## Tuesday, January 19, 2010

### Logicomix

Logicomix is a graphic novel which features a fictionalized life story of the Philosopher/Logician Bertrand Russell. I thought the story was quite charming and did manage to give some nice hints about the controversies in the foundations of mathematics, including Russells' Paradox, one of my favorites. I've never tried to read Principia Mathematica (Russell and Whitehead's magnum opus) myself, though I once had a meeting at a professor's office who showed me a copy - he was a big fan of the work - it looked pretty hairy!

### Nucleon Excitations

Nucleon Excitations
The mass pattern of The mass pattern of nucleon and Delta resonances is compared with predictions based on quark models, the Skyrme model, AdS/QCD, and the conjecture of chiral symmetry restoration. and $\Delta$ resonances is compared with predictions based on quark models, the Skyrme model, AdS/QCD, and the conjecture of chiral symmetry restoration.

Baryon Spectroscopy and the Origin of Mass
The proton mass arises from spontaneous breaking of chiral symmetry and the formation of constituent quarks. Their dynamics cannot be tested by proton tomography but only by studying excited baryons. However, the number of excited baryons is much smaller than expected within quark models; even worse, the existence of many known states has been challenged in a recent analysis which includes - compared to older analyses - high-precision data from meson factories. Hence $\pi N$ elastic scattering data do not provide a well-founded starting point of any phenomenological analysis of the baryon excitation spectrum. Photoproduction experiments now start to fill in this hole. Often, they confirm the old findings and even suggest a few new states. These results encourage attempts to compare the pattern of observed baryon resonances with predictions from quark models, from models generating baryons dynamically from meson-nucleon scattering amplitudes, from models based on gravitational theories, and with the conjecture that chiral symmetry may be restored at high excitation energies. Best agreement is found with a simple mass formula derived within AdS/QCD. Consequences for our understanding of QCD are discussed as well as experiments which may help to decide on the validity of models.