NA62e+: dark sector searches with high intensity positron beams in ECN3

Abstract: Dark sector models present a rich phenomenology that requires high-intensity beams and precision detectors for thorough exploration. The NA62 experiment has already published several constraints on dark sector candidates, leveraging proton beam dump and meson decay techniques. This proposal seeks to significantly enhance NA62's discovery potential for dark sector candidates by using the positron-on-target technique. High intensity and high-energy positron beams, reaching up to $\sim$150 GeV energy, have already been produced at the SPS extracted beam lines. If a positron beam with an intensity in the range of 2$\times10^{14}$ positrons on target per year is delivered to the present K12 beam line, the NA62 detector would be ideal for searches of dark sector particles in both visible and invisible decay channels. Additionally, this approach would enable precision measurement of key standard model observable, including a detailed scan of $\sigma(e^+e^-\to\pi^+\pi^-$) and $\sigma(e^+e^-\to\mu^+\mu^-)$ at the di-pion and di-muon production threshold, with discovery potential for the True Muonium ($\mu^+\mu^-$) bound state.