ELEMENTARY PARTICLE PHYSICS

The Universe as we know it is highly sensitive to the size of the n - p mass difference. If it would be larger than the binding energy of the deuteron, no fusion would take place. If it would be only a little smaller, all hydrogen would have been burned to helium. Though it is one of the most consequential parameters of physics, the n - p mass difference is not a primary quantity. The relevant theories for the calculation are Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED). With strong and electromagnetic effects being of the same order of magnitude and strongly correlated, this makes a nonperturbative evaluation necessary. For the first time, scientists at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, included both QCD and QED in the same nonperturbative calculation, which allowed the physicists to predict isospin breaking effects in the meson, baryon and quark sectors from first principles, and in particular the n - p mass difference.

The nucleon mass difference of M_n – M_p = 1.35(18)(8) MeV and the electromagnetic contribution to the pion splitting M_{π +} – M_{π 0} = 4.60(20) MeV is obtained in good agreement with experiment (Figure 1). Further the scientists report a first determination of the separation between strong and electromagnetic contributions in the MSbar scheme (Figure 2).

Acknowledgements:

The calculations were made possible due to a generous grant of compute time assigned to the DESY research team with the 9^th GCS Call for Large-Scale Projects. High performance computing system JUQUEEN of the Jülich Supercomputing Centre served as computing platform for this project.

Scientific Contact:

Prof. Dr. Gerrit Schierholz
Deutsches Elektronen-Synchrotron DESY
Notkestraße 85, D-22607 Hamburg (Germany)
e-mail: gerrit.schierholz [at] desy.de