Table of contents

The International Conference on Precision Physics of Simple Atomic Systems (PSAS2018) took place in Vienna, Austria, from Monday 14th to Friday 18th of May 2018. The 10th edition of this biannual conference series was organized by the Stefan Meyer Institute (SMI) of the Austrian Academy of Sciences (ÖAW).

More details including conference photo, committees and sponsors can be found in the PDF.

The editors.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

The anomalous magnetic moment of the electron is one of the physical quantities measured with the highest precision. Such high precision demands a similar precision in the theoretical evaluations in order to obtain stringent tests of QED. In this talk I will summarize the situation of the theoretical calculations of the contributions to the electron g-2; then, I will describe in detail the results of the twenty-year long project of the evaluation of all the 891 mass-independent four-loop QED Feynman diagrams contributing to the electron g-2 [11], with the 1100-digits result ${a}_{e}^{QED}(4-{\rm{loop}})=-1.912245764926445574152647167439830054060873390658725345\ldots {(\frac{\alpha }{\pi })}^{4}$ and high-precision analytical fits. The consequences of this result on the QED tests and the determination of the fine structure constant will be also discussed.

Recent years have witnessed a remarkable improvement in the theoretical description of bound-electron g factors, paralleled with a quantum jump in the experimental accuracy in the investigation of these quantities. In the present article we give a brief summary of the latest developments, with emphasis on the influence of quantum electrodynamic and nuclear effects on the g factor of few-electron highly charged ions, and on the possible determination of fundamental constants.

Theoretical calculations of the interelectronic-interaction and QED corrections to the g factor of the ground state of boronlike ions are presented. The first-order interelectronic-interaction and the self-energy corrections are evaluated within the rigorous QED approach in the effective screening potential. The second-order interelectronic interaction is considered within the Breit approximation. The nuclear recoil effect is also taken into account. The results for the ground-state g factor of boronlike ions in the range Z = 10-20 are presented and compared to the previous calculations.

We describe a new experiment that aims at a parts per billion measurement of the nuclear magnetic moment of ³He²+ and a 100 parts per trillion measurement of the Zeeman effect of the ground-state hyperfine splitting of ³He+. To enable ultrafast and efficient experiment cycles the experiment relies on new technologies such as sympathetic laser cooling of single ³He-ions coupled to a cloud of Doppler-cooled ⁹Be-ions in a Penning trap or a novel spin-state detection scheme.

Positronium spectroscopy is of continuing interest as a high-precision test of our understanding of binding in QFT. Spectroscopic studies of low-lying states (n = 1 hyperfine splitting, n = 2 fine structure, and the 1S — 2S interval) have reached a precision of order 1MHz, and ongoing experimental efforts give the promise of improved results. Theoretical calculations of positronium energies at order mα⁶ ∼ 18.7MHz are complete, but only partial results are known at order mα⁷ ∼ 0.14MHz. We report on the status of the positronium energy calculations and give some details of the methods employed.

Currently, the experimental uncertainty for the determination of the ortho-positronium (o-Ps) decay rate is at 150 ppm precision; this is two orders of magnitude lower than the theoretical one, at 1 ppm level. Here we propose a new proof of concept experiment aiming for an accuracy of 100 ppm to be able to test the second-order correction in the calculations, which is $\simeq 45{(\frac{a}{\pi })}^{2}\approx 200{\rm{ppm}}$. The improvement relies on a new technique to confine the o-Ps in a vacuum cavity. Moreover, a new method was developed to subtract the time dependent pick-off annihilation rate of the fast backscattered positronium from the o-Ps decay rate prior to fitting the distribution. Therefore, this measurement will be free from the systematic errors present in the previous experiments. The experimental setup developed for our recent search for invisible decay of ortho-positronium is being used. The precision will be limited by the statistical uncertainty, thus, if the expectations are fulfilled, this experiment could pave the way to reach the ultimate accuracy of a few ppm level to confirm or confront directly the higher order QED corrections. This will provide a sensitive test for new physics, e.g. a discrepancy between theoretical prediction and measurements could hint at the existence of a hidden sector which is a possible dark matter candidate.

We present the current status for the direct measurement of the positronium hyperfine structure using the 2³S₁ → 2¹S₀ transition. This experiment, currently being commissioned at the slow positron beam facility at ETH Zurich, will be the first measurement of this transition and the first positronium hyperfine splitting experiment conducted in vacuum altogether. This experiment will be free of systematic effects found in earlier experiments, namely the inhomogeneity in static magnetic fields and the extrapolation from dense gases to vacuum. The achievable precision is expected to be on the order of 10 ppm while the systematic uncertainty is estimated to be within a few ppm. This would allow to check recent bound state QED calculations and a 3σ discrepancy with earlier experiments.

