Recently a $6.8\sigma$ anomaly has been reported in the opening angle and invariant mass distributions of $e^{+}{e}^{-}$ pairs produced in ${}^8\mathrm{Be}$ 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, ${}^8{\mathrm{Be}}^{*}\to {}^8\mathrm{Be}X$, and then decays through $X\to e^{+}{e}^{-}$. The $X$ boson mediates a fifth force with a characteristic range of 12 fm and has millicharged 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 $6.8\sigma$ anomaly in excited ${}^8\mathrm{Be}$ 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 ${}^8\mathrm{Be}$ decays and the characteristics of the observed anomaly. To develop its particle interpretation, we provide an effective operator analysis for excited ${}^8\mathrm{Be}$ 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 ${}^8\mathrm{Be}$ anomaly. In the first model, the new force carrier is a ${\mathrm{U}(1)}_B$ gauge boson that kinetically mixes with the photon; in the second model, it is a $\mathrm{U}(1{)}_{B-L}$ gauge boson with a similar kinetic mixing. In both cases, the models predict relatively large charged lepton couplings $\sim 0.001$ 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.

Neutron stars capture dark matter efficiently. The kinetic energy transferred during capture heats old neutron stars in the local galactic halo to temperatures detectable by upcoming infrared telescopes. We derive the sensitivity of this probe in the framework of effective operators. For dark matter heavier than a GeV, we find that neutron star heating can set limits on the effective operator cutoff that are orders of magnitude stronger than possible from terrestrial direct detection experiments in the case of spin-dependent and velocity-suppressed scattering.

We present simplified models for the galactic center $\gamma$-ray excess where Dirac dark matter annihilates into pairs or triplets of on-shell bosonic mediators to the standard model. These annihilation modes allow the dark matter mass to be heavier than those of conventional effective theories for the $\gamma$-ray excess. Because the annihilation rate is set by the dark matter–mediator coupling, the standard model coupling can be made parametrically small to “hide” the dark sector by suppressing direct detection and collider signals. We explore the viability of these models as a thermal relic and on the role of the mediators for controlling the $\gamma$-ray spectral shape. We comment on ultraviolet completions for these simplified models and novel options for standard model final states.

Dark matter may interact with the Standard Model through the kinetic mixing of dark photons, $A^{\prime }$, with Standard Model photons. Such dark matter will accumulate in the Sun and annihilate into dark photons. The dark photons may then leave the Sun and decay into pairs of charged Standard Model particles that can be detected by the Alpha Magnetic Spectrometer (AMS). The directionality of this “dark sunshine” is distinct from all astrophysical backgrounds, providing an opportunity for unambiguous dark matter discovery by AMS. We perform a complete analysis of this scenario including Sommerfeld enhancements of dark matter annihilation and the effect of the Sun’s magnetic field on the signal, and we define a set of cuts to optimize the signal probability. With the three years of data already collected, AMS may discover dark matter with mass $1\mathrm{TeV}\lesssim m_X\lesssim 10\mathrm{TeV}$, dark photon masses $m_{A^{\prime }}\sim \mathcal{O}(100)\mathrm{MeV}$, and kinetic mixing parameters ${10}^{-11}\lesssim ϵ\lesssim {10}^{-8}$. The proposed search extends beyond existing beam dump and supernova bounds, and it is complementary to direct detection, probing the same region of parameter space.

The center of the Milky Way is predicted to be the brightest region of $\gamma$-rays generated by self-annihilating dark matter particles. Excess emission about the Galactic center above predictions made for standard astrophysical processes has been observed in $\gamma$-ray data collected by the Fermi Large Area Telescope. It is well described by the square of a Navarro, Frenk, and White dark matter density distribution. Although other interpretations for the excess are plausible, the possibility that it arises from annihilating dark matter is valid. In this paper, we characterize the excess emission as annihilating dark matter in the framework of an effective field theory. We consider the possibility that the annihilation process is mediated by either pseudoscalar or vector interactions and constrain the coupling strength of these interactions by fitting to the Fermi Large Area Telescope data for energies 1–100 GeV in the $15°\times 15°$ region about the Galactic center using self-consistently derived interstellar emission models and point source lists for the region. The excess persists and its spectral characteristics favor a dark matter particle with a mass in the range approximately from 50 to 190 (10 to 90) GeV and annihilation cross section approximately from $1\times {10}^{-26}$ to $4\times {10}^{-25}$ ($6\times {10}^{-27}$ to $2\times {10}^{-25}$) ${\mathrm{cm}}^3/\mathrm{s}$ for pseudoscalar (vector) interactions. We map these intervals into the corresponding WIMP-neutron scattering cross sections and find that the allowed range lies well below current and projected direct detection constraints for pseudoscalar interactions, but are typically ruled out for vector interactions.