Periodic Reporting for period 4 - STEP (Symmetry Tests in Experiments with Portable Antiprotons)
Reporting period: 2024-01-01 to 2025-06-30
Summary of the context and overall objectives of the project
The origin of the universe, its development and its fate has inspired the research of curious minds through the ages of mankind. Today, particle physics plays an essential role in our current understanding of cosmology. As the early universe was in a hot dense state that we can approach in particle accelerators, particle physics helps us to model the evolution of our universe. However, there are still many mysteries that elude satisfying scientific answers in cosmology. One of them is the asymmetry between matter and antimatter that we observe in our universe. Every object that we encounter in the cosmos appears to be made from matter, whereas our particle physics theory expects equal amounts of matter and antimatter remaining from the Big Bang today. To resolve this question, we are comparing the properties fundamental building blocks of matter and antimatter to test if we really find the symmetric behavior predicted by our particle physics theory.
This ERC project called STEP (Symmetry Tests in Experiments with Portable antiprotons) aims to improve the methods that are used to compare properties of the proton and the antiproton, in particular the measurements of their magnetic moments and charge-to-mass ratios.
The most precise measurements of these properties are conducted in an ion trap, where a single proton or antiproton is confined in a combination of magnetic and electric fields. Here, the trapped particles are isolated in a high vacuum at liquid helium temperature (4.2 K / -269 C°) and stored for longer than one year to precisely measure their fundamental properties. Protons are loaded into the trap using an electron beam that creates a proton, e.g. from a water molecule that is frozen on the walls of the ion trap. The antiproton – as antiparticle – can only be produced in a pair creation reaction by a high energy proton beam and required a particle accelerator to be connected to the ion trap. Currently, the world’s only facility capable of providing antiprotons for ion traps is the Antimatter Factory of CERN (European Organization for Nuclear Research, Geneva, Switzerland), and all antiproton precision measurements have been conducted inside this facility.
The antiproton precision measurements at CERN by the BASE collaboration have provided the most precise limits on the symmetry of protons and antiprotons so far. However, the magnetic noise of the accelerator in the environment perturbs the ion trap and sets a limit to the precision that can be reached in this facility. Therefore, we aim to develop a transportable antiproton trap in this project, and conduct antiproton measurements in a laboratory with a calm magnetic environment to reach a higher measurement precision.
The development of the transportable trap is a technical challenge, that requires a novel ion trap design using a robust superconducting magnet that can be transported on the road while in operation. It requires also to design an injection channel for the antiproton between the accelerator and the ion trap that reduces the pressure by a factor of at least one million to prevent the antiprotons from annihilation by colliding with residual gas. The development of this apparatus, and the demonstration of transporting and storing antiprotons is an objective of this project.
Further, we also aim to improve the measurement methods for comparing protons and antiprotons. One challenge is systematic shifts of the measured values due to the residual motion of the trapped particle, and state-of-art methods conduct in many cases measurements where the trapped particle has 300 K or higher temperature during the measurement, or requires excitation to large amplitudes during the measurement. We aim to implement a novel measurement concept that is based on a two-trap measurement scheme that determines small changes of the cyclotron energy in a magnetic bottle trap, and has the potential to conduct measurements at 10-fold or even 100-fold lower energies. This reduces the systematic shifts and enables precision measurements without the temperature limits of state-of-art ion traps.
With the new methods, we intend to contribute to improving the charge-to-mass ratio comparison of protons and antiprotons, to make a test of Lorentz symmetry with antiprotons by conducting simultaneous frequency measurements in spatially separated experiments, and test whether antiprotons have an anomalous gravitational behavior by measuring the antiproton charge-to-mass ratio in at locations with different gravitational potential. All these measurements contribute limits to symmetry violations between protons and antiprotons, and improve our understanding of the origin of the matter-antimatter asymmetry in our universe.
This ERC project called STEP (Symmetry Tests in Experiments with Portable antiprotons) aims to improve the methods that are used to compare properties of the proton and the antiproton, in particular the measurements of their magnetic moments and charge-to-mass ratios.
The most precise measurements of these properties are conducted in an ion trap, where a single proton or antiproton is confined in a combination of magnetic and electric fields. Here, the trapped particles are isolated in a high vacuum at liquid helium temperature (4.2 K / -269 C°) and stored for longer than one year to precisely measure their fundamental properties. Protons are loaded into the trap using an electron beam that creates a proton, e.g. from a water molecule that is frozen on the walls of the ion trap. The antiproton – as antiparticle – can only be produced in a pair creation reaction by a high energy proton beam and required a particle accelerator to be connected to the ion trap. Currently, the world’s only facility capable of providing antiprotons for ion traps is the Antimatter Factory of CERN (European Organization for Nuclear Research, Geneva, Switzerland), and all antiproton precision measurements have been conducted inside this facility.
