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The AEgIS experiment aims at performing the first test of the Weak Equivalence Principle of General Relativity in the antimatter sector by measuring the gravitational acceleration acting on a beam of cold antihydrogen to a precision of 1 %. The installation of the apparatus is making good progress and large parts were taken into operation. Parasitic detector tests during the beamtime in December 2012 gave essential input for an optimal moir´e deflectometer and the detector layout necessary to perform the gravity measurement.

The main goal of the AEgIS experiment at CERN is to test the weak equivalence principle for antimatter. We will measure the Earth's gravitational acceleration g with antihydrogen atoms being launched in a horizontal vacuum tube and traversing a moire deflectometer. We intend to use a position sensitive device made of nuclear emulsions (combined with a time-of-flight detector such as silicon μ� strips) to

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., Measuring forces on antimatter is vital to testing our understanding of fundamental physics. Towards this aim, Aghion et al. present a method to measure the deflection of antiprotons based on an atom optical tool, the moiré deflectometer, which could be extended to future antihydrogen gravity measurements.

The goal of the AEḡIS experiment at the Antiproton Decelerator (AD) at CERN, is to measure directly the Earth’s gravitational acceleration on antimatter by measuring the free fall of a pulsed, cold antihydrogen beam. The final position of the falling antihydrogen will be detected by a position sensitive detector. This detector will consist of an active silicon part, where the annihilations take place, followed by an emulsion part. Together, they allow to achieve 1% precision on the measurement of ḡ with about 600 reconstructed and time tagged annihilations. We present here the prospects for the development of the AEḡIS silicon position sentive detector and the results from the first beam tests on a monolithic silicon pixel sensor, along with a comparison to Monte Carlo simulations. publishedVersion

Precision spectroscopic comparison of hydrogen and antihydrogen holds the promise of a sensitive test of the Charge-Parity-Time theorem and matter-antimatter equivalence. The clearest path towards realising this goal is to hold a sample of antihydrogen in an atomic trap for interrogation by electromagnetic radiation. Achieving this poses a huge experimental challenge, as state-of-the-art magnetic-minimum atom traps have well depths of only ∼1 T (∼0.5 K for ground state antihydrogen atoms). The atoms annihilate on contact with matter and must be ‘born’ inside the magnetic trap with low kinetic energies. At the ALPHA experiment, antihydrogen atoms are produced from antiprotons and positrons stored in the form of non-neutral plasmas, where the typical electrostatic potential energy per particle is on the order of electronvolts, more than 104 times the maximum trappable kinetic energy. In November 2010, ALPHA published the observation of 38 antiproton annihilations due to antihydrogen atoms that had been trapped for at least 172 ms and then released—the first instance of a purely antimatter atomic system confined for any length of time (Andresen et al., Nature 468:673, 2010). We present a description of the main components of the ALPHA traps and detectors that were key to realising this result. We discuss how the antihydrogen atoms were identified and how they were discriminated from the background processes. Since the results published in Andresen et al. (Nature 468:673, 2010), refinements in the antihydrogen production technique have allowed many more antihydrogen atoms to be trapped, and held for much longer times. We have identified antihydrogen atoms that have been trapped for at least 1,000 s in the apparatus (Andresen et al., Nature Physics 7:558, 2011). This is more than sufficient time to interrogate the atoms spectroscopically, as well as to ensure that they have relaxed to their ground state.

AEgIS (Antimatter experiment: gravity, interferometry, spectroscopy) is an experiment approved by CERN with the goal of studying antihydrogen physics. In AEgIS, antihydrogen will be produced by charge exchange reactions of cold antiprotons with positronium atoms excited in a Rydberg state (n > 20). In the first phase of the experiment, controlled acceleration by an electric field gradient (Stark effect) and subsequent measurement of free fall in a Moiré deflectometer will allow a test of the weak equivalence principle. In a second phase, the antihydrogen will be slowed, confined, and laser-cooled to perform CPT studies and detailed spectroscopy. In the present work, after a general description of the experiment, the present status of advancement will be reviewed, with special attention to the production and excitation of positronium atoms.

