Kaonic Atoms Research Articles

Single-and multi-nucleon K− interactions with nuclei near threshold

Six recent SU(3) chiral-model EFT approaches to the K̅-nucleon interaction near threshold, constrained by K− p low-energy scattering and reaction data and by the kaonic hydrogen SIDDHARTA experiment, are used as input in kaonic atom calculations. Good agreement with the world-data on kaonic atoms is achieved with optical potentials built on the above models only when K̅N amplitudes are supplemented by a phenomenological multi-nucleon term. Comparing predictions with experimental single-nucleon absorption-at-rest fractions on nuclei, only two of the models together with their associated phenomenological term are acceptable. The information content of K−-nucleus data near threshold is discussed and the topic of deeply-bound kaonic atoms is re-visited.

The kaonic atoms research program at DAΦNE: from SIDDHARTA to SIDDHARTA-2

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.

AMADEUS Experiment: Studies on Antikaon Interactions with Nucleons and Nuclei

The understanding of the low-energy strong interaction involving strangeness is a challenging topic due to resonances and predicted kaonic nuclear bound states. The K- nucleon interaction is strongly attractive at low energies verified in kaonic hydrogen studied in the SIDDHARTA experiment at the DA[Formula: see text]NE electron-positron collider of LNF-INFN (Frascati/Italy). Hyperon resonances like the elusive [Formula: see text] in the s-wave impose questions about its nature. According to theoretical studies it can be described as a dynamically generated resonance with two poles or a quasi-bound [Formula: see text]N state, which could lead to kaonic nuclear bound states (e.g. K-pp). An insight in many open facets of the antikaon interactions can be provided by the AMADEUS experiment at DA[Formula: see text]NE based on the analysis of the data collected in 2004/2005 by the KLOE collaboration, and of the dedicated data set collected in 2012 by AMADEUS in collaboration with KLOE. As a first step data from antikaon-induced reactions in the drift chamber of KLOE were analyzed and yielded new results on antikaon absorption on nuclei. Recent results of the experimental studies and an outlook to the future possibilities within AMADEUS are presented.

Kaonic deuterium and low-energy antikaon-nucleon interaction

A new evaluation of the 1s level shift and width of kaonic deuterium is presented based on an accurate K̅ NN three-body calculation, using as input a realistic antikaon-nucleon interaction constrained by the SIDDHARTA kaonic hydrogen data. The three-body Schrödinger equation is solved with a superposition of a large number of correlated Gaussian basis functions extending over distance scales up to several hundred fm. The resulting energy shift and width of the kaonic deuterium 1s level are △E ≃ 0:67 keV and Γ ≃ 1.02 keV, with estimated uncertainties at the 10 % level.

Dynamically generated hadronic states in the K̅N and ηN coupled-channels interactions

We compare and review the theoretical predictions for the K̅N and ηN elastic amplitudes where sizable variations are found among the considered approaches, especially at subthreshold energies relevant for studies of kaonic atoms and meson-nuclear quasi-bound states. Conditions for an appearance of dynamically generated states in meson-baryon multi-channel interactions are established and discussed for the K̅N and ηN systems.

Constraining the K¯N interaction from the 1S level shift of kaonic deuterium

Motivated by the precise measurement of the 1S level shift of kaonic hydrogen, we perform accurate three-body calculations for the spectrum of kaonic deuterium using a realistic antikaon-nucleon (K¯N) interaction. In order to describe both short- and long-range behaviors of the kaonic atomic states, we solve the three-body Schrodinger equation with a superposition of a large number of correlated Gaussian basis functions covering distances up to several hundreds of fm. Transition energies between 1S,2P, and 2S states are determined with high precision. The complex energy shift of the 1S level of kaonic deuterium is found to be ΔE−iΓ/2=(670−i508) eV. The sensitivity of this level shift with respect to the isospin I=1 component of the K¯N interaction is examined. It is pointed out that an experimental determination of the kaonic deuterium level shift within an uncertainty of 25% will provide a constraint for the I=1 component of the K¯N interaction significantly stronger than that from kaonic hydrogen.

K− nuclear states: Binding energies and widths

$K^-$ optical potentials relevant to calculations of $K^-$ nuclear quasi-bound states were developed within several chiral meson-baryon coupled-channel interaction models. The applied models yield quite different $K^-$ binding energies and widths. Then, the $K^-$ multinucleon interactions were incorporated by a phenomenological optical potential fitted recently to kaonic atom data. Though the applied $K^-$ interaction models differ significantly in the $K^-N$ subthreshold region, our self-consistent calculations of kaonic nuclei across the periodic table lead to conclusions valid quite generally. Due to $K^-$ multinucleon absorption in the nuclear medium the calculated widths of $K^-$ nuclear states are sizable, $\Gamma_{K^-} \geq 90$ MeV, and exceed substantially their binding energies in all considered nuclei.

