3 nucleons are stronger than 2, says Feynman

It's written in the Feynman lectures in physics. The story goes that Sir Richard took the task of preparing a lecture on spin as a challenge from a colleague, but quickly found the big challenges of the endeavor. One of the mantras behind his lecture series was "if we really understand a topic in Physics we should be able to explain it at a level accessible to undergraduates". It seems he still didn't fully comprehend spin.

Contrast that to the instructions we received from the organizers as you might find in the PASI webpage: make a presentation at the Advanced Undergraduate level, and spend like 75% of the talk discussing the open questions in the topic. A clear clash with Feynman's ideas! So lets be fair to the speakers in this my first lecture blog (it took only a week!). Most have made the effort to keep talks at the students level; at least for the first few slides. There were of course a few who couldn't resist and showed off their results right from the start, even though I think we're not the most receptive audience for that kind of stuff... but I digress.

Achim Schwenk talk last Friday started at the (graduate) student level; or so it seemed to my highly under-caffeinated and under-slept experimentalist mind*. The main clear message was: 3-body forces are frontier, they impact the structure and existence of neutron-rich nuclei and neutron-rich matter in astrophysics. Very easy to get it, as it was written explicitly in the second slide.

The highest point of Achim's advanced undergraduateness was a intuitive explanation of three body forces with such mundane objects as the sun, and the moon, and the tides. But we know intuitive explanations can only take us so far in nuclear physics, so soon we were dealing with pion exchange, V_low,k, and non-local potentials. I got a bit lost with what sounded like technicalities (cut offs, next to next to next leading order), yet it was still possible for me to keep track to where the talk was going until we reached the results.

When the results of the NNNNMNNLO calculations showed up, we saw again how 3 body interactions are there to help us describe nuclear structure of semi light isotopes (Jason Holt's earlier talk), and their effect in the equation of state (EOS) of nuclear matter. Achim can calculate the problem until nuclear matter density, and afterwards he asked his nephew to finish painting the picture.... But the nephew had fever that day, so he had to use polytrops. From these emerged constrained in the EOS that resulted in a neutron star radius of about 12 km +/- a couple of km. The curves shoot straight up from this point in the mass-radius diagram, so the NS mass basically is whatever it wants to be (that's your task astronomer!). He mentioned the results agree nicely with recent constrains from observations (a completely different method) published by another bright young mind, A. Steiner and collaborators (arxiv). Should be fun to watch how the NS community digests these constraints.

My big question at the end was why calculations stop at nuclear density. It seems you have to give the drawing to your nephew right when things take a turn in the mass-radius diagram!


* I had stayed up making the last figure for my talk, which I decided to migrated to the "extra slide section" on the morning!