Back to the Lithium Plateau with J0023+0307 with [Fe/H]<-6
  We present an analysis of the UVES high-resolution spectroscopic observations
at the 8.2m VLT of J0023+0307, a main-sequence extremely iron-poor dwarf star.
We are unable to detect iron lines in the spectrum but derive [Fe/H]<-6.1 from
the Ca II resonance lines assuming [Ca/Fe]>0.40. The chemical abundance pattern
of J0023+0307, with very low [Fe/Mg] and [Ca/Mg] abundance ratios, but
relatively high absolute Mg and Si abundances, suggests J0023+0307 is a second
generation star formed from a molecular cloud polluted by only one supernova in
which the fall-back mechanism played a role. We measure a carbon abundance of
A(C)=6.2 that places J0023+0307 on the low band in the A(C)-[Fe/H] diagram,
suggesting no contamination from a binary companion. This star is also unique
having a lithium abundance (A(Li)=2.02+-0.08) close to the level of the Lithium
Plateau, in contrast with lower Li determinations or upper limits in all other
extremely iron-poor stars. The upper envelope of the lithium abundances in
unevolved stars spanning more than three orders of magnitude in metallicity
(-6<[Fe/H]<-2.5) defines a nearly constant value. We argue that it is unlikely
that such uniformity is the result of depletion processes in stars from a
significantly higher initial Li abundance, but suggests instead a lower
primordial production, pointing to new physics such as decaying massive
particles, varying fundamental constants, or nuclear resonances, that could
have affected the primordial 7Li production.
