Abstract
We report the results of a search for a new vector boson ( A') decaying into two dark matter particles chi _1 chi _2 of different mass. The heavier chi _2 particle subsequently decays to chi _1 and an off-shell Dark Photon A'^* rightarrow e^+e^-. For a sufficiently large mass splitting, this model can explain in terms of new physics the recently confirmed discrepancy observed in the muon anomalous magnetic moment at Fermilab. Remarkably, it also predicts the observed yield of thermal dark matter relic abundance. A detailed Monte-Carlo simulation was used to determine the signal yield and detection efficiency for this channel in the NA64 setup. The results were obtained re-analyzing the previous NA64 searches for an invisible decay A'rightarrow chi overline{chi } and axion-like or pseudo-scalar particles a rightarrow gamma gamma . With this method, we exclude a significant portion of the parameter space justifying the muon g-2 anomaly and being compatible with the observed dark matter relic density for A' masses from 2m_e up to 390 MeV and mixing parameter varepsilon between 3times 10^{-5} and 2times 10^{-2}.
Highlights
Despite its great success, the Standard Model (SM) does not provide a complete description of nature
We focus on a Dark Matter model that extends the SM symmetry group with a dark sector U (1)D, which is spontaneously broken by a dark Higgs field h D [15]
We present a direct search for the A semivisible signature using the NA64 experiment located at CERN SPS
Summary
The Standard Model (SM) does not provide a complete description of nature. The results were obtained reanalyzing the previous NA64 searches for an invisible decay A → χ χ and axion-like or pseudo-scalar particles a → γ γ . Considering models with a diagonal A coupling to DM and SM fields, two decay modes are possible, A → e+e− (visible mode) [8] and A → χ χ (invisible mode) [9].
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