Periodic Reporting for period 4 - FunI (Revealing Fundamental Interactions and their Symmetries at the highest Precision and the lowest Energies)
Reporting period: 2023-11-01 to 2024-10-31
The FunI project aimed to conduct highly precise tests of fundamental interactions and their symmetries, crucial to understanding the basic structure of the universe. We rigorously tested predictions from underlying theories, such as bound-state quantum electrodynamics (BS-QED), with an unprecedented level of accuracy. To that end, we measured among others the g-factor of the bound electron in hydrogenlike 118Sn49+ and achieved together with state-of-the-art theory calculations a test of the underlying QED to about 0.012%, yielding a stringent test of QED in the strong-field regime [Morgner et al., Nature 622, 53 (2023)]. Advancing these tests beyond current precision presents a significant challenge for modern measurement techniques and required the development of innovative experimental methods, including new approaches for manipulating, cooling, storing, and producing the particles under study. Here, we succeeded to demonstrate for the first time sympathetic laser cooling of protons reaching an axial temperature of only about 100 mK, more than a factor of 40 improved compared to the precious best value [Will et al., Phys. Rev. Lett. 133, 023002 (2024)].
Furthermore, there are numerous unresolved questions that the Standard Model cannot answer, such as the mystery of why neutrinos have mass. We contributed to these questions by e.g. measuring the Q-value of the electron capture decay in 163Ho with a more than a factor of 50 improved precision [Schweiger et al., Nature Physics, 20, 921 (2024)]. Addressing these questions and exploring physics beyond the Standard Model is of paramount importance to both scientific progress and societal advancement. Equally crucial is the knowledge and technology transfer, exemplified by the close collaboration with the Physikalische Technische Bundesanstalt in Braunschweig.
Furthermore, there are numerous unresolved questions that the Standard Model cannot answer, such as the mystery of why neutrinos have mass. We contributed to these questions by e.g. measuring the Q-value of the electron capture decay in 163Ho with a more than a factor of 50 improved precision [Schweiger et al., Nature Physics, 20, 921 (2024)]. Addressing these questions and exploring physics beyond the Standard Model is of paramount importance to both scientific progress and societal advancement. Equally crucial is the knowledge and technology transfer, exemplified by the close collaboration with the Physikalische Technische Bundesanstalt in Braunschweig.
WP1a:
Penning-trap mass spectrometers around the world are focused on measuring the masses of exotic nuclides, which are typically available in extremely small quantities, often as low as 1E14 atoms or even fewer. This challenge is also faced in the measurements outlined in WP2a and 2b of the FunI project. To enable the efficient injection of these rare species into an electron beam ion trap (EBIT) for charge breeding, we have developed a specialized metallic wire probe. Through this approach, we have successfully demonstrated the continuous production of highly charged ions (HCI) from the stable isotope 165Ho, starting with samples as small as 1E12 atoms (approximately 300 pg). These ions were produced in charge states up to 45+ [Schweiger et al., Rev. Sci. Instrum. 90, 123201 (2019)] und used for the electron capture Q-value of 163Ho [Schweiger et al., Nature Physics, 20, 921 (2024)]. WP1a has been successfully completed.
WP1b:
Efficient cooling and coupling of trapped charged particles are crucial for fundamental physics experiments, high-precision metrology, and quantum technology applications. Traditionally, coupling techniques and sympathetic cooling have relied on close-range Coulomb interactions between ions. This has led to a strong interest in extending laser cooling methods to particles stored in spatially separated traps. In this work package, we have, for the first time, demonstrated the coupling of a proton and a highly charged ion with a cloud of laser-cooled beryllium ions, which are stored in two spatially separated traps within an advanced Penning-trap system. The energy exchange between these particles occurs through a shared superconducting LC circuit [Tu et al., Adv. Quantum Techn. 2100029 (2021) and Bohman et al., Nature 596, 514 (2021)]. The final goal for WP1b, the determination of the ultimate temperature of the species being cooled, has been reached in [Will et al., Phys. Rev. Lett. 133, 023002 (2024)] with an axial proton temperature of ~100mK.
WP2a:
The proposed test of special relativity requires high-precision mass measurements on 35,36Cl, i.e. on rare light mass species. As test case served 20,22Ne, where relative mass uncertainties of 1E-11 have been demonstrated [Heiße et al., Phys. Rev. Lett. 131, 253002 (2023)]. While the collection of the 35Cl/36Cl sample at the research reactor ILL in Grenoble was very successful and charge breeding demonstrated, the required measurements of the gamma-ray wavelengths needed to improve the limits on a special relativity test could not be carried out mainly due to a severe delay caused by the Corona pandemic. Thus, also the Q-value measurements could not be performed and the main goal in this WP was not achieved.
WP2b:
A relative mass uncertainty of less than 1E-11 was achieved for the nuclides involved (163Ho-163Dy), leading to a Q_EC-value of 2863.2(0.6) eV/c² [Schweiger et al., Nature Physics, 20, 921 (2024)]. This represents an improvement of more than a factor of 50 over the previous best Q_EC-value, marking a significant achievement in this ERC project and enhancing its international visibility. The WP-goal of an improvement factor on the uncertainty of the Q_EC value by >10 has been successfully reached.
WP3a:
Remarkable results were achieved here, including several world-record measurements on hydrogenlike systems [Sailer et al., Nature 606, 467 (2022), Heisse et al., Phys. Rev. Lett. 131, 253002 (2023), Morgner et al., Nature 622, 53 (2023)], as well as lithiumlike [Morgner et al., Science, in print (2025)] and boronlike systems [Morgner et al., Phys. Rev. Lett., in print (2025)]. The project has successfully investigated systems up to Z = 50 (tin), reaching a Zα ~ 0.4. Unfortunately, due to a severe delay in the manufacturing and installation of a new high-field electron beam ion trap (HYPER-EBIT), hydrogenlike lead (208Pb81+) could not be addressed.
