Periodic Reporting for period 1 - ANGRAM (Antimatter gravity measurement: How does antihydrogen fall?)
Berichtszeitraum: 2017-07-01 bis 2019-06-30
Baryon asymmetry is considered to be one of the greatest unsolved problems in modern physics. One of the methods towards resolving this issue is through direct comparisons of the physics properties between matter and antimatter. This is achieved via production and study of antihydrogen, which is the only stable, neutral antimatter system available for laboratory study. This project addressed the R&D required to measure the antimatter coupling to gravity, which has never been directly measured and could greatly impact our understanding of the early history of the Universe. We proposed a new approach for measuring the sign of the gravitational acceleration for antihydrogen, i.e. to carry out a 30% measurement of g. The relevance of such statistically significant direct measurement on the hitherto unprobed property of antihydrogen is manifold. The experimental result would provide a direct test to the applicability of the Principle of Equivalence to antimatter and would validate the different theoretical predictions (e.g. quantum gravity and antigravity theories) that have led to the idea that antimatter may react with a slightly different magnitude. Ultimately, a gravity measurement for antihydrogen will reveal if gravitational interaction is the key to the matter-antimatter imbalance in the Universe and whether this is the direction for to seek for the answers of the very fundamental questions not only in physics, but also deeper philosophical questions about our mere existence.
The methodology of the proposed measurement was based on a three-gratings moiré deflectometer, which does not rely on time-of-flight information, but the signal is obtained by observing the phase shift (pattern shift) via rotating the whole grating system. The project was based on three key steps that would lead to the final result:
1) Production of antihydrogen in the AEgIS apparatus, by three-body recombination of antiprotons and positrons,
2)Development of moiré deflectometer and detection system and
3)Data taking for the gravity measurement.
The work in the project was divided in three work packages, with WP1 being dedicated to the antihydrogen production, WP2 to the R&D of the moiré deflectometer and the detection system and WP3 to the data taking and data analysis for the gravity measurement.
The first step required a common effort from all members of the AEgIS collaboration. An ample amount of experimental work was performed in WP1 by the researcher, towards achieving of the main requirement (production and characterization of antihydrogen).
Despite the completion of the necessary work, this intermediate goal (production of antiydrogen) was not timely achieved. A statistically significant signal observed immediately after the antihydrogen production protocol, which is consistent with production of antihydrogen atoms was reported only after the end of the beam time. The produced low flux requires deeper analysis on the antihydrogen formation data, which are still ongoing within the AEgIS Collaboration. A draft paper is in preparation and the final result is expected to be published within this year.
The lack of antihydrogen data ceased the possibility to perform the gravity measurement and also shifted the experimental work towards measurements with antiprotons. Therefore, some of the main tasks of this project were adapted to produce deliverables that can be exploited for the same kind of measurement that would happen at a later time, e.g. during the next antiproton beam time at CERN (2021).
WP2 consisted of R&D on the detection system and the moiré deflectometer, and included multiple tasks related to exploration and optimization of different detection technologies. The Timepix3 technology was selected for further investigation due to its outstanding capabilities and all the milestones in WP2 were successfully completed.
Investigations about the grating system raised questions on the feasibility to use gratings made of matter. The crucial finding was that the Rydberg state antihydrogen interacts heavily with the atoms from the gratings, and the study led to the conclusion that material gratings could not be used to measure the vertical deflection of Rydberg state antihydrogen in Earth’s gravitational field. Therefore, the gratings were finally not produced, but some other options were proposed.
Taking the above into consideration, a great part of the work in WP2 to the R&D of the detection system and its efficiency. As precise modelling of the background contribution is one of the key elements for efficient antihydrogen detection, the work on the final detection system described in WP2 was extended with a dedicated physics study measuring antiproton-nucleus annihilation at rest and comparing it with the current simulation models (GEANT4, FLUKA).
