Rydberg Positronium Research Articles

AEgIS experiment: Towards antihydrogen beam production for antimatter gravity measurements

AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is an experiment that aims to perform the first direct measurement of the gravitational acceleration g of antihydrogen in the Earth’s field. A cold antihydrogen beam will be produced by charge exchange reaction between cold antiprotons and positronium excited in Rydberg states. Rydberg positronium (with quantum number n between 20 and 30) will be produced by a two steps laser excitation. The antihydrogen beam, after being accelerated by Stark effect, will fly through the gratings of a moire deflectometer. The deflection of the horizontal beam due to its free fall will be measured by a position sensitive detector. It is estimated that the detection of about 103 antihydrogen atoms is required to determine the gravitational acceleration with a precision of 1%. In this report an overview of the AEgIS experiment is presented and its current status is described. Details on the production of slow positronium and its excitation with lasers are discussed.

AE$\overline {\rm{g}}$IS Experiment: Measuring the accelerationgof the earth’s gravitational field on antihydrogen beam

The AEIS experiment [1] aims at directly measuring the gravitational acceleration g on a beam of cold antihydrogen () to a precision of 1%, performing the first test with antimatter of the (WEP) Weak Equivalence Principle. The experimental apparatus is sited at the Antiproton Decelerator (AD) at CERN, Geneva, Switzerland. After production by mixing of antiprotons with Rydberg state positronium atoms (Ps), the atoms will be driven to fly horizontally with a velocity of a few 100 ms−1 for a path length of about 1 meter. The small deflection, few tens of μm, will be measured using two material gratings (of period ∼ 80 μm) coupled to a position-sensitive detector working as a moiré deflectometer similarly to what has been done with matter atoms [2]. The shadow pattern produced by the beam will then be detected by reconstructing the annihilation points with a spatial resolution (∼ 2 μm) of each antiatom at the end of the flight path by the sensitive-position detector. During 2012 the experimental apparatus has been commissioned with antiprotons and positrons. Since the AD will not be running during 2013,during the refurbishment of the CERN accelerators, the experiment is currently working with positrons, electrons and protons, in order to prepare the way for the antihydrogen production in late 2014.

Calculated photoexcitation spectra of positronium Rydberg states

Calculations of the photoexcitation spectra of ortho-positronium Rydberg states with principal quantum numbers between 10 and 30 are presented. The effects of Doppler broadening and saturation of the corresponding electric-dipole transitions are studied, together with the role of static and motionally induced electric fields. This is done in the context of recent measurements reported by Cassidy et al. [Phys. Rev. Lett. 108, 043401 (2012)], and with regard to experiments involving the production of antihydrogen by charge-exchange between Rydberg positronium and cold antiprotons.

Efficient Production of Rydberg Positronium

We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2(3)P state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2(3)P atoms to Rydberg levels occurs very efficiently (~90%) and that the ~25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (3)S-2(3)P transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps.

Towards the production of an ultra cold antihydrogen beam with the AEGIS apparatus

The AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment is an international collaboration, based at CERN, with the experimental goal of performing the first direct measurement of the Earth’s gravitational acceleration on antihydrogen. In the first phase of the experiment, a gravity measurement with 1% precision will be performed by passing a beam of ultra cold antihydrogen atoms through a classical Moire deflectometer coupled to a position sensitive detector. The key requirements for this measurement are the production of ultra cold (T∼100 mK) Rydberg state antihydrogen and the subsequent Stark acceleration of these atoms. The aim is to produce Rydberg state antihydrogen by means of the charge exchange reaction between ultra cold antiprotons (T∼100 mK) and Rydberg state positronium. This paper will present details of the developments necessary for the successful production of the ultra cold antihydrogen beam, with emphasis on the detector that is required for the development of these techniques. Issues covered will include the detection of antihydrogen production and temperature, as well as detection of the effects of Stark acceleration.

The AEGIS detection system for gravity measurements

The main scientific goal of the AEGIS experiment ( Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is the direct measurement of the Earth's gravitational acceleration g on a beam of cold antihydrogen ( H ¯ ). The production of an antihydrogen beam is achieved by a charge exchange reaction between Rydberg positronium and cold antiprotons. The H ¯ beam will be accelerated up to a velocity of a few 100 m/s and the gravitational acceleration will be obtained by measuring the small vertical deflection of the beam (a few tens μm) using a Moire' deflectometer.