Antihydrogen spectroscopy promises precise tests of the symmetry of matter and antimatter, and can possibly offer new insights into the baryon asymmetry of the universe. Antihydrogen is, however, difficult to synthesize and is produced only in small quantities. The ALPHA collaboration is therefore pursuing a path towards trapping cold antihydrogen to permit the use of precision atomic physics tools to carry out comparisons of antihydrogen and hydrogen. ALPHA has addressed these challenges. Control of the plasma sizes has helped to lower the influence of the multipole field used in the neutral atom trap, and thus lowered the temperature of the created atoms. Finally, the first systematic attempt to identify trapped antihydrogen in our system is discussed. This discussion includes special techniques for fast release of the trapped anti-atoms, as well as a silicon vertex detector to identify antiproton annihilations. The silicon detector reduces the background of annihilations, including background from antiprotons that can be mirror trapped in the fields of the neutral atom trap. A description of how to differentiate between these events and those resulting from trapped antihydrogen atoms is also included.

Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom. Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom.

The creation of atoms of antihydrogen under controlled conditions has opened up a new era in physics with antimatter. We describe the experimental realisation of low energy antihydrogen, via the mixing of carefully prepared clouds of positrons and antiprotons, and some of the progress that has been made in the last few years in characterising properties of the nascent anti-atoms. Ongoing efforts aimed at trapping the anti-atoms in magnetic field minima are discussed. Some of the motivations for undertaking experiments with antihydrogen are presented.

Objective : To evaluate whether smooth pursuit eye movements differed between patients with long-lasting whiplash-associated disorders and controls when using a purely computerized method for the eye movement analysis. Design : Cross-sectional study comparing patients with whiplash-associated disorders and controls who had not been exposed to head or neck trauma and had no notable neck complaints. Methods : Smooth pursuit eye movements were registered while the subjects were seated with and without rotated cervical spine. Subjects : Thirty-four patients with whiplash-associated disorders with symptoms more than six months after a car collision and 60 controls. Results : Smooth pursuit eye movements were almost identical in patients with chronic whiplash-associated disorders and controls, both when the neck was rotated and in the neutral position. Conclusion : Disturbed smooth pursuit eye movements do not appear to be a distinct feature in patients with chronic whiplash-associated disorders. This is in contrast to results of previous studies and may be due to the fact that analyses were performed in a computerized and objective manner. Other possible reasons for the discrepancy to previous studies are discussed.

The goal of the ALPHA collaboration at CERN is to test CPT conservation by comparing the 1S–2S transitions of hydrogen and antihydrogen. To reach the ultimate accuracy of 1 part in 10 18 , the (anti)atoms must be trapped. Using current technology, only magnetic minimum traps can confine (anti)hydrogen. In this paper, the design of the ALPHA antihydrogen trap and the results of measurements on a prototype system will be presented. The trap depth of the final system will be 1.16 T, corresponding to a temperature of 0.78 K for ground state antihydrogen.

Antihydrogen, the atomic bound state of an antiproton and a positron, was produced at low energy for the first time by the ATHENA experiment, marking an important first step for precision studies of atomic antimatter. This paper describes the first production and some subsequent developments., Invitated Talk at COOL03, International Workshop on Beam Cooling and Related Topics, to be published in NIM A

The production and observation of cold antihydrogen atoms have been recently reported by the ATHENA experiment at the CERN Antiproton Decelerator. The antiatoms were produced by mixing low-energy antiprotons and positrons in an electromagnetic trap. The antihydrogen detection is based on the observation of a characteristic signature in the annihilation of the neutral antiatoms on the trap walls by means of an imaging particle detector. An overview of the apparatus is given and the results obtained are discussed.

The ATHENA experiment recently produced the first atoms of cold antihydrogen. This paper gives a brief review of how this was achieved., Invited talk at Int. Conf. on Low Energy Antiprotons 2003 (LEAP03), to be published in NIM B

A positron accumulator has been constructed for use in the ATHENA anti-hydrogen experiment in CERN. Employing a solid neon moderator plated on to a 50 mCi 22 Na source, a low energy beam of 7 10 6 positrons/s is guided into a 0.14 T magnetic field where they are trapped and cooled down to room temperature using nitrogen as a buffer gas. Plasmas of up to 2 10 8 positrons in 450 s, with an FWHM of 4 mm after compressing with the rotating electrical wall technique have been observed. In order to transfer the plasma to the main ATHENA (3 T) magnet, where the recombination trap is situated, a transfer section has been constructed consisting of a valve and a pulsed magnet with a pumping restriction inside. This magnet pulses to 1.2 T during the transfer. Preliminary tests have yielded transfer efficiencies in the order of 50%. # 2002 Elsevier Science B.V. All rights reserved.

We present here a new application of silicon sensors aimed at the direct detection of antinucleons annihilations taking place inside the sensor׳s volume. Such detectors are interesting particularly for the measurement of antimatter properties and will be used as part of the gravity measurement module in the AE g ¯ IS experiment at the CERN Antiproton Decelerator. One of the goals of the AE g ¯ IS experiment is to measure the gravitational acceleration of antihydrogen with 1% precision. Three different silicon sensor geometries have been tested with an antiproton beam to investigate their properties as annihilation detection devices: strip planar, 3D pixels and monolithic pixel planar. In all cases we were successfully detecting annihilations taking place in the sensor and we were able to make a first characterization of the clusters and tracks.

Antihydrogen holds the promise to test, for the first time, the universality of free-fall with a system composed entirely of antiparticles. The AEgIS experiment at CERN’s antiproton decelerator aims to measure the gravitational interaction between matter and antimatter by measuring the deflection of a beam of antihydrogen in the Earths gravitational field (g¯¯¯). The principle of the experiment is as follows: cold antihydrogen atoms are synthesized in a Penning-Malberg trap and are Stark accelerated towards a moiré deflectometer, the classical counterpart of an atom interferometer, and annihilate on a position sensitive detector. Crucial to the success of the experiment is the spatial precision of the position sensitive detector. We propose a novel free-fall detector based on a hybrid of two technologies: emulsion detectors, which have an intrinsic spatial resolution of 50 nm but no temporal information, and a silicon strip / scintillating fiber tracker to provide timing and positional information. In 2012 we tested emulsion films in vacuum with antiprotons from CERN’s antiproton decelerator. The annihilation vertices could be observed directly on the emulsion surface using the microscope facility available at the University of Bern. The annihilation vertices were successfully reconstructed with a resolution of 1–2 μmon the impact parameter. If such a precision can be realized in the final detector, Monte Carlo simulations suggest of order 500 antihydrogen annihilations will be sufficient to determine g¯¯¯with a 1 % accuracy. This paper presents current research towards the development of this technology for use in the AEgIS apparatus and prospects for the realization of the final detector., Hyperfine Interactions, 228 (1-3), ISSN:0304-3843, ISSN:0304-3834, ISSN:1572-9540, Proceedings of the 11th International Conference on Low Energy Antiproton Physics (LEAP 2013) held in Uppsala, Sweden, 10–15 June, 2013

The motivation of the AEgIS experiment is to test the universality of free fall with antimatter. The goal is to reach a relative uncertainty of 1% for the measurement of the earth's gravitational acceleration [Formula: see text] on an antihydrogen beam. High vertex position resolution is required for a position detector. An emulsion based detector can measure the annihilation vertex of antihydrogen atoms with a resolution of 1-2 μm, which if realized in the actual experiment will enable a 1% measurement of [Formula: see text] with less than 1000 [Formula: see text] atoms. Developments and achievements on emulsion detectors for the AEgIS experiment are presented here.

The AEg¯ $\overline {\rm{g}}$IS experiment [1] aims at directly measuring the gravitational acceleration g on a beam of cold antihydrogen (H¯$\overline {\rm{H}}$) 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 H¯$\overline {\rm{H}}$ 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 H¯$\overline {\rm{H}}$ 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.

The ATHENA experiment at the Antiproton Decelerator facility at CERN aims at testing CPT symmetry with antihydrogen. An overview of the experiment, together with preliminary results of development towards the production of slow antihydrogen are reported.

