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Defect and diffusion forum/Diffusion and defect data, solid state data. Part A, Defect and diffusion forum

Positron Manipulation and Positronium Laser Excitation in AEgIS

S. Mariazzi, R. Caravita, Stefano Aghion, C. Amsler, A. Ariga, Tomoko Ariga, G. Bonomi, P. Bräunig, J. Bremer, Roberto Sennen Brusa, L. Cabaret, M. Caccia, F. Castelli, G. Cerchiari, K. Chlouba, S. Cialdi, D. Comparat, Giovanni Consolati, Andrea Demetrio, L. Di Noto, M. Doser, Alexey Dudarev, Antonio Ereditato, Craig Evans, R. Ferragut, J. Fesel, A. Fontana, Ola Kenji Forslund, S. Gerber, Marco G. Giammarchi, A. Gligorova, Sergei N. Gninenko, F. Guatieri, Stefan Haider, H. Holmestad, Torkjell Huse, Ine L. Jernelv, Elena Jordan, T. Kaltenbacher, A. Kellerbauer, Mitsuhiro Kimura, Torsten Koetting, D. Krasnický, Vittorio Lagomarsino, P. Lansonneur, P. Lebrun, Sebastian Lehner, J. Liberadzka, C. Malbrunot, L. Marx, V. Matveev, Z. Mazzotta, Giancarlo Nebbia, P. Nédélec, Markus K. Oberthaler, N. Pacifico, Davide Pagano, Luca Penasa, V. Petráček, C. Pistillo, F. Prelz, M. Prevedelli, Luca Ravelli, Laura Resch, B. Rienäcker, Ole Myren Rohne, S. Rosenberger, Alberto Rotondi, M. Sacerdoti, H. Sandaker, R. Santoro, P. Scampoli, Fiodor Sorrentino, Michal Špaček, James William Storey, Izabela M. Strojek, G. Testera, Ingmari C. Tietje, S. Vamosi, E. Widmann, P. Yzombard, Sandra Zavatarelli, Johann Zmeskal

Published Mar 24, 2017• 1 citation
Publisher

Abstract

Production of antihydrogen by using the charge exchange reaction, as proposed by AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy), requires the formation of a dense cloud of positronium atoms excited to Rydberg states. In this work, the recent advances in AEgIS towards this result are described. Namely, the manipulation of positrons to produce bunches containing more than 108 particles and the laser excitation of positronium to Rydberg states, using n=3 as intermediate level, are presented.