Beamline Test of a Transition-Edge-Sensor Spectrometer in Preparation for Kaonic-Atom Measurements

We are developing a new technique to apply transition-edge sensors (TESs) to X-ray spectroscopy of exotic atoms, especially of kaonic atoms. To demonstrate the feasibility of this pioneering project, performance of a TES-based X-ray detector was evaluated in pion- and kaon-beam environments at particle accelerators. We successfully observed X-rays from pionic-carbon atoms with a resolution as good as 7 eV FWHM at 6 keV. Also at a kaon beamline, we confirmed that the TES spectrometer will be able to achieve our resolution goal, 6 eV, in our first scientific campaign to measure X-rays from kaonic-helium atoms.

Are there any narrow K−-nuclear states?

We performed self-consistent calculations of K−-nuclear quasi-bound states using a single-nucleon K− optical potential derived from chiral meson–baryon coupled-channel interaction models, supplemented by a phenomenological K− multinucleon potential introduced recently to achieve good fits to kaonic atom data [1]. Our calculations show that the effect of K− multinucleon interactions on K− widths in nuclei is decisive. The resulting widths are considerably larger than corresponding binding energies. Moreover, when the density dependence of the K−-multinucleon interactions derived in the fits of kaonic atoms is extended to the nuclear interior, the only two models acceptable after imposing as additional constraint the single-nucleon fraction of K− absorption at rest do not yield any kaonic nuclear bound state in majority of considered nuclei.

Experimental studies of the kaon-nucleus interaction at low energy with x-ray spectroscopy of kaonic atoms

In the exotic atoms in which one electron is replaced by a negatively charged kaon, the kaon-nucleus hadronic interaction introduces an energy shift and broadening of the low-lying states of the kaonic atoms. The shift and width 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.

K−N amplitudes below threshold constrained by multinucleon absorption

Six widely different subthreshold K−N scattering amplitudes obtained in SU(3) chiral-model EFT approaches by fitting to low-energy and threshold data are employed in optical-potential studies of kaonic atoms. Phenomenological terms representing K− multinucleon interactions are added to the EFT-inspired single-nucleon part of the K−-nucleus optical potential in order to obtain good fits to kaonic-atom strong-interaction level shifts and widths across the periodic table. Introducing as a further constraint the fractions of single-nucleon K− absorption at rest from old bubble-chamber experiments, it is found that only two of the models considered here reproduce these absorption fractions. Within these two models, the interplay between single-nucleon and multinucleon K− interactions explains features observed previously with fully phenomenological optical potentials. Radial sensitivities of kaonic atom observables are re-examined, and remarks are made on the role of ‘subthreshold kinematics’ in absorption-at-rest calculations.

Kaonic atoms and strangeness in nuclei: SIDDHARTA-2 and AMADEUS experiments

The dynamics of the strong interaction processes in the non-perturbative regime is currently approached by lattice calculations and effective field theories (ChPT), still lacking several experimental results, fundamental for a good understanding of the strangeness sector. Among these, the information provided by the low-energy kaon nucleon/nuclei interaction, accessible through the study of kaonic atoms and kaonic nuclear processes, plays a key-role. The lightest atomic systems, the kaonic hydrogen and the kaonic deuterium, deliver, in a model-independent way, the isospin-dependent kaon-nucleon scattering lengths, through the X- ray spectroscopy of the exotic atoms de-exciting to the fundamental level. The most precise kaonic hydrogen measurement to-date, together with an exploratory measurement of kaonic deuterium, were carried out in 2009 at the DAΦNE collider, by the SIDDHARTA collaboration. Nowadays, an upgraded setup was built, for a precise measurement of kaonic deuterium and, eventually, of heavier exotic atoms. A correlated study of the kaon-nuclei interaction at momenta below 130 MeV/c is carried out by the AMADEUS collaboration, using the KLOE detector and dedicated targets inserted near the collider interaction point. Preliminary results of the study of charged antikaons interacting with nuclei are shown, including a discussion of the still controversial Λ(1405).

First application of superconducting transition-edge sensor microcalorimeters to hadronic atom X-ray spectroscopy

High-resolution pionic atom X-ray spectroscopy was performed with an X-ray spectrometer based on a 240 pixel array of superconducting transition-edge sensor (TES) microcalorimeters at the πM1 beam line of the Paul Scherrer Institute. X-rays emitted by pionic carbon via the 4f→3d transition and the parallel 4d→3p transition were observed with a full width at half maximum energy resolution of 6.8 eV at 6.4 keV. The measured X-ray energies are consistent with calculated electromagnetic values which considered the strong interaction effect assessed via the Seki–Masutani potential for the 3p energy level, and favor the electronic population of two filled 1s electrons in the K-shell. Absolute energy calibration with an uncertainty of 0.1 eV was demonstrated under a high-rate hadron beam condition of 1.45 MHz. This is the first application of a TES spectrometer to hadronic atom X-ray spectroscopy and is an important milestone towards next-generation high-resolution kaonic atom X-ray spectroscopy.

