Antihydrogen, the bound state of a positron and an antiproton, is a uniquely well-suited system for testing fundamental symmetries between matter and antimatter. The Antihydrogen Laser Physics Apparatus (ALPHA collaboration) synthesises antihydrogen atoms by slowly merging cold non-neutral positron and antiproton plasmas and traps the antiatoms in a magnetic minimum trap. While trapping efficiency has improved since its first demonstration in 2010, it plateaued after 2017.In ALPHA’s experimental conditions, antihydrogen is predominantly formed via three-body recombination, which has been theoretically and experimentally shown to strongly depend on the temperature of the positron plasma: lower positron temperatures yield higher antihydrogen trapping rates. In the ALPHA-2 trap, positrons alone reach a lower temperature limit of 15−20 K. The implementation of sympathetic cooling of positrons through collisions with laser-cooled Be+ions enabled stable lowering of these temperatures to below 10 K. This thesis describes the work towards antihydrogen synthesis with positrons prepared using this cooling method. A novel ion plasma preparation technique was developed to improve reproducibility, and a sophisticated control system with automated beam steering was implemented to ensure long-term operational stability. Moreover, the first demonstration of sympathetic cooling of positrons to < 10 K in a radially asymmetric magnetic field was carried out. Culminating in the first Be+-assisted antihydrogen synthesis and trapping. Careful optimisation of this technique resulted in a near eightfold increase in the antihydrogen trapping rate, and allowed for systematic studies of three-body recombination that were not possible before its implementation. Overall, this technique represents a paradigm shift for the ALPHA physics programme and provides a deeper understanding of antihydrogen synthesis, opening new avenues for precision studies of antimatter.
Antihydrogen, the bound state of a positron and an antiproton, is a uniquely well-suited system for testing fundamental symmetries between matter and antimatter. The Antihydrogen Laser Physics Apparatus (ALPHA collaboration) synthesises antihydrogen atoms by slowly merging cold non-neutral positron and antiproton plasmas and traps the antiatoms in a magnetic minimum trap. While trapping efficiency has improved since its first demonstration in 2010, it plateaued after 2017.In ALPHA’s experimental conditions, antihydrogen is predominantly formed via three-body recombination, which has been theoretically and experimentally shown to strongly depend on the temperature of the positron plasma: lower positron temperatures yield higher antihydrogen trapping rates. In the ALPHA-2 trap, positrons alone reach a lower temperature limit of 15−20 K. The implementation of sympathetic cooling of positrons through collisions with laser-cooled Be+ions enabled stable lowering of these temperatures to below 10 K. This thesis describes the work towards antihydrogen synthesis with positrons prepared using this cooling method. A novel ion plasma preparation technique was developed to improve reproducibility, and a sophisticated control system with automated beam steering was implemented to ensure long-term operational stability. Moreover, the first demonstration of sympathetic cooling of positrons to < 10 K in a radially asymmetric magnetic field was carried out. Culminating in the first Be+-assisted antihydrogen synthesis and trapping. Careful optimisation of this technique resulted in a near eightfold increase in the antihydrogen trapping rate, and allowed for systematic studies of three-body recombination that were not possible before its implementation. Overall, this technique represents a paradigm shift for the ALPHA physics programme and provides a deeper understanding of antihydrogen synthesis, opening new avenues for precision studies of antimatter.
Answering the question of why we live in a matter-dominated universe is of great interest to contemporary physicists, as the Standard Model of Particle Physics predicts that matter and an-timatter should only ever be produced in equal parts. Antihydrogen is a good candidate for searches for asymmetries between matter and antimatter as it is the simplest antimatter bound state, and it has an extremely well-understood matter counterpart: the hydrogen atom. The AL-PHA collaboration at CERN can now routinely trap several hundred antihydrogen atoms in a magnetic trap, allowing precise measurements of the fundamental properties of antihydrogen.ALPHA currently traps around 20 antihydrogen atoms every few minutes, so accumulat-ing enough antiatoms to perform precision measurements can take many hours. Increasing this trapping rate would allow for faster or more precise measurements. Simulations and experimen-tal data show that there is a strong correlation between the temperature of the positron plasma used in antihydrogen formation and the trapping rate that can be achieved. This thesis describes work towards using laser-cooled beryllium ions to obtain colder positron plasmas in the ALPHA trapping apparatus. Singly-charged beryllium ions are liberated from the surface of a solid beryl-lium target inside the apparatus via laser ablation, where the ions can be trapped, laser-cooled and mixed with positron plasmas. By mixing the ions and positrons together, they can exchange energy, and the ions can provide cooling to the positrons. Careful tuning of laser and trap pa-rameters, as well as use of other techniques, has allowed for the successful sympathetic cooling of positron plasmas in ALPHA.
