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Theory of the Strong Interaction Verified

Supercomputer calculates mass difference between neutron and proton

Jülich, 27. März 2015 – The fact that the neutron is slightly more massive than the proton is the reason why atomic nuclei have exactly those properties that make our world and ultimately our existence possible. Eighty years after the discovery of the neutron, a team of physicists from France, Germany, and Hungary headed by Zoltán Fodor, a researcher from Wuppertal, has finally calculated the tiny neutron-proton mass difference. The findings, which have been published in the current edition of Science, are considered a milestone by many physicists and confirm the theory of the strong interaction. As one of the most powerful computers in the world, JUQUEEN at Forschungszentrum Jülich was decisive for the simulation.

The existence and stability of atoms relies heavily on the fact that neutrons are slightly more massive than protons. The experimentally determined masses differ by only around 0.14 percent. A slightly smaller or larger value of the mass difference would have led to a dramatically different universe, with too many neutrons, not enough hydrogen, or too few heavier elements. The tiny mass difference is the reason why free neutrons decay on average after around ten minutes, while protons – the unchanging building blocks of matter – remain stable for a practically unlimited period.

In 1972, about 40 years after the discovery of the neutron by Chadwick in 1932, Harald Fritzsch (Germany), Murray Gell-Mann (USA), and Heinrich Leutwyler (Switzerland) presented a consistent theory of particles and forces that form the neutron and the proton known as quantum chromodynamics. In 2004, David Gross, Frank Wilczek, and David Politzer received the Nobel Prize for their discovery of "asymptotic freedom", the pivotal property of the theory: unlike in electrodynamics, consider for example two approaching magnets, the force in quantum chromodynamics weakens at shorter distances.

Today, we know that protons and neutrons are composed of "up quarks" and "down quarks". The proton is made of one down and two up quarks, while the neutron is composed of one up and two down quarks. Simulations on supercomputers over the last few years confirmed that most of the mass of the proton and neutron results from the energy carried by their quark constituents in accordance with Einstein’s formula E=mc2. However, a small contribution from the electromagnetic field surrounding the electrically charged proton should make it about 0.1 percent more massive than the neutral neutron. The fact that the neutron mass is measured to be larger is evidently due to the different masses of the quarks, as Fodor and his team have now shown in extremely complex simulations.

For the calculations, the team developed a new class of simulation techniques combining the laws of quantum chromodynamics with those of quantum electrodynamics in order to precisely determine the effects of electromagnetic interactions. By controlling all error sources, the scientists successfully demonstrated how finely tuned the forces of nature are.

Professor Kurt Binder is Chairman of the Scientific Council of the John von Neumann Institute for Computing (NIC) and member of the German Gauss Centre for Supercomputing. Both organizations allocate computation time on JUQUEEN to users in a competitive process. "Only using world-class computers, such as those available to the science community at Forschungszentrum Jülich, was it possible to achieve this milestone in computer simulation," says Binder. JUQUEEN was supported in the process by its "colleagues" operated by the French science organizations CNRS and GENCI as well as by the computing centres in Garching (LRZ) and Stuttgart (HLRS).

The results of this work by Fodor’s team of physicists from Bergische Universität Wuppertal, Centre de Physique Théorique de Marseille, Eötvös University Budapest, and Forschungszentrum Jülich open the door to a new generation of simulations that will be used to determine the properties of quarks, gluons, and nuclear particles. According to Professor Kálmán Szabó from Forschungszentrum Jülich, "In future, we will be able to test the standard model of elementary particle physics with a tenfold increase in precision, which could possibly enable us to identify effects that would help us to uncover new physics beyond the standard model."

JUQUEENSupercomputer JUQUEEN at Forschungszentrum Jülich.
Copyright: Forschungszentrum Jülich

"Forschungszentrum Jülich is supporting the work of excellent researchers in many areas of science with its supercomputers. Basic research such as elementary particle physics is an area where methods are forged, and the resulting tools are also welcomed by several other users," says Prof. Dr. Sebastian M. Schmidt, member of the Board of Directors at Jülich who has supported and encouraged these scientific activities for years.

Original publication

Sz. Borsanyi, S. Durr, Z. Fodor, C. Hoelbling, S. D. Katz, S. Krieg, L. Lellouch, T. Lippert, A. Portelli, K. K. Szabo, B. C. Toth
Ab initio calculation of the neutron-proton mass difference
Science 27 March 2015, DOI: 10.1126/science.1257050

Commentary from Nobel laureate Frank Wilczek in Nature News & Views
"Particle physics: A weighty mass difference"
Nature (2015), DOI: 10.1038/nature14381, published online 08 April 2015


Dr. Stefan Krieg, Jülich Supercomputing Centre, Forschungszentrum Jülich
Tel. +49 2461 61-8715

Prof. Kalman Szabo, Jülich Supercomputing Centre, Forschungszentrum Jülich
Tel. + 49 2461 61-96471

Press contact

Tobias Schlößer, Corporate Communications, Forschungszentrum Jülich
Tel. +49 2461 61-4771

Further Information

University of Wuppertal, Faculty of Mathematics und Natural Sciences

Eötvös University, TTK Physics

Centre de Physique Théorique

John von Neumann-Institut für Computing (NIC)

The John von Neumann Institute for Computing (NIC) provides supercomputer capacity for projects in science, research, and industry in the fields of modelling and computer simulation. As a joint foundation of the three Helmholtz centres Forschungszentrum Jülich, Deutsches Elektronensynchrotron DESY, and GSI Helmholtzzentrum für Schwerionenforschung, it supports computational science in Germany and Europe. Its Scientific Council decides autonomously on the allocation of computation time based on scientific considerations only.

Gauss Centre for Supercomputing (GCS)

GCS provides the most powerful high-performance computing infrastructure in Europe to serve a broad range of research and industrial activities in various disciplines. GCS is the alliance of the three national supercomputing centres High Performance Computing Center Stuttgart (HLRS), Jülich Supercomputing Centre (JSC), and Leibniz Supercomputing Centre, Garching near Munich (LRZ).

Jülich Supercomputing Centre (JSC)

JSC at Forschungszentrum Jülich has operated the first German supercomputing centre since 1987. JSC operates one of the most powerful supercomputers in Europe, JUQUEEN. About 200 experts and contacts for all aspects of supercomputing and simulation sciences work at JSC. JSC's research and development concentrates on mathematical modelling and numerical, especially parallel algorithms for quantum chemistry, molecular dynamics, and Monte Carlo simulations. The focus in the computer sciences is on cluster computing, performance analysis of parallel programs, visualization, computational steering and grid computing. As a member of the German Gauss Centre for Supercomputing, JSC has coordinated the construction of the European research infrastructure PRACE – Partnership for Advanced Computing in Europe since 2008.

Das Forschungszentrum Jülich

Forschungszentrum Jülich pursues cutting-edge interdisciplinary research on pressing issues facing society today. With its competence in materials science and simulation, and its expertise in physics, nanotechnology, and information technology, as well as in the biosciences and brain research, Jülich is developing the basis for the key technologies of tomorrow. In this way, Forschungszentrum Jülich helps to solve the grand challenges facing society in the fields of energy and the environment as well as information and the brain. Forschungszentrum Jülich is breaking new ground in the form of strategic partnerships with universities, research institutions, and industry in Germany and abroad. With more than 5,000 employees, Jülich – a member of the Helmholtz Association – is one of the large interdisciplinary research centres in Europe.