Neutron-Proton Mass Difference Calculated Using JUQUEEN and JUROPA

In a recent paper published in the 27 March 2015 issue of Science, physicists from Budapest, Marseille, Wuppertal, and Jülich calculated the neutron-proton mass difference using the JUQUEEN and JUROPA supercomputers at JSC. This work was described as a 'landmark calculation' by Nobel laureate Frank Wilczek

The mass difference results from two counteracting effects: for equal light quark masses, electrodynamics renders the proton slightly heavier due to the energy stored in the electromagnetic field that surrounds it. Differences in the light quark masses, conversely, increase the neutron mass, since the neutron contains two of the heavier down-quarks compared to one in the case of the proton. The interplay of these effects has significant implications for the stability of matter: if the neutron-proton mass splitting only differed slightly from the value found in nature, the universe as we know it could not exist.

Due to the complicated non-linear interaction between quarks and gluons, combined with the long-range nature of the electromagnetic interaction, this mass difference could not, until now, be calculated from the underlying theory, the Standard Model of elementary particle physics. Through a combination of new algorithms, analytical calculations and simulations on JUQUEEN and JUROPA, with contributions from machines at HLRS, LRZ, and IDRIS in France, the authors made 'breakthrough progress', according to Wilczek, some 80 years after the discovery of the neutron and 40 years after the first formulation of the theory.

The Science article is available at, and Wilczek's comments at
(Contact: Dr. Stefan Krieg,

JSC News, 22 April 2015

Last Modified: 16.08.2022