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KAMEO (Kaonic Atoms Measuring Nuclear Resonance Effects Ob-servables) is a proposal for an experiment aiming to perform the first consistent measurement of the E2 nuclear resonance effects in kaonic molybdenum A=94,96,98,100 isotopes. The E2 nuclear resonance mixes atomic states, due to the electrical quadrupole excitation of nuclear rotational states. It occurs in atoms having the energy of a nuclear excitation state closely matching an atomic de-excitation state energy, and affects the rates of X-ray atomic transitions matching the energy of the resonance. The measurement E2 nuclear resonance effect in KMO isotopes allows the study of the strong kaon-nucleus interaction in a rotational excited nuclear state. Moreover, the effect enables the K- to access an inner atomic level not easily reachable by the kaon normal cascade, due to the nuclear absorption. The KAMEO proposed apparatus consists of 4 enriched Mo A=94,96,98,100 isotope strips, exposed to the kaons produced by the DAφNE collider, for kaonic atoms formation, with a high-purity germanium detector, cooled with liquid nitrogen, used to measure the X-ray atomic transitions. The DAφNE collider is located at the National Laboratories of Frascati (LNF-INFN), in Italy. It is already suited for kaonic atoms measurement by the SIDDHARTA-2 collaboration.

The E2 nuclear resonance effect in kaonic atoms occurs when the energy of atomic de-excitation closely matches the energy of nuclear excitation, leading to the attenuation of some X-ray lines in the resonant isotope target. This phenomenon provides crucial information on the strong interaction between kaons and nuclei. The only nuclear E2 resonance effect observed so far was in the K− −9842Mo isotope, measured by G. L. Goldfrey, G-K. Lum, and C. E. Wiegand at Lawrence Berkeley Laboratory in 1975. However, the 25 hours of data taking were not sufficient to yield conclusive results. In four kaonic Molybdenum isotopes (9442Mo, 9642Mo, 9842and Mo, and 10042Mo), the nuclear E2 resonance effect is expected to occur at the same transition with similar energy values. To investigate this, the KAMEO (Kaonic Atoms Measuring Nuclear Resonance Effects Observables) experiment plans to conduct research on kaonic Molybdenum isotopes at the DAΦNE e+e− collider during the SIDDHARTA-2 experiment. The experimental strategy involves exposing four solid strip targets, each enriched with one Molybdenum isotope, to negatively charged kaons and using a germanium detector to measure X-ray transitions. In addition, a non-resonant 9242Mo isotope solid strip target will be used as a reference for standard non-resonant transitions.

Light kaonic atoms spectroscopy provides a unique approach to study the low-energy strong interaction in the strangeness sector. Precise measurements of X-ray emission from light kaonic atoms provide valuable information on kaon-nucleus interaction at threshold without the need for extrapolation as required in scattering experiments. The SIDDHARTA-2 experiment at the DAΦNE collider of INFN-LNF is now poised to perform the challenging measurements of the K−- d 2p → 1s transition to extract the isospin-dependent antikaonnucleon scattering lengths. To achieve this goal, the background reduction is a crucial factor. This paper provides an overview of the SIDDHARTA-2 Veto-1 system, which uses scintillators outside the vacuum chamber to detect charged particles produced by K− absorption by the nucleus. The arrival time of these particles is correlated with the position where the kaonic atom has been created inside the setup, allowing for the rejection of kaons stopped outside the target cell, which is a critical component for reducing the background and improve the accuracy of the measurement.

The SIDDHARTA-2 experiment aims to perform the first measurement of the kaonic deuterium 2p → 1s x-ray transition energy. Such measurement, together with the measurement of kaonic hydrogen 2p → 1s x-ray energy transition performed by the SIDDHARTA experiment in 2011, allows the determination of kaon proton and kaon neutron scattering lengths and represents a fundamental input for the low energies QCD in the strangeness sector theory. The SIDDHARTA-2 experiment is presently installed at the DAΦNE electronpositron collider at the National Laboratories of Frascati, in Italy. In May 2022, the kaonic 4He x-ray transitions measurement was performed by the complete SIDDHARTA-2 setup, by using a gaseous target. The result of this measurement is presented in this paper, with a specific focus on the background rejection performed by the kaon trigger system.

