$X_{17}$ anomaly and prospects for its verification with NA64 experiment

The ATOMKI collaboration reported a 7$\sigma$ excess in the angular distribution of $e^{+}e^{-}$ events in the nuclear de-excitation of $^8$Be, which was recently confirmed by new results on other nuclei, $^4$He. This anomaly could be explained by the existence of a new boson $X_{17}$ with a mass around 17 MeV and spin 0 or 1. In this work, we summarize the latest results on the searches performed by NA64 of a vector $X_{17}$, an example of which could be the so-called protophobic gauge boson, with predicted coupling strength with electrons $2\times10^{-4} \lesssim \epsilon \lesssim 1.4\times10^{-3}$ and lifetime in the range $10^{-14} \lesssim \tau_{X17} \lesssim 10^{-12} \mathrm{s}$. NA64 experiment is a fixed target experiment at the CERN Super Proton Synchrotron (SPS) capable to perform an independent measurement. If such a particle exists, it can be produced via Bremsstrahlung after high energy secondary SPS electron beam collides with the active target nuclei. In the analysis of the combined 2017-2018 run, corresponding to $8.4\times10^{10}$ electrons on target, no signal-like events have been found excluding a large fraction of the coupling parameter space able to explain the anomaly: $\epsilon < 6.8 \times 10^{-4}$. We will also briefly discuss prospects for further searches aiming to cover parameter space for models suggesting enabled explanation of the anomaly.