We discuss the formation of antihydrogen atoms ($\bar{\rm H}$) in an ultra-cold positron-antiproton plasma. For positron densities $n_p\agt 10^8$ cm$^{-3}$ the characteristic formation time of stable $\bar{H}$ is determined by collisional relaxation of highly excited atoms produced in the process of 3-body Thompson recombination. Relying on the mechanisms of ``replacement collisions'' and ``transverse collisional drift'' we find a bottleneck in the relaxation kinetics and analyze the physical consequences of this phenomenon.
The antihydrogen formation by charge exchange between cold antiprotons and Rydberg positronium Ps is studied by using the Classical Trajectory Monte Carlo (CTMC) method.
I give an overview of experimental and theoretical results for antiproton and antihydrogen scattering with atoms and molecules (in particular H, He). At low energies ($\lesssim1$~keV) there are practically no experimental data available. Instead I compare the results from different theoretical calculations, of various degrees of sophistication. At energies up to a few 10:s of eV, I focus on simple approximations that give reasonably accurate results, as these allow quick estimates of collision rates without embarking on a research project.
A quantum mechanical approach is proposed for the formation of antihydrogen in the ground and excited states (2s, 2p) via the mechanism of three body recombination (TBR) inside a trapped plasma of anti proton and positron or in the collision between the two beams of them. Variations of the differential (DCS) as well as the total (TCS) formation cross sections are studied as a function of the incident energies of both the active and the spectator positrons. Significantly large cross sections are found at very low incident energies in the TBR process as compared to other processes leading to ant…
Antihydrogen is produced by mixing an antiproton and a positron plasma in a cryogenic electromagnetic trap. The dominant antihydrogen formation mechanism is three-body recombination, while the subsequent level population evolution is governed by various processes, mainly collisional (de)excitation, ionisation and radiative decay. In this work the impact of various positron plasma lengths on the level population evolution is investigated. The main interest is the ground-state antihydrogen atom yield. It is found that the ground state level population shows different power-law behaviour at short…
Antihydrogen production has reached such a level that precision spectroscopic measurements of its properties are within reach. In particular, the ground-state level population is of central interest for experiments aiming at antihydrogen spectroscopy. The positron density and temperature dependence of the ground-state yield is a result of the interplay between recombination, collisional, and radiative processes. Considering the fact that antihydrogen atoms with the principal quantum number n=15 or lower quickly cascade down to the ground state within 1ms, the number of such states are adopted …
Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilated. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be co…
High precision radio-frequency, microwave and infrared spectroscopic measurements of the antihydrogen molecular ion $\bar{H}_{2}^{-}$ ($\bar{p}\bar{p}e^{+}$) compared with its normal matter counterpart provide direct tests of the CPT theorem. The sensitivity to a difference between the positron/antiproton and electron/proton mass ratios, and to a difference between the positron-antiproton and electron-proton hyperfine interactions, can exceed that obtained by comparing antihydrogen with hydrogen by several orders of magnitude. Practical schemes are outlined for measurements on a single $\bar{H…
The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration plans to measure the ground-state hyperfine splitting of antihydrogen in a beam at the CERN Antiproton Decelerator with initial relative precision of 10-6 or better, to test the fundamental CPT (combination of charge conjugation, parity transformation and time reversal) symmetry between matter and antimatter. This challenging goal requires a polarised antihydrogen beam with a sufficient number of antihydrogen atoms in the ground state. The first measurement of the quantum state distribution of antihydrogen at…
This paper describes measurements of the hyperfine structure of two antiprotonic atoms that are planned at the Antiproton Decelerator (AD) at CERN. The first part deals with antiprotonic helium, a three-body system of alpha-particle, antiproton and electron that was previously studied at LEAR. A measurement will test existing three-body calculations and may - through comparison with these theories - determine the magnetic moment of the antiproton more precisely than currently available, thus providing a test of CPT invariance. The second system, antihydrogen, consisting of an antiproton and a …
Laser- assisted antihydrogen formation cross sections (differential and total) via the three - body recombination (TBR) process in a plasma of antiprotons and positrons are studied for the ground and excited states . Significantly large cross sections are obtained for low energies. The ground state antihydrogen cross section dominates throughout the energy range, essential for high precision spectroscopics studies in order to test the CPT theorem. The formation cross section are found to be much higher for unequal energies of the two positrons than for equal energies.