The strong-interaction effects both in pionic hydrogen and deuterium atoms have been re-determined with improved precision. The hadronic shift and width in pionic hydrogen together with the hadronic shift in pionic deuterium constitute a one-fold constraint for the two independent pion-nucleon scattering lengths. Furthermore, the hadronic width in pionic deuterium measures the transition strength of s-wave pions on an isoscalar nucleon-nucleon pair which is an independent quantity not related to the pion-nucleon scattering lengths. The experiment was performed at the Paul Scherrer Institute by stopping a high-intensity low-energy pion beam in gaseous targets using the cyclotron trap. The X-rays emitted by the πH and πD atoms were analysed with a high resolution Bragg spectrometer equipped with spherically bent crystals. The pion-nucleon scattering lengths and other physical quantities extracted from the atom data are in good agreement with the results obtained from pionnucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.

The ASACUSA collaboration produces a beam of antihydrogen atoms by mixing pure positron and antiproton plasmas in a strong magnetic field with a double cusp geometry. The positrons cool via cyclotron radiation inside the cryogenic trap. Low positron temperature is essential for increasing the fraction of antihydrogen atoms which reach the ground state prior to exiting the trap. Many experimental groups observe that such plasmas reach equilibrium at a temperature well above the temperature of the surrounding electrodes. This problem is typically attributed to electronic noise and plasma expansion, which heat the plasma. The present work reports anomalous heating far beyond what can be attributed to those two sources. The heating seems to be a result of the axially open trap geometry, which couples the plasma to the external (300 K) environment via microwave radiation.

The low-energy, non-perturbative regime of QCD can be studied directly by X-ray spectroscopy of light kaonic atoms. The SIDDHARTA-2 experiment, located at the DAΦNE collider, aims to measure the 2p → 1s transition in kaonic deuterium for the first time to extract the antikaon-nucleon scattering lengths. This measurement is impeded, inter alia, by the low K−d X-ray yield. Hence, several updates have been implemented on the apparatus to increase the signal-to-background ratio, which are discussed in detail in this paper: a lightweight gas target cell, novel Silicon Drift Detectors for the X-ray detection with excellent performance, and a veto system for active background suppression. The experiment has undergone a first preparatory run during DAΦNE’s commissioning phase in 2021, concluding with a successful kaonic helium measurement.

A quite simple procedure for the generation of a polarized antiproton beam could be worked out if antiprotons are produced with some polarization. In order to investigate this possibility measurements of the polarization of produced antiprotons have been started at a CERN/PS test beam. The polarization will be determined from the asymmetry of the elastic antiproton scattering at a liquid hydrogen target in the CNI region for which the analyzing power is well known. The data are under analysis and an additional measurement is done in 2018. Details on the experiment and the ongoing data analysis will be given.

The AMADEUS experiment at the DAΦNE collider of LNF-INFN deals with the investigation of the at-rest, or low-momentum, K− interactions in light nuclear targets, with the aim to constrain the low energy QCD models in the strangeness sector. The 0 step of the experiment consisted in the reanalysis of the 2004/2005 KLOE data, exploiting K− absorptions in H, 4He, 9Be and 12C, leading to the first invariant mass spectroscopic study with very low momentum (about 100 MeV) in-flight K− captures. With AMADEUS step 1 a dedicated pure Carbon target was implemented in the central region of the KLOE detector, providing a high statistic sample of pure at-rest K− nuclear interaction. The first measurement of the non-resonant transition amplitude | AK−n→Λπ− |$\left| {{A_{{K^ - }n \to \Lambda {\pi ^ - }}}} \right|$ at s=33 MeV$\sqrt s = 33\,MeV$ below the K̄N threshold is presented, in relation with the Λ(1405) properties studies. The analysis procedure adopted in the serarch for K− multi-nucleon absorption cross sections and Branching Ratios will be also described.

The goal of the P-349 experiment is to test whether 3.5 GeV/c antiprotons produced in high-energy proton-proton collisions are polarized in view of the preparation of a polarized antiproton beam. In this article, we present the details of the ongoing analysis focused on the drift chambers calibration and particle identification with DIRC.

The interaction of antikaons (K−) with nucleons and nuclei in the low-energy regime represents a very active research field in hadron physics. A unique and rather direct experimental access to the antikaon-nucleon scattering lengths is provided by precision X-ray spectroscopy of transitions in low-lying states in the lightest kaonic atoms (i.e. kaonic hydrogen and deuterium). In the SIDDHARTA experiment at the electron-positron collider DAFNE of LNFINFN we measured the most precise values of the strong interaction observables in conic hydrogen. The strong interaction on the 1s ground state of the electromagnetically bound K-p atom causes an energy shift and broadening of the 1s state. SIDDHARTA will extend the spectroscopy to kaonic deuterium to get access to the antikaon-neutron interaction and thus the isospin dependent scattering lengths. At J-PARC a kaon beam is used in a complementary experiment with a different setup for spectroscopy of kaonic deuterium atoms. The talk will give an overview of the of the upcoming experiments SIDDHARTA and the complementary experiment at J-PARC.Furthermore, the implications of the experiments for the theory of low-energy strong interaction with strangeness will be discussed.