A description is given of an electrostatically guided positron beam set-up designed for testing the performance of a remoderation stage in a positron micro-beam facility. In this set-up positrons from a 15 mCi 22Na source are moderated in a 3.5 lm thick tungsten foil and focused onto the entrance of a 903 cylindrical mirror de#ector. At the exit of the de#ector the positrons are focused onto a micro-channel plate with phosphor screen assembly in order to characterise the beam diameter and intensity at the entrance of the remoderation stage. ( 1999 Elsevier Science B.V. All rights reserved. PACS: 0778; 7870.B

Two different methods of measuring the reflection coefficient of positrons impinging on tungsten foils with incident energies below the positron workfunction for tungsten have been employed to study reflection. One involves using a 1 keV incident positron energy and monitoring the build-up in annihilation signal as a function of time after a grid in front of the sample is switched to a positive potential to return re-emitted positrons to the sample. The other method involves using very low-energy incident positrons (20 eV) and observing the dying out of the annihilation signal over time after the grid is switched to a potential in excess of +20 V to reflect re-emitted positrons and switch off the primary beam. A reflection coefficient of approximately 0.55 is found from the first experiment and a value of 0.58 is derived from the second experiment.

The thermal annealing of 1.5 μm thick SiO 2 layers deposited on Si with a High Density Plasma (HDP) has been monitored by Thermal Desorption Spectrometry (TDS), Fourier Transform Infra-Red absorption (FTIR) and Positron Annihilation Doppler Broadening (PADB). Two samples were prepared with different production conditions (substrate temperature, Ar flow, O 2 /SiH 4 ratio) in order to get oxide layers rich in Si-H bonds or rich in Si-OH bonds. The deposition temperatures of these layers were 545 K and 575 K, respectively. The formation of Si-H rich and Si-OH rich oxides was confirmed by FTIR absorption measurements. Desorption measurements showed that at a heating rate of 5 K/s release of hydrogen takes place around 1100 K. For the layer deposited at 575 K additional release of hydrogen is observed at 900 K. In both samples argon release is seen around 1200 K. The release of hydrogen at 1100 K is consistent with FTIR measurements at annealed samples. The absorption peaks associated with the Si-H and Si-OH bonds only disappear after heating above 1000 K. For the PADB measurements thermal anneals were carried out from 300 to 1300 K in steps of 50 and 100 K of 16 min duration. The positron measurements show different annealing behavior of the two oxides. It is observed that up to 1200 K the 545 K oxide shows a significant higher value for the defect parameter. The argon and hydrogen release is observed by an increase of the defect parameter at 1000 K.

For the first time the AEgIS (Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy) experiment will measure the Earth's local gravitational acceleration g on antimatter through the evaluation of the vertical displacement of an antihydrogen horizontal beam. This will be a model independent test of the Weak Equivalence Principle at the base of the general relativity. The initial goal of a g measurement with a relative uncertainty of 1% will be achieved with less than 1000 detected antihydrogens, provided that their vertical position could be determined with a precision of a few micrometers. An emulsion based detector is very suitable for this purpose featuring an intrinsic sub-micrometric spatial resolution. Nevertheless, the AEgIS experiment requires unprecedented operational conditions for this type of detector, namely vacuum environment and very low temperature. An intense R&D; activity is presently going on to optimize the detector for the AEgIS experimental requirements with rather encouraging results.

see paper for full list of authors; International audience; The AEgIS experiment is an international collaboration with the main goal of performing the first direct measurement of the Earth's gravitational acceleration on antimatter. Critical to the success of AEgIS is the production of cold antihydrogen (View the MathML source) atoms. The FACT detector is used to measure the production and temperature of the View the MathML source atoms and for establishing the formation of a View the MathML source beam. The operating requirements for this detector are very challenging: it must be able to identify each of the thousand or so annihilations in the 1 ms period of pulsed View the MathML source production, operate at 4 K inside a 1 T solenoidal field and not produce more than 10 W of heat. The FACT detector consists of two concentric cylindrical layers of 400 scintillator fibres with a 1 mm diameter and a 0.6 mm pitch. The scintillating fibres are coupled to clear fibres which transport the scintillation light to 800 silicon photomultipliers. Each silicon photomultiplier signal is connected to a linear amplifier and a fast discriminator, the outputs of which are sampled continuously by Field Programmable Gate Arrays (FPGAs). In the course of the developments for the FACT detector we have established the performance of scintillating fibres at 4 K by means of a cosmic-ray tracker operating in a liquid helium cryostat. The FACT detector was installed in the AEgIS apparatus in December 2012 and will be used to study the View the MathML source formation when the low energy antiproton physics programs resume at CERN in the Summer of 2014. This paper presents the design requirements and construction methods of the FACT detector and provides the first results of the detector commissioning.

