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Search for the X17 particle in $^{7}\mathrm{Li}(\mathrm{p},\mathrm{e}^+ \mathrm{e}^{-}) ^{8}\mathrm{Be}$ processes with the MEG II detector
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abstract
The observation of a resonance structure in the opening angle of the electron-positron pairs in the $^{7}$Li(p,\ee) $^{8}$Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving $^{4}$He and $^{12}$C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the $\mu^+ \rightarrow \mathrm{e}^+ \gamma$ decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft-Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The $\gamma$ and the e$^{+}$e$^{-}$ pair emerging from the $^8\mathrm{Be}^*$ transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value \SI{17.6}{\mega\electronvolt} and \SI{18.1}{\mega\electronvolt}. No significant signal was found, and limits at \SI{90}{\percent} C.L. on the branching ratios (relative to the $\gamma$ emission) of the two resonances to X17 were set, $R_{17.6} < 1.8 \times 10^{-6} $ and $R_{18.1} < 1.2 \times 10^{-5} $.
Forward citations
Cited by 6 Pith papers
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Possible Evidence for Neutral Color-Singlet $q\bar q$ Quark Matter from High-Energy Pb-Emulsion Collisions
The complex e+e- invariant mass spectrum below 50 MeV is attributed to thermal annihilation and bound states of QED-deconfined and confined neutral q bar q quark matter.
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Updated E141 constraints on a long-lived $X_{17}$ vector boson
A refined simulation of SLAC E141 shows the experiment's old exclusion region does not cover the 16.9 MeV X17, leaving a window at ε_e between 6.5e-5 and 1.1e-4.
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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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Probing the ATOMKI X17 vector boson using Dalitz decays $V\to Pe^+e^-$
In a quark-model calculation, X17 contributions to D*, B*, and J/psi Dalitz decays are too small to explain the BESIII D*0 e+e- excess unless quark couplings are far outside ATOMKI-favored ranges.
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Potential for the discovery of the protophobic boson at the STCF
Detector-level simulations indicate the proposed STCF drift chamber could discover the 17 MeV protophobic X17 boson through displaced electron-positron vertices if Standard Model backgrounds stay below about 10^4 even...
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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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