CERN Accelerating science

The $\Apr$ energy distribution for $e^+e^-$ annihilation events, for different values of $M_\Apr$. The parameters $\alpha_D=0.1$, $r=1/3$ were used.

The ECAL signal efficiency curve for LDM production via $e^+e^-$ annihilation as a function of $E_{ECAL}^{THR}$, for different values of the dark photon mass.

The new NA64 exclusion limit including the resonant $A^\prime$ via $e^+e^-$ annihilation, comparing the scalar and the fermionic LDM cases. Existing limits from BaBar~\cite{Lees:2017lec}, E787 and E949~\cite{Essig:2013vha}, and NA62~\cite{CortinaGil:2019nuo} are shown, as well as the favored area from the muon $g-2$ anomaly~\cite{Fayet:2007ua}, also including the new result that takes into account the latest results from Fermilab~\cite{Abi:2021gix} (red lines). The dashed cyan line report the previous NA64 result, without including the contribution from $e^+e^-$ annihilation. The orange dashed line is the sensitivity projection for a NA64-like experiment with an $e^+$ beam, assuming the same run conditions and accumulated statistics.

The new NA64 exclusion limit in the $(y,m_\chi)$ plane, including the $e^+e^-$ annihilation process, in the $(m_\chi,y)$ plane, for $\alpha_D=0.1$ (left) and $\alpha_D=0.5$ (right). The other curves and shaded areas report already-existing limits in the same parameters space from E137~\cite{Batell:2014mga}, LSND~\cite{deNiverville:2011it,Batell:2009di}, MiniBoone~\cite{Batell:2009di}, and BaBar~\cite{Lees:2017lec}. The black lines show the favored parameter combinations for the observed dark matter relic density for different variations of the model.

The new NA64 exclusion limit in the $(y,m_\chi)$ plane, including the $e^+e^-$ annihilation process, in the $(m_\chi,y)$ plane, for $\alpha_D=0.1$ (left) and $\alpha_D=0.5$ (right). The other curves and shaded areas report already-existing limits in the same parameters space from E137~\cite{Batell:2014mga}, LSND~\cite{deNiverville:2011it,Batell:2009di}, MiniBoone~\cite{Batell:2009di}, and BaBar~\cite{Lees:2017lec}. The black lines show the favored parameter combinations for the observed dark matter relic density for different variations of the model.

The new NA64 exclusion limit in the $(y,m_\chi)$ plane, including the $e^+e^-$ annihilation process, in the $(m_\chi,y)$ plane, for $\alpha_D=0.1$ (left) and $\alpha_D=0.5$ (right). The other curves and shaded areas report already-existing limits in the same parameters space from E137~\cite{Batell:2014mga}, LSND~\cite{deNiverville:2011it,Batell:2009di}, MiniBoone~\cite{Batell:2009di}, and BaBar~\cite{Lees:2017lec}. The black lines show the favored parameter combinations for the observed dark matter relic density for different variations of the model.

The new NA64 exclusion limit in the $(y,m_\chi)$ plane, including the $e^+e^-$ annihilation process, in the $(m_\chi,y)$ plane, for $\alpha_D=0.1$ (left) and $\alpha_D=0.5$ (right). The other curves and shaded areas report already-existing limits in the same parameters space from E137~\cite{Batell:2014mga}, LSND~\cite{deNiverville:2011it,Batell:2009di}, MiniBoone~\cite{Batell:2009di}, and BaBar~\cite{Lees:2017lec}. The black lines show the favored parameter combinations for the observed dark matter relic density for different variations of the model.