Gaia halo stars; neutron lifetime

In the morning, Lauren Anderson (Flatiron) and I discussed Gaia DR2 projects. First we talked about things we could do with David Blei (Columbia) and his group, who have variational methods for extremely large inferences of the types we would like to do. We drew some graphical models (and posted them on twitter). Then we looked at halo red giants selected by parallax and color. Sagittarius shows up beautifully, and now it is time to start to look at other features. The data are incredibly rich.

Alberto Sirlin (NYU) gave the brown-bag talk, on the neutron lifetime. He showed that the neutron lifetime and a certain coupling are related, and showed that measurements of each, and their combination, are consistent, for at least some measurements. There are interesting puzzles though: Some kinds of lifetime measurements disagree with other kinds, and there was a step change in the coupling measurements in 2002-ish. So there are hints of new physics, but also a consistent no-new-physics story. He also showed that the simplest new-physics scenarios are not sensible. The neutron lifetime is important for many things, but especially big-bang nucleosynthesis.

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