Antihydrogen Spectroscopy Research Articles

Hyperfine Spectroscopy of Antihydrogen, Hydrogen, and Deuterium

The prospects of tests of CPT symmetry using precision spectroscopy of antihydrogen are discussed with special emphasis on the ground-state hyperfine structure, a measurement of which is the aim of the ASACUSA collaboration at the AD/ELENA facility of CERN. Ongoing parallel experiments using hyperfine spectroscopy of hydrogen and deuterium aiming at studying Lorentz invariance by determining coefficients of the Standard Model Extension framework are described.

Cold and stable antimatter for fundamental physics.

The field of cold antimatter physics has rapidly developed in the last 20 years, overlapping with the period of the Antiproton Decelerator (AD) at CERN. The central subjects are CPT symmetry tests and Weak Equivalence Principle (WEP) tests. Various groundbreaking techniques have been developed and are still in progress such as to cool antiprotons and positrons down to extremely low temperature, to manipulate antihydrogen atoms, to construct extremely high-precision Penning traps, etc. The precisions of the antiproton and proton magnetic moments have improved by six orders of magnitude, and also laser spectroscopy of antihydrogen has been realized and reached a relative precision of 2 × 10−12 during the AD time. Antiprotonic helium laser spectroscopy, which started during the Low Energy Antiproton Ring (LEAR) time, has reached a relative precision of 8 × 10−10. Three collaborations joined the WEP tests inventing various unique approaches. An additional new post-decelerator, Extra Low ENergy Antiproton ring (ELENA), has been constructed and will be ready in 2021, which will provide 10–100 times more cold antiprotons to each experiment. A new era of the cold antimatter physics will emerge soon including the transport of antiprotons to other facilities.

Hyperfine spectroscopy of hydrogen and antihydrogen in ASACUSA

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 reviewed and progress of ASACUSA is presented.

Precision measurements on trapped antihydrogen in the ALPHA experiment.

Both the 1S-2S transition and the ground state hyperfine spectrum have been observed in trapped antihydrogen. The former constitutes the first observation of resonant interaction of light with an anti-atom, and the latter is the first detailed measurement of a spectral feature in antihydrogen. Owing to the narrow intrinsic linewidth of the 1S-2S transition and use of two-photon laser excitation, the transition energy can be precisely determined in both hydrogen and antihydrogen, allowing a direct comparison as a test of fundamental symmetry. The result is consistent with CPT invariance at a relative precision of around 2×10-10 This constitutes the most precise measurement of a property of antihydrogen. The hyperfine spectrum of antihydrogen is determined to a relative uncertainty of 4×10-4 The excited state and the hyperfine spectroscopy techniques currently both show sensitivity at the few 100 kHz level on the absolute scale. Here, the most recent work of the ALPHA collaboration on precision spectroscopy of antihydrogen is presented together with an outlook on improving the precision of measurements involving lasers and microwave radiation. Prospects of measuring the Lamb shift and determining the antiproton charge radius in trapped antihydrogen in the ALPHA apparatus are presented. Future perspectives of precision measurements of trapped antihydrogen in the ALPHA apparatus when the ELENA facility becomes available to experiments at CERN are discussed.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

Measurements of Properties of Antihydrogen

The ALPHA project at the CERN AD is testing fundamental symmetries between matter and antimatter using trapped antihydrogen atoms. The spectrum of the antihydrogen atom may be compared to ordinary hydrogen where it has been measured very precisely. CPT conservation, which underpins our current theoretical framework, requires equality of the masses and charges of matter and its antimatter partners, so antihydrogen spectroscopy presents a path to precision CPT tests. I will discuss the techniques used by ALPHA to trap more than 8000 antihydrogen atoms in 2016, and interrogate them for 600s. The 1S-2S transition in antihydrogen has been observed for the first time, and it agrees with its hydrogen counterpart within an uncertainty of 400 kHz or 0.2 ppb. The charge of the antihydrogen atom has been bounded below [Formula: see text]. A value of 1420.4 0.5MHz for the hyperfine splitting has been obtained from observation of the positron spin resonance spectrum.

In-beam measurement of the hydrogen hyperfine splitting and prospects for antihydrogen spectroscopy

Antihydrogen, the lightest atom consisting purely of antimatter, is an ideal laboratory to study the CPT symmetry by comparison with hydrogen. With respect to absolute precision, transitions within the ground-state hyperfine structure (GS-HFS) are most appealing by virtue of their small energy separation. ASACUSA proposed employing a beam of cold antihydrogen atoms in a Rabi-type experiment, to determine the GS-HFS in a field-free region. Here we present a measurement of the zero-field hydrogen GS-HFS using the spectroscopy apparatus of ASACUSA’s antihydrogen experiment. The measured value of νHF=1,420,405,748.4(3.4) (1.6) Hz with a relative precision of 2.7 × 10−9 constitutes the most precise determination of this quantity in a beam and verifies the developed spectroscopy methods for the antihydrogen HFS experiment to the p.p.b. level. Together with the recently presented observation of antihydrogen atoms 2.7 m downstream of the production region, the prerequisites for a measurement with antihydrogen are now available within the ASACUSA collaboration.

Cryogenic mount for mirror and piezoelectric actuator for an optical cavity.

We present the development of a mount that accommodates a mirror and a piezoelectric actuator with emphasis on physical needs for low temperature operation. The design uses a monolithic construction with flexure features that allow it to steadily hold the mirror and the piezoelectric actuator without glue and accommodate differential thermal contraction. The mount is small and lightweight, adding little heat capacity and inertia. It provides a pre-loading of the piezoelectric actuator as well as a good thermal connection to the mirror and a thermal short across the piezoelectric actuator. The performance of the assemblies has been tested by thermally cycling from room temperature down to 3 K more than a dozen times and over one hundred times to 77 K, without showing any derating. Such mounts are proposed for the cryogenic optical enhancement cavities of the ALPHA experiment at CERN for laser spectroscopy of antihydrogen and for hydrogen spectroscopy in our laboratory at UFRJ.

A sensitive detection method for high resolution spectroscopy of trapped antihydrogen, hydrogen and other trapped species

A method for detection of the weak 1s–2s laser excitation of a few trapped antihydrogen atoms is described. It involves the typical antihydrogen trapping environment that combines a magnetic trap for the atoms as well as a Penning trap for its constituent particles. By photoionization of the excited state the photoion can be kept in a weak Penning trap and at a suitable time be ejected towards a charged particle detector such as a microchannel plate or a channel electron multiplier. Since it does not rely on annihilation, the method is also suitable for trapped hydrogen and may find application with other species when a weak transition to a metastable state is intended and only a few trapped atoms or molecules are available.

Towards a precise measurement of the antihydrogen ground state hyperfine splitting in a beam: the case of in-flight radiative decays

The ASACUSA antihydrogen setup at the CERN Antiproton Decelerator (AD) consists of an antihydrogen source (cusp magnet coupled to a positron source and an antiproton catching magnet) followed by a spectrometer beamline. After production in the cusp, the antihydrogen atoms decay while they escape the trap leading to changes in their effective magnetic moment which in turn affect their trajectories in the beamline. Those sequential decays in the presence of a varying magnetic field strength from their production point in the cusp to their detection at the end of the spectrometer line can in principle greatly affect the prospects for a precision measurement of the antihydrogen hyperfine splitting given the so-far relatively low number of available anti-atoms. The impact of the antihydrogen decay in this context has for the first time been simulated. The implementation of atomic radiative decay has been done in Geant4 to extend the particle tracking capabilities originally embedded in Geant4 to excited atoms, and to allow studies of the effect of dynamic atomic properties on trajectories. This new tool thus allows the study of particle–matter interaction via the Geant4 toolkit while properly taking into account the atomic nature of the object under study. The implementation as well as impacts on the experimental sensitivity for antihydrogen spectroscopy are discussed in this paper.

Scaling behavior of the ground-state antihydrogen yield as a function of positron density and temperature from classical-trajectory Monte Carlo simulations

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 1 ms, the number of such states is adopted as a measure of useful antihydrogen atoms. It has been found that the scaling behavior of the useful antihydrogen yield is different depending on the positron density and positron temperature.

Antimatter, the SME, and gravity

A general field-theoretic framework for the analysis of CPT and Lorentz violation is provided by the Standard-Model Extension (SME). This work discusses a number of SME-based proposals for tests of CPT and Lorentz symmetry, including antihydrogen spectroscopy and antimatter gravity tests.

Microwave-plasma interactions studied via mode diagnostics in ALPHA

The goal of the ALPHA experiment is the production, trapping and spectroscopy of antihydrogen. A direct comparison of the ground state hyperfine spectra in hydrogen and antihydrogen has the potential to be a high-precision test of CPT symmetry. We present a novel method for measuring the strength of a microwave field for hyperfine spectroscopy in a Penning trap. This method incorporates a non-destructive plasma diagnostic system based on electrostatic modes within an electron plasma. We also show how this technique can be used to measure the cyclotron resonance of the electron plasma, which can potentially serve as a non-destructive measurement of plasma temperature.

Towards antihydrogen trapping and spectroscopy at ALPHA

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 method of identifying antiproton annihilations and effectively rejecting the cosmic-ray background.