The main steps for the analysis of the Λp correlated production in low-energy K− captures on Carbon nuclei, performed by the AMADEUS collaboration, are presented. The goal is to perform the first comprehensive study of the K− absorption on two, three and four nucleons, exploiting the lowmomentum K−s (pK ~ 127 MeV/c) produced at the DAΦNE collider, and to measure the low-energy cross sections and branching ratios of the K− multinucleon absorption processes in both the Λp and Σ0p channels. An integrated luminosity of 1.74 fb−1 collected during the KLOE 2004/2005 data campaign is analysed by reconstructing the Λp final state.

In the VIP (VIolation of Pauli exclusion principle) and its follow-up VIP- 2 experiments at the Laboratori Nazionali del Gran Sasso, we test the validity of the Pauli Exclusion Principle, by searching for x-rays from copper atomic transitions from a 2p orbit electron to the ground state which is already occupied by two electrons. Such transitions are prohibited by the Pauli Exclusion Principle. The physics run of the VIP-2 experiment started in late 2016 and will collect data for three years. From the first data taking period of two months we have obtained a new limit better than the VIP result from three years of running. In this article we present the published first physics result from the VIP-2 experiment and discuss about the future perspectives.

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, in which one electron is replaced by a negatively charged kaon, is a unique tool to provide precise information on this interaction; the energy shift and the broadening of the low-lying states of such atoms, induced by the kaon-nucleus hadronic interaction, can be determined with high precision from the atomic X-ray spectroscopy, and this experimental method provides unique information to understand the low energy kaon-nucleus interaction at the production threshold. The lightest atomic systems, like the kaonic hydrogen and the kaonic deuterium deliver, in a model-independent way, 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 in 2009 by the SIDDHARTA collaboration at the DAΦNE electron-positron collider of LNF-INFN, combining the excellent quality kaon beam delivered by the collider with new experimental techniques, as fast and very precise X-ray detectors, like the Silicon Drift Detectors. The SIDDHARTA results triggered new theoretical work, which achieved major progress in the understanding of the low-energy strong interaction with strangeness reflected by the antikaon-nucleon scattering lengths calculated with the antikaon-proton amplitudes constrained by the SIDDHARTA data. The most important open question is the experimental determination of the hadronic energy shift and width of kaonic deuterium; presently, a major upgrade of the setup, SIDDHARTA-2, is being realized to reach this goal. In this paper, the results obtained in 2009 and the proposed SIDDHARTA-2 upgrades are presented.

The AMADEUS collaboration is performing experimental investigations in the sector of the low energy strangeness hadron physics by taking advantage of the DAΦNE collider, which provides a unique source of monochromatic low-momentum kaons and exploiting the KLOE detector as an active target. The K− single and multi-nuclear absorption on H, 4He, 9Be and 12C, both at-rest and in-flight, are studied with the aim to determine the nature of the controversial Λ(1405), the non-resonant hyperon pion formation amplitude below the K̅N threshold, the yield and cross sections of K− multi-nucleon absorptions intimately connected to the antikaon multi-nucleon clusters properties and the K− scattering cross sections on light nuclear targets. These studies have a strong impact on the non-perturbative strangeness QCD with implications ranging from nuclear physics to the equation of state of neutron stars.

The validity of the Weak Equivalence Principle (WEP) as predicted by General Relativity has been tested up to astounding precision using ordinary matter. The lack hitherto of a stable source of a probe being at the same time electrically neutral, cold and stable enough to be measured has prevented highaccuracy testing of the WEP on anti-matter. The AEg̅IS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) experiment located at CERN's AD (Antiproton Decelerator) facility aims at producing such a probe in the form of a pulsed beam of cold anti-hydrogen, and at measuring by means of a moiré deflectometer the gravitational force that Earth's mass exerts on it. Low temperature and abundance of the H̅ are paramount to attain a high precision measurement. A technique employing a charge-exchange reaction between antiprotons coming from the AD and excited positronium atoms is being developed at AEg̅IS and will be presented hereafter, alongside an overview of the experimental apparatus and the current status of the experiment

The AMADEUS collaboration aims to provide unique quality results from K− hadronic interactions in light nuclear targets, in order to solve fundamental open questions in the non-perturbative strangeness QCD sector, like the controversial nature of the Λ(1405) state, the yield of hyperon formation below threshold, the yield and shape of multi-nucleon K− absorption, processes which are intimately connected to the possible existence of exotic antikaon multi-nucleon clusters and to the role of strangeness in neutron stars. AMADEUS takes advantage of the DAΦNE collider, which provides a unique source of monochromatic low-momentum kaons and exploits the KLOE detector as an active target, in order to obtain excellent acceptance and resolution data for K− nuclear capture on H, 4He, 9Be and 12C, both at-rest and in-flight.

