100\mathrm{MeV}$ as an explanation of the muon $g_{\mu }-2$ anomaly.

Figure 1

Schematic illustration of the setup to search for $A^{\prime }\to \text{invisible}$ decays of the bremsstrahlung $A^{\prime }\mathrm{s}$ produced in the reaction $eZ\to eZ{A}^{\prime }$ of 100 GeV $e^{-}$ incident on the active ECAL target.

Figure 2

The left panel shows the measured distribution of events in the ($E_{\mathrm{ECAL}}$; $E_{\mathrm{HCAL}}$) plane from the combined BGO and PbSc run data at the earlier phase of the analysis. Another plot shows the same distribution after applying all selection criteria. The dashed area is the signal box region which is open. The side bands $A$ and $C$ are the ones used for the background estimate inside the signal box. For illustration purposes the size of the signal box along the $E_{\mathrm{HCAL}}$ axis is increased by a factor of 5.

Figure 3

The NA64 90% C.L. exclusion region in the ($m_{A^{\prime }}$, $\epsilon$) plane. Constraints from the BABAR [48, 56], and $\mathrm{E}787+\mathrm{E}949$ experiments [47, 58], as well as the muon ${\alpha }_{\mu }$ favored area are also shown. Here, ${\alpha }_{\mu }=(g_{\mu }-2)/2$. For more limits obtained from indirect searches and planned measurements see, e.g., Ref. [5].