MuSEUM (Muonium Spectroscopy Experiment Using Microwave) collaboration aims to measure the muonium hyperfine structure (MuHFS,vhfs) with a few ppb (parts per billion). MuHFS spectroscopy is a stringent test of the bound-state QED. From this measurement, the muon-proton magnetic moment ratio (μ_μ/μ_P) and the muon-electron mass ratio (m_μ/m_e) can also be determined by applying a high magnetic field. In the previous MuHFS measurement carried out at Los Alamos Meson Physics Facility (LAMPF), the main uncertainty was caused by the lack of the statistics. MuSEUM collaboration uses the high intense pulsed muon beam at Japan Proton Accelerator Research Complex (J-PARC) to improve the statistical uncertainty. We are also developing our own experimental setups for the improvement of the systematic uncertainties. From June 2016, MuSEUM collaboration have carried out the MuHFS measurement in an extremely low magnetic field within 100 nT and now we are preparing for the measurement in a 1.7 T high magnetic field.

Muonic hydrogen is a bound state of a proton and a negative muon. Its Bohr radius is 200 times smaller than that of an electronic hydrogen atom. Therefore, a spectroscopy of the muonic hydrogen is highly sensitive to the finite size effect of proton. Recent years, the proton charge radius was determined by the laser spectroscopy of the Lamb shifts in muonic hydrogen atom. The experiment determined the proton charge radius significantly smaller than the results of past measurements. This anomaly is called "proton radius puzzle" and it has been an important unsolved problem in subatomic physics. Towards solving the puzzle, a new measurement of the ground-state hyperfine splitting in muonic hydrogen was proposed. The hyperfine splitting of muonic hydrogen derives the proton Zemach radius, which is defined as a convolution of the charge distribution with the magnetic moment distribution. This experiment aims to determine the proton Zemach radius with 1% precision by a measurement of the decay electron angular asymmetry. In order to test the feasibility of the laser spectroscopy, a preliminary experiment to measure the hyperfine quenching rate was proposed.

Precision spectroscopy of light muonic atoms provides unique information about the atomic and nuclear structure of these systems and thus represents a way to access fundamental interactions, properties and constants. One application comprises the determination of absolute nuclear charge radii with unprecedented accuracy from measurements of the 2S - 2P Lamb shift. Here, we review recent results of nuclear charge radii extracted from muonic hydrogen and helium spectroscopy and present experiment proposals to access light muonic atoms with Z ≥ 3. In addition, our approaches towards a precise measurement of the Zemach radii in muonic hydrogen (μp) and helium (μ³He⁺) are discussed. These results will provide new tests of bound-state quantum-electrodynamics in hydrogen-like systems and can be used as benchmarks for nuclear structure theories.

The interaction of antikaons with nucleons and nuclei in the low-energy regime represents an active research field in hadron physics with still many important open questions. The investigation of light kaonic atoms is, in this context, a unique tool to obtain precise information on this interaction. The energy shift and broadening of the lowest-lying states of such atoms, induced by the kaon-nucleus strong interaction, can be determined with high precision from atomic X-ray spectroscopy. This experimental method provides unique information to understand the low energy kaon-nucleus interaction at threshold. The lightest atomic systems, kaonic hydrogen and kaonic deuterium, deliver the isospin-dependent kaon-nucleon scattering lengths. The most precise kaonic hydrogen measurement to date, together with an exploratory measurement of kaonic deuterium, were carried out by the SIDDHARTA collaboration at the DAΦNE electron-positron collider of LNF-INFN, by combining the excellent quality kaon beam delivered by the collider with new experimental techniques, as fast and precise X-ray detectors: Silicon Drift Detectors. The measurement of kaonic deuterium will be realized in the near future by SIDDHARTA-2, a major upgrade of SIDDHARTA. In this paper an overview of the main results obtained by SIDDHARTA together with the future plans, are given.

The antikaon-nucleon interaction close to threshold provides crucial information on the interplay between spontaneous and explicit chiral symmetry breaking in low-energy QCD. In this context, the importance of kaonic deuterium x-ray spectroscopy has been well recognized, but no experimental results have yet been obtained due to the difficulty of the measurement. To measure the shift and width of the kaonic deuterium 1s state with an accuracy of 30 eV and 75 eV, respectively, an apparatus is under construction at the Laboratori Nazionali di Frascati. A detailed Monte Carlo simulation has shown that an increase of the signal to background ratio by a factor of ten will be required compared to the successfully performed kaonic hydrogen measurement (SIDDHARTA). Three pillars are essential for the newly developed experimental apparatus: a large area x-ray detector system (consisting of Silicon Drift Detectors), a lightweight cryogenic target system and a veto system, consisting of an outer veto detector (Veto-1) for active shielding and an inner veto detector (Veto-2) for charged particle suppression. For both veto systems, an excellent time resolution is required to distinguish kaons stopping in gas from direct kaon stops in the entrance window or side wall of the target. First test measurements on the Veto-2 system were performed. An average time resolution of (54 ± 2) ps and detection efficiencies of ~ 99 % were achieved.

