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Protophobic Fifth Force Interpretation of the Observed Anomaly in $^8$Be Nuclear Transitions

Jonathan L. Feng, Bartosz Fornal, Iftah Galon, Susan Gardner, Jordan Smolinsky, Tim M. P. Tait, Philip Tanedo

TL;DR

The protophobic X boson may also alleviate the current 3.6σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

Abstract

Recently a 6.8$σ$ anomaly has been reported in the opening angle and invariant mass distributions of $e^+ e^-$ pairs produced in $^8\text{Be}$ nuclear transitions. The data are explained by a 17 MeV vector gauge boson $X$ that is produced in the decay of an excited state to the ground state, $^8\text{Be}^* \to {}^8\text{Be} \, X$, and then decays through $X \to e^+ e^-$. The $X$ boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The protophobic $X$ boson may also alleviate the current 3.6$σ$ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

Protophobic Fifth Force Interpretation of the Observed Anomaly in $^8$Be Nuclear Transitions

TL;DR

The protophobic X boson may also alleviate the current 3.6σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

Abstract

Recently a 6.8 anomaly has been reported in the opening angle and invariant mass distributions of pairs produced in nuclear transitions. The data are explained by a 17 MeV vector gauge boson that is produced in the decay of an excited state to the ground state, , and then decays through . The boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The protophobic boson may also alleviate the current 3.6 discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

Paper Structure

This paper contains 10 equations, 2 figures, 1 table.

Figures (2)

  • Figure 1: The required charges to explain the ${}^8\text{Be}$ anomaly in the $(\varepsilon_u, \varepsilon_d)$ (top) and $(\varepsilon_{\nu}, \varepsilon_e)$ (bottom) planes, along with the leading constraints discussed in the text. Top: The $n$-Pb and NA48/2 constraints are satisfied in the shaded regions. On the protophobic contour, $\varepsilon_d / \varepsilon_u = -2$. The width of the ${}^8\text{Be}$ bands corresponds to requiring the signal strength to be within a factor of 2 of the best fit. Bottom: The E141, KLOE-2, $(g-2)_e$, and $\nu-e$ scattering constraints exclude their shaded regions, whereas $(g-2)_{\mu}$ favors its shaded region. The ${}^8\text{Be}$ signal imposes a lower bound on $| \varepsilon_e |$.
  • Figure 2: The ${}^8\text{Be}$ signal region, along with current constraints discussed in the text (gray) and projected sensitivities of future experiments in the $(m_X, \varepsilon_e)$ plane. For the ${}^8\text{Be}$ signal, the other couplings are assumed to be in the ranges given in Eq. (\ref{['erange']}); for all other contours, the other couplings are those of a dark photon.