Prof. Dr. Thomas Mannel
It is probably the most important and best-tested theory in human history and describes the fundamental building blocks of matter and the forces between them: the standard model of elementary particle physics. Researchers in Siegen are constantly improving the calculations that result from this model in order to detect even tiny deviations from experimental data. Because these could be clues to a world beyond known physics.
If you search the Internet for a gift for physics students or physics teachers, you will find T-shirts with a print like "Nobody is perfect. But as a physicist, you're damn close". Or ones with the formula of the standard model emblazoned legibly on them. It's amazing: mathematically, all the properties of fundamental particles and their interactions can actually be summarized in four short lines. It takes a lot more words.
The standard model recognizes quarks as fundamental particles. But didn't we learn at school that our material world is made up of atoms? And that an atom consists of an atomic nucleus and electrons, with the atomic nucleus in turn being made up of neutrons and protons? Where are the quarks?
Professor Dr. Thomas Mannel, theoretical particle physicist at the University of Siegen, is used to closing this gap between school knowledge and his subject: "The neutrons and protons in the atomic nucleus each consist of three quarks: the neutrons of one up quark and two down quarks, the protons of two up quarks and one down quark." This means that the visible matter known to us appears to consist of up and down quarks as well as electrons.
Focus on the heaviest quarks
But according to the standard model, that is by no means all. In fact, heavier copies of the up and down quark as well as the electron, which are referred to as first-generation particles, exist for fractions of a second. And there are even heavier copies of these second-generation particles. "We in Siegen are working specifically on this third generation of particles," says Mannel. "We are using increasingly precise calculations to find out whether the standard model is also strictly valid for this generation."
The two quark types of the third generation are called top and bottom. Just like the quarks of the second generation, they are unstable and decay into lighter quarks. The quarks of the second and third generation initially appear to be unnecessary duplications of nature - of which nobody knows exactly why they exist. But the standard model describes their existence. And at huge accelerators such as the Large Hadron Collider (LHC) at the CERN research center in Geneva, where protons collide at almost the speed of light, they have been detected - usually indirectly - as products of these crashes. Indirectly because individual free quarks do not exist. Just as the up and down quarks are locked up as first-generation particles in protons or neutrons, for example, the heavier quarks, with the exception of the top quark, also always occur in bound states. The top quark and many other particles produced in the crash decay so quickly that researchers have to reconstruct their properties on the basis of the decay products.
For decades, all theoretical calculations based on the standard model agreed with the experimental data. When scientists were able to detect the so-called Higgs particle at CERN in 2012, this was another triumph for the model, as physicists had already predicted the existence of this particle on its basis in the 1960s. It is part of the mechanism that explains the mass of elementary particles.
However, for around ten years, discrepancies have been found between calculations and experiments. "The specialization here in Siegen on the third generation has something to do with the fact that their particles are so heavy. That's why they have the strongest coupling to the mass-imparting Higgs particle and have very special properties," explains Mannel. "In fact, this is where the observed, apparent deviations between theoretical calculations and measurement results preferentially occur." The topic is therefore "currently the hot topic in particle physics".
"However, the deviations are not large enough to be able to be definitively sure whether there really is something behind it that can only be explained by the existence of previously unknown particles and forces," says Mannel's colleague Professor Dr. Alexander Lenz. This is because all values from the calculations of elementary particle physicists - just like measurement results - are subject to a certain degree of uncertainty. Scientists use the term uncertainty in a special way: Uncertainty delimits a range of values in which there is a very high probability that the true value lies. Mannel, Lenz and the other theorists of the Transregional Collaborative Research Center (TRR/SFB) "Phenomenological Elementary Particle Physics after the Higgs Discovery" of the German Research Foundation have therefore set themselves the task of reducing the uncertainties in the calculated values and thus increasing confidence in the results. In addition to the researchers from Siegen, the TRR/SFB included particle physicists from the Karlsruhe Institute of Technology and RWTH Aachen University.
However, the Siegen theorists are not only working on ever more precise calculations, they are already going one step further: they are trying to develop alternative models. "If something is wrong with the standard model, then you need something else. But constructing such alternatives is anything but easy, because there is also a lot of data that agrees extremely precisely with the standard model," says Mannel.
Elementary particle physicists around the world are searching hard for new models because the standard model does not provide explanations for certain phenomena in the universe. In addition to the mystery of dark matter, this also includes gravity. Many particle physicists therefore see the standard model as a theory that functions as a borderline case of a more far-reaching theory in the area of comparatively low energies. We already know something similar from mechanics, for example: Classical mechanics, the principles of which were formulated by Isaac Newton as early as the end of the 17th century, correctly describes all physical phenomena at speeds and distances in everyday life. Today, however, we know that classical mechanics is only a borderline case of more comprehensive theories - the theory of relativity and quantum mechanics.
The ideal would be to explain the largest and smallest phenomena of our world comprehensively with a single theory. The theorists from Siegen play an important role in the global search for this world formula. A T-shirt with the slogan "The standard model is not perfect. But Siegen calculations are very close to perfection" would be apt. But the scientists prefer to draw attention to themselves through outstanding specialist publications.