A light particle solution to the cosmic lithium problem
  We point out that the cosmological abundance of ${}^7$Li 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 and lifetimes of
such particles are bounded by $ 1.6~{\rm MeV}\leq m_X \leq 20~{\rm MeV}$ and $
{\rm few}~100~{\rm s} \lesssim \tau_X \lesssim 10^4~{\rm s}$, and the abundance
times the absorption cross section by either deuterium or ${}^7$Be are
comparable to the Hubble rate, $n_X \sigma_{\rm abs} v \sim H$, at the time of
${}^7$Be 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 ${}^7$Li+${}^7$Be, and find specific model
realizations of this scenario. Concentrating on the axion-like-particle case,
$X=a$, we show that all these conditions can be satisifed if the coupling to
$d$-quarks is in the range of $f_d^{-1} \sim {\rm TeV}^{-1}$, which can be
probed at intensity frontier experiments.
