Can galactic cosmic rays account for solar⁶Li without overproducing gamma rays?

Cosmic-ray interactions with interstellar gas produce both ⁶Li, which accumulates in the interstellar medium (ISM), and π⁰ mesons, which decay to gamma rays that propagate throughout the cosmos. Local ⁶Li abundances and extragalactic gamma rays thus have a common origin that tightly links them. We exploit this connection to use gamma-ray observations to infer the contribution to ⁶Li nucleosynthesis by standard Galactic cosmic-ray (GCR) interactions with the ISM. Our calculation uses a carefully propagated cosmic-ray spectrum and accounts for ⁶Li production from both fusion reactions (α α → ⁶Li) as well as spallation channels (p, α + CNO → ⁶Li). We find that, although extreme assumptions yield a consistent picture, more realistic ones indicate that solar ⁶Li cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays. Implications for the primordial ⁶Li production by decaying dark matter and cosmic rays from cosmological structure formation are discussed. Upcoming gamma-ray observations by the Gamma-Ray Large Area Space Telescope will be crucial for determining the resolution of this problem.