The goal of the AMADEUS experiment is to shed light on unsolved fundamental issues in the non-perturbative strangeness QCD sector through the study of lowenergy K- hadronic interactions with light nuclear targets. The main open questions are the controversial nature of the Δ(1405) state, which is investigated in hyperon-pion correlation studies, and the possible existence of exotic antikaon multi-nucleon clusters, whose search in K- induced reactions is intimately related to the studies of the K- multi-nucleon absorption processes in hyperon-nucleon/nucleus channels. The DAΦNE collider at the INFN-LNF provides unique monochromatic low-momentum kaons from the φ meson decay almost at-rest, suitable for the AMADEUS studies. The KLOE detector is exploited as an active target, in order to obtain excellent acceptance and resolution data for K- nuclear capture on H, 4He, 9Be and 12C, both at-rest and in-flight.

The ASACUSA Collaboration at CERNs Antiproton Decelerator aims to measure the ground state hyperfine splitting of antihydrogen with high precision to test the fundamental symmetry of CPT (combination of charge conjugation, parity transformation, and time reversal). For this purpose an antihydrogen detector has been developed. Its task is to count the arriving antihydrogen atoms and therefore distinguish backgroundevents (mainly cosmics) from antiproton annihilations originating from antihydrogen atoms which are produced only in small amounts. A central BGO crystal disk with position sensitive read-out detects the annihilation and a surrounding two-layered hodoscope is used for tracking charged secondaries. The hodoscope has been recently upgraded to allow precise vertex reconstruction. A machine learning analysis based on measured antiproton annihilations and cosmic rays has been developed to identify antihydrogen events.

The AEgIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is a CERN based experiment with the central aim to measure directly the gravitational acceleration of antihydrogen. Antihydrogen atoms will be produced via charge exchange reactions which will consist of Rydberg-excited positronium atoms sent to cooled antiprotons within an electromagnetic trap. The resulting Rydberg antihydrogen atoms will then be horizontally accelerated by an electric field gradient (Stark effect), they will then pass through a moiré deflectometer. The vertical deflection caused by the Earth's gravitational field will test for the first time the Weak Equivalence Principle for antimatter. Detection will be undertaken via a position sensitive detector. Around 103 antihydrogen atoms are needed for the gravitational measurement to be completed. The present status, current achievements and results will be presented, with special attention toward the laser excitation of positronium (Ps) to the n=3 state and the production of Ps atoms in the transmission geometry.

The AMADEUS experiment aims to provide unique quality data of K− hadronic interactions with light nuclear targets, in order to solve fundamental open questions in the non-perturbative strangeness QCD sector, like the controversial nature of the Λ(1405) state, the yield of hyperon formation below threshold, the yield and shape of multi-nucleon K− absorption, processes which are intimately connected to the possible existence of exotic antikaon multi-nucleon clusters. AMADEUS takes advantage of the DAΦNE collider, which provides a unique source of monochromatic low-momentum kaons and exploits the KLOE detector as an active target, in order to obtain excellent acceptance and resolution data for K− nuclear capture on H, 4He, 9Be and 12C, both at-rest and in-flight.

The AMADEUS collaboration studied the K− absorptions at low momentum in light nuclei leading to Σ0p final state. Those events were recorded by the KLOE detector, used as an active target, installed in the the DAΦNE collider. The results show that it is possible to isolate the process where the K− is absorbed by two nucleons and the decay products are emitted without any further final state interactions among other contributions involving more than two nucleons. Further, the possible contribution of a ppK− bound state was investigated. The best fit gives space to a yield of ppK−/Kstop− = (0.044 ± 0.009 stat−0.005+0.004) × 10−2 corresponding to a binding energy and a width of 45 and 30 MeV/c2, respectively. A statistical analysis of this result shows although that its significance is only at the level of 1σ.

The AMADEUS experiment deals with the investigation of the low-energy kaon-nuclei hadronic interaction at the DAΦNE collider at LNF-INFN. This study is fundamental to solve longstanding questions concerning interactions of strange quarks in the non-perturbative QCD. AMADEUS step 0 consisted in the reanalysis of the 2004/2005 KLOE data, exploiting K− absorptions in H, 4He, 9Be and 12C, leading to the first invariant mass spectroscopy study with very low momentum (100 MeV/c) in-flight K− captures. In this paper, we present an overview of the analysis strategy, with particular emphasis on the results obtained in the analyses of the events with correlated Σ0 and p.

