Measurement of the antiproton-nucleus annihilation cross-section at very low energies

The ASACUSA collaboration at the Antiproton Decelerator at CERN is planning to measure the hyperfine splitting of the ground state of antihydrogen using an atomic beam line. This will be a measurement of the antiproton magnetic moment, and also a test of the CPT invariance. The planned experimental method and setup, including the radiofrequency resonance cavity, are described, and results of Monte Carlo simulations are shown. These simulations predict that the antihydrogen ground-state hyperfine splitting can be determined with a relative precision of ～10^???7.     

Antihydrogen atom formation in a CUSP trap towards spin polarized beams

The ASACUSA collaboration has been making a path to realize high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atom in flight for stringent test of the CPT symmetry. For this purpose, an efficient extraction of a spin polarized antihydrogen beam is essential. In 2010, we have succeeded in synthesizing our first cold antihydrogen atoms employing a CUSP trap. The CUSP trap confines antiprotons and positrons simultaneously with its axially symmetric magnetic field to form antihydrogen atoms. It is expected that antihydrogen atoms in the low-field-seeking states are preferentially focused along the cusp magnetic field axis whereas those in the high-field-seeking states are defocused, resulting in the formation of a spin-polarized antihydrogen beam.     

The ASACUSA CUSP: an antihydrogen experiment

In order to test CPT symmetry between antihydrogen and its counterpart hydrogen, the ASACUSA collaboration plans to perform high precision microwave spectroscopy of ground-state hyperfine splitting of antihydrogen atom in-flight. We have developed an apparatus (“cusp trap”) which consists of a superconducting anti-Helmholtz coil and multiple ring electrodes. For the preparation of slow antiprotons and positrons, Penning-Malmberg type traps were utilized. The spectrometer line was positioned downstream of the cusp trap. At the end of the beamline, an antihydrogen beam detector was located, which comprises an inorganic Bismuth Germanium Oxide (BGO) single-crystal scintillator housed in a vacuum duct and surrounding plastic scintillators. A significant fraction of antihydrogen atoms flowing out the cusp trap were detected.     

Production of relativistic antihydrogen atoms by pair production with positron capture and measurement of the Lamb shift

A beam of relativistic antihydrogen atoms — the bound state ( e⁺) — can be created by circulating the beam of an antiproton storage ring through an internal gas target. An antiproton which passes through the Coulomb field of a nucleus will create e⁺e⁻ pairs, and antihydrogen will form when a positron is created in a bound instead of continuum state about the antiproton. The cross section for this process is roughly 3Z ² pb for antiproton momenta about 6 GeV/c. A sample of 600 antihydrogen atoms in a low-emittance, neutral beam will be made in 1995 as an accidental byproduct of Fermilab experiment E760. We describe a simple experiment, Fermilab Proposal P862, which can detect this beam, and outline how a sample of a few-10⁴ atoms can be used to measure the antihydrogen Lamb shift to 1 %. Work supported in part by Department of Energy contract DE-AC03-76SF00515 (SLAC). Work supported by Fondo Nacional de Investigación Científica y Tecnológica, Chile.     

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 Moiré 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.     

Control of Non-Neutral Plasmas and Generation of Ultra-Slow Antiproton Beam

The Antiproton Decelerator (AD) devoted primarily to atomic physics experiments has been stably operated since 2000. Until now, three proposals have been approved, two of which are on the production and spectroscopy of antihydrogen, and the third one is on atomic collisions and precision spectroscopy of antiprotonic atoms, ASACUSA collaboration. One of the unique features of the ASACUSA collaboration is to develop intense slow and ultra slow antiproton beams of high quality, which will open a new multidisciplinary field involving atomic physics, nuclear physics and elementary particle physics. The ultra slow antiprotons will be prepared by combining the AD (down to 5.3 MeV), the RFQD (Radio Frequency Quadrupole Decelerator) (down to several tens keV), and an electron cooling device which will be called “MUSASHI” (Monoenergetic Ultra Slow Antiproton Source for High-precision Investigations) (down to several eV). MUSASHI produces the eV antiproton beam through an electron cooling of trapped antiprotons and a radial compression followed by an extraction through a transport beam line. The transport beam line is specially designed so that the pressure at the trap region can be maintained more than six orders of magnitude better than the collision region and at the same time the transport efficiency is kept at almost 100%. The ultra slow antiproton beam allows for the first time to study collision dynamics such as antiprotonic atom formation and ionization processes under single collision conditions, and also to study spectroscopic nature of various metastable antiprotonic atoms such as p, He⁺, He⁺⁺, etc. Metastable p are particularly interesting because they allow to make high precision spectroscopy of two body exotic atoms. Production and spectroscopy of antiprotonic atoms consisting of unstable exotic nuclei will also be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spectroscopy apparatus for the measurement of the hyperfine structure of antihydrogen

The ASACUSA CUSP collaboration at the Antiproton Decelerator (AD) of CERN is planning to measure the ground-state hyperfine splitting of antihydrogen (H?) using an atomic spectroscopy beamline. We describe here the latest developments on the spectroscopy apparatus developed to be coupled to the H? production setup (CUSP).     

Measurement of the gravitational acceleration of antihydrogen

The gravitational force acting on antiparticles has never been directly measured to date. A method for measuring the gravitational effects on antihydrogen by equilibrating the gravitational force with a magnetic gradient is discussed. The systematic and statistical errors inherent to the measurement will be presented. It will be shown that a measurement of gravity at 1% can be realised using ∼ 5 × 10⁵ antihydrogen atoms. The production of antihydrogen atoms in conditions suitable for the measurement is also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observation of antihydrogen production in flight at CERN

The observation of the production of antihydrogen atoms \overlineH ⁰\equiv\barpe⁺, the simplest atomic bound state of antimatter, is presented. A method has been used by the PS210 collaboration at LEAR which assumes that the production of \overlineH⁰ is predominantly mediated by the e⁺e^--pair creation via the two-photon mechanism in the antiproton--nucleus interaction. Neutral \overlineH⁰ atoms are indentified by a unique sequence of characteristics. In principle \overlineH⁰ is well suited for investigations of fundamental CPT violation studies under different forces, however, in our investigations we concentrate on the production of this antimatter object, since so far it had not been observed. The production of eleven antihydrogen atoms is reported including possibly 2± 1 background signals, the observed yield agrees with theoretical predictions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measuring the gravitational acceleration of antimatter with an antihydrogen interferometer

The gravitational force on antimatter has never been directly measured. A method is suggested for making this measurement by directing a low-energy beam of neutral antihydrogen atoms through a transmission-grating interferometer and measuring the gravitationally-induced phase shift in the interference pattern. A 1% measurement of the acceleration due to the Earth's gravitational field (¯ g) should be possible from a beam of about 10⁵ or 10⁶ atoms. If more antihydrogen can be made, a much more precise measurement of¯ g would be possible. A method is suggested for producing an antihydrogen beam appropriate for this experiment.     

Ground-state hyperfine splitting of antihydrogen

ASACUSA collaboration at CERN’s antiproton decelerator (CERN AD) plans to measure the ground-state hyperfine splitting (GS-HFS) of antihydrogen () to test the CPT symmetry to high precision. Our scheme is to produce an (anti-) atomic beam with a novel two-frequency superconducting Paul trap, and to use sextupole magnets and a 1.4-GHz cavity to analyze the HFS resonance frequency.      

Antihydrogen production and precision experiments. The ATHENA collaboration

The study of CPT invariance with the highest achievable precision in all particle sectors is of fundamental importance for physics. Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved at the Low Energy Antiproton Ring (LEAR) at CERN in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and in storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low energy positron or positronium beams have been generated. Finally, steady progress has been made in trapping and cooling neutral atoms. Thus the ingredients to form antihydrogen at rest are at hand. We propose to investigate the different methods to form antihydrogen at low energy, and to utilize the best of these methods to capture a number of antihydrogen atoms sufficient for spectroscopic studies in a magnetostatic trap. Once antihydrogen atoms have been captured at low energy, spectroscopic methods can be applied to interrogate their atomic structure with extremely high precision and compare it to its normal matter counterpart, the hydrogen atom. Especially the 1S-2S transition, with a lifetime of the excited state of 122 ms and thereby a natural linewidth of 5 parts in 10¹⁶, offers in principle the possibility to directly compare matter and antimatter properties at a level of 1 part in 10¹⁸. Additionally, comparison of the gravitational masses of hydrogen and antihydrogen, using either ballistic or spectroscopic methods, can provide direct experimental tests of the Weak Equivalence Principle for antimatter at a high precision. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fast antihydrogen beam spectroscopy

The motivation for production and precision spectroscopy of antihydrogen atoms is outlined. An experimental configuration is considered, concerning laser-microwave spectroscopy of a fast hydroten beam with characteristics similar to those of an antihydrogen beam emanating from an antiproton-positron overlap region in an antiproton storage ring. In particular, a possible experiment for the measurement of the ground state hyperfine structure splitting is described.     

