Towards measuring gravity on neutral antimatter

One of the antimatter community current biggest efforts is to measure gravity on a neutral antimatter system, towards a first test of Equivalence Principle - a cornerstone of General Relativity. Any deviation from the expected identical behaviour with respect to normal matter would be an indication of new physics. This thesis explores the possibility to measure gravity with cold antihydrogen (Hbar) produced with a novel pulsed scheme based on a charge-exchange reaction between antiprotons and Rydberg positronium (Ps). Hbar and Ps are currently the only neutral antimatter systems available in quantities at low temperatures, and both are being considered for measuring gravity. Several results towards producing pulsed charge-exchange Hbar were obtained in the context of the AEgIS experiment. Positronium was produced with 35% effciency in a dedicated setup, where spectroscopy of the 13S - 33P transition and subsequent excitation to Rydberg levels were performed with ~ 15% effciency (bandwidth limited). Ps was produced also in the cryogenic Hbar production region of AEgIS (with ~25% effciency), where ~10^5 antiprotons were also stored for minutes after capture, cooling, radial compression and transfer. An estimate showed that 3-40 Hbar atoms per measurement run could be formed in the present setup. The experimental work motivated two detailed theoretical studies. A numerical code for calculating Rydberg Ps energy levels and optical spectra in arbitrary (strong) external fields, based on a non-perturbative approach, was developed. Results well reproduced high-resolution spectroscopic data from experiments, and also demonstrated the feasibility of measuring both Ps axial and transverse velocity components using only transverse laser probes. A novel numerical method for calculating single-species non-neutral plasma equilibria in cylindrical trap at low T was also developed, showing higher robustness and faster convergence to solution (T^-1) than currently available methods (T^-2). Finally, the combined availability of the intense Ps source and the 13S - 33P laser made possible to observe the 23S metastable state of positronium by optical decay from 33P, with a measured effciency of ~1%. This result represents a first step towards a gravity measurement on metastable 23S positronium

AEgIS: Status and Prospects

The progresses of the AEgIS collaboration on its way towards directly measuring the gravitational free-fall of neutral antimatter atoms are reviewed. The experiment recently developed the first pulsed cold antihydrogen source and entered in its second phase, aiming at the first proof-of-concept gravitational measurement. Several major upgrades were deployed, including an upgraded antihydrogen production scheme and a fully-redesigned antiproton trap. AEgIS re-started its operation on the new CERN ELENA decelerator in late 2021, capturing its first antiprotons and commissioning its new antiproton energy degrading system and hardware/software control systems

Toward inertial sensing with a 23^{3}S positronium beam

In this work, we discuss the possibility of inertial sensing with positronium in the 2$^{3}$S metastable state for the measurement of optical dipole, relativistic and gravitational forces on a purely leptonic matter-antimatter system. Starting from the characteristics of an available 2$^{3}$S beam, we estimate the time necessary to measure accelerations ranging from ~10$^{5}$ m/s$^{2}$ to 9.1 m/s$^{2}$ with two different inertial sensitive devices: a classical moiré deflectometer and a Mach–Zehnder interferometer. The sensitivity of the Mach–Zehnder interferometer has been estimated to be several tens of times better than that of the moiré deflectometer, for the same measurement time. Different strategies to strengthen the 2$^{3}$S beam flux and to improve the sensitivity of the devices are proposed and analyzed. Among them, the most promising are reducing the divergence of the positronium beam through 2D laser Doppler cooling and coherent positronium Raman excitation from the ground state to the 2$^{3}$S level. If implemented, these improvements promise to result in the time required to measure an acceleration of 9.1 m/s$^{2}$ of few weeks and 100 m/s$^{2}$ of a few hours. Different detection schemes for resolving the fringe pattern shift generated on 2$^{3}$S positronium crossing the deflectometer/interferometer are also discussed

