Strong interaction reconfirmed

Two studies confirm validity of strong force holding atomic nuclei together

The strong interaction is one of the four fundamental forces in the universe. Around 50 years after it was first described, its theoretical foundations have now been tested with unprecedented accuracy in two recent studies. Scientists from the Max Planck Institute (MPP) for Physics were involved in both of them. These precision tests of the strong interaction are crucial for further developing theories of particle physics and evaluating future experimental results.

The theory of strong interaction is also known as quantum chromodynamics (QCD). In simple terms, the strong force uses gluons to bind elementary quarks, which can only exist in a bound state. Like many other parameters in particle physics, the strong interaction is subject to continuous precision calculations. The International Particle Data Group publishes the latest findings of the international research community every two years in a review – the next on September 5, 2025. This will also include both studies with major contributions of the MPP. 

The nature of asymptotic freedom

The strong interaction was first described in 1973. Since then, the properties of the strong force have been gradually calculated with increasing precision and tested with experiments. Nevertheless, the parameters of the strong interaction remain among the least accurately determined fundamental quantities in physics. 

“The unusual thing about this force is that its strength increases with the distance between the particles,” explains Daniel Britzger, a scientist at the ATLAS group at the MPP. “The closer quarks are to each other, the less they ‘feel’ each other.” This property is called asymptotic freedom and is a central concept of QCD. Its discovery was awarded the Nobel Prize in 2004.

Tests in the largest energy range to date 

The current study is based on new precision calculations on the formation of dijets, i.e., two jets, for example, after proton-proton collisions at the LHC. In their study, the researchers were able to determine the value of the strong interaction constant with an accuracy of 1.8 percent. “This parameter indicates the strength of the force, and the new result agrees very well with all previous measurements,” says Daniel Britzger. 

Dijet events are directly related to the interactions between quarks and gluons. This enabled the scientists to precisely confirm the predictions of asymptotic freedom, i.e., how the strength of the interaction behaves at different energies. “According to theory, the higher the energy, the closer the quarks move together,” says Daniel Britzger. “We have tested this prediction for a huge energy range from 7 to 7,000 gigaelectronvolts – and confirmed it. For the first time, we also included very high energies, which are particularly important for testing models of particle physics.” 

Second study provides consistent results 

Another study was recently published, authored by Giulia Zanderighi, director at the Max Planck Institute for Physics, together with MPP Humboldt fellow Paolo Nason, a scientist at the University of Milan. This study comes to consistent results. The two researchers investigated the probability of three-jet events (trijets) occurring after electron-positron collisions. 

Their calculations are based on a large data sample from various electron-positron experiments at energies from 22 to 207 gigaelectronvolts that were performed, e.g. at the LEP accelerator and the JADE experiment. By applying innovative calculation methods, the two researchers were able to improve their precision calculations compared to the 2023 report of the Particle Data Group (PDG). 

New data after the upgrade of the LHC 

For the two studies, the researchers had access to an enormous treasure trove of data generated over the past 45 years in the LEP, SLC, HERA, and LHC experiments. “In our work, we have applied the most comprehensive calculations to date and compared them with the entire available data set. However, for even more accurate studies, we need to include additional existing and future data. New data is expected as soon as measurements begin with the High-Luminosity upgrade of the LHC, a project with substantial contributions from the MPP,” concludes Daniel Britzger.

In parallel, researchers at the MPP are continuing to refine the theoretical foundations in order to gain a better understanding of the fundamental forces of nature in the universe.