Speaker: 
Antonin Portelli
Speaker affiliation: 
University of Southampton
Date: 
Thursday, November 13, 2014 - 13:00
Room: 
Building 46, seminar room (5th floor)
Title: 
Ab initio calculation of the neutron-proton mass difference
Abstract: 

(based on arXiv:1406.4088)
The existence and stability of atoms relies on the fact that neutrons are more massive than protons. The mass difference is only 0.14% of the average. This tiny mass splitting has significant astrophysical and cosmological implications. A slightly smaller or larger value would have led to a dramatically different universe. Here we show, how this difference results from the competition between electromagnetic and mass isospin breaking effects. We compute the neutron-proton mass splitting and show that it is greater than zero by five standard deviations. Furthermore, splittings in the \Sigma, \Xi, D and \Xi_{cc} isospin multiplets are determined providing also predictions. We perform lattice Quantum-Chromodynamics plus Quantum-Electrodynamics computations with four, non-degenerate Wilson fermion flavours. Four lattice spacings and pion masses down to 195 MeV are used.