The antiproton precision measurements at CERN by the BASE collaboration have provided the most precise limits on the symmetry of protons and antiprotons so far. However, the magnetic noise of the accelerator in the environment perturbs the ion trap and sets a limit to the precision that can be reached in this facility. Therefore, we aim to develop a transportable antiproton trap in this project, and conduct antiproton measurements in a laboratory with a calm magnetic environment to reach a higher measurement precision.
The development of the transportable trap is a technical challenge, that requires a novel ion trap design using a robust superconducting magnet that can be transported on the road while in operation. It requires also to design an injection channel for the antiproton between the accelerator and the ion trap that reduces the pressure by a factor of at least one million to prevent the antiprotons from annihilation by colliding with residual gas. The development of this apparatus, and the demonstration of transporting and storing antiprotons is an objective of this project.
Further, we also aim to improve the measurement methods for comparing protons and antiprotons. One challenge is systematic shifts of the measured values due to the residual motion of the trapped particle, and state-of-art methods conduct in many cases measurements where the trapped particle has 300 K or higher temperature during the measurement, or requires excitation to large amplitudes during the measurement. We aim to implement a novel measurement concept that is based on a two-trap measurement scheme that determines small changes of the cyclotron energy in a magnetic bottle trap, and has the potential to conduct measurements at 10-fold or even 100-fold lower energies. This reduces the systematic shifts and enables precision measurements without the temperature limits of state-of-art ion traps.
With the new methods, we intend to contribute to improving the charge-to-mass ratio comparison of protons and antiprotons, to make a test of Lorentz symmetry with antiprotons by conducting simultaneous frequency measurements in spatially separated experiments, and test whether antiprotons have an anomalous gravitational behavior by measuring the antiproton charge-to-mass ratio in at locations with different gravitational potential. All these measurements contribute limits to symmetry violations between protons and antiprotons, and improve our understanding of the origin of the matter-antimatter asymmetry in our universe.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
We have conducted a technical design study to construct the transportable trap system, which includes the several novel parts for cryogenic precision measurement traps. First, we designed a transportable superconducting magnet with a flexible hybrid cooling concept to adept between transport and storage operation, and a rigid mechanical design to withstand forces and vibrations during the transport. The magnet bore contains an advanced multi Penning-trap system optimized for catching, transferring and storing of antiprotons, and an image-current detection unit for non-destructive detection. Further, we produced a differential pumping section that connects the accelerator vacuum to the cryogenic trap region, which requires typically a factor one million lower pressure. To reach this, the differential pumping section consists of a cryogenic inlet valve and a thin long transfer channel for the injection and ejection of antiprotons. To increase the protection of the antiprotons from the interaction with residual gas even more, we designed two cryogenic motorized trap electrodes that act as an extremely low-pressure airlock for the antiprotons. The cryogenic inlay of the transportable trap system has been completed and tested, and presently we are waiting for the delivery of the superconducting magnet to start the operation of the transport trap system.
To load antiprotons into the transportable trap, we have presented the project plan of the transportable trap, and the plans to load antiprotons in the Antimatter Factory to the SPSC committee at CERN. The project was welcomed in the Antimatter Factory of CERN, and is recognized as BASE-STEP as new project within the BASE collaboration. CERN provided a new experiment area for the project, and we have installed and tested the injection beam line that is needed to steer and focus the antiproton beam into the transportable trap.
Regarding the improvement of measurement methods, we have reached a major breakthrough by demonstrating a novel sympathetic cooling technique that is applicable for both protons and antiprotons. The method uses a cloud of beryllium ions in a second separate trap that cools the proton through the image-currents in a wire between the two traps. We managed to cool a single proton to a fraction of 15% of its initial energy. The cooling limit is presently defined by the noise of an LC circuit that we use to amplify the image currents, however, we have explored in simulations how to reduce the cooling limit in the most advantageous way, and anticipate that we can reach 0.1% of the LC circuit noise temperature. Presently, we are working on the implementation of the simulated cooling sequence in a multi-Penning trap system at the University of Mainz. The development of this cooling method was selected to be among the Top 10 breakthroughs in 2021 by Physics World as innovative particle cooling technique.
We also contributed to advance the precision comparisons of protons and antiprotons with conventional methods in the BASE collaboration. A new result of a proton-to-antiproton charge-to-mass ratio comparison was published that improves the uncertainty by a factor of 4 compared to the previous best value. The measurement was conducted over a time span of more than a year, which allowed us to compare the proton and antiproton charge-to-mass along different positions on the elliptic orbit of the earth around the sun. Since the gravitational potential at the measurement location varied during our measurement, we were able to constrain anomalous antiproton gravitation for the first time with a differential method, setting a limit on the violation of the weak equivalence principle for antiprotons with a differential test to less than 3% of the gravitational interaction.