A fiber detector to monitor ortho-Ps formation and decay

We describe a novel method to use a scintillating fiber detector similar to the Fast Annihilation Cryogenic Tracking (FACT) used at the antimatter experiment AEḡIS to monitor the presence of ortho-positronium. A single scintillating fiber was coupled to a photomultiplier tube and irradiated by flashes of about 6 × 106 gamma quanta with 511keV energy, produced by approximately 10ns long positron pulses. The results were used to demonstrate the ability to track the creation and annihilation of ortho-positronium atoms over time in cryogenic and highly magnetic environments by using the FACT detector as a “digital calorimeter”.We describe a novel method to use a scintillating fiber detector similar to the Fast Annihilation Cryogenic Tracking (FACT) used at the antimatter experiment AE$\bar{g}$IS to monitor the presence of ortho-positronium. A single scintillating fiber was coupled to a photomultiplier tube and irradiated by flashes of about 6 x 10$^{6}$ 511 keV $\gamma$-rays produced by $\approx10\,\text{ns}$ long positron pulses. The results were used to demonstrate the ability to track the creation and annihilation of ortho-positronium atoms over time in cryogenic and highly magnetic environments by using the FACT detector as a "digital calorimeter"

Experiments with mid-heavy antiprotonic atoms in AEgIS

ments which provide the most precise data on the strong interaction between protons and antiprotons and of the neutron skin of many nuclei thanks to the clean annihilation signal. In most of these experiments, the capture process of low energy antiprotons was done in a dense target leading to a significant suppression of specific transitions between deeply bound levels that are of particular interest. In particular, precise measurements of specific transitions in antiprotonic atoms with Z>2 are sparse. We propose to use the pulsed production scheme developed for antihydrogen and protonium for the formation of cold antiprotonic atoms. This technique has been recently achieved experimentally for the production of antihydrogen at AE$\overline{\rm g}$IS. The proposed experiments will have sub-ns synchronization thanks to an improved control and acquisition system. The formation in vacuum guarantees the absence of Stark mixing or annihilation from high n states and together with the sub-ns synchronization would resolve the previous experimental limitations. It will be possible to access the whole chain of the evolution of the system from its formation until annihilation with significantly improved signal-to-background ratio

Rydberg-positronium velocity and self-ionization studies in a 1T magnetic field and cryogenic environment

We characterized the pulsed Rydberg-positronium production inside the Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AE¯gIS) apparatus in view of antihydrogen formation by means of a charge exchange reaction between cold antiprotons and slow Rydberg-positronium atoms. Velocity measurements on the positronium along two axes in a cryogenic environment (≈10K) and in 1T magnetic field were performed. The velocimetry was done by microchannel-plate (MCP) imaging of a photoionized positronium previously excited to the n=3 state. One direction of velocity was measured via Doppler scan of this n=3 line, another direction perpendicular to the former by delaying the exciting laser pulses in a time-of-flight measurement. Self-ionization in the magnetic field due to the motional Stark effect was also quantified by using the same MCP-imaging technique for Rydberg positronium with an effective principal quantum number neff ranging between 14 and 22. We conclude with a discussion about the optimization of our experimental parameters for creating Rydberg positronium in preparation for an efficient pulsed production of antihydrogen

Toward a pulsed antihydrogen beam for WEP tests in AEgIS

The AEg̅IS collaboration at CERN’s AD produces antihydrogen atoms in the form of a pulsed, isotropic source with a precisely defined formation time. AEg̅IS has recently undergone major upgrades to fully benefit from the increased number of colder antiprotons provided by the new ELENA decelerator and to move toward forming a horizontal beam to directly investigate the influence of gravity on the H̅ atoms, thereby probing the Weak Equivalence Principle for antimatter. This contribution gives an overview of these upgrades as well as subsequent results from the first beam times with ELENA

Pulsed production of antihydrogen

Antihydrogen atoms with K or sub-K temperature are a powerful tool to precisely probe the validity of fundamental physics laws and the design of highly sensitive experiments needs antihydrogen with controllable and well defined conditions. We present here experimental results on the production of antihydrogen in a pulsed mode in which the time when 90% of the atoms are produced is known with an uncertainty of ~250 ns. The pulsed source is generated by the charge-exchange reaction between Rydberg positronium atoms\u2014produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses\u2014and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the tunability of the Rydberg states, their de-excitation by pulsed lasers and the manipulation through electric field gradients. The production of pulsed antihydrogen is a major landmark in the AEgIS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter