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

TL;DR

This work analyzes the 6.8σ anomaly in excited Be-8 decays, showing that a protophobic ~17 MeV vector boson can explain the IPC excess while satisfying diverse experimental constraints. It develops an effective-field-theory description of ^8Be* → ^8Be X, demonstrates scalar and many other candidates are disfavored, and identifies a viable vector scenario whose couplings align with isospin considerations and nuclear mixing effects. The authors present two anomaly-free SM extensions, U(1)_B and U(1)_{B-L} with kinetic mixing, including anomaly-cancelling vectorlike fermions and potential collider/cosmology signals, and discuss how such models can also address the muon g−2 anomaly. A comprehensive constraint survey across quark, lepton, and neutrino sectors is provided, along with near-term experimental prospects (low-energy experiments and LHC tests) to probe the proposed protophobic gauge boson parameter space.

Abstract

The 6.8$σ$ 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.

Figures (6)

• Figure 1: The most relevant $^8$Be states, our naming conventions for them, and their spin-parities $J^P$, isospins $T$, excitation energies $E$, and decay widths $\Gamma$ from Ref. nndc. Asterisks on isospin assignments indicate states with significant isospin mixing. Decays of the $^8\text{Be}\xspace^*$ (18.15) state to the ground state $^8$Be exhibit anomalous internal pair creation; decays of the $^8\text{Be}\xspace^*{}'$ (17.64) state do not Krasznahorkay:2015iga.
• Figure 2: Schematic depiction of the Atomki pair spectrometer experiment Gulyas:2015miaKrasznahorkay:2015iga, interpreted as evidence for the production of a new boson $X$. The proton beam's energy is tuned to excite lithium nuclei into the $^8\text{Be}\xspace^*$ state, which subsequently decays into the $^8$Be ground state and $X$. The latter decays into an electron--positron pair whose opening angle and invariant mass are measured.
• Figure 3: The ratio $\Gamma_X / \Gamma_\gamma$ in the case of perfect isospin ($\alpha_1 = \kappa = 0$) (left) and isospin violation ($\alpha_1 = 0.21$, $\kappa = 0.549$) (right) in the $(\varepsilon_p, \varepsilon_n)$ plane for $m_X = 16.7~\text{MeV}\xspace$. The effects of isospin violation may be significant in general, but for the viable protophobic regions of parameter space consistent with NA48/2 constraints (shaded), their effects are small. The best fit value of $\Gamma_X / \Gamma_\gamma = 5.8 \times 10^{-6}$ is highlighted. The dark photon scenario corresponds to $\varepsilon_n=0$.
• Figure 4: Contours of $\Gamma_X/\Gamma_\gamma$ in the $(\varepsilon_p, \varepsilon_n)$ plane for the parameterization of isospin violation in Eq. (\ref{['eq:widthratioisospinnumerical']}). Also shown are the dark photon axis $(\varepsilon_n = 0)$ and the protophobic region with $|\varepsilon_p| \le 1.2 \times 10^{-3}$ allowed by NA48/2 constraints on $\pi^0 \to X \gamma$. The $m_X$ values are fixed to $m_X =16.1~\text{MeV}\xspace$ (left) and 17.3 MeV (right), corresponding to the $\pm 1\sigma$ (statistical) range of $m_X$.
• Figure 5: Summary of constraints and target regions for the leptonic couplings of a hypothetical $X$ gauge boson with $m_X \approx 17$ MeV. Updated from Ref. Feng:2016jff.