Hadron–hadron correlation and interaction from heavy–ion collisions

We investigate the ΛΛ and K−p intensity correlations in high-energy heavy–ion collisions. First, we examine the dependence of the ΛΛ correlation on the ΛΛ interaction and the ΛΛ pair purity probability λ. For small λ, the correlation function needs to be suppressed by the ΛΛ interaction in order to explain the recently measured ΛΛ correlation data. By comparison, when we adopt the λ value evaluated from the experimentally measured Σ0/Λ ratio, the correlation function needs to be enhanced by the interaction. We demonstrate that these two cases correspond to the two analyses which gave opposite signs of the ΛΛ scattering length. Next, we discuss the K−p correlation function. By using the local K¯N potential which reproduces the kaonic hydrogen data by SIDDHARTA, we obtain the K−p correlation function. We find that the K−p correlation can provide a complementary information to the K−p elastic scattering amplitude.

A high-resolution x-ray spectrometer for a kaon mass measurement

The ASPECT consortium (Adaptable Spectrometer Enabled by Cryogenic Technology) is currently constructing a generalised cryogenic platform for cryogenic detector work which will be able to accommodate a wide range of sensors. The cryogenics system is based on a small mechanical cooler with a further adiabatic demagnetisation stage and will work with cryogenic detectors at sub-Kelvin temperatures. The commercial aim of the consortium is to produce a compact, user-friendly device with an emphasis on reliability and portability which can easily be transported for specialised on-site work, such as beam-lines or telescope facilities. The cryogenic detector platform will accommodate a specially developed cryogenic sensor, either a metallic magnetic calorimeter or a magnetic penetration-depth thermometer. The detectors will be designed to work in various temperatures regions with an emphasis on optimising the various detector resolutions for specific temperatures. One resolution target is of about 10eV at the energies range typically created in kaonic atoms experiments (soft x-ray energies). A following step will see the introduction of continuous, high-power, sub-Kelvin cooling which will bring the cryogenic basis for a high resolution spectrometer system to the market. The scientific goal of the project will produce an experimental set-up optimised for kaon-mass measurements performing high-resolution x-ray spectroscopy on a beam-line provided foreseeably by the J-PARC (Tokai, Japan) or DAΦNE (Frascati, Italy) facilities.

K-series X-ray yield measurement of kaonic hydrogen atoms in a gaseous target

We measured the K-series X-rays of the K−p exotic atom in the SIDDHARTA experiment with a gaseous hydrogen target of 1.3 g/l, which is about 15 times the ρSTP of hydrogen gas. At this density, the absolute yields of kaonic X-rays, when a negatively charged kaon stopped inside the target, were determined to be 0.012−0.003+0.004 for Kα and 0.043−0.011+0.012 for all the K-series transitions Ktot. These results, together with the KEK E228 experiment results, confirm for the first time a target density dependence of the yield predicted by the cascade models, and provide valuable information to refine the parameters used in the cascade models for the kaonic atoms.

Structure ofΛ(1405)and construction ofK¯Nlocal potential based on chiral SU(3) dynamics

We develop the single-channel local potential for the KbarN system, which is applicable to quantitative studies of Kbar bound states in nuclei. Because the high precision measurement of the kaonic hydrogen by SIDDHARTA reduces the uncertainty of the KbarN amplitude below the KbarN threshold, the local potential should be calibrated in a wide energy region. We establish a new method to construct the local potential focusing on the behavior of the scattering amplitude in the complex energy plane. Applying this method, we construct the KbarN potential based on the chiral coupled-channel approach with the SIDDHARTA constraint. The wave function from the new potential indicates the KbarN molecular structure of Lambda(1405).

Strong interaction studies with kaonic atoms

The strong interaction of antikaons 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. 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 DAFNE 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 kaonic hydrogen atom leading to a hadronic shift and a hadronic broadening 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.

Precision X-ray spectroscopy of kaonic atoms as a probe of low-energy kaon-nucleus interaction

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.

K+-nucleon amplitudes in the medium below 800 MeV/c

Simple in-medium meson-nucleon kinematics has been applied recently in calculations of strong interaction effects in kaonic atoms, pionic atoms and elastic scattering of low energy pions by nuclei. More sensitive tests of this approach are possible with K + -nucleus interactions below 800 MeV/c because of the superior penetration of kaons into nuclei. The energy dependence of calculated reaction and total cross sections for K + on 6 Li, C, Si and Ca agrees better with experiment when this approach is used. Calculated absolute scales are in full agreement with experiment for the very low density nucleus 6 Li whereas calculations are 23±4 % too low for C, Si and Ca, thus quantifying phenomenologically the enhancement in the nuclear medium observed before.