Low-energy antimatter experiments at the antiproton decelerator at CERN: Testing CPT invariance and the WEP

The riddle of the baryon asymmetry, i.e. the matter antimatter imbalance in the universe can be addressed by comparing matter particles with their antimatter counterparts. At the antiproton decelerator (AD) at CERN several antimatter experiments investigate whether CPT (charge-parity-time reversal) invariance and the WEP (weak equivalence principle) hold. The systems probed are antihydrogen (H¯), antiprotonic helium and individual antiprotons (p¯). This article is meant to give an overview of the experiments located at the AD, discuss some commonly used experimental techniques and point out what the different experimental approaches entail. The research done on low-energy antimatter systems can be seen as complementary to the high energy research carried out at CERN and elsewhere: It provides bounds on CPT invariance and directly addresses the question of whether the WEP holds for antimatter. It is noted that the AD - at the moment - is the only low-energy antiproton source on earth

Optical Atomic Clock aboard an Earth-orbiting Space Station (OACESS): Enhancing searches for physics beyond the standard model in space

We present a concept for a high-precision optical atomic clock (OAC) operating on an Earth-orbiting space station. This pathfinder science mission will compare the space-based OAC with one or more ultra-stable terrestrial OACs to search for space-time-dependent signatures of dark scalar fields that manifest as anomalies in the relative frequencies of station-based and ground-based clocks. This opens the possibility of probing models of new physics that are inaccessible to purely ground-based OAC experiments such as models where a dark scalar field is strongly screened near Earth's surface

Charakterisierung und Kontrolle von Antiprotonen-Elektronen-Plasmen zur Antiwasserstoff-Produktion

The highly collaborative result of pulsed antihydrogen production in a 80 ns (FWHM) time window [1] presents a landmark in the AEḡIS experimental scheme of testing the weak equivalence principle with a pulsed horizontal beam of antihydrogen [2]. In the highly-complex experimental protocol, the manipulation and characterisation of non-neutral pure electron and combined antiproton-electron plasmas play a crucial part. The physics of non-neutral plasmas is marked by high self-electric fields, excellent equilibrium confinement properties in Penning or Penning-Malmberg traps, and the possibility to cool the plasmas down to cryogenic temperatures [3, 4]. In the high magnetic field strengths of the AeḡIS apparatus the electron and antiproton-electron plasmas can typically be treated in analogy to two-dimensional fluids [4, 5]. The low-energy antiprotons, provided by the antiproton decelerator at CERN, are caught in the AEḡIS apparatus, stored, cooled and manipulated by means of electron plasmas. The implementation of the strong drive evaporative cooling (SDREVC) technique into the AeḡIS apparatus has offered a more precise control of the density, radius and particle number of pure electron plasmas. We were able to reduce electron number fluctuations to merely 1.4 % from previously 15 − 20 %. Building on SDREVC, which was pioneered in [6], we devised an absolute plasma length measurement method. A qualitative analysis of a Kelvin-Helmholtz like diocotron instability evolving in a thin hollow antiproton ring, gives insight into the linear, non-linear and final states of the evolution as well as its time scales. While beyond the scope of this thesis, measures for a future quantitative analysis are motivated. To measure the free fall of antihydrogen, the antiatom is required to be sufficiently cold. The antihydrogen temperature is mainly determined by the antiproton plasma temperature. The parallel energy analyser method [7] revealed antiproton plasma temperatures as low as 50 K and 300 K in the trapping region (4.46 T) and the antihydrogen production region (1 T) of the AEḡIS apparatus, respectively. In the presentation of the key points of the pulsed antihydrogen production analysis, we motivate possible explanations for a discrepancy between the measured and expected antihydrogen annihilation time window. The time window is longer than expected, which might be due to multiple reflections of the produced Rydberg antihydrogen atoms off the trap wall before they annihilate. The cause could also lie in limitations of the parallel energy analyser method in the relatively weak 1 T magnetic field of the antihydrogen production trap. An underlying theme of this thesis concerns the reduction of fluctuations in the electron plasma particle number. The implementation of the SDREVC technique, is a key in providing an overall improvement for the reproducibility of experimental protocols including the diocotron instability, the temperature measurements as well as pulsed antihydrogen production. In view of the electron number fluctuation reduction as well as the start of the ELENA decelerator ring operation at CERN we discuss the anticipated efficiencies for pulsed antihydrogen production as well as prospects for the pulsed antihydrogen beam formation.Die Demonstration gepulster Antiwasserstoffproduktion innerhalb eines Zeitfensters von nur 80 ns (FWHM) [1], ist ein Meilenstein im experimentellen Vorhaben der AEḡIS Kollaboration das schwache Äquivalenzprinzip mittels eines gepulsten, horizontalen Antiwasserstoffstrahls zu überprüfen [2]. Für das hochkomplexe experimentelle Protokoll spielt die Kontrolle und Charakterisierung nicht-neutraler, reiner Elektronenplasmen sowie Antiprotonen-Elektronenplasmen eine entscheidende Rolle. Nicht-neutrale Plasmen zeichnen sich aus durch starke Raumladungen, hervorragende Speichereigenschaften in Penning bzw. Penning-Malmberg Fallen, sowie der Möglichkeit sie auf kryogene Temperaturen herabzukühlen [3, 4]. In den recht hohen Magnetfeldstärken des AeḡIS Experiments, können die Elektronen und Antiprotonen-Elektronenplasmen typischerweise analog zu zweidimensionalen Fluiden beschrieben werden [4, 5]. Der Antiprotonenentschleuniger am CERN stellt niederenergetische Antiprotonen bereit, welche im experimentellen Aufbau des AeḡIS Experiments gefangen, gespeichert, gekühlt und mithilfe von Elektronenplasmen kontrolliert werden. Die Realisierung der experimentellen Methode strong drive evaporative cooling (SDREVC) im AEḡIS Experiment erlaubt eine präzise Kontrolle der Dichte, des Radius sowie der Teilchenzahl eines reinen Elektronenplasmas. Auf der Methode SDREVC aufbauend, welche zuerst in [6] eingeführt wurde, konzipierten wir eine Methode zur absoluten Plasmalängenmessung. Eine qualitative Analyse einer Diokotroninstabilität, welche sich in einem dünnen Antiprotonenring entwickelt, gewährt Einblick in den linearen, nicht-linearen Abschnitt der Evolution, in ihre Zeitskalen sowie ihr Endstadium. Schritte für eine quantitative Analyse der Kelvin-Helmholtz ähnlichen Instabilität werden motiviert, gehen jedoch in ihrer Ausführung über den Rahmen dieser Arbeit hinaus. Um den freien Fall von Antiwasserstoff zu messen, muss dieser langsam bzw. kalt genug sein. Die Temperatur des Antiatoms ist hauptsächlich durch die Antiprotonenplasmatemperatur bestimmt. Die niedrigsten Temperaturen von Antiprotonenplasmen, die wir mit der parallel energy analyser Methode [7] gemessen haben, waren 50 K in der Antiprotonen-Einfangregion (4.46 T) sowie 300 K in der Region, in welcher Antiwasserstoff produziert wird (1 T). In der Darlegung der Produktion und Analyse von gepulstem Antiwasserstoff gehen wir näher auf eine Diskrepanz ein, zwischen dem gemessenen und erwarteten Zeitfenster der Antiwasserstoffannihilation. Dass sich das Zeitfenster länger hinzieht als erwartet, könnte durch eine mehrfache Reflexion der Rydberg-Antiwasserstoffe von der Fallenwand bedingt sein [1]. Ein Grund könnte auch darin liegen, dass die parallel energy analyser Messmethode im relativ schwachen 1 T Magnetfeld eingeschränkt angewendet werden kann. Ein grundlegendes Motiv dieser Arbeit zielt ab auf eine Reduktion der Fluktuationen in der Elektronenplasmateilchenzahl. Die Einführung der SDREVC Methode ist ein Schlüsselpunkt, um eine bessere Reproduzierbarkeit zu gewährleisten für die experimentellen Protokolle der Diokotroninstabilität, der Temperaturmessungen sowie der gepulsten Antiwasserstoffproduktion. Im Hinblick auf die Reduktion der Elektronenfluktuationen sowie der in Betriebnahme des ELENA Entschleunigerrings am CERN, diskutieren wir die voraussichtlichen Effizienzen der Antiwasserstoffproduktion sowie Perspektiven für die gepulste Antiwasserstoff-Strahlformation

