New perspectives in the charge radii determination for light nuclei

28 Jul 2025, 09:00 → 1 Aug 2025, 19:00 Europe/Rome

Aula Renzo Leonardi (ECT*)

Franziska Hagelstein (JGU Mainz & PSI), Loredana Gastaldo (Universität Heidelberg), Nancy Paul (Laboratoire Kastler Brossel), Randolf Pohl (Uni Mainz)

The charge radius is a fundamental property of the nucleus, whose knowledge has implications in the development of nuclear structure theory, precision QED tests and searches for physics beyond the Standard model (BSM). Spectroscopy of muonic atoms has produced charge radii with unprecedented precision. Laser spectroscopy is the leading technology to determine charge radii of H and He, while low temperature microcalorimeters as metallic magnetic calorimeters allow a ten-fold improvement for the lightest nuclei (up to Z~10). On the theory side, progress depends critically on a better understanding of nucleon and nuclear polarizabilities, which traditionally constitute the main systematic limitation.
The aim of this workshop is to discuss the innovative technologies for high-resolution X-ray spectroscopy, and to discuss necessary improvements on the theoretical side, required to match the experimental accuracies. In addition, perspectives for testing BSM will be presented. 

             

Poster.pdf

Monday 28 July

1 Registration

2 Welcome

Speaker: Ubirajara van Kolck (ECT*, Trento)

3 Light muonic atoms - fruitful collaboration between experiment and theory

Muonic atoms are the ideal tool to study nuclear properties. Close collaboration of theory and experiment is crucial for achieving the best possible precision for the charge radii.

Speaker: Randolf Pohl (Uni Mainz)

12_Pