Light Particle Solution to the Cosmic Lithium Problem

We point out that the cosmological abundance of Li7 can be reduced down to observed values if during its formation, big bang nucleosynthesis is modified by the presence of light electrically neutral particles X that have substantial interactions with nucleons. We find that the lithium problem can be solved without affecting the precisely measured abundances of deuterium and helium if the following conditions are satisfied: the mass (energy) and lifetimes of such particles are bounded by 1.6 MeV≤mX(EX)≤20 MeV and few100s τX 104 s, and the abundance times the absorption cross section by either deuterium or Be7 are comparable to the Hubble rate, nXσabsv∼H, at the time of Be7 formation. We include X-initiated reactions into the primordial nucleosynthesis framework, observe that it leads to a substantial reduction of the freeze-out abundances of Li7+Be7, and find specific model realizations of this scenario. Concentrating on the axionlike-particle case, X=a, we show that all these conditions can be satisfied if the coupling to d quarks is in the range of fd-1∼TeV-1, which can be probed at intensity frontier experiments.