Further, we contributed to a new dark matter search concept for ion traps that aims to detect millicharged particles, which are hypothetical particles that carry a small fraction (<1/1000) of the electron’s elementary charge. Heating rate measurements from ion traps that may origin from Coulomb interaction with the millicharge particles can set very sensitive limits, and we evaluated the limits from antiproton measurements in a trap that has reported the so far lowest heating rates. We also contributed to enhance the image-current detectors and the plans to utilize them to search for axion-like particles, which are another promising dark matter candidate.
To load antiprotons into the transportable trap, we have presented the project plan of the transportable trap, and the plans to load antiprotons in the Antimatter Factory to the SPSC committee at CERN. The project was welcomed in the Antimatter Factory of CERN, and is recognized as BASE-STEP as new project within the BASE collaboration. CERN provided a new experiment area for the project, and we have installed and tested the injection beam line that is needed to steer and focus the antiproton beam into the transportable trap.
Regarding the improvement of measurement methods, we have reached a major breakthrough by demonstrating a novel sympathetic cooling technique that is applicable for both protons and antiprotons. The method uses a cloud of beryllium ions in a second separate trap that cools the proton through the image-currents in a wire between the two traps. We managed to cool a single proton to a fraction of 15% of its initial energy. The cooling limit is presently defined by the noise of an LC circuit that we use to amplify the image currents, however, we have explored in simulations how to reduce the cooling limit in the most advantageous way, and anticipate that we can reach 0.1% of the LC circuit noise temperature. Presently, we are working on the implementation of the simulated cooling sequence in a multi-Penning trap system at the University of Mainz. The development of this cooling method was selected to be among the Top 10 breakthroughs in 2021 by Physics World as innovative particle cooling technique.
We also contributed to advance the precision comparisons of protons and antiprotons with conventional methods in the BASE collaboration. A new result of a proton-to-antiproton charge-to-mass ratio comparison was published that improves the uncertainty by a factor of 4 compared to the previous best value. The measurement was conducted over a time span of more than a year, which allowed us to compare the proton and antiproton charge-to-mass along different positions on the elliptic orbit of the earth around the sun. Since the gravitational potential at the measurement location varied during our measurement, we were able to constrain anomalous antiproton gravitation for the first time with a differential method, setting a limit on the violation of the weak equivalence principle for antiprotons with a differential test to less than 3% of the gravitational interaction.
Further, we contributed to a new dark matter search concept for ion traps that aims to detect millicharged particles, which are hypothetical particles that carry a small fraction (<1/1000) of the electron’s elementary charge. Heating rate measurements from ion traps that may origin from Coulomb interaction with the millicharge particles can set very sensitive limits, and we evaluated the limits from antiproton measurements in a trap that has reported the so far lowest heating rates. We also contributed to enhance the image-current detectors and the plans to utilize them to search for axion-like particles, which are another promising dark matter candidate.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
We have designed a novel trap system for catching, transporting, and storing antiprotons. This includes technical innovations such as the transportable superconducting magnet, the cryogenic inlet valve, the cryogenic motorized electrodes that form the low-pressure airlock for the antiprotons, and the transport channel to inject and eject antiprotons from the trap. We anticipate the completion of the transportable antiproton trap within the next year 2023, and expect to report the transport of antiproton outside the Antimatter Factory of CERN and the characterization of the transport trap system in the scope of this project.
The cooling of trapped particles is essential for any kind of measurement, as the motion of the particle causes systematic shifts due to the residual trap imperfections that the particle encounters along this orbit. The state-of-art cooling method in cryogenic multi Penning trap system is resistive cooling with an image-current detector with the temperature limit close to liquid helium temperature. Our novel sympathetic cooling methods goes beyond the state of art as it provides the opportunity to cool protons and antiprotons to the temperature of laser-cooled ions. This temperature range was in particular not accessible yet for antiprotons, as direct sympathetic cooling with a co-trapped ion has not been sucessfully realized yet.
We expect to optimize the sympathetic cooling technique, and demonstrate a measurement scheme for the cyclotron frequency of trapped ions at much lower energies compared to the state of art. This method is described in the project proposal and uses a magnetic bottle trap to detect tiny excitations in the cyclotron mode to probe the cyclotron frequency. If we use the sympathetic cooling method for precooling, we expect that this method is only limited by magnetic field fluctuations, and for antiproton measurements, these will be reduced by using the transportable antiproton trap. Consequently, the cornerstones for the next antiproton precision measurements will be set within this project.
