Study of annihilations with slow extracted antiprotons

It is well known that everything we observe, from the smallest objects in our everyday lives to distant stars and galaxies, including all known life forms is made of matter. However, according to the leading theory about the creation of our universe, the Big Bang should have produced equal amounts of matter and antimatter, as particle creation out of energy can only happen in pairs. Each pair consists of a particle and its corresponding antiparticle with the same mass but opposite electric charge. When such partners come into contact again, they cancel each other out, releasing energy, as they mutually annihilate. If matter and antimatter were created and destroyed together, it seems the universe should contain nothing but leftover energy. The reason why todays universe is dominated by matter is still one of the biggest mysteries in modern physics. Driven by this unsolved problem, in the past few decades physicists have successfully produced and studied antimatter particles in laboratory. Today antiprotons, the antimatter counterparts of protons are routinely produced at the Antiproton Decelerator at CERN, and even antihydrogen, the only antimatter atom synthesized hitherto is being thoroughly examined in various experiments. The measurements of the different properties and interactions of antimatter particles are compared to the corresponding matter particles or to physics models, in search for subtle differences that could point to the reason why our universe is matter-filled. The antiproton annihilation process, where an antiproton annihilates with a proton or with an atomic nucleus and new particles are produced is one of the key mechanisms in matter- antimatter interaction. Moreover, the only way to detect antihydrogen in the antimatter experiments at CERN is through its annihilation. At present, the features of the very low energy antiproton annihilation with nuclei are still not well known, and the different models give different predictions. This FWF project aims to reveal the full image of the antiproton annihilation with various atomic nuclei, by detecting all the charged particles that emerge from such interaction. This will explore some of the annihilations unknown traits and will help the validation of different physics models, while potentially identifying novel nuclear physics processes not yet included in these models. The measurements will take place at the ASACUSA experiment at CERN, which will be upgraded to guide very slow antiprotons towards different nuclear targets. Part of the project will be devoted to simulate and to estimate the feasibility of a new experiment in which a very rare, exotic annihilation occurring between one antiproton and helium-3 would be observed for the first time.

In this project, we set out to understand one of the most fascinating processes in physics: what happens when antimatter meets ordinary matter. We focused on antiprotons-the antimatter counterparts of protons-and studied how they interact with atomic nuclei when they come to rest in very thin solid materials. In most cases, the antiproton and part of the nucleus annihilate each other, i.e. disappear releasing a significant amount of energy and producing a bunch of new particles. These particles can be very different, depending on the mass of the nucleus. Their detection and identification can provide information about the mechanism of the annihilation process itself. Despite being studied for decades, this process is still not fully understood, especially at very low energies and within complex nuclei. Our goal has been to observe these annihilations with unprecedented precision and to use the data to improve existing theoretical and simulation models. To achieve this, we developed a new experimental setup and analysis method using state-of-the-art pixel detectors known as Timepix4. By arranging several detectors in a box-like geometry around a thin foil target, we recorded detailed images of the charged particles produced in each annihilation, allowing us to reconstruct the three-dimensional point where the annihilation occurred inside the target. We were able to detect particles coming out in all directions, which allowed us to study how they spread after the annihilation. With the advanced algorithms that we developed we were able to track these particles and identify the annihilation vertex with sub-millimeter accuracy. Our measurements were carried out at CERN's Antimatter Factory, currently the only place in the world that can deliver low energy antiprotons for physics studies, using the antiproton trap and beamline of the AEgIS experiment and nine different target materials. The experiments allowed us to record the number of emitted charged particles, their energies, and to reveal how the emitted particles depend on the target nucleus. The analysis phase of the project is underway, and first quantitative and qualitative results are expected soon. To obtain reliable final results, we validate the data analysis methods such as algorithms, carefully calibrate the detectors, and compare the findings with theoretical simulations. This project provides one of the most comprehensive datasets to date on low-energy antiproton annihilations in different materials, facilitating the improvement of the current models and a deeper understanding of how antimatter reacts with matter. More broadly, antiproton annihilation is a basic process used in the study of antihydrogen, making this research a significant contribution to one of the most fundamental questions in particle physics on the matter-antimatter asymmetry, demonstrating how modern experimental techniques continue to reveal new details even in processes we thought we already understood.