We apply a new highly efficient method of solving Faddeev-Merkuriev equations to multi-channel scattering calculations of the antihydrogen formation cross section for antiproton scattering off the ground and excited states of the positronium. Our results demonstrate good agreement with the known data on total and partial cross sections for all the reaction channels. Using moderate computational resources we have achieved a supreme energy resolution.
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-se…
Three-charge-particle collisions with participation of ultra-slow antiprotons ($\bar{\rm{p}}$) is the subject of this work. Specifically we compute the total cross sections and corresponding thermal rates of the following three-body reactions: $\bar{\rm p}+(e^+e^-) \rightarrow \bar{\rm{H}} + e^-$ and $\bar{\rm p}+(μ^+μ^-) \rightarrow \bar{\rm{H}}_μ + μ^-$, where $e^-(μ^-)$ is an electron (muon) and $e^+(μ^+)$ is a positron (antimuon) respectively, $\bar{\rm{H}}=(\bar{\rm p}e^+)$ is an antihydrogen atom and $\bar{\rm{H}}_μ=(\bar{\rm p}μ^+)$ is a muonic antihydrogen atom, i.e. a bound state of $…
The AEgIS collaboration is underway to directly measure the gravitational free-fall of neutral antimatter atoms. The experiment recently succeded in producing a pulsed cold antihydrogen source for the first time, and has now entered into its second phase, which aims at the formation of a slow antihydrogen beam and subseuqently a first proof-of-concept gravitational measurement. Major upgrades have been made, such as an improved antihydrogen production scheme and a new state-of-the-art antiproton trap. AEgIS was also connected to CERN's new antiproton deceleration facility ELENA and achieved fi…
First measurement of the antihydrogen production cross section through the charge-exchange reaction of low-energy antiprotons with orthopositronium
National Centre for Nuclear Research (NCBJ)（国家核研究中心）；IRFU, CEA, Université Paris-Saclay（巴黎萨克雷大学CEA IRFU）；Pohang University of Science and Technology (POSTECH)（浦项科技大学）；Laboratoire Kastler Brossel, Sorbonne Université, CNRS, ENS-Université PSL, Collège de France（索邦大学卡斯勒-布罗塞尔实验室）；Université d’Evry-Val d’Essonne, Université Paris-Saclay（埃夫里-瓦莱德埃松大学，巴黎萨克雷大学）；Université Paris-Saclay, CNRS/IN2P3, IJCLab（巴黎萨克雷大学，CNRS/IN2P3，IJCLab）；Conservatoire National des Arts et Métiers（国立工艺院）
The present theoretical work addresses the study of the formation of a positive Antihydrogen ion in a single charge exchange reaction where a positronium ( ground or excited state) interacts with a ground state anti-hydrogen atom, being embedded in a dense Debye plasma. The calculations are carried out in the frame work of a Coulomb Distorted Eikonal Approximation. The result shows that Ps(3S) is the most interesting state to enhance the anti H^+ production at the energy close to the reaction threshold. Regarding the Debye screening, it may be inferred that in lower energy regime, the screenin…
Spectroscopy of antihydrogen has the potential to yield high-precision tests of the CPT theorem and shed light on the matter-antimatter imbalance in the Universe. The ALPHA antihydrogen trap at CERN's Antiproton Decelerator aims to prepare a sample of antihydrogen atoms confined in an octupole-based Ioffe trap and to measure the frequency of several atomic transitions. We describe our techniques to directly measure the antiproton temperature and a new technique to cool them to below 10 K. We also show how our unique position-sensitive annihilation detector provides us with a highly sensitive m…
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.
