Positronium Laser Cooling via the 1

We report on laser cooling of a large fraction of positronium (Ps) in free flight by strongly saturating the 1^{3}S-2^{3}P transition with a broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is produced in a magnetic and electric field-free environment. We observe two different laser-induced effects. The first effect is an increase in the number of atoms in the ground state after the time Ps has spent in the long-lived 2^{3}P states. The second effect is one-dimensional Doppler cooling of Ps, reducing the cloud's temperature from 380(20) to 170(20) K. We demonstrate a 58(9)% increase in the fraction of Ps atoms with v_{1D}<3.7×10^{4} ms^{-1}.

Formal calculation of exchange correlation effects on annihilation lifetimes of positronium confined in small cavities.

Positronium atoms (Ps) are widely used as a probe to characterize voids or vacancies in non-metallic materials. The annihilation lifetime of trapped Ps is strongly modified by pickoff, depending on the cavity size and on the appropriate external electron density. The connection between these material characteristics and Ps annihilation lifetimes is usually based on models that do not consider the requirements of full electron indistinguishability, that must be taken into account for a correct description of pickoff annihilation processes. In this report, we provide a formal theoretical framework in which exchange correlation effects between confined Ps and surrounding electrons are introduced in a natural way, giving a clear and versatile picture of the various contributions to pickoff annihilation. Moreover, our results provide a simple explanation of the lowering of the contact density (the Ps-electron density at the positron position) as a direct consequence of the electrons indistinguishability, at variance with previous interpretation based on spatial deformations of Ps wavefunction. Calculations are performed within the 'symmetry adapted perturbation theory' approach, and the results are compared with available experimental data on Ps lifetimes for polymers and molecular solids. Finally, our analysis gives a formal justification to the approximations involved in early models based on the well known Tao-Eldrup approach, and gives a simple interpretation of Ps properties in subnanometric voids.

AEgIS at ELENA: outlook for physics with a pulsed cold antihydrogen beam.

The efficient production of cold antihydrogen atoms in particle traps at CERN's Antiproton Decelerator has opened up the possibility of performing direct measurements of the Earth's gravitational acceleration on purely antimatter bodies. The goal of the AEgIS collaboration is to measure the value of g for antimatter using a pulsed source of cold antihydrogen and a Moiré deflectometer/Talbot-Lau interferometer. The same antihydrogen beam is also very well suited to measuring precisely the ground-state hyperfine splitting of the anti-atom. The antihydrogen formation mechanism chosen by AEgIS is resonant charge exchange between cold antiprotons and Rydberg positronium. A series of technical developments regarding positrons and positronium (Ps formation in a dedicated room-temperature target, spectroscopy of the n=1-3 and n=3-15 transitions in Ps, Ps formation in a target at 10 K inside the 1 T magnetic field of the experiment) as well as antiprotons (high-efficiency trapping of [Formula: see text], radial compression to sub-millimetre radii of mixed [Formula: see text] plasmas in 1 T field, high-efficiency transfer of [Formula: see text] to the antihydrogen production trap using an in-flight launch and recapture procedure) were successfully implemented. Two further critical steps that are germane mainly to charge exchange formation of antihydrogen-cooling of antiprotons and formation of a beam of antihydrogen-are being addressed in parallel. The coming of ELENA will allow, in the very near future, the number of trappable antiprotons to be increased by more than a factor of 50. For the antihydrogen production scheme chosen by AEgIS, this will be reflected in a corresponding increase of produced antihydrogen atoms, leading to a significant reduction of measurement times and providing a path towards high-precision measurements.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

Positronium Confinement in Small Cavities: A Two-Particle Model for the Lowering of Contact Density.

Positronium (Ps) is widely used as a probe for studying nanometric porosities in condensed matter. Accessible experimental measurements concern annihilation rates by pickoff processes and contact densities (the electron density at the positron position). Existing models for describing Ps properties in small cavities do not justify the lowering of the contact density with respect to that of Ps in vacuum, as found in most materials. We formulate a two-particle model in which only the electron is confined in the cavity, while the positron is moving freely and feels the medium via a positive work function. Our calculation fully explains experimental data for a large class of materials and suggests a way to gain information on pore sizes and positron work functions.

A moiré deflectometer for antimatter.

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics--the moiré deflectometer--for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.

Polarization of positronium in amorphous polar polymers: a case study.

The features of positronium in an amorphous copolymer (polyvinyl acetate-crotonic acid) in a range of temperatures including the glass transition were investigated by means of positron annihilation lifetime spectroscopy. In particular, para-positronium lifetime was found to be longer than in a vacuum and to decrease with the temperature. This was attributed to the electron density at the positron (contact density), which is lower than in vacuo due to the presence of polar groups in the copolymer. A three quantum yield experiment confirmed the lifetime results.

Temperature dependence on free volume in cured natural rubber and styrene-butadiene rubber blends.

A systematic study on the evolution of free volume as a function of the temperature in vulcanized at 433 K natural rubber (NR) and styrene butadiene rubber (SBR) in 25-75, 50-50, 75-25 NR-SBR (percent content of pure NR and SBR, respectively) blends was studied by positron annihilation lifetime spectroscopy. All samples were prepared with sulfur and TBBS (n-t-butyl-2-benzothiazole sulfenamide) as accelerator. The glass transition temperatures of the samples studied were determined by differential scanning calorimetry (DSC) and from lifetime data. In general, a sigmoidal-like complex behavior of the long-lived lifetime component, linked to the nanohole free volume, as a function of the temperature was found. For SBR, the slope of the ortho-positronium lifetime against temperature curves could be well-fitted using a linear function. For blends and also for NR, two different linear functions were necessary. This last behavior is explained in terms of the supercooled process involving a reconfiguration of the elastomeric chains. In the case of blends, the state of cure of NR and SBR in each NR-SBR sample was also taken into account in the discussion of the results obtained. Besides, thermal expansion coefficients of the free volumes in the transition and glassy region of all compounds were estimated. The differences observed in the values of this parameter are discussed by taking into account the morphology and formulation of each blend, the crosslink densities, and the role of the interphases formed between both NR and SBR elastomers.

On the thermal expansion of nanohole free volume in perfluoropolyethers.

To determine the free volume in polymers, positron annihilation lifetime spectroscopy data are transformed into nanohole volumes by modeling the cavities as spheres or, more generally, using geometries assuming an isotropic thermal expansion. However, this guess could be unrealistic owing to the irregular shape of nanoholes and constrained movements of the macromolecules. In this work, it is shown that a comparison of hole-lattice theory with positron and dilatometric data for a homologous series of perfluoropolyethers supplies information on the anisotropic expansion of nanoholes; the relation between volume and typical unconstrained size of the cavities can be expressed by a power law with noninteger exponents.