New Constraints on Light Vectors Coupled to Anomalous Currents

We derive new constraints on light vectors coupled to standard model (SM) fermions, when the corresponding SM current is broken by the chiral anomaly. The cancellation of the anomaly by heavy fermions results, in the low-energy theory, in Wess-Zumino–type interactions between the new vector and the SM gauge bosons. These interactions are determined by the requirement that the heavy sector preserves the SM gauge groups and lead to $(\mathrm{energy}/\mathrm{vector}\mathrm{mass}{)}^2$ enhanced rates for processes involving the longitudinal mode of the new vector. Taking the example of a vector coupled to a vector coupled to SM baryon number, $Z$ decays and flavor-changing neutral current meson decays via the new vector can occur with $(\mathrm{weak}\mathrm{scale}/\mathrm{vector}\mathrm{mass}{)}^2$ enhanced rates. These processes place significantly stronger coupling bounds than others considered in the literature, over a wide range of vector masses.

Figure 1

Left panel: Constraints on a vector $X$ coupling to a vector coupled to SM baryon number, assuming a kinetic mixing with the SM photon $\epsilon \sim e{g}_X/(4\pi {)}^2$ and no additional invisible $X$ decay channels. Colored regions with solid borders indicate constraints from visible decays, dashed borders correspond to missing energy searches, and dotted borders denote projections based on current expected sensitivities. The gray regions indicate constraints from the previous literature. The new constraints come from searches for $K\to \pi X$ (green) [41, 42, 43], $B\to KX$ (blue) [40, 44, 45, 46], $Z\to X\gamma$ (red) [32, 33, 34, 35, 36], and very displaced decays at the CHARM proton beam dump experiment [47]. For the latter, the enhanced $K\to \pi X$ decays result in larger $X$ production than computed in naive analyses [48, 49]. The “anomalon” line shows the approximate region in which anomaly-canceling fermions would be light enough to have been detected [27]. The other gray constraints are from $\varphi$ and $\eta$ decays [21] and $\mathrm{\Upsilon }$ decays [9] (left to right). Right panel: As above, but with the assumption that $X$ dominantly decays invisibly.