The ASACUSA collaboration at the Antiproton Decelerator of CERN aims at a precise measurement of the antihydrogen ground-state hyperfine structure as a test of the fundamental CPT symmetry. A beam of antihydrogen atoms is formed in a CUSP trap, undergoes Rabi-type spectroscopy and is detected downstream in a dedicated antihydrogen detector. In parallel measurements using a polarized hydrogen beam are being performed to commission the spectroscopy apparatus and to perform measurements of parameters of the Standard Model Extension (SME). The current status of antihydrogen spectroscopy is reviewe…
We present the commissioning of the Fast Annihilation Cryogenic Tracker detector (FACT), installed around the antihydrogen production trap inside the 1 T superconducting magnet of the AEgIS experiment. FACT is designed to detect pions originating from the annihilation of antiprotons. Its 794 scintillating fibers operate at 4 K and are read out by silicon photomultipliers (MPPCs) at near room temperature. FACT provides the antiproton/antihydrogen annihilation position information with a few ns timing resolution. We present the hardware and software developments which led to the successful opera…
A brief survey is made to highlight the recent interests in positron, positronium and antimatter physics. Positron is the first antiparticle observed which was predicted by Dirac. Positronium is itself its antiparticle and bi-positronium molecule is recently observed in laboratory which was predicted by Wheeler. The simplest antiatom i.e. antihydrogen is observed in the laboratory and the process to achieve the stable confinement of antihydrogen within the trap are in progress to test the standard model.
A few-body formalism is applied for computation of two different three-charge-particle systems. The first system is a collision of a slow antiproton, $\bar{\rm{p}}$, with a positronium atom: Ps$=(e^+e^-)$ $-$ a bound state of an electron and a positron. The second problem is a collision of $\bar{\rm{p}}$ with a muonic muonium atom, i.e. true muonium $-$ a bound state of two muons one positive and one negative: Ps$_μ=(μ^+μ^-)$. The total cross section of the following two reactions: $\bar{\rm p}+(e^+e^-) \rightarrow \bar{\rm{H}} + e^-$ and $\bar{\rm p}+(μ^+μ^-) \rightarrow \bar{\rm{H}}_μ + μ^-$…
Antihydrogen atoms are routinely formed at the Antiproton Decelerator at CERN in a wide range of Rydberg states. To perform precision measurements, experiments rely on ground state antimatter atoms which are currently obtained only after spontaneous decay. In order to enhance the number of atoms in ground state, we propose and assess the efficiency of different methods to stimulate their decay. First, we investigate the use of THz radiation to simultaneously couple all n-manifolds down to a low lying one with sufficiently fast spontaneous emission toward ground state. We further study a deexci…
The ASACUSA CUSP collaboration at the Antiproton Decelerator (AD) of CERN is planning to measure the ground-state hyperfine splitting of antihydrogen using an atomic spectroscopy beamline. We describe here the latest developments on the spectroscopy apparatus developed to be coupled to the antihydrogen production setup (CUSP).
