Cosmological Lithium Solution from Discrete Gauged Baryon Minus Lepton Number

The cosmological lithium problem -- that theory predicts a primordial abundance far higher than the observed value -- has resisted decades of attempts by cosmologists, nuclear physicists, and astronomers alike to root out systematics. We reconsider this problem in the setting of the Standard Model extended by gauged baryon minus lepton number, which we spontaneously break by a scalar with charge $2 N_c N_g$. Cosmic strings from this breaking can support interactions converting three protons into three positrons, and we argue that an `electric'-`magnetic' interplay can give this process an amplified, strong-scale cross-section in an analogue of the Callan-Rubakov effect. We suggest such cosmic strings have disintegrated $\mathcal{O}(1)$ of the primordial lithium nuclei, and lay out what is necessary for this scheme to succeed. To our knowledge this is the first new physics mechanism with microphysical justification for the abundance of lithium uniquely to be modified after Big Bang Nucleosynthesis.