Positron beam experiments have been performed to establish the diffusion behavior and surface branching of positrons implanted with energies varying from 0–25 keV into an epitaxially grown layer of β-SiC on a silicon substrate. The diffusion length of the positrons amounted to 42 nm. The surface branching was as follows: 30% positron emission, 30% positronium formation and 40% trapping at the surface. The energy distribution of re-emitted positrons consisted of a narrow contribution centered around 0.5 eV with a wider contribution up to 3 eV. The use of this wide band gap material for field assisted positron moderation is discussed.

We describe an experiment performed in the ATHENA apparatus in which there is evidence that the antiproton-proton bound state, protonium, has been produced at very low energies in vacuum following the interaction of cold antiprotons with a trapped cloud of molecular hydrogen ions. The latter were confined in a centrifugally separated belt outside a positron plasma used for antihydrogen formation. Studies have been performed at low positron plasma temperatures in which the protonium annihilation signal has been identified along with that from antihydrogen, and we discuss how their contributions can be disentangled. With the positron plasma heated to around 10,000 K the ions become distributed in the positrons, and the majority of the annihilation signal can be explained in terms of protonium formationion, as antihydrogen creation is heavily suppressed. In this case we compare the observed protonium formation rate with expectations from theory and find reasonable accord, when experimental systematics are taken into account. The effect on the annihilation signals of the passage of an electron current through a pre-loaded positron plasma has been studied in detail, and the results are presented here for the first time.

The AEGIS experiment, currently being set up at the Antiproton Decelerator at CERN, has the objective of studying the free fall of antimatter in the Earth's gravitational field by means of a pulsed cold atomic beam of antihydrogen atoms. Both duration of free fall and vertical displacement of the horizontally emitted atoms will be measured, allowing a first test of the WEP with antimatter.

After the first production of cold antihydrogen by the ATHENA and ATRAP experiments ten years ago, new second-generation experiments are aimed at measuring the fundamental properties of this anti-atom. The goal of AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is to test the weak equivalence principle by studying the gravitational interaction between matter and antimatter with a pulsed, cold antihydrogen beam. The experiment is currently being assembled at CERN’s Antiproton Decelerator. In AEGIS, antihydrogen will be produced by charge exchange of cold antiprotons with positronium excited to a high Rydberg state (n > 20). An antihydrogen beam will be produced by controlled acceleration in an electric-field gradient (Stark acceleration). The deflection of the horizontal beam due to its free fall in the gravitational field of the earth will be measured with a moiré deflectometer. Initially, the gravitational acceleration will be determined to a precision of 1%, requiring the detection of about 10^5 antihydrogen atoms. In this paper, after a general description, the present status of the experiment will be reviewed.

The ALPHA experiment has succeeded in trapping antihydrogen, a major milestone on the road to spectroscopic comparisons of antihydrogen with hydrogen. An annihilation vertex detector, which determines the time and position of antiproton annihilations, has been central to this achievement. This detector, an array of double-sided silicon microstrip detector modules arranged in three concentric cylindrical tiers, is sensitive to the passage of charged particles resulting from antiproton annihilation. This article describes the method used to reconstruct the annihilation location and to distinguish the annihilation signal from the cosmic ray background. Recent experimental results using this detector are outlined.

ALPHA is one of the experiments situated at CERN's Antiproton Decelerator (AD). A Silicon Vertex Detector (SVD) is placed to surround the ALPHA atom trap. The main purpose of the SVD is to detect and locate antiproton annihilation events by means of the emitted charged pions. The SVD system is presented with special focus given to the design, fabrication and performance of the modules.

We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ~30 ms. After a three-week experimental run in 2009 involving mixing of 10^7 antiprotons with 1.3 10^9 positrons to produce 6 10^5 antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally., Comment: 12 pages, 7 figures

Characterization of a positronium (Ps) gas target to be used for the charge conjugate reaction for antihydrogen formation [1], namely proton + Ps-atom → hydrogen + positron, has been performed [2–4]. In the above collision the Ps target utilised is produced efficiently on the transmission side of a hot thin (1900 A) Ag(100) foil with 12% efficiency at 800 K foil temperature, with a mean velocity of thermally activated Ps of 1.2×105 m/s and with a maximum kinetic energy of prompt emitted Ps of 1.5±0.2 eV.

