Protophobic Fifth Force Interpretation of the Observed Anomaly in $^8$Be Nuclear Transitions (1604.07411v2)

Abstract: Recently a 6.8$\sigma$ 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$\sigma$ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.

• The paper identifies a potential protophobic vector boson that could explain the 6.8σ anomaly observed in 8Be nuclear transitions.
• It employs detailed analysis of angular correlations and invariant mass distributions to constrain the boson’s milli-charged couplings.
• The research suggests the new force might not only clarify the 8Be anomaly but also impact the muon (g-2) discrepancy and future experiments.

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

The paper "Protophobic Fifth Force Interpretation of the Observed Anomaly in $^8$Be Nuclear Transitions" presents an intriguing investigation into a potential new fundamental interaction mediated by a light vector boson. This analysis is motivated by a reported 6.8$\sigma$ anomaly in the angular correlation and invariant mass distributions of $e^+ e^-$ pairs emerging from the transitions of excited states of $^8$Be. This anomaly provides a compelling opportunity to explore the existence of a fifth force beyond the known electromagnetic, weak, strong, and gravitational forces.

$^8$Be Anomaly: Experimental Observations

The recent experimental work by Krasznahorkay et al. found significant deviations from expected patterns in the $e^+ e^-$ pairs resulting from transitions between excited states of $^8$Be. Specifically, peaks were observed at angle $\Theta \approx 140^\circ$ and invariant mass $m_{e^+ e^-} \approx 17~MeV$, suggesting the presence of an intermediate boson with the mass of 17 MeV. This anomaly is not accounted for by known nuclear processes, leading to the hypothesis of a new vector gauge boson, herein termed the $X$ boson.

Theoretical Framework and Protophobic Vector Boson

The vector boson proposed in the paper is characterized by its protophobic nature, meaning its coupling to protons is suppressed relative to neutrons and other fermions. The theoretical framework suggests that the $X$ boson induces a fifth force with a range of approximately 12 femtometers, extending the landscape of fundamental forces. The boson exhibits unique milli-charged couplings to up and down quarks, electrons, and neutrons, positing milli-charge values that satisfy multiple experimental constraints.

Constraints and Model Viability

The comprehensive analysis in the paper evaluates the $X$ boson model against existing experimental constraints, including those from electron anomalous magnetic moment $(g-2)_e$, neutron-proton scattering experiments, and numerous particle decay processes. Notably, the analysis identifies specific parameter ranges for the charges on up and down quarks and electrons, enabling the proposed $X$ boson to evade these constraints while explaining the $^8$Be anomaly. For instance, the electron coupling is constrained to be between 0.0002 and 0.0014, while certain ratios of quark charges are necessary to ensure compatibility with observed phenomena.

Implications and Future Prospects

The implications of this research are profound. If verified, the existence of an $X$ boson might not only explain the $^8$Be anomaly but also contribute to reducing the discrepancy in the muon's anomalous magnetic moment, currently at 3.6$\sigma$. Moreover, the research provides a platform for future experimental verifications. Suggested methods include the investigation of electromagnetic nuclear transitions with energy above 17 MeV and various experimental programs aiming to search for light vector bosons, such as those planned at DarkLight and LHCb. These investigations could provide critical tests of the proposed fifth force scenario.

Conclusion

This work efficiently scrutinizes a potential fifth force manifested through a novel protophobic vector boson, grounded in the anomalous $^8$Be nuclear transitions. The evaluated experimental and theoretical landscape suggests a range of viable parameters for this hypothesis, bolstered by a robust analysis of existing bounds and constraints. The potential broader implications for the muon magnetic moment and the formulation of ultraviolet complete models present exciting avenues for continued research in this domain. Further experimental exploration will be crucial to substantiate or refute this intriguing proposal.