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

Imaging a positronium cloud in a 1 Tesla

We report on recent developments in positronium work in the frame of antihydrogen production through charge exchange in the AEgIS collaboration [1]. In particular, we present a new technique based on spatially imaging a cloud of positronium by collecting the positrons emitted by photoionization. This background free diagnostic proves to be highly efficient and opens up new opportunities for spectroscopy on antimatter, control and laser manipulation of positronium clouds as well as Doppler velocimetry

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

Compression of a mixed antiproton and electron non-neutral plasma to high densities

We describe a multi-step “rotating wall” compression of a mixed cold antiproton–electron non-neutral plasma in a 4.46 T Penning–Malmberg trap developed in the context of the AE¯gIS experiment at CERN. Such traps are routinely used for the preparation of cold antiprotons suitable for antihydrogen production. A tenfold antiproton radius compression has been achieved, with a minimum antiproton radius of only 0.17 mm. We describe the experimental conditions necessary to perform such a compression: minimizing the tails of the electron density distribution is paramount to ensure that the antiproton density distribution follows that of the electrons. Such electron density tails are remnants of rotating wall compression and in many cases can remain unnoticed. We observe that the compression dynamics for a pure electron plasma behaves the same way as that of a mixed antiproton and electron plasma. Thanks to this optimized compression method and the high single shot antiproton catching efficiency, we observe for the first time cold and dense non-neutral antiproton plasmas with particle densities n ≥ 1013 m−3 , which pave the way for an efficient pulsed antihydrogen production in AE¯gIS

Imaging a positronium cloud in a 1 Tesla

We report on recent developments in positronium work in the frame of antihydrogen production through charge exchange in the AEgIS collaboration [1]. In particular, we present a new technique based on spatially imaging a cloud of positronium by collecting the positrons emitted by photoionization. This background free diagnostic proves to be highly efficient and opens up new opportunities for spectroscopy on antimatter, control and laser manipulation of positronium clouds as well as Doppler velocimetry

Monte-Carlo simulation of positronium laser excitation and anti-hydrogen formation via charge exchange

The AEgIS experiment aims at producing antihydrogen (and eventually measuring the effects of the Earth gravitational field on it) with a method based on the charge exchange reaction between antiproton and Rydberg positronium. To be precise,antiprotons are delivered by the CERN Antiproton Decelerator (AD) and are trapped in a multi-ring Penning trap, while positronium is produced by a nanoporous silica target and is excited to Rydberg states by means of a two steps laser excitation. New Monte Carlo simulations are presented in this paper in order to investigate the current status of the AEgIS experiment [1] and to interpret the recently collected data [2]