With the use of surface charging the efficiency of a rare gas solid (RGS) positron (e+) moderator has been determined as a function of applied electric field. This technique relies on electron (e−) capture by adsorbed molecular species such as oxygen. These studies have shown that the emitted e+ yield from an RGS moderator can be enhanced by at least a factor of three by negative surface charging or almost totally reduced by positive surface charging. Even under ultra high vacuum conditions significant surface charging of RGS films can occur and such effects should be considered if moderating materials of this type are to be used.

A positron accumulator based on the modified Penning–Malmberg design of Surko and co-workers at UCSD has been constructed and undergone testing in preparation for the ATHENA experiment now under way at CERN. This experiment aims to produce and characterize atomic anti-hydrogen. The positron accumulator utilises nitrogen buffer gas to cool and trap a continuous beam of positrons emanating from a Na radioactive source. A solid neon moderator slows the positrons from the source down to epithermal energies of a few eV before being injected into the trap. It is estimated that around 10 positrons can be trapped and cooled to ambient temperature within a couple of minutes in this scheme using a 6 mCi source. Preliminary tests have so far demonstrated trapping of approximately 3 x 10 positrons and an efficiency of the Ne moderator of nearly 1 %. INTRODUCTION In order to produce low energy antihydrogen via recombination it is necessary to have copious amounts of cold positrons available. To attain this a positron accumulator based on the design of the Surko Group at the University of California San Diego [1-3] has been constructed and undergone preliminary testing in the United Kingdom before being shipped to CERN in Geneva to be a part of the ATHENA (AnTiHydrogEN Apparatus) experiment [4-5]. The accumulator is an ideal source of positrons in this case as it is capable of supplying large quantities (>10 in the later stages of the experiment) of positrons in short well defined bursts, with a short cycle time, in the order of 5 minutes. EXPERIMENTAL The positron accumulator is an instrument for trapping and cooling a continuous beam of slow positrons. A continuous beam of slow positrons injected into the accumulator is generated by moderating β+ particles from a 6 mCi Na radioactive source and guiding them into the trapping region using a magnetic field. A cryogenic cold head capable of reaching 5.5 K cools down the source and makes it possible to grow a solid neon moderator directly on the source. The slow positrons emanating from the source/moderator are magnetically guided through a kink and a long narrow pumping restriction into the main trapping region (see [6] for details). Placing a NaIdetector close to the gate valve between the source end and the main trapping region made it possible to optimise the moderator growth. During moderator growth this gate valve would be closed and the positrons would annihilate on the closed valve. To calibrate the NaI signal to absolute beam strength a channeltron detector was briefly inserted here and coincidence Figure 1. Schematic of the trapping scheme showing the electrode array and the pressures and electric potentials along the array. measurements performed. The channeltron detector with a retarding grid in front of it was also used to measure the energy distribution of the emitted positrons. The main trapping chamber is situated within a 0.1 T magnet and contains an electrode array. This consists of a set of eight separate goldplated aluminium electrodes with an appropriate potential applied which will confine the positrons in the axial direction after the initial trapping (Fig.1). The 0.1 T axial magnetic field supplies the radial confinement and combined with the electric potentials this constitutes our Penning-Malmberg trap. The physical dimensions of the electrodes are designed to allow a pressure gradient to be developed along their length. Nitrogen gas can be introduced midway along electrode II and is pumped out at either end or through a set of three vents located at the end of the same electrode. The positrons are trapped and cooled within the array via a buffer gas method. The pressure of the nitrogen gas used as buffer gas is tuned such that on average, a positron entering from the source region will experience one inelastic collision with a nitrogen molecule whilst traversing electrode II. Thus confined and unable to escape the array a second collision typically occurs within a millisecond further confining the positron to between electrodes III and VI. Typically, a third collision after some 10ms will then finally restrict the positron to electrodes IV and V. Electrode IV is split into 6 segments, making it possible to use it to compress the plasma by applying a rotating electric field (‘rotating wall’) [7]. This method has been shown recently to work well also for positrons plasmas [8]. In our case the electrodes used have a significantly larger radius (~10 cm) than in earlier experiments but preliminary tests have shown that we can still influence the positron plasma from that distance. The detection system used for our experiment consists of 3 detectors: (1) A segmented Faraday cup detector consisting of 9 plates to get information about the size and position of the plasma. (2) A CsI-diode detector to detect the positron annihilation signal. (3) A ‘Gauss’ Law’ signal from one of the confining electrodes caused by charge being induced on the electrode as the confined plasma is being dumped on the Faraday cup. Presently the whole experimental set-up including the data acquisition from the detectors is being automated to make it possible to run the experiment remotely. RESULTS A study of moderator growth was conducted using a channeltron and a NaI detector in coincidence in order to ascertain the moderator efficiencies for different moderator thicknesses etc. These detectors were placed at the entrance to the main vacuum system. Using this method we have been able to detect more than 1.6 μ 10 e+ s, giving a moderator efficiency of 0.62 % based on a source strength of 7 mCi at the time of the measurement. However, using a measurement of the current of positrons from the source taken during an electronically quiet period as the base for the moderator efficiency a real efficiency of 0.76 % is reached. The study of the energy distribution of 01020340506 10 20 30 40 50 CsI Output (mV) Holding Tme (s) 01020340506 10 20 304 50 CsI Output (mV)

Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7 × 10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4 ± 1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.

The main scientific goal of the AEGIS experiment ( A ntimatter E xperiment: G ravity, I nterferometry, S pectroscopy) 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 μ m) using a Moire' deflectometer.

A comparison of the 1S-2S transitions of hydrogen and antihydrogen will yield a stringent test of CPT conservation. Necessarily, the antihydrogen atoms need to be trapped to perform high precision spectroscopy measurements. Therefore, an approximately 0.75 T deep neutral atom trap, equivalent to about 0.5 K for ground state (anti)hydrogen atoms, has been superimposed on a Penning-Malmberg trap in which the anti-atoms are formed. The antihydrogen atoms are produced following a number of steps. A bunch of antiprotons from the CERN Antiproton Decelerator is caught in a Penning-Malmberg trap and subsequently sympathetically cooled and then compressed using rotating wall electric fields. A positron plasma, formed in a separate accumulator, is transported to the main system and also compressed. Antihydrogen atoms are then formed by mixing the antiprotons and positrons. The velocity of the anti-atoms, and their binding energies, will strongly depend on the initial conditions of the constituent particles, for example their temperatures and densities, and on the details of the mixing process. In this paper the complete lifecycle of antihydrogen atoms will be presented, starting with the production of the constituent antiparticles and the description of the manipulations necessary to prepare them appropriately for antihydrogen formation. The latter will also be described, as will the possible fates of the anti-atoms.

The field assisted moderation of positrons (e+) has been achieved by charging the surfaces of argon and krypton rare gas solid (RGS) films. Following deposition the RGS surface was coated with a thin layer of molecular oxygen which serves to trap some of the charge which was then applied by low energy electron bombardment. Energy spectra of both the low energy e+s and secondary electrons emitted from a charged argon surface were found to be consistent with the production of a surface potential of approximately -20 V. This resulted in a threefold enhancement of the slow e+ yield and a reduction of around a factor of five in the emission of secondary electrons. Electron bombardment of RGS films not exposed to oxygen showed no such surface charge effects.

Cold antihydrogen has been produced at CERN (Amoretti et al. (Nature, 419, 456 (2002)), Gabrielse et al. (Phys. Rev. Lett. 89, 213401 (2002))), with the aim of performing a high-precision spectroscopic comparison with hydrogen as a test of the CPT symmetry. Hydrogen, a unique system used for the development of quantum mechanics and quantum electrodynamics, has been continuously used to produce high-precision tests of theories and measurements of fundamental constants and can lead to a very sensitive search for CPT violation. After the initial production of cold antihydrogen atoms by the ATHENA group, the ALPHA Collaboration ( http://alpha.web.cern.ch/ ) has set forth on an experiment to trap and perform high-resolution laser spectroscopy on the 1S-2S transition of both atoms. In this contribution, we will review the motivations, goals, techniques, and recent developments towards this fundamental physics test. We present new discussion on predicted lineshapes for the 1S-2S spectroscopy of trapped atoms in a regime not discussed before.

