Implications of the Oklo phenomenon in a chiral approach to nuclear
  matter
  It has been customary to use data from the Oklo natural nuclear reactor to
place bounds on the change that has occurred in the electromagnetic fine
structure constant $\alpha$ over the last 2 billion years. Alternatively, an
analysis could be based on a recently proposed expression for shifts in
resonance energies which relates them to changes in both $\alpha$ and the
average $m_q$ of the $u$ and $d$ current quark masses, and which makes explicit
the dependence on mass number $A$ and atomic number $Z$. (Recent model
independent results on hadronic $\sigma$-terms suggest sensitivity to the
strange quark mass is negligible.) The most sophisticated analysis, to date, of
the quark mass term invokes a calculation of the nuclear mean-field within the
Walecka model of quantum hadrodynamics. We comment on this study and consider
an alternative in which the link to low-energy quantum chromodynamics (QCD) and
its pattern of chiral symmetry-breaking is more readily discernible.
Specifically, we investigate the sensitivity to changes in the pion mass
$M_\pi$ of a single nucleon potential determined by an in-medium chiral
perturbation theory ($\chi$PT) calculation which includes virtual
$\mathrm{\Delta}$-excitations. Subject to some reasonable assumptions about
low-energy constants (LECs), we confirm that the $m_q$-contribution to
resonance shifts is enhanced by a factor of 10 or so relative to the
$\alpha$-term and deduce that the Oklo data for Sm imply that
$|m_q(\mathrm{Oklo})- m_q(\mathrm{now})| \lesssim 10^{-9}m_q(\mathrm{now})$.
