Researchers "weigh" matter building blocks

Supercomputers calculate the exact nucleon mass for the first time

Supercomputers help to understand the processes in atomic nuclei and their masses. The forces between three quarks in the nucleon, explain the visible portion of the mass in the universe. © Forschungszentrum Jülich / Site plan with material from NASA, ESA and AURA / Caltech)
Read out

An international team of researchers has for the first time succeeded in calculating the mass of the most important building blocks of matter - protons and neutrons - in a theoretical way. Physicists' most important tool: the supercomputer JUGENE at Forschungszentrum Jülich. The elaborate simulations confirm the accuracy of a fundamental physical theory, quantum chromodynamics, according to scientists in the current issue of Science.

Matter is made up of atoms, and atoms in turn consist of a nucleus of protons and neutrons that revolve around the electrons. "More than 99.9 percent of the mass of visible matter comes from the protons and neutrons, " explains Zoltan Fodor, a Hungarian physicist currently working at Bergische Universität Wuppertal, who led the research project at the Jülich supercomputer JUGENE. These particles, summarized by the physicists under the term "nucleons", are each made up of three quarks.

Three quarks make a nucleon

The mass of the three quarks, however, only adds up to about five percent of the mass of a core building block - so from where do the nucleons have their mass? The answer to this question can be found in the famous formula E = m × c2 by Albert Einstein: energy and mass are equivalent to each other, and 95 percent of the nucleon mass has its origin in the kinetic energy of the quarks and between them exchanged particles.

The three quarks of a nucleon are bound together by the strong interaction, a force which is important only in the range of the elementary particles, but which, as their name implies, is very strong. Physicists have long had a theoretical description of this interaction, quantum chromodynamics. "In principle, it should be possible to calculate the mass of the nucleons from quantum chromodynamics, " says Fodor.

However, such calculations are enormously complicated. Just as the electromagnetic forces are mediated by photons - light particles - there are also carrier particles in the strong interaction, the so-called gluons. But these gluons can - unlike photons - also attract each other. On the one hand, this self-interaction leads to quarks attracting each other so strongly that they never occur alone, but always form larger particles in pairs or threes. On the other hand, the self-interaction makes the calculation of the mass of these particles so complex that it has so far exceeded the possibilities of researchers. display

The fastest computer in Europe

Thanks to the JUGENE supercomputer at J Forschungslich Research Center, Fodor and his colleagues were able to overcome this obstacle, for the first time correctly describing the strong interaction even for larger quark distances and thus the masses of protons, nucleons and other quarks. JUGENE can perform 180 trillion arithmetic operations every second, making it the fastest computer in Europe.

For their calculations, Fodor and his team have divided space and time into a close-meshed four-dimensional grid and solved the complex equations of quantum chromodynamics on the points of this grid. Then the researchers gradually made the spacing of the grid points smaller and smaller, in order to adhere more and more to reality, the continuous space-time. "It is one of the most computationally intensive works in the history of humanity, " says Fodor.

Quantum chromodynamics are correct

As a result, the scientists finally obtained values ​​for the masses of the nucleons, which correspond exactly to the values ​​measured in experiments. "That's how we showed that quantum chromodynamics is actually a correct description of the strong interaction, " Fodor says happily.

"The origin of the vast majority of the mass of visible matter is thus clarified, " the researcher continues. But not all riddles are solved. Because the visible matter makes up only a small part of the total mass of the universe about 80 percent of this mass is dark and consists of previously unknown elementary particles. Wherefore this dark matter has its mass, so far we have no explanation.

(Forschungszentrum J lich, 21.11.2008 - DLO)