Can Galactic Cosmic Rays Account for Solar 6Li 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 6 Li 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 (αα → 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.