Pulsed antihydrogen production for direct gravitational measurement on antimatter

One of the biggest efforts of the antimatter community is to measure gravity on a neutral antimatter system towards a direct test of the validity of the Weak Equivalence Principle (WEP). WEP is a cornerstone of the General Relativity and such a test has never been carried out on antimatter. Electrically neutral antimatter systems are expected to behave in exactly the same way as their matter counterparts. Several indirect arguments indicate that possible differences between the free fall of matter and antimatter, if existing, wouldn't be greater than 10^-6 g. To date, however, the question of whether antimatter falls in the Earth's gravitational field with the same acceleration g as ordinary matter does not yet have a direct experimental answer. Antihydrogen is the bound state of an antiproton and a positron. It is the simplest antimatter atom - and the only one ever synthesised. In AEgIS, antihydrogen is formed through charge exchange reaction between laser-excited positronium and cold trapped antiprotons. Positronium (Ps) is a purely leptonic hydrogen-like bound state composed of an electron and a positron. In this thesis, I present the first pulsed formation of antihydrogen, achieved in aegis as a core topic of my work in the last three years, from late 2016 to early 2020. The work here presented was carried out between the end of the Run 2 and the beginning of the Long Shut down 2 (LS2). During this period, I was a member of the aegis Collaboration and part of the aegis Genoa Group. The experimental activities involving antiprotons are expected to resume in 2021. Pulsed antihydrogen production is the crucial achievement to validate the aegis experimental approach to perform the gravity measurement on antimatter. A key feature of this result is the knowledge, within few hundred ns, of the Hbar production time. Previous production schemes provide a quasi-continuous source of antihydrogen without the possibility of precisely tagging the time of formation. The achievement of a pulsed production of antihydrogen opens the possibility to measure the atoms' time-of-flight, unavailable from currently available trap-based methods. This is an important accomplishment and a paper with the results presented in this thesis was submitted to Nature Physics. The aegis Genoa Group is responsible for the trap system, detection and data acquisition systems. The trap system is the main system of the experimental apparatus. The electronics of all the other systems depend on that of the trap system. I strongly contributed to the developments on antiproton manipulation procedures and related detection techniques. Among the results of this thesis work, an outstanding antiproton plasma compression was obtained, allowing the movement of antiproton clouds into the production trap in suitable conditions for the formation of antihydrogen and its detection. Remarkably, with a minimum cloud radius of 0.17 mm, such compression is the best ever reached for antiprotons. A number of stored antiprotons about 10 times larger than expected in the original aegis proposal was stored in the production trap for macroscopic times, despite the design limitations of the electrodes. Several novel detection techniques were implemented in the positronium diagnostics, also with my contribution. In particular, the use of a kicker pulser to detect the charge induced by the passage of the positron bunch was developed, tested and used as a main technique to monitor the condition of the positrons entering the main apparatus. This work was carried out by the Genoa Group, also responsible for the pos transfer line, its mechanics and detection system. On the top of this, the conversion of the MCP into a position sensitive detector for slow positronium allowed to characterise the Rydberg state of this unstable atom in magnetic field. The work for the fine tuning and characterisation of the new detection techniques was part of the activities I carried out during the data taking period. Finally, I evaluated the possibility to use a novel scintillating material in the detection of excited positronium produced in the Hbar production region to fully overcome the present existing limitations. I completed an extensive calibration of the material and I assembled a detector coupling such scintillator with a fast response photomultiplier tube. Such detector succeeded in detecting the formation of positronium and gave encouraging results for the detection of the elusive Rydberg-Ps states in magnetic field. I wrote a paper on this activity for the submission to a technical newspaper. At the moment, the article is under internal review.