The latest measurement of the antiproton charge-to-mass ratio sets a limit on the violation of the weak equivalence principle for antiprotons with a sensitivity of 3% of the gravitational interaction. This is the first time that we set a differential limit, i.e. using changes of the gravitational potential along the elliptic orbit of the sun, and not one that is based on elusive assumptions, such as the total binding energy of our galaxy super cluster, that involves distance scales on which gravitation is not understood. Consequently, our work put the test of anomalous antiproton gravitation based on antiproton frequency measurements on a solid ground and is even sensitive enough to exclude antigravity for antiprotons. Using the transportable trap, we aspire to repeat this test in the future with active modifications of the gravitational potential by measuring in different altitudes.
We expect to conduct simultaneous antiproton frequency measurements that are conducted in the transportable antiproton trap and the BASE experiment at CERN. These measurements will constitute a direct test of Lorentz symmetry for antiprotons. So far, it was not possible to conduct such measurements because there were never two antiproton precision experiments simultaneously in operation.
We also expect to report on the use of permanent magnet traps as antiproton transport or storage traps, and we are presently testing a permanent magnet trap with protons at the University of Mainz. Permanent magnet traps provide a cost-efficient trap system that becomes essential to have additional backup traps for offline operation, or to economize transport traps to be able supply multiple laboratories with antiproton reservoirs and increase the number of potential antiproton experiments.
We have also engaged in developing an image-current detector based on the high-temperature superconductor YBCO. We anticipate that this technology can replace the hand-made NbTi coils that are presently used for the image-current detectors. We intend to use the YBCO detector to improve the counting of the number of trapped antiprotons, and this will improve our methods to search for unexpected interactions or decays of antiprotons with particles beyond the Standard Model.
The cooling of trapped particles is essential for any kind of measurement, as the motion of the particle causes systematic shifts due to the residual trap imperfections that the particle encounters along this orbit. The state-of-art cooling method in cryogenic multi Penning trap system is resistive cooling with an image-current detector with the temperature limit close to liquid helium temperature. Our novel sympathetic cooling methods goes beyond the state of art as it provides the opportunity to cool protons and antiprotons to the temperature of laser-cooled ions. This temperature range was in particular not accessible yet for antiprotons, as direct sympathetic cooling with a co-trapped ion has not been sucessfully realized yet.
We expect to optimize the sympathetic cooling technique, and demonstrate a measurement scheme for the cyclotron frequency of trapped ions at much lower energies compared to the state of art. This method is described in the project proposal and uses a magnetic bottle trap to detect tiny excitations in the cyclotron mode to probe the cyclotron frequency. If we use the sympathetic cooling method for precooling, we expect that this method is only limited by magnetic field fluctuations, and for antiproton measurements, these will be reduced by using the transportable antiproton trap. Consequently, the cornerstones for the next antiproton precision measurements will be set within this project.
The latest measurement of the antiproton charge-to-mass ratio sets a limit on the violation of the weak equivalence principle for antiprotons with a sensitivity of 3% of the gravitational interaction. This is the first time that we set a differential limit, i.e. using changes of the gravitational potential along the elliptic orbit of the sun, and not one that is based on elusive assumptions, such as the total binding energy of our galaxy super cluster, that involves distance scales on which gravitation is not understood. Consequently, our work put the test of anomalous antiproton gravitation based on antiproton frequency measurements on a solid ground and is even sensitive enough to exclude antigravity for antiprotons. Using the transportable trap, we aspire to repeat this test in the future with active modifications of the gravitational potential by measuring in different altitudes.
We expect to conduct simultaneous antiproton frequency measurements that are conducted in the transportable antiproton trap and the BASE experiment at CERN. These measurements will constitute a direct test of Lorentz symmetry for antiprotons. So far, it was not possible to conduct such measurements because there were never two antiproton precision experiments simultaneously in operation.
We also expect to report on the use of permanent magnet traps as antiproton transport or storage traps, and we are presently testing a permanent magnet trap with protons at the University of Mainz. Permanent magnet traps provide a cost-efficient trap system that becomes essential to have additional backup traps for offline operation, or to economize transport traps to be able supply multiple laboratories with antiproton reservoirs and increase the number of potential antiproton experiments.
We have also engaged in developing an image-current detector based on the high-temperature superconductor YBCO. We anticipate that this technology can replace the hand-made NbTi coils that are presently used for the image-current detectors. We intend to use the YBCO detector to improve the counting of the number of trapped antiprotons, and this will improve our methods to search for unexpected interactions or decays of antiprotons with particles beyond the Standard Model.