New Constraints (and Motivations) for Abelian Gauge Bosons in the MeV-TeV Mass Range

TL;DR

The paper analyzes constraints on a light Abelian gauge boson $X$ with mass $M_X$ spanning MeV to TeV and couplings comprising direct fermion interactions and kinetic mixing with hypercharge. By diagonalizing the mixed kinetic/mass system, it derives physical couplings and oblique parameters, then applies a wide array of experimental bounds—from high-energy precision electroweak data to low-energy measurements and cosmology—to map allowed regions in the $(M_X,\alpha_X,\eta)$ parameter space. The results show that even small kinetic mixing or direct couplings can be tightly constrained, but in certain regimes (e.g., small $\eta$ or near specific mass scales) complementary bounds can be evaded or softened, highlighting the importance of considering the full coupling structure. The study emphasizes UV motivations for such bosons (e.g., extra dimensions and Green-Schwarz anomaly cancellation) and demonstrates how proton-stability considerations can naturally lead to light, weakly coupled gauge bosons with distinctive phenomenology across laboratory and cosmological probes.

Abstract

We survey the phenomenological constraints on abelian gauge bosons having masses in the MeV to multi-GeV mass range (using precision electroweak measurements, neutrino-electron and neutrino-nucleon scattering, electron and muon anomalous magnetic moments, upsilon decay, beam dump experiments, atomic parity violation, low-energy neutron scattering and primordial nucleosynthesis). We compute their implications for the three parameters that in general describe the low-energy properties of such bosons: their mass and their two possible types of dimensionless couplings (direct couplings to ordinary fermions and kinetic mixing with Standard Model hypercharge). We argue that gauge bosons with very small couplings to ordinary fermions in this mass range are natural in string compactifications and are likely to be generic in theories for which the gravity scale is systematically smaller than the Planck mass - such as in extra-dimensional models - because of the necessity to suppress proton decay. Furthermore, because its couplings are weak, in the low-energy theory relevant to experiments at and below TeV scales the charge gauged by the new boson can appear to be broken, both by classical effects and by anomalies. In particular, if the new gauge charge appears to be anomalous, anomaly cancellation does not also require the introduction of new light fermions in the low-energy theory. Furthermore, the charge can appear to be conserved in the low-energy theory, despite the corresponding gauge boson having a mass. Our results reduce to those of other authors in the special cases where there is no kinetic mixing or there is no direct coupling to ordinary fermions, such as for recently proposed dark-matter scenarios.