Fundamental physics with ESPRESSO: a new determination of the D/H ratio towards PKS1937-101

Primordial abundances of light elements are sensitive to the physics of the early Universe and can directly constrain cosmological quantities, such as the baryon-to-photon ratio eta(10), the baryon density, and the number of neutrino families. Deuterium is especially suited for these studies: its primordial abundance is sensitive and monotonically dependent on eta(10), allowing an independent measurement of the cosmic baryon density that can be compared, for instance, against the Planck satellite data. The primordial deuterium abundance can be measured in high H I column density absorption systems towards distant quasars. We report here a new measurement, based on high-resolution ESPRESSO data, of the primordial D I abundance of a system at redshift z similar to 3.572, towards PKS1937-101. Using only ESPRESSO data, we find a D /H ratio of 2.638 +/- 10(-5), while including the available UVES data improves the precision, leading to a ratio of 2.608 +/- 10(-5). The results of this analysis agree with those of the most precise existing measurements. We find that the relatively low column density of this system (log N-HI/cm(-2)similar to 18) introduces modelling uncertainties, which become the main contributor to the error budget.