Non-detection of $^6$Li in Spite plateau stars with ESPRESSO
  The detection of $^6$Li in Spite plateau stars contradicts the standard Big
Bang nucleosynthesis prediction, known as the second cosmological lithium
problem. We measure the isotopic ratio $^6$Li/$^7$Li in three Spite plateau
stars: HD 84937, HD 140283, and LP 815-43. We use 3D NLTE radiative transfer
and for the first time apply this to high resolution, high-S/N data from the
ultra-stable VLT/ESPRESSO spectrograph. These are amongst the best spectra ever
taken of any metal-poor stars. As the measurement of $^6$Li/$^7$Li is
degenerate with other physical stellar parameters, we employ Markov chain Monte
Carlo methods to find the probability distributions of measured parameters. As
a test of systematics we also use three different fitting methods. We do not
detect $^6$Li in any of the three stars, and find consistent results between
our different methods. We estimate 2$\sigma$ upper limits to $^6$Li/$^7$Li of
0.7%, 0.6%, and 1.7% respectively for HD 84937, HD 140283, and LP 815-43. Our
results indicate that there is no second cosmological lithium problem, as there
is no evidence of $^6$Li in Spite Plateau stars.
