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Lattice QCD Investigations of Nuclear and Hadronic Properties

Principal Investigator: Zoltán Fodor, Institut für Theoretische Physik, FB-C, Universität Wuppertal (Germany)
HPC Platform: JUQUEEN of JSC

The typical scale of Quantum Chromodynamics (QCD) is on the level of GeV (giga electron volt), but QCD should also describe nuclear physics, with has a typical scale of MeV (mega electron volt). This three orders of magnitude difference is a precision challenge, which scientists now were able to tackle in the proton-neutron system.

Since the proton and the neutron, the two constituents of atomic nuclei, play the central role in nuclear physics, their study is of crucial importance. The neutron is more massive than the proton by just 0.14% and therefore unstable with a half-life of about 10 minutes. Already a slight change in this relation could lead to a world that is dramatically different from ours: the list of stable atomic nuclei would vary greatly and stars would not have ignited the way they do in our universe.

In a large-scale effort utilizing GSC supercomputing resources, an international team of scientists has computed how this MeV scale mass difference arises as a residual of two competing effects: Electromagnetic interactions, which would make the proton heavier, are offset by the tiny difference in the mass of the quarks, which constitute both the proton and the neutron, by just the right amount. From this calculation one can infer how finely tuned two fundamental parameters - the electromagnetic coupling and the mass difference between the light quarks - need to be to allow for the existence of a universe similar to ours.

In addition, precise values for the mass difference between other particles - some of them yet undiscovered - could also be computed. The project required the numerical solution of the theory of the strong interaction (QCD) as well as the electromagnetic interaction with a very high precision. The large scale supercomputing resources from GCS were an indispensable tool for carrying out these calculations with the necessary precision. This same precision is also required to pin down the nuclear forces.

Lattice QCD Investigations of Nuclear and Hadronic Properties

Fig. 01: This figure displays the variation of the mass difference between the neutron and the proton when two fundamental parameters are changed with respect to the values they have in our universe (the blue cross). Along the horizontal axis, the value of the coupling strength of electromagnetism is changed from zero to twice its actual value and on the vertical axis the mass difference between the two light quarks (d and u) is varied. The region in which simple hydrogen atoms are unstable and therefore hydrogen could not exist in large quantities is shaded blue. Bands give the mass difference in units of MeV, where 1 MeV is approximately one permil of the mass of a hydrogen atom. The actual mass difference between neutrons and protons in our universe is approximately 1.29MeV.
© Universität Wuppertal, Fachbereich C

Lattice QCD Investigations of Nuclear and Hadronic Properties

Fig. 02: The mass differences between several sets of subatomic particles. The red dots represent results of our calculation whereas the black lines and grey bands indicate experimental measurements and their errors where applicable. Mass differences are given in units of MeV, where 1 MeV is approximately one permil of the mass of a hydrogen atom.
© Universität Wuppertal, Fachbereich C

This project was also published in SCIENCE (Science 27 March 2015: Vol. 347 no. 6229 pp. 1452-1455, DOI: 10.1126/science.1257050)

Scientific Contact:
Dr. Christian Hölbling
Theoretische Physik, Fachbereich C - Bergische Universität Wuppertal
D-42097 Wuppertal/Germany
e-mail: hch [at]

March 2015