The primary goal of the AEgIS collaboration at CERN is to measure the gravitational acceleration on neutral antimatter. Positronium (Ps), the bound state of an electron and a positron, is a suitable ...candidate for a force-sensitive inertial measurement by means of deflectometry/interferometry. In order to conduct such an experiment, the impact position and time of arrival of Ps atoms at the detector must be detected simultaneously. The detection of a low-velocity Ps beam with a spatial resolution of (88 ± 5) μm was previously demonstrated 1. Based on the methodology employed in 1 and 2, a hybrid imaging/timing detector with increased spatial resolution of about 10 μm was developed. The performance of a prototype was tested with a positron beam. The concept of the detector and first results are presented.
A detailed overview of the conventional (van Hove) neutron scattering theory and its specification to the Impulse Approximation (IA) are given. The IA constitutes the basis of all interpretations of ...results from neutron scattering experiments in the Compton regime, in which is widely believed that single-particle properties are probed. Here the validity of this approximation is carefully scrutinized and it is shown that there are several different steps in the derivation, whose validity can be questioned: (1) the scattering system is a closed system (in the quantum mechanical sense); (2) all entanglement in the scattering system is absent (N-body operators are replaced by single particle operators); (3) consequently the one-body momentum distribution n(p) is introduced ad hoc; (4) the δ-function in energy assumed in the derivation is not valid when the interaction times underlying the IA are so short that the uncertainty relation allows a noticeable spread in the energy balance. Additionally, (5) the entanglement due to the direct interaction of the neutron with a nucleus is completely absent in the general formalism. The concrete experimental context of recent neutron Compton scattering (NCS) experiments at the ISIS neutron spallation source is considered and emphasis is put on the ultrafast (attosecond) scattering time for the neutron-proton scattering. Recent modern theoretical developments are shortly mentioned in the discussion, which take into account the neutron's coherence length being larger than the de Broglie wavelength, or decoherence phenomena during the short but finite scattering time.
In this work we describe a high-resolution position-sensitive detector for positronium. The detection scheme is based on the photoionization of positronium in a magnetic field and the imaging of the ...freed positrons with a Microchannel Plate assembly. A spatial resolution of (88±5) μm on the position of the ionized positronium –in the plane perpendicular to a 1.0 T magnetic field– is obtained. The possibility to apply the detection scheme for monitoring the emission into vacuum of positronium from positron/positronium converters, imaging positronium excited to a selected state and characterizing its spatial distribution is discussed. Ways to further improve the spatial resolution of the method are presented.
In general, nuclei and electrons in condensed matter and/or molecules are entangled, due to the prevailing (electromagnetic) interactions. As a matter of fact, the "environment" interacting with a ...microscopic system of interest (e.g. a proton) causes the destruction of the entanglement. This process, called decoherence, has until now prevented experimenters from directly accessing atomic and/or nuclear entanglement effects in real experiments. However, our neutron and electron Compton scattering experiments from protons (H-atoms) in several condensed systems and molecules at ambient conditions demonstrated a new striking effect, i.e. an "anomalous" decrease of scattering intensity from protons, which seem to become partially "invisible" to the neutrons. This effect, which has no interpretation within conventional neutron scattering theory, is proposed to be caused by the non-unitary time evolution (due to decoherence) during the ultrashort, but finite, time-window of the neutron-proton scattering process. Due to the large energy (several eV) and momentum (20-200 Å−1) transfers of these experiments the collisional time between the probe particle and a struck proton is 100–1000 attoseconds long. It is shown that, due to this short timescale, the scattering process must be theoretically treated within quantum dynamics of open quantum systems. New experimental neutron Compton scattering results from a single crystal KHCO3 are presented. They provide the first direct evidence for a connection between the momentum distribution and the "anomalous" scattering intensity of H. Additionally, recent experiments with electron-atom Compton scattering at momentum transfers are mentioned, which reveal the presence of this effect even in scattering from single H2 molecules in the gas phase. Theoretical discussions "from first principles" are presented, also in relation to the quantum Zeno effect, which underline the crucial role of decoherence in the considered experiments. The experimental results and their qualitative interpretation show that epithermal neutrons being available at spallation sources (e.g. ISIS/UK, SNS/USA or ESS/Sweden), and electron spectrometers with large scattering angles, provide tools for investigating new physical and chemical phenomena in the sub-femtosecond timescale.
From the experimental point of view, very little is known about the gravitational interaction between matter and antimatter. In particular, the Weak Equivalence Principle, which is of paramount ...importance for the General Relativity, has not yet been directly probed with antimatter. The main goal of the AEgIS experiment at CERN is to perform a direct measurement of the gravitational force on antimatter. The idea is to measure the vertical displacement of a beam of cold antihydrogen atoms, traveling in the gravitational field of the Earth, by the means of a moiré deflectometer. An overview of the physics goals of the experiment, of its apparatus and of the first results is presented.
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 , radial compression to sub-millimetre radii of mixed plasmas in 1 T field, high-efficiency transfer of 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'.
PDF
