Implications of the Oklo phenomenon in a chiral approach to nuclear matter

TL;DR

This paper explores how the Oklo natural reactor data constrains variations in quark masses over billions of years using a chiral perturbation theory approach linked to low-energy QCD, offering an alternative to previous models.

Contribution

It introduces an in-medium chiral perturbation theory calculation including virtual Δ-excitations to analyze resonance energy shifts related to quark mass changes, providing a more direct connection to QCD.

Findings

Quark mass contribution to resonance shifts is about ten times larger than the electromagnetic term.

Oklo data constrains the change in quark mass to less than 10^{-9} of its current value.

The analysis supports a very stable quark mass over geological timescales.

Abstract

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 $|m_q(\mathrm{Oklo})- m_q(\mathrm{now})| \lesssim 10^{-9}m_q(\mathrm{now})$.