Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays (1608.03591v2)

Abstract: The 6.8$\sigma$ anomaly in excited 8Be nuclear decays via internal pair creation is fit well by a new particle interpretation. In a previous analysis, we showed that a 17 MeV protophobic gauge boson provides a particle physics explanation of the anomaly consistent with all existing constraints. Here we begin with a review of the physics of internal pair creation in 8Be decays and the characteristics of the observed anomaly. To develop its particle interpretation, we provide an effective operator analysis for excited 8Be decays to particles with a variety of spins and parities and show that these considerations exclude simple models with scalar particles. We discuss the required couplings for a gauge boson to give the observed signal, highlighting the significant dependence on the precise mass of the boson and isospin mixing and breaking effects. We present anomaly-free extensions of the Standard Model that contain protophobic gauge bosons with the desired couplings to explain the 8Be anomaly. In the first model, the new force carrier is a U(1)B gauge boson that kinetically mixes with the photon; in the second model, it is a U(1)(B-L) gauge boson with a similar kinetic mixing. In both cases, the models predict relatively large charged lepton couplings ~ 0.001 that can resolve the discrepancy in the muon anomalous magnetic moment and are amenable to many experimental probes. The models also contain vectorlike leptons at the weak scale that may be accessible to near future LHC searches.

• The paper introduces a novel protophobic 17 MeV gauge boson model to explain the 8Be decay anomaly observed at 6.8σ significance.
• It employs an effective operator framework to systematically exclude scalar and pseudoscalar candidates while exploring U(1)_B and U(1)_{B-L} model extensions.
• The study rigorously evaluates experimental constraints from π0 decays, beam dump, and collider experiments, guiding future validations of the Standard Model extension.

Analysis of Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays

The paper presented by Feng et al. elucidates a theoretical framework for addressing the observed anomaly in the excited state of beryllium-8 ($^{8}$Be) nuclear decays, which reveals an excess of internal pair creation at a significance of 6.8$\sigma$. The anomaly is described using a particle physics perspective which introduces the existence of a novel 17 MeV gauge boson. This discussion explores the gauge boson's properties, the implications of such a particle within the structure of the Standard Model (SM), and possible extensions necessary to accommodate the particle under the existing experimental constraints.

The initial section of the paper sets the stage for interpreting the anomaly through nuclear transitions. The beryllium nuclear states and transitions that are primarily relevant to the discussion are meticulously reviewed—emphasizing the $^{8}$Be excited state transitions, which could feasibly produce such a gauge boson. The paper then critically advances the search for a particle interpretation, deploying an effective operator framework that examines transitions to states with different quantum properties (spin and parity). This analysis notably excludes several simple particle candidates, including scalars and pseudoscalars, but posits a viable candidate in a new spin-1 gauge boson.

In its core sections, the paper presents extensions to the SM that integrate a protophobic gauge boson—an essential feature to accommodate empirical observations while remaining consistent with known constraints. Specifically, the protophobic characteristic minimizes the boson's coupling to protons, allowing for current limits on $\pi^0$ decays to uphold. Two distinct models highlight this investigation: a U(1)$_B$ gauge boson with kinetic mixing with the photon, and a U(1)$_{B-L}$ gauge boson. Each model presents unique implications and requires the addition of other particles to achieve anomaly cancellation. The authors effectively utilize these models to describe the anomaly, delineating parameter spaces that accommodate the experimental data.

Moreover, nuclear physics effects such as isospin mixing and breaking are rigorously evaluated to uncover their potential impacts on the beryllium nuclear transitions and the required couplings. The analysis ascertains that although isospin effects could significantly disturb signal interpretation, the protophobic limit mitigates these impacts adequately.

From a future experimental analysis perspective, the paper provides a robust discussion on constraints and signals in various experimental frameworks. These include stringent tests from $\pi^0 \to X\gamma$ decay searches, electron coupling constraints from beam dump experiments, neutrino coupling constraints from scattering measurements, and potential signals that could be accessible in collider experiments like the LHC in vector-like lepton searches. The premise is that such future survey experiments have great potential to validate or exclude the existence of the proposed $X$ boson.

To conclude, Feng et al.'s exploration within the particle physics domain highlights necessary conditions and resultant constraints that any credible theory explaining the $^{8}$Be anomaly must satisfy. Their research integrates particle phenomenology with robust theoretical models, providing a comprehensive view of the steps needed to validate the existence of a new force carrier particle in nuclear interactions. Future developments in high-energy and nuclear physics, relying on both experimental validation and theoretical refinement, stand poised to expand the bounds of the SM to potentially incorporate these intriguing findings. Such integrations will shape core aspects of dark matter studies, $B-L$ symmetries, and grand unification theories if corroborated by ensuing data.