The kaonic deuterium measurement at J-PARC and DAΦNE will provide a piece of information still missing to the antikaon-nucleon interaction close to threshold, providing valuable information to answer one of the most fundamental problems in hadron physics today - to the yet unsolved puzzle of how the hadron mass is generated. For this a new X-ray detector system has been developed to measure the shift and width of the 2p → 1s transition of kaonic deuterium with a precision of 60 eV and 140 eV, respectively.

The Casimir And Non-Newtonian force Experiment(Cannex) was designed to detect Casimir and hypothetical fifth forces between truly parallel plates of cm size, set 10–30 μm apart. With sub-pN sensitivity and large interacting areas, the experiment aims to settle a long-standing question of Casimir physics regarding the role of dissipation at zero frequency in the description of dielectric functions. Active measurement and control of parallelism allows to accurately probe non-standard geometries, such as crossed cylinders or a cylinder opposing a plate. If the designed precision could also be reached in gas at pressures up to 500 mbar,Cannex has been predicted to rule out completely the so-called chameleon model as explanation for dark energy. After a 6-year construction phase, the experiment has reached a first operational prototype state. In the present article, we give an overview of the setup and applied methods, present proofs of principle for key-technologies, and discuss technical hurdles yet to be overcome. Finally, we present first force gradient measurements between parallel plates in the range 6–40 μm.

We study, using the formalism proposed by Dalibard, Dupont-Roc, and Cohen-Tannoudji, the resonance interatomic energy (RIE) of two identical two-level static atoms in a symmetric/antisymmetric entangled state, which are coupled to massless scalar fields, in a number of different spacetimes. We first show that the presence of a boundary in a flat Minkowski spacetime can dramatically modify the RIE of the two static atoms, resulting in an enhanced or weakened and even nullified RIE, as compared with that in the unbounded case; we then show that the RIE of the two atoms in the spacetime of a Schwarzschild black hole can be sharply affected by the spacetime curvature on one hand, but on the other hand it is surprisingly undisturbed by the Hawking radiation of the black hole; we finally show that the RIE of the two static atoms in the spacetime with an infinite and straight cosmic string (the so called cosmic string spacetime) is sensitive to the nontrivial topological structure of the spacetime, making the RIE of the two static atoms behave in a manner very similar to that near a perfectly reflecting boundary in a flat Minkowski spacetime.

The Spherical Neutral Detector (SND) is an experiment for e⁺e⁻ annihilation study at low energies 0.2−2 GeV. The light quark anti−quark bound states are the main subject of study at these energies. They express themselves as resonances in the e+e⁻ hadronic cross sections. Hadronic cross sections could be recalculated to hadronic vacuum polarization (HVP). The Standard Model predictions today are limited by HVP which is not calculable with modern QCD theory. In this talk we present the review of the hadronic cross sections measurements at SND and new measurements: e⁺e⁻ → π⁺π⁻, π⁰γ, π⁺π⁻π⁰, ωπ⁰, K⁺K⁻η, η, K_sK_Lπ⁰, π⁺π⁻π⁰η, ωπ⁰η e.t.c.

The main goal of the FAMU experiment is the measurement of the hyperfine splitting (hfs) in the 1S state of muonic hydrogen ΔE_hfs (μ^-p)1S. The physical process behind this experiment is the following: μp are formed in a mixture of hydrogen and a higher-Z gas. When absorbing a photon at resonance-energy ΔE_hfs ≈ 0.182 eV, in subsequent collisions with the surrounding H₂ molecules, the μp is quickly de-excited and accelerated by ∼ 2/3 of the excitation energy. The observable is the time distribution of the K-lines X-rays emitted from the μZ formed by muon transfer (μp) + Z → (μZ)* + p, a reaction whose rate depends on the μp kinetic energy. The maximal response, to the tuned laser wavelength, of the time distribution of X-ray from K-lines of the (μZ)* cascade indicate the resonance. During the preparatory phase of the FAMU experiment, several measurements have been performed both to validate the methodology and to prepare the best configuration of target and detectors for the spectroscopic measurement. We present here the crucial study of the energy dependence of the transfer rate from muonic hydrogen to oxygen (Λ_μp → _μ0), precisely measured for the first time.