A microchannel plate (MCP)/phosphor screen assembly has been used to destructively measure the radial profile of cold, confined antiprotons, electrons, and positrons in the ALPHA experiment, with the goal of using these trapped particles for antihydrogen creation and confinement. The response of the MCP to low energy (10-200 eV

We report results from a novel diagnostic that probes the outer radial profile of trapped antiproton clouds. The diagnostic allows us to determine the profile by monitoring the time-history of antiproton losses that occur as an octupole field in the antiproton confinement region is increased. We show several examples of how this diagnostic helps us to understand the radial dynamics of antiprotons in normal and nested Penning-Malmberg traps. Better understanding of these dynamics may aid current attempts to trap antihydrogen atoms. We report results from a novel diagnostic that probes the outer radial profile of trapped antiproton clouds. The diagnostic allows us to determine the profile by monitoring the time-history of antiproton losses that occur as an octupole field in the antiproton confinement region is increased. We show several examples of how this diagnostic helps us to understand the radial dynamics of antiprotons in normal and nested Penning-Malmberg traps. Better understanding of these dynamics may aid current attempts to trap antihydrogen atoms.

We demonstrate temporally controlled modulation of cold antihydrogen production by periodic RF heating of a positron plasma during antiproton-positron mixing in a Penning trap. Our observations have established a pulsed source of atomic antimatter, with a rise time of about 1 s, and a pulse length ranging from 3 to 100 s. Time-sensitive antihydrogen detection and positron plasma diagnostics, both capabilities of the ATHENA apparatus, allowed detailed studies of the pulsing behavior, which in turn gave information on the dependence of the antihydrogen production process on the positron temperature T. Our data are consistent with power law scaling T (-1.1+/-0.5) for the production rate in the high temperature regime from approximately 100 meV up to 1.5 eV. This is not in accord with the behavior accepted for conventional three-body recombination.

Control of the radial profile of trapped antiproton clouds is critical to trapping antihydrogen. We report the first detailed measurements of the radial manipulation of antiproton clouds, including areal density compressions by factors as large as ten, by manipulating spatially overlapped electron plasmas. We show detailed measurements of the near-axis antiproton radial profile and its relation to that of the electron plasma. Control of the radial profile of trapped antiproton clouds is critical to trapping antihydrogen. We report the first detailed measurements of the radial manipulation of antiproton clouds, including areal density compressions by factors as large as ten, by manipulating spatially overlapped electron plasmas. We show detailed measurements of the near-axis antiproton radial profile and its relation to that of the electron plasma.

We have demonstrated production of antihydrogen in a 1$,$T solenoidal magnetic field. This field strength is significantly smaller than that used in the first generation experiments ATHENA (3$,$T) and ATRAP (5$,$T). The motivation for using a smaller magnetic field is to facilitate trapping of antihydrogen atoms in a neutral atom trap surrounding the production region. We report the results of measurements with the ALPHA (Antihydrogen Laser PHysics Apparatus) device, which can capture and cool antiprotons at 3$,$T, and then mix the antiprotons with positrons at 1$,$T. We infer antihydrogen production from the time structure of antiproton annihilations during mixing, using mixing with heated positrons as the null experiment, as demonstrated in ATHENA. Implications for antihydrogen trapping are discussed. We have demonstrated production of antihydrogen in a 1$,$T solenoidal magnetic field. This field strength is significantly smaller than that used in the first generation experiments ATHENA (3$,$T) and ATRAP (5$,$T). The motivation for using a smaller magnetic field is to facilitate trapping of antihydrogen atoms in a neutral atom trap surrounding the production region. We report the results of measurements with the ALPHA (Antihydrogen Laser PHysics Apparatus) device, which can capture and cool antiprotons at 3$,$T, and then mix the antiprotons with positrons at 1$,$T. We infer antihydrogen production from the time structure of antiproton annihilations during mixing, using mixing with heated positrons as the null experiment, as demonstrated in ATHENA. Implications for antihydrogen trapping are discussed.

In the last three years of data taking, the ATHENA experiment at the CERN Antiproton Decelerator facility has been able to produce large amounts of antihydrogen atoms and to study the formation process in detail. Moreover, in 2004, ATHENA tried to produce antihydrogen by means of laser stimulation. In this contribution we summarize the main results obtained and the progress made in analysing the results of the laser experiment.

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.