X-ray spectroscopy of light kaonic atoms is a unique tool to provide precise information on the fundamental K̄N interaction at the low-energy limit and the in-medium nuclear interaction of K−. The future experiments of kaonic deuterium strong-interaction shift and width (SIDDHARTA-2 and J-PARC E57) can extract the isospin dependent K−N interaction at threshold. The high-resolution X-ray spectroscopy of kaonic helium with microcalorimeters (J-PARC E62) has the possibility to solve the long-standing potential-strength problem of the attractive K−-nucleus interaction. Here, the recent experimental results and the future projects of X-ray spectroscopy of light kaonic atoms are presented.

The P-349 experiment aims to test whether for antiprotons the production process itself can be a source of polarization in view of the preparation of a polarized antiproton beam. In this article we present the details of performed measurements and report on the status of the ongoing analysis.

In the exotic atoms where one atomic 1s electron is replaced by a K−, the strong interaction between the K− and the nucleus introduces an energy shift and broadening of the low-lying kaonic atomic levels which are determined by only the electromagnetic interaction. By performing X-ray spectroscopy for Z = 1,2 kaonic atoms, the SIDDHARTA experiment determined with high precision the shift and width for the 1s state of K− p and the 2p state of kaonic helium-3 and kaonic helium-4. These results provided unique information of the kaon-nucleus interaction in the low energy limit.

The strong interaction of antikaons (K−) with nucleons and nuclei in the low-energy regime represents an active research field connected intrinsically with few-body physics. There are important open questions like the question of antikaon nuclear bound states - the prototype system being K−pp. A unique and rather direct experimental access to the antikaon-nucleon scattering lengths is provided by precision X-ray spectroscopy of transitions in low-lying states of light kaonic atoms like kaonic hydrogen isotopes. In the SIDDHARTA experiment at the electron-positron collider DAΦNE of LNF-INFN we measured the most precise values of the strong interaction observables, i.e. the strong interaction on the 1s ground state of the electromagnetically bound K−p atom leading to a hadronic shift ϵ1s and a hadronic broadening Γ1s of the 1s state. The SIDDHARTA result triggered new theoretical work which achieved major progress in the understanding of the low-energy strong interaction with strangeness. Antikaon-nucleon scattering lengths have been calculated constrained by the SIDDHARTA data on kaonic hydrogen. For the extraction of the isospin-dependent scattering lengths a measurement of the hadronic shift and width of kaonic deuterium is necessary. Therefore, new X-ray studies with the focus on kaonic deuterium are in preparation (SIDDHARTA2). Many improvements in the experimental setup will allow to measure kaonic deuterium which is challenging due to the anticipated low X-ray yield. Especially important are the data on the X-ray yields of kaonic deuterium extracted from a exploratory experiment within SIDDHARTA.

The weak equivalence principle states that the motion of a body in a gravitational field is independent of its structure or composition. This postulate of general relativity has been tested to very high precision with ordinary matter, but no relevant experimental verification with antimatter has ever been carried out. The AEGIS experiment will measure the gravitational acceleration of antihydrogen to ultimately 1% precision. For this purpose, a pulsed horizontal antihydrogen beam with a velocity of several 100 m s−1 will be produced. Its vertical deflection due to gravity will be detected by a setup consisting of material gratings coupled with a position-sensitive detector, operating as a moiré deflectometer or an atom interferometer. The AEGIS experiment is installed at CERN’s Antiproton Decelerator, currently the only facility in the world which produces copious amounts of low-energy antiprotons. The construction of the setup has been going on since 2010 and is nearing completion. A proof-of-principle experiment with antiprotons has demonstrated that the deflection of antiparticles by a few μm due to an external force can be detected. Technological and scientific development pertaining to specific challenges of the experiment, such as antihydrogen formation by positronium charge exchange or the position-sensitive detection of antihydrogen annihilations, is ongoing.

The AMADEUS experiment deals with the investigation of the low-energy kaon-nuclei hadronic interaction at the DAΦNE collider at LNF-INFN, fundamental to respond to longstanding open questions in the non-perturbative QCD in the strangeness sector. One of the most interesting aspects is to understand how hadron masses and interactions change in the nuclear environment. The antikaon-nucleon potential is investigated searching for signals from possible bound kaonic clusters, which would imply a strongly attractive antikaon-nucleon potential. AMADEUS step 0 consists in the analysis of 2004/2005 KLOE data, exploring K− absorptions in H, 4He, 9Be and 12C present in setup materials. The status of the various preliminary analyses is presented, together with future perspectives.