Search for a Hypothetical 16.7 MeV Gauge Boson and Dark Photons in the NA64 Experiment at CERN

We report the first results on a direct search for a new 16.7 MeV boson ($X$) which could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the excited ${}^8{\mathrm{Be}}^{*}$ nucleus decays. Because of its coupling to electrons, the $X$ could be produced in the bremsstrahlung reaction $e^{-}Z\to e^{-}ZX$ by a 100 GeV $e^{-}$ beam incident on an active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through the subsequent decay into a $e^{+}{e}^{-}$ pair. With $5.4\times {10}^{10}$ electrons on target, no evidence for such decays was found, allowing us to set first limits on the $X-e^{-}$ coupling in the range $1.3\times {10}^{-4}\lesssim {\epsilon }_e\lesssim 4.2\times {10}^{-4}$ excluding part of the allowed parameter space. We also set new bounds on the mixing strength of photons with dark photons ($A^{\prime }$) from nonobservation of the decay $A^{\prime }\to e^{+}{e}^{-}$ of the bremsstrahlung $A^{\prime }$ with a mass $\lesssim 23\mathrm{MeV}$.

Figure 1

Schematic illustration of the NA64 setup to search for the $A^{\prime }$, $X\to e^{+}{e}^{-}$ decays.

Figure 2

Distribution of selected e.m. neutral (presumably photon) and signal events in the ($E_{\mathrm{WCAL}}$; $E_{\mathrm{ECAL}}$) plane from the combined 30 $X_0$ and 40 $X_0$ runs. Neutral e.m. events are shown as blue squares. The only signal-like event is shown as a red square. The dashed band represents the signal box.

Figure 3

The 90% C.L. exclusion areas in the ($m_X;\epsilon$) plane from the NA64 experiment (blue area). For the mass of 16.7 MeV, the $X-e^{-}$ coupling region excluded by NA64 is $1.3\times {10}^{-4}<{\epsilon }_e<4.2\times {10}^{-4}$. The allowed range of ${\epsilon }_e$ explaining the ${}^8\mathrm{Be}$ anomaly (red area) [2, 3], constraints on the mixing $\epsilon$ from the experiments E141 [22], E774 [25], BABAR [40], KLOE [45], HADES [47], PHENIX [48], NA48 [50], and bounds from the electron anomalous magnetic moment $(g-2{)}_e$ [71] are also shown.