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New evidence supporting the existence of the hypothetic X17 particle
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abstract
We observed electron-positron pairs from the electro-magnetically forbidden M0 transition depopulating the 21.01 MeV 0$^-$ state in $^4$He. A peak was observed in their $e^+e^-$ angular correlations at 115$^\circ$ with 7.2$\sigma$ significance, and could be described by assuming the creation and subsequent decay of a light particle with mass of $m_\mathrm{X}c^2$=16.84$\pm0.16 (stat) \pm 0.20 (syst)$ MeV and $\Gamma_\mathrm{X}$= $3.9\times 10^{-5}$ eV. According to the mass, it is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)] for describing the anomaly observed in $^8$Be.
Forward citations
Cited by 5 Pith papers
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Flavor specific chiral $U(1)_X$ framework for explaining the ATOMKI anomaly
A gauged, flavor-specific U(1)_X two-Higgs-doublet model can realize the axial-vector Z' couplings needed to explain the ATOMKI 8Be and 4He anomalies while evading current bounds.
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The X17 with Chiral Couplings
A spin-1 X17 with chiral couplings fits the ATOMKI nuclear anomalies at 99% CL only in a parameter region that atomic parity violation and KLOE-2/NA64 exclude, driven mainly by the 12C(17.23) transition.
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Constraints on Axion-Like Particles with the Silicon Detector at a Nuclear Reactor
New 90% C.L. limits on the ALP–photon coupling in the 0.1–100 keV range are derived from Connie and Atucha-II reactor data via plasmon excitation in silicon; a 30 kg·yr Oscura-style run could improve on NEON by about tenfold.
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Where to find $X(17)$?
Combining existing bounds with a new parity-violating Møller asymmetry constraint, the paper's body claims no surviving parameter space for scalar, pseudoscalar, vector, axial-vector, or V±A X(17)-electron couplings.
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Simulation of dark photon sensitivity in $\eta \rightarrow \gamma e^+e^-$ at HIAF
Simulations of a proposed HIAF eta factory project a dark photon branching ratio limit of 10^-6 to 10^-7 and a kinetic mixing sensitivity of epsilon^2 about 10^-7 for masses up to 0.4 GeV.
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