Antihydrogen, the bound state of an antiproton and a positron, is of interest for use in precision tests of nature's fundamental symmetries. Antihydrogen formed by carefully merging cold plasmas of positrons and antiprotons has recently been trapped in magnetic traps. The efficiency of trapping is strongly dependent on the temperature of the nascent antihydrogen, which, to be trapped, must have a kinetic energy less than the trap depth of . In the conditions in the ALPHA experiment, the antihydrogen temperature seems dominated by the temperature of the positron plasma used for the synthesis. Cold positrons are therefore of paramount interest in that experiment. In this paper, we propose an alternative route to make ultra-cold positrons for enhanced antihydrogen trapping. We investigate theoretically how to extend previously successful sympathetic cooling of positrons by laser-cooled positive ions to be used for antihydrogen trapping. Using simulations, we investigate the effectiveness of such cooling in conditions similar to those in ALPHA, and discuss how the formation process and the nascent antihydrogen may be influenced by the presence of positive ions. We argue that this technique is a viable alternative to methods such as evaporative and adiabatic cooling, and may overcome limitations faced by these. Ultra-cold positrons, once available, may also be of interest for a range of other applications.
One of the best ways to study antimatter is to investigate antihydrogen, the bound state of an antiproton and a positron. Antihydrogen atoms do not exist naturally and must be synthesized in the lab by merging carefully-prepared plasmas of positrons and antiprotons. If the atoms are created in a magnetic trap like the one used by the ALPHA experiment at CERN, then a fraction of the coldest atoms remain trapped, while the rest escape and annihilate on the trap walls. The trapped atoms may then be probed using microwaves or lasers to make high-precision comparisons with hydrogen. Increasing the trapping rate would allow us to perform precision measurements on antihydrogen in a shorter period of time and with better systematics. Particle simulations indicate that by sympathetically cooling positrons using laser-cooled beryllium ions, we have the ability to improve the antihydrogen trapping rate by up to two orders of magnitude. This thesis describes the effort to design and qualify a beryllium ion source that is compatible with the extreme environment of the ALPHA trapping apparatus. To produce the ions, pulsed laser ablation of a beryllium target is investigated and the ion plume is characterized. By carefully choosing the laser parameters, a plume with a suitable number of ions and kinetic energy distribution can be created for subsequent trapping and laser-cooling in a Penning trap. This thesis reports the successful demonstration of the ion source, in particular its compatibility with the requirements of the ALPHA experiment. The ion source has been installed into the main apparatus at ALPHA and is expected to be commissioned early next year.
Antihydrogen, the bound state of a positron and an antiproton, is the only pure anti-atomic system ever studied. It is produced exclusively in the laboratory, as it has never been observed in nature. This unique system is of great interest for searching for tentative differences between matter and antimatter. Antihydrogen has been routinely trapped since 2010 and accumulated since 2017, enabling, for example, the first precision spectroscopic study of the anti-atom in 2018 and the first observation of the influence of gravity in 2023. Here we report an eight-fold increase in the trapping rate of antihydrogen, enabled by sympathetic cooling of positrons with laser-cooled beryllium ions. With beryllium sympathetic cooling, we now accumulate over 15000 antihydrogen atoms in under seven hours. This technique transforms our ability to study systematic and sidereal effects in existing experiments while paving the way for studies that would otherwise remain out of reach.
Abstract The positron, the antiparticle of the electron, predicted by Dirac in 1931 and discovered by Anderson in 1933, plays a key role in many scientific and everyday endeavours. Notably, the positron is a constituent of antihydrogen, the only long-lived neutral antimatter bound state that can currently be synthesized at low energy, presenting a prominent system for testing fundamental symmetries with high precision. Here, we report on the use of laser cooled Be + ions to sympathetically cool a large and dense plasma of positrons to directly measured temperatures below 7 K in a Penning trap for antihydrogen synthesis. This will likely herald a significant increase in the amount of antihydrogen available for experimentation, thus facilitating further improvements in studies of fundamental symmetries.
The merging of antiprotons with a positron plasma is the predominant and highest efficient method for cold antihydrogen formation used to date. We present experimental evidence that this method has serious disadvantages for producing antihydrogen cold enough to be trapped. Antihydrogen is neutral but may be trapped in a magnetic field minimum. However, the depth of such traps are of order 1 K, shallow compared to the kinetic energies in current antihydrogen experiments. Studying the spatial distribution of the antihydrogen emerging from the ATHENA positron plasma we have, by comparison with a simple model, extracted information about the temperature of the antihydrogen formed. We find that antihydrogen is formed before thermal equilibrium is attained between the antiprotons and the positrons, and thus that further positron cooling may not be sufficient for producing antihydrogen cold enough to be trapped. We discuss the implications for trapping of antihydrogen in a magnetic trap, important for ongoing work by the ALPHA collaboration.