The measurements were carried out during two beam campaigns at the ASACUSA experimental facility at CERN, using slow extracted antiproton beam. The data analysis and comparison against the performed GEANT4 simulations are still ongoing, but the initial results suggest that the current physics models do not precisely reflect the measurements in terms of multiplicity and energy distribution of the annihilation progns. The researcher is engaged in ongoing discussions with the developers of these models (in particular FTFP in GEANT4), identifying together the drawbacks as well as ways to include the results from the measurements to improve the precision of these models. The final results will be submitted in a peer-reviewed journal. In general, the results of this project were so far published in several peer-reviewed publications and presented to six international conferences and workshops. Some of the outcomes were presented to the general public, at events such as the European Researcher's Night in Vienna.
1) Production of antihydrogen in the AEgIS apparatus, by three-body recombination of antiprotons and positrons,
2)Development of moiré deflectometer and detection system and
3)Data taking for the gravity measurement.
The work in the project was divided in three work packages, with WP1 being dedicated to the antihydrogen production, WP2 to the R&D of the moiré deflectometer and the detection system and WP3 to the data taking and data analysis for the gravity measurement.
The first step required a common effort from all members of the AEgIS collaboration. An ample amount of experimental work was performed in WP1 by the researcher, towards achieving of the main requirement (production and characterization of antihydrogen).
Despite the completion of the necessary work, this intermediate goal (production of antiydrogen) was not timely achieved. A statistically significant signal observed immediately after the antihydrogen production protocol, which is consistent with production of antihydrogen atoms was reported only after the end of the beam time. The produced low flux requires deeper analysis on the antihydrogen formation data, which are still ongoing within the AEgIS Collaboration. A draft paper is in preparation and the final result is expected to be published within this year.
The lack of antihydrogen data ceased the possibility to perform the gravity measurement and also shifted the experimental work towards measurements with antiprotons. Therefore, some of the main tasks of this project were adapted to produce deliverables that can be exploited for the same kind of measurement that would happen at a later time, e.g. during the next antiproton beam time at CERN (2021).
WP2 consisted of R&D on the detection system and the moiré deflectometer, and included multiple tasks related to exploration and optimization of different detection technologies. The Timepix3 technology was selected for further investigation due to its outstanding capabilities and all the milestones in WP2 were successfully completed.
Investigations about the grating system raised questions on the feasibility to use gratings made of matter. The crucial finding was that the Rydberg state antihydrogen interacts heavily with the atoms from the gratings, and the study led to the conclusion that material gratings could not be used to measure the vertical deflection of Rydberg state antihydrogen in Earth’s gravitational field. Therefore, the gratings were finally not produced, but some other options were proposed.
Taking the above into consideration, a great part of the work in WP2 to the R&D of the detection system and its efficiency. As precise modelling of the background contribution is one of the key elements for efficient antihydrogen detection, the work on the final detection system described in WP2 was extended with a dedicated physics study measuring antiproton-nucleus annihilation at rest and comparing it with the current simulation models (GEANT4, FLUKA).
The measurements were carried out during two beam campaigns at the ASACUSA experimental facility at CERN, using slow extracted antiproton beam. The data analysis and comparison against the performed GEANT4 simulations are still ongoing, but the initial results suggest that the current physics models do not precisely reflect the measurements in terms of multiplicity and energy distribution of the annihilation progns. The researcher is engaged in ongoing discussions with the developers of these models (in particular FTFP in GEANT4), identifying together the drawbacks as well as ways to include the results from the measurements to improve the precision of these models. The final results will be submitted in a peer-reviewed journal. In general, the results of this project were so far published in several peer-reviewed publications and presented to six international conferences and workshops. Some of the outcomes were presented to the general public, at events such as the European Researcher's Night in Vienna.
The implementation of this project managed to provide deliverables that can and will be used regardless of the method for carrying out a gravity measurement. The optimization of the simulation models for annihilation at rest will be beneficial for every antimatter experiments that involves detection of antiprotons/antihydrogen, in particular via improvements of the detector design, which relies heavily on the simulation models, as well as improvement of the tagging efficiency via more precise modeling of the background contribution. Finally, despite the substantial efforts made by few experiments aiming at measuring the gravitational acceleration of antihydrogen at the Antiproton Decelerator at CERN, in the last couple of years, such measurement has not been carried out yet. I expect the findings of this project to be broadly exploitation by any experiment that studies antimatter in lab.