DOI
10.4028/www.scientific.net/ddf.373.11

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Production of antihydrogen by using the charge exchange reaction, as proposed by AEgIS (Antimatter Experiment: gravity, Interferometry, Spectroscopy), requires the formation of a dense cloud of positronium atoms excited to Rydberg states. In this work, the recent advances in AEgIS towards this result are described. Namely, the manipulation of positrons to produce bunches containing more than 108 particles and the laser excitation of positronium to Rydberg states, using n=3 as intermediate level, are presented.
Cold antihydrogen atoms are a powerful tool to probe the validity of fundamental physics laws, and it's clear that colder atoms, generally speaking, allow an increased level of precision. After the first production of cold antihydrogen (Hˉ\bar{H}Hˉ) in 2002, experimental efforts have progressed continuously (trapping, beam formation, spectroscopy), with competitive results already achieved by adapting to cold antiatoms techniques previously well developed for ordinary atoms. Unfortunately, the number of Hˉ\bar{H}Hˉ atoms that can be produced in dedicated experiments is many orders of magnitude smaller than available hydrogen atoms, which are at hand in large amount, so the development of novel techniques that allow the production of Hˉ\bar{H}Hˉ with well defined conditions (and possibly control its formation time and energy levels) is essential to improve the sensitivity of the methods applied by the different experiments. We present here the first experimental results concerning the production of Hˉ\bar{H}Hˉ in a pulsed mode where the time when 90\% of the atoms are produced is known with an uncertainty of around 250~ns. The pulsed Hˉ\bar{H}Hˉ source is generated by the charge-exchange reaction between Rydberg positronium atoms (PsPsPs) and trapped antiprotons (pˉ\bar{p}pˉ​), cooled and manipulated in an electromagnetic trap: pˉ+Ps∗→Hˉ∗+e− \bar{ p}+Ps^* \rightarrow \bar{H}^* + e^- pˉ​+Ps∗→Hˉ∗+e− where Rydberg positronium atoms, in turn, are produced through the implantation of a pulsed positron beam into a mesoporous silica target, and are excited by two subsequent laser pulses, the first to n=3n=3n=3, the second to the needed Rydberg level (n≃17n \simeq 17n≃17). The pulsed production allows the control of the antihydrogen temperature, and facilitates the tunability of the Rydberg states, their de-excitation by pulsed lasers and the manipulation through electric field gradients. In fact, the production of pulsed antihydrogen is a major milestone in the AEgIS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.
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.
Manipulation et refroidissement laser de l'antimatière, au sein de l'expérience AEgIS Ma thèse s’est déroulée dans le cadre de la collaboration AEgIS, une des expériences étudiant l’antimatière au CERN. L’objectif final est de mesurer l’effet de la gravité sur un faisceau froid d’antihydrogène (Hbar). AEgIS se propose de créer les Hbar froids par échange de charges entre un atome de Positronium (Ps) excité (état de Rydberg) et un antiproton piégé : 〖Ps〗^*+ pbar → (H^*)⁻ + e⁻. L’étude de la physique du Ps est cruciale pour AEgIS, et demande des systèmes lasers adaptés. Pendant ma thèse, ma première tâche a été de veiller au bon fonctionnement des systèmes lasers de l’expérience. Afin d’exciter le positronium jusqu’à ses états de Rydberg (≃20) en présence d’un fort champ magnétique (1 T), deux lasers pulsés spectralement larges ont été spécialement conçu. Nous avons réalisé la première excitation par laser du Ps dans son niveau n=3, et prouvé une excitation efficace du nuage de Ps vers les niveaux de Rydberg n=16-17. Ces mesures, réalisées dans la chambre à vide de test d’AEgIS, à température ambiance et pour un faible champ magnétique environnant, sont la première étape vers la formation d’antihydrogène. Le prochain objectif est de répéter ces résultats dans l’enceinte du piège à 1 T, où les antihydrogènes seront formés. Pour autant, malgré l’excitation Rydberg des Ps pour accroître la section efficace de collision, la production d’antihydrogène restera faible, et la température des H bar formés sera trop élevée pour toute mesure de gravité. Pendant ma thèse, j’ai installé au CERN un autre système laser prévu pour pratiquer une spectroscopie précise des niveaux de Rydberg du Ps. Ce système excite des transitions optiques qui pourraient convenir à un refroidissement Doppler : la transition n=1 ↔ n=2. J’ai étudié la possibilité d’un tel refroidissement, en procédant à des simulations poussées pour déterminer les caractéristiques d’un système laser adapté La focalisation du nuage de Ps grâce au refroidissement des vitesses transverses devrait accroitre le recouvrement des positroniums avec les antiprotons piégés, et ainsi augmenter grandement la production d’Hbar. Le contrôle du refroidissement et de la compression du plasma d’antiprotons est aussi essentiel pour la formation des antihydrogènes. Pendant les temps de faisceaux d’antiprotons de 2014 et 2015, j’ai contribué à la caractérisation et l’optimisation des procédures pour attraper et manipuler les antiprotons, afin d’atteindre des plasmas très denses, et ce, de façon reproductible. Enfin, j’ai participé activement à l’élaboration d’autre projet à l’étude AEgIS, qui vise aussi à augmenter la production d’antihydrogène : le projet d’un refroidissement sympathique des antiprotons, en utilisant un plasma d’anions refroidis par laser. J’ai étudié la possibilité de refroidir l’ion moléculaire C₂⁻, et les résultats de simulations sont encourageants. Nous sommes actuellement en train de développer au CERN le système expérimental qui nous permettra de faire les premiers tests de refroidissement sur le C₂⁻. Si couronné de succès, ce projet ne sera pas seulement le premier résultat de refroidissement par laser d’anions, mais ouvrira aussi les portes à une production efficace d’antihydrogènes froids.
Laser-controlled charge exchange is a promising method for producing cold antihydrogen. Caesium atoms in Rydberg states collide with positrons and create positronium. These positronium atoms then interact with antiprotons, forming antihydrogen. Laser excitation of the caesium atoms is essential to increase the cross section of the charge-exchange collisions. This method was demonstrated in 2004 by the ATRAP collaboration by using an available copper vapour laser. For a second generation of charge-exchange experiments we have designed a new semiconductor laser system that features several improvements compared to the copper vapour laser. We describe this new laser system and show the results from the excitation of caesium atoms to Rydberg states within the strong magnetic fields in the ATRAP apparatus.
The formation of antihydrogen atoms from three-body collisions between positronium atoms and antiprotons plays a crucial role in the gravitational behavior of antihydrogen at rest (GBAR) experiment devoted to the measure of the action of gravity on antimatter. One of the main challenges facing this experiment is to find the means for increasing the production of antihydrogens which are produced by the reaction $\overline{p}+\text{Ps}\ensuremath{\rightarrow}\overline{\text{H}}+{e}^{\ensuremath{-}}$ with an extremely low rate. Along these lines, we investigate here the possibility to influence the collision process by using a laser field. A perturbative approach combining the Coulomb-Born approximation for modeling the charged-particle collision and a first-order perturbation theory for describing laser-atom interactions is employed to estimate laser-mediated charge-exchange cross sections. By carefully considering the laser specifications compatible with the experimental constraints, we present an extensive study of the influence of the laser parameters (laser wavelength and intensity) on the cross sections in the energy range of interest for GBAR. We show that under special irradiation conditions the rate of antihydrogen production may be significantly increased by the presence of the laser field.
Abstract 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—produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses—and 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 AE gˉ\bar{g}gˉ​ ḡ IS experiment to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.