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It would appear as a narrow bump in electron-positron pair masses or as missing energy.","posed_since":"1986","precise":"Determine whether a $U(1)'$ gauge boson $A'$ with mass $1\\,\\mathrm{MeV}$ to $10\\,\\mathrm{GeV}$ and kinetic mixing $\\epsilon \\sim 10^{-6}\\ \\text{to}\\ 10^{-3}$ exists, in visible ($A' \\to \\ell+ \\ell-$) or invisible decay modes. 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