Periodic Reporting for period 1 - ColdPbar (Cold antiProtons for Better Antimatter Research)
Berichtszeitraum: 2023-09-01 bis 2025-08-31
The discovery of antimatter – the finding that each subatomic particle has a counterpart with the same mass but opposite charge, with which it annihilates on contact – is a landmark in the physics of the 20th century. According to the Standard Model (SM) of particle physics, particles and antiparticles show equivalent physical interactions and dynamics. This symmetry leads to one of the most prominent open questions in physics: Why do we – and everything around us – exist?!?
If matter and antimatter are equivalent, we would expect equal amounts of each having been created at the beginning of the universe, followed by immediate mutual annihilation. However, a part of the matter has survived and we live now in a world dominated by matter. This matter-antimatter asymmetry is not explained by the SM.
A promising way to study it is by measuring antiparticle properties and compare them with their matter counterparts. Two efforts to that end are currently pursued: investigating the gravitational interaction of antimatter, and performing precision spectroscopy of bound systems containing antiparticles.
The AEgIS experiment at CERN pursues both these approaches. It makes use of slow antiprotons provided by CERN’s AD/ELENA complex (Antiproton Decelerator / Extra Low ENergy Antiproton ring). Those are combined with positrons (“anti-electrons”) to form antihydrogen atoms. Antihydrogen atoms can then be used in free-fall experiments to measure their gravitational acceleration, or as starting point to form more complex antiprotonic bound systems that can then be studied spectroscopically.
However, the sensitivity – and thus the discovery potential – of these measurements is limited by the thermal motion of antiprotons. In free-fall experiments, the random thermal motion limits the achievable flux of antihydrogen atoms and blurs the atom’s trajectories. In spectroscopic studies, it leads to limited storage time of the particles and Doppler broadening of spectroscopic transitions.
The antiproton temperature could be lowered drastically by thermalization with laser-cooled ions. Laser-cooling – a standard technique for neutral atoms and positive ions – however, has never been realized with negative ions (anions). The focus of this project was to progress towards this goal of laser cooling anions to ultimately provide ultracold antiprotons.
If matter and antimatter are equivalent, we would expect equal amounts of each having been created at the beginning of the universe, followed by immediate mutual annihilation. However, a part of the matter has survived and we live now in a world dominated by matter. This matter-antimatter asymmetry is not explained by the SM.
A promising way to study it is by measuring antiparticle properties and compare them with their matter counterparts. Two efforts to that end are currently pursued: investigating the gravitational interaction of antimatter, and performing precision spectroscopy of bound systems containing antiparticles.
The AEgIS experiment at CERN pursues both these approaches. It makes use of slow antiprotons provided by CERN’s AD/ELENA complex (Antiproton Decelerator / Extra Low ENergy Antiproton ring). Those are combined with positrons (“anti-electrons”) to form antihydrogen atoms. Antihydrogen atoms can then be used in free-fall experiments to measure their gravitational acceleration, or as starting point to form more complex antiprotonic bound systems that can then be studied spectroscopically.
However, the sensitivity – and thus the discovery potential – of these measurements is limited by the thermal motion of antiprotons. In free-fall experiments, the random thermal motion limits the achievable flux of antihydrogen atoms and blurs the atom’s trajectories. In spectroscopic studies, it leads to limited storage time of the particles and Doppler broadening of spectroscopic transitions.
The antiproton temperature could be lowered drastically by thermalization with laser-cooled ions. Laser-cooling – a standard technique for neutral atoms and positive ions – however, has never been realized with negative ions (anions). The focus of this project was to progress towards this goal of laser cooling anions to ultimately provide ultracold antiprotons.
The main activity of this project focused on the production and selection of anions, the guidance of the anion beam and the deceleration of that beam, as well as, finally, the trapping of anions originating from that decelerated beam in an ion trap. A dedicated setup to develop techniques for these steps, learn their challenges, and optimize the necessary operations had previously been developed at the AEgIS collaboration. First, modest, results of negative ion generation and trapping had been demonstrated.
This project set out by improving upon these previous results. In particular, the number of negative ions trapped and the lifetime of the ions in the trap needed to be increased significantly.
During the project, significant limitations of the existing setup in both these aspects have been identified, preventing serious attempts of laser manipulation of trapped anions with that setup. Therefore, it was decided to take a leap forward and move on from that “test bed” setup to the actual antimatter experiment apparatus of the AEgIS collaboration.
For that endeavor, the existing anion source and parts of the ion-beam apparatus had to be moved from the previous laboratory to the main AEgIS experimental facility. These parts of the previous setup were integrated into the larger AEgIS apparatus. This means, they had to be mechanically and electronically adapted for operation with that apparatus. Also, they had to be adapted to the different environment conditions, particularly for operation in a high-magnetic-field environment. Thereafter, the anion source was operated again and a beam of anions was injected into the AEgIS apparatus.
This project set out by improving upon these previous results. In particular, the number of negative ions trapped and the lifetime of the ions in the trap needed to be increased significantly.
During the project, significant limitations of the existing setup in both these aspects have been identified, preventing serious attempts of laser manipulation of trapped anions with that setup. Therefore, it was decided to take a leap forward and move on from that “test bed” setup to the actual antimatter experiment apparatus of the AEgIS collaboration.
For that endeavor, the existing anion source and parts of the ion-beam apparatus had to be moved from the previous laboratory to the main AEgIS experimental facility. These parts of the previous setup were integrated into the larger AEgIS apparatus. This means, they had to be mechanically and electronically adapted for operation with that apparatus. Also, they had to be adapted to the different environment conditions, particularly for operation in a high-magnetic-field environment. Thereafter, the anion source was operated again and a beam of anions was injected into the AEgIS apparatus.
While actual laser cooling could not be achieved due to the challenges mentioned above, considerable progress towards that long-term goal could be achieved. The challenges with production, injection and trapping of anions have been identified. A new approach to trap anions, and especially to co-trap them with antiprotons, has been developed. The anion source from the original experimental setup has been integrated into the main experimental apparatus of the AEgIS experiment. The source was successfully operated in that environment and anions were injected into the AEgIS apparatus for future co-trapping with antiprotons.
Moreover, a novel, less complex, scheme for cooling anions has been developed. Together, these achievements pave the way for future cooling of anions and thus for preparing cold antiprotons and cold antihydrogen.
In the long run, this will lead to substantially more sensitive measurements of the gravitational acceleration of antihydrogen and to novel experiments with bound antiprotonic systems – and thus ultimately to novel insight in the puzzling matter-antimatter asymmetry.
Moreover, a novel, less complex, scheme for cooling anions has been developed. Together, these achievements pave the way for future cooling of anions and thus for preparing cold antiprotons and cold antihydrogen.
In the long run, this will lead to substantially more sensitive measurements of the gravitational acceleration of antihydrogen and to novel experiments with bound antiprotonic systems – and thus ultimately to novel insight in the puzzling matter-antimatter asymmetry.