WP3b:
Improved CPT symmetry tests by comparing the properties of protons and antiprotons were carried out in two measurement campaigns. The initial results, which achieved world-record precision for the charge-to-mass ratio of the proton and antiproton, were published in [Borchert et al., Nature 601, 53 (2022)], demonstrating a tenfold improvement in the CPT test. A second campaign, which included a significant enhancement in the measurement of the g-factors, has been completed end of 2024, and data analysis is still ongoing. The final results are expected to appear end of 2025.
Penning-trap mass spectrometers around the world are focused on measuring the masses of exotic nuclides, which are typically available in extremely small quantities, often as low as 1E14 atoms or even fewer. This challenge is also faced in the measurements outlined in WP2a and 2b of the FunI project. To enable the efficient injection of these rare species into an electron beam ion trap (EBIT) for charge breeding, we have developed a specialized metallic wire probe. Through this approach, we have successfully demonstrated the continuous production of highly charged ions (HCI) from the stable isotope 165Ho, starting with samples as small as 1E12 atoms (approximately 300 pg). These ions were produced in charge states up to 45+ [Schweiger et al., Rev. Sci. Instrum. 90, 123201 (2019)] und used for the electron capture Q-value of 163Ho [Schweiger et al., Nature Physics, 20, 921 (2024)]. WP1a has been successfully completed.
WP1b:
Efficient cooling and coupling of trapped charged particles are crucial for fundamental physics experiments, high-precision metrology, and quantum technology applications. Traditionally, coupling techniques and sympathetic cooling have relied on close-range Coulomb interactions between ions. This has led to a strong interest in extending laser cooling methods to particles stored in spatially separated traps. In this work package, we have, for the first time, demonstrated the coupling of a proton and a highly charged ion with a cloud of laser-cooled beryllium ions, which are stored in two spatially separated traps within an advanced Penning-trap system. The energy exchange between these particles occurs through a shared superconducting LC circuit [Tu et al., Adv. Quantum Techn. 2100029 (2021) and Bohman et al., Nature 596, 514 (2021)]. The final goal for WP1b, the determination of the ultimate temperature of the species being cooled, has been reached in [Will et al., Phys. Rev. Lett. 133, 023002 (2024)] with an axial proton temperature of ~100mK.
WP2a:
The proposed test of special relativity requires high-precision mass measurements on 35,36Cl, i.e. on rare light mass species. As test case served 20,22Ne, where relative mass uncertainties of 1E-11 have been demonstrated [Heiße et al., Phys. Rev. Lett. 131, 253002 (2023)]. While the collection of the 35Cl/36Cl sample at the research reactor ILL in Grenoble was very successful and charge breeding demonstrated, the required measurements of the gamma-ray wavelengths needed to improve the limits on a special relativity test could not be carried out mainly due to a severe delay caused by the Corona pandemic. Thus, also the Q-value measurements could not be performed and the main goal in this WP was not achieved.
WP2b:
A relative mass uncertainty of less than 1E-11 was achieved for the nuclides involved (163Ho-163Dy), leading to a Q_EC-value of 2863.2(0.6) eV/c² [Schweiger et al., Nature Physics, 20, 921 (2024)]. This represents an improvement of more than a factor of 50 over the previous best Q_EC-value, marking a significant achievement in this ERC project and enhancing its international visibility. The WP-goal of an improvement factor on the uncertainty of the Q_EC value by >10 has been successfully reached.
WP3a:
Remarkable results were achieved here, including several world-record measurements on hydrogenlike systems [Sailer et al., Nature 606, 467 (2022), Heisse et al., Phys. Rev. Lett. 131, 253002 (2023), Morgner et al., Nature 622, 53 (2023)], as well as lithiumlike [Morgner et al., Science, in print (2025)] and boronlike systems [Morgner et al., Phys. Rev. Lett., in print (2025)]. The project has successfully investigated systems up to Z = 50 (tin), reaching a Zα ~ 0.4. Unfortunately, due to a severe delay in the manufacturing and installation of a new high-field electron beam ion trap (HYPER-EBIT), hydrogenlike lead (208Pb81+) could not be addressed.
WP3b:
Improved CPT symmetry tests by comparing the properties of protons and antiprotons were carried out in two measurement campaigns. The initial results, which achieved world-record precision for the charge-to-mass ratio of the proton and antiproton, were published in [Borchert et al., Nature 601, 53 (2022)], demonstrating a tenfold improvement in the CPT test. A second campaign, which included a significant enhancement in the measurement of the g-factors, has been completed end of 2024, and data analysis is still ongoing. The final results are expected to appear end of 2025.
Tremendous progress beyond state of the art has been achieved by the ever first sympathetic laser cooling of protons, reaching a final temperature of 100mK in the axial motion. This result was selected as one of the 10 Physics Highlights of the Year by Physics World in 2021. With the Penning-trap mass spectrometer PENTATRAP relative mass uncertainties of 4E-12 have been demonstrated, making this the world best balance for atomic mass measurements on stable as well as long-lived radioactive species. With the measurement of a long-lived low-lying excited electronic state in 187Re and 187Os, a new field of research has been opened, namely the search for possible clock transitions in highly-charged ions by Penning-trap mass spectrometry. The comparison of the experimental g-factor of the bound electron in hydrogenlike 118Sn49+ with state-of-the-art bound state QED calculations allowed for the most stringent test of QED in strong fields to date. Last but not least, the charge-to-mass ratio comparison of the proton and antiproton allowed for a record test of CPT symmetry in the baryon sector.