Search for trapped antihydrogen in ALPHAThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at École de Physique, les Houches, France, 30 May – 4 June, 2010.

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.

Hydrogen spectroscopy with a Lamb-shift polarimeter

A Lamb-shift polarimeter, which has been built for a fast determination of the polarization of protons and deuterons of an atomic-beam source and which is frequently used in the ANKE experiment at COSY-J\"ulich, is shown to be an excellent device for atomic-spectroscopy measurements of metastable hydrogen isotopes. It is demonstrated that magnetic and electric dipole transitions in hydrogen can be measured as a function of the external magnetic field, giving access to the full Breit-Rabi diagram for the $2^2S_{1/2}$ and the $2^2P_{1/2}$ states. This will allow the study of hyperfine structure, $g$ factors and the classical Lamb shift. Although the data are not yet competitive with state-of-the-art measurements, the potential of the method is enormous, including a possible application to anti-hydrogen spectroscopy.

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

The main fields of investigation of the ASACUSA Experiment at the Antiproton Decelerator at CERN concern test of CPT violation through spectroscopy of antihydrogen and antiprotonic helium, atomic collision and nuclear physics experiments. The nuclear physics program foresees measurements of ̅p-nuclei cross sections at low energies (from few MeV down to the keV region) using a cylindrical detector with 6 layers of scintillating fibers surrounding the target. These measurements will contribute to understand the dynamic of the annihilation process and in particular to explain why the antiproton-nucleus annihilation rate at very low energy appears independent from the size of the target, as measured by the Obelix Collaboration at CERN for H, D and He. The preliminary results of the measurements performed during 2008 are presented.

Aspects of neutral atom traps for antihydrogen spectroscopy

We discuss the need for neutral atom traps instead of atomic beams for the high-precision spectroscopic comparison of anithydrogen and hydrogen. We also describe recent proposals and advances towards a new trap for hydrogen and briefly discuss traps for antihydrogen.

Ultracold antiprotons by indirect laser cooling

The performance of proposed antihydrogen spectroscopy or gravity experiments will crucially depend on the temperature of the initial antihydrogen sample. Measurements by ATRAP and ATHENA have shown that antihydrogen produced with the nested-trap technique is much hotter than the temperature of the surrounding trap. Therefore, novel schemes for antihydrogen recombination as well as for the pre-cooling of antiprotons are being considered. We are investigating a possible antiproton cooling technique based on the laser cooling of negative osmium ions. If demonstrated to be successful, it will allow the sympathetic cooling of antiprotons—or any negatively charged particles—to microkelvin temperatures. As a first milestone toward the laser cooling of negative ions, we have performed collinear laser spectroscopy on negative osmium and determined the transition frequency and the cross-section of the relevant bound–bound electric-dipole transition.

Antiproton mass measurements

The most accurate mass measurements by far are measurements of charge-to-mass ratios, with the assumption that there is only one fundamental unit of charge. The most accurate of such antimatter mass measurements, by orders of magnitude, is an extremely high precision comparison of the charge-to-mass ratio of the antiproton and proton. Mass measurements with antimatter particles require the solution of unique problems—owing to the need to obtain the antimatter from unusual sources, and because antimatter particles annihilate upon interacting with matter. For the future, the most accurate antimatter mass measurements are likely to arise from even more accurate comparisons of the frequency of laser transitions of antihydrogen and hydrogen. The techniques to slow, trap and cool antiprotons that were developed to make the q / m measurements possible, have now made it possible to produce slow antihydrogen, an important step toward the eventual laser spectroscopy of antihydrogen.

Strong equivalence, Lorentz and CPT violation, anti-hydrogen spectroscopy and gamma-ray burst polarimetry

The strong equivalence principle, local Lorentz invariance and CPT symmetry are fundamental ingredients of the quantum field theories used to describe elementary particle physics. Nevertheless, each may be violated by simple modifications to the dynamics while apparently preserving the essential fundamental structure of quantum field theory itself. In this paper, we analyse the construction of strong equivalence, Lorentz and CPT violating Lagrangians for QED and review and propose some experimental tests in the fields of astrophysical polarimetry and precision atomic spectroscopy. In particular, modifications of the Maxwell action predict a birefringent rotation of the direction of linearly polarised radiation from synchrotron emission which may be studied using radio galaxies or, potentially, gamma-ray bursts. In the Dirac sector, changes in atomic energy levels are predicted which may be probed in precision spectroscopy of hydrogen and anti-hydrogen atoms, notably in the Doppler-free, two-photon 1s–2s and 2s– nd ( n ∼ 10 ) transitions.