  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.