We consider the spin-exchange in ultracold collisions of hydrogen ($H$) and antihydrogen ($\bar{H}$) atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in $\bar{H}-H$ collisions is basically driven by the strong force between proton and antiproton, that spin-exchange cross-sections are proportional to the difference of singlet and triplet strong force S-wave scattering lengths, while the strong force effects are enhanced about $10^9$ times due to long range atomic forces. We show that any difference in hyperfine …
Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only…
The GBAR experiment aims to directly test the Weak Equivalence Principle of ultracold antihydrogen in Earth's gravitational field. The gravitational acceleration $\bar{g}$ will be measured to a precision of $1\,\%$ using a classical free fall of the anti-atoms from a fixed height. Reaching a precision of $10^{-6}$ is planned by performing a "quantum free fall" experiment by detecting quantum interference of $\mathrm{\bar{H}}$ due to quantum gravitational bound states above a reflecting surface. Additionally, a $\mathrm{\bar{H}}$ Lamb shift measurement is being prepared in parallel that will al…
The formation of positive ions of antihydrogen $\bar{\rm{H}}^+$ via the three body reaction (i) $\rm{e}^+ + \rm{e}^- + \bar{\rm{H}} \rightarrow \rm{e}^- + \bar{\rm{H}}^+$ is considered. In reaction (i), free positrons $\rm{e}^+$ are incident on antihydrogen $\bar{\rm{H}}$ embedded in a gas of low-energy ($\sim $ meV) electrons and, due to the positron-electron interaction, a positron is attached to $\bar{\rm{H}}$ whereas an electron carries away the energy excess. We compare reaction (i) with two radiative attachment mechanisms. One of them is (ii) spontaneous radiative attachment, in which th…
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 i…
Different experiments are ongoing to measure the effect of gravity on cold neutral antimatter atoms such as positronium, muonium and antihydrogen. Among those, the project GBAR in CERN aims to measure precisely the gravitational fall of ultracold antihydrogen atoms. In the ultracold regime, the interaction of antihydrogen atoms with a surface is governed by the phenomenon of quantum reflection which results in bouncing of antihydrogen atoms on matter surfaces. This allows the application of a filtering scheme to increase the precision of the free fall measurement. In the ultimate limit of smal…
Predicted twenty years ago, positron binding to neutral atoms has not yet been observed experimentally. A new scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for Ps charge …
We discuss aspects of antihydrogen studies, that relate to particle physics ideas and techniques, within the context of the ALPHA experiment at CERN's Antiproton Decelerator facility. We review the fundamental physics motivations for antihydrogen studies, and their potential physics reach. We argue that initial spectroscopy measurements, once antihydrogen is trapped, could provide competitive tests of CPT, possibly probing physics at the Planck Scale. We discuss some of the particle detection techniques used in ALPHA. Preliminary results from commissioning studies of a partial system of the AL…
Charges in cold, multiple-species, non-neutral plasmas separate radially by mass, forming centrifugally-separated states. Here, we report the first detailed measurements of such states in an electron-antiproton plasma, and the first observations of the separation dynamics in any centrifugally-separated system. While the observed equilibrium states are expected and in agreement with theory, the equilibration time is approximately constant over a wide range of parameters, a surprising and as yet unexplained result. Electron-antiproton plasmas play a crucial role in antihydrogen trapping experime…
The potential splitting approach incorporated into the framework of Faddeev-Merkuriev equations in the differential form is used for calculations of multichannel scattering in $e^-e^+\bar{p}$ and $e^+e^-\mbox{He}^{++}$ systems. Detailed calculations of all possible S-wave cross-sections are performed %in systems $e^+e^-\bar{p}$ and $e^+e^-\mbox{He}^{++}$ in the low-energy region which supports up to seven open channels including the rearrangement channels of ground and excited states of antihydrogen, positronium and helium ion formations. All known sharp resonances of the systems obtained and …
We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature as low as 9~K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise \emph{CPT} test on trapped antihydrogen is a long-standing goal.
In this chapter we analyze the multiple ionization by impact of |Z|=1 projectiles: electrons, positrons, protons and antiprotons. Differences and similarities among the cross sections by these four projectiles allows us to have an insight on the physics involved. Mass and charge effects, energy thresholds, and relative importance of collisional and post-collisional processes are discussed. For this purpose, we performed a detailed theoretical-experimental comparison for single up to quintuple ionization of Ne, Ar, Kr and Xe by particles and antiparticles. We include an extensive compilation of…
The Antiproton Decelerator of CERN began operation in 1999 to serve experiments for studies of CPT invariance by precision laser and microwave spectroscopy of antihydrogen ($\bar{\rm H}$) and antiprotonic helium ($\bar{p}{\rm He}^+$). The first 12 years of operation saw cold $\bar{\rm H}$ synthesized by overlapping clouds of positrons ($e^+$) and antiprotons ($\bar{p}$) confined in magnetic Penning traps. Cold $\bar{\rm H}$ was also produced in collisions between Rydberg positronium atoms and $\bar{p}$. Ground-state $\bar{\rm H}$ was later trapped for up to $\sim 1000$ s in a magnetic bottle t…
