What is the key to understanding physics

Breakthrough in nuclear physics

Using collision data from the ALICE detector at the Large Hadron Collider at CERN, the strong interaction between a proton (right) and the rarest of the hyperons, the omega hyperon (left), which contains three strange quarks, has succeeded with high precision to eat. D. Dominguez / CERN

Precise measurements of the strong interaction between stable and unstable particles

The positively charged protons in atomic nuclei should actually repel one another, and yet even heavy nuclei with many protons and neutrons stick together. The so-called strong interaction is responsible for this. Prof. Laura Fabbietti and her research group at the Technical University of Munich (TUM) have now developed a method to precisely measure the strong interaction in particle collisions in the ALICE experiment at CERN in Geneva.

The strong interaction is one of the four basic forces in physics. It is essentially responsible for the existence of atomic nuclei that consist of several protons and neutrons. Protons and neutrons, in turn, consist of smaller particles called quarks. And this, too, is held together by the strong interaction.

As part of the ALICE (A Large Ion Collider Experiment) project at CERN in Geneva, Prof. Laura Fabbietti and her research group at the Technical University of Munich have now developed a method to determine with high precision the forces that act between protons and hyperons, unstable Particles with so-called strange quarks.

The measurements are not only groundbreaking in the field of nuclear physics, but also the key to understanding neutron stars, one of the most enigmatic and fascinating objects in our universe.

One of the greatest challenges of modern nuclear physics is to understand the strong interaction between particles with different quark content using first principles, i.e. to derive this from the forces between the components of the particles, the quarks and the gluons that convey the force.

However, the theory of strong interaction does not allow reliable predictions for normal nucleons with up and down quarks, but only for nucleons that contain heavy quarks, such as hyperons.

Experiments to measure the force are very difficult because hyperons are unstable particles that, hardly produced, immediately decay again. A meaningful comparison between theory and experiment has therefore not been possible so far. The method used by Laura Fabbietti now opens a door for high-precision studies of the dynamics of the strong interaction at the Large Hadron Collider (LHC) particle accelerator.

Four years ago, Professor Laura Fabbietti, Professor of Density and Strange Hadronic Matter at TUM, suggested using femtoscopy to investigate the strong interaction in the ALICE experiment. This technique enables examinations with a spatial resolution close to a femtometer (10-15 Meter). This corresponds roughly to the size of a proton and also to the spatial order of magnitude in which the strong interaction is effective.

Since then, Prof. Fabbietti's group has not only succeeded in examining the collision data for most hyperon-nucleon combinations, but also in determining the strong interaction for the rarest of all hyperons, the omega, which consists of three strange quarks.

In addition, the physicists have also developed a theoretical framework that can provide predictions. -My TUM group has opened up a new way for nuclear physics at the LHC to measure the strong interaction that includes all types of quarks - and this with an unexpected precision and in a place that no one has seen before, says Fabbietti. In the work now published in -Nature-, only a part of the interactions that were examined for the first time are presented.

Understanding the interaction between hyperons and nucleons is also extremely important for testing the hypothesis of whether neutron stars contain hyperons. The forces between the particles have a direct influence on the size of a neutron star.

So far it is unknown what relationship exists between the mass and the radius of a neutron star. In the future, Prof. Fabbietti's work will therefore also help to solve the riddle of neutron stars.

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