ATRAP antihydrogen experiments

AbstractAntihydrogen (Hbar) was first produced at CERN in 1996. Over the past decade our ATRAP collaboration has made massive progress toward our goal of producing large numbers of cold Hbar atoms that will be captured in a magnetic gradient trap for precise comparison between the atomic spectra of matter and antimatter. The AD at CERN provides bunches of 3 × 107 low energy Pbars every 100 seconds. We capture and cool to 4 K, 0.1% of these in a cryogenic Penning trap. By stacking many bunches we are able to do experiments with 3 × 105 Pbars. ∼100 e+/sec from a 22Na radioactive source are captured and cooled in the trap, with 5 × 106 available experiments.We have developed 2 ways to make Hbar from these cold ingredients, namely 3‐body collisions, and 2‐stage Rydberg charge exchange. In the first case, Pbars are injected into a nested trap containing e+. Hbar is formed when 2 e+ and 1 Pbar collide. In 2‐stage Rydberg charge exchange, laser‐excited caesium (Cs) enters the trap through a small hole. Rydberg positronium is formed when a e+ captures an e‐from a Cs. These atoms exit the trap, some passing through a nearby cloud of cold Pbars. A 2nd charge‐exchange results when a Pbar captures the e+, forming Hbar. We have also developed techniques to measure the excited‐state distribution of the Hbar and measure their velocity. I will present results from these experiments and discuss the next generation of apparatus to be commissioned this year. This new apparatus includes a e+ accumulator built at York University providing many more e+. The new Pbar annihilation detector provides spatial information of annihilations. Windows allow lasers to enter the trap for spectroscopic measurements and for laser cooling of the Hbar. Possibly the most exciting inclusion in this new apparatus is the inclusion of a neutral particle trap which may, for the first time, capture the Hbar and lead to the first atomic spectrum from antimatter. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser-controlled production of Rydberg positronium via charge exchange collisions

Lasers are used to choose the binding energies of highly excited positronium atoms (Ps*) created via charge exchange collisions. The lasers directly excite cesium (Cs) atoms to highly excited states (Cs*). These large Cs*atoms have a large cross section for resonant charge exchange collisions with trapped positrons. Highly excited Ps* is formed with a binding energy that is determined by the initial laser excitation. These Ps* should have a large cross section for resonant charge exchange collisions with trapped antiprotons—suggesting a possible new way to produce cold antihydrogen.

Antihydrogen formation by collisions of antiprotons with positronium in a magnetic field

Using the classical trajectory Monte Carlo method, we calculated the charge-transfer cross section for antiprotons colliding with Rydberg positronium, leading to antihydrogen formation. The results show a significant influence of an externally applied magnetic field which causes a reduction of the cross section.

Can we measure the gravitational free fall of cold Rydberg state positronium?

In this paper we examine the possibilities for detecting the free fall of Rydberg positronium atoms. In our scheme, cold positronium atoms are emitted from a “point” source and excited to the n=25 circular Rydberg state with L= n−1. The positronium atoms are allowed to travel horizontally 10 m in a field free vacuum and focused onto a detector using an elliptical Van der Waals mirror. A free fall distance of order 50 μm and a few detected atoms per hour are anticipated. Various extraneous influences on the positronium, such as collisions with residual gas atoms, Stark mixing in stray electric and magnetic fields, photoionization due to thermal radiation, and accelerations due to patch potentials are estimated.

Field Ionization of Strongly Magnetized Rydberg Positronium: A New Physical Mechanism for Positron Accumulation

Magnetized Rydberg positronium forms when an energetic positron ( e(+)) slows within a tungsten crystal and picks up an electron ( e(-)) as it emerges in a strong magnetic field. The signature is equal numbers of e(+) and e(-) when a weak electric field is applied, either of which can be accumulated and counted. The new e(+) accumulation technique is simple, robust, and much more efficient than any other demonstrated to be compatible with a cryogenic vacuum. Possible applications include the study of cold single component plasmas of e(+) and the formation of cold antihydrogen.