High-power Ti:sapphire lasers for spectroscopy of antiprotonic atoms and radioactive ions

The ASACUSA collaboration has developed injection-seeded Ti:sapphire lasers of linewidth Γ_pl ～ 6 MHz, pulse energy 50–100 mJ, and output wavelength λ?=?726–941?nm. They are being used in two-photon spectroscopy experiments of antiprotonic helium atoms at the Antiproton Decelerator (AD) of CERN. Ti:sapphire lasers of larger linewidth Γ_pl ～ 100 MHz but more robust design will also be used in collinear resonance ionization spectroscopy (CRIS) experiments of neutron-deficient francium ions at the ISOLDE facility.     

Anomalous dependence of the magnetic resonance signal of cesium atoms in the afterglow in an N<Subscript>2</Subscript>-Ar mixture

The investigation of spin-exchange collisions between optically oriented cesium atoms in the ground ² S _1/2 state and nitrogen atoms in the ground ⁴ S _3/2 state reveals an anomalous behavior of the magnetic resonance signal of cesium atoms in the afterglow in an N₂-Ar mixture, namely, the magnetic resonance signal is slowly enhanced during the time interval between the high-frequency pulses exciting a discharge in the absorption cell. It is found that such a behavior of the magnetic resonance signal is explained by a slow change in the concentration of nitrogen atoms in the absorption cell, which affects the magnetic resonance of cesium atoms via efficient spin exchange.     

Two photon laser spectroscopy of antiprotonic helium atoms at CERN’s AD

The ASACUSA collaboration of CERN has carried out two-photon laser spectroscopy of antiprotonic helium atoms using counter-propagating ultraviolet laser beams. This excited some non-linear transitions of the antiproton at the wavelengths λ = 139.8–197.0 nm, in a way that reduced the thermal Doppler broadening of the observed resonances. The resulting narrow spectral lines allowed the measurement of three transition frequencies with fractional precisions of 2.3–5 parts in 10⁹. By comparing these values with three-body QED calculations, the antiproton-to-electron mass ratio was derived as 1836.1526736(23). We briefly review these results.     

Antimatter plasmas in a multipole trap for antihydrogen

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.     

The role of the nuclear spin in optical pumping withD 2-light

Optical pumping withD ₂-light provides an excellent means for studying collisional relaxation in the excited² P _3/2-state of alkali atoms. Collisional relaxation of orientations in that state very sensitively affects the spin orientation in the ground state. All these orientations may be easily created by absorption of σ⁺- or σ{?{-light. At a certain strength of the relaxation realized by a certain buffer gas pressure, the spin orientation in the ground state even vanishes, providedD ₂-light is used for excitation. The condition for this situation is derived from the set of rate equations which governs the evolution of all the orientations involved. These conditions very markedly depend on the nuclear spin valueI. The validity of this dependence has been checked by magnetic decoupling of the nuclear spin and observing the associated shift of the pressure for vanishing spin orientation.     

Designing an RFQ ring trap for antiproton confinement and trapping and laser cooling Mg+ ions

Summary In the true sense of Bernie's approach to physics, an old idea (the Radiofrequency Quadrupole Trap) was taken and upgraded to be applied to a new area of physics (formation of an antihydrogen beam). During the course of the development work, new applications were identified and immediately put to use. While the collaboration is still pursuing its original goal, formation of antihydrogen by Bernie's reaction, collisions between positronium atoms and antiprotons, many new experiments have been found possible and are actively pursued. These include atomic and nuclear physics studies with ultra-low energy antiprotons ejected from the initial catching trap of the antihydrogen project, and the formation and study of exotic atoms and molecules in ultra-thin targets using trapped antiprotons (an extension of the work by the PS205 collaboration at CERN described elsewhere in this volume [10]). A large physics community has grown around these ideas and may even succeed in obtaining its very own antiproton source, which is a true sign of the recognition of the importance of this field. The work in this area will hopefully continue for many years to come, but we will truly miss the motivation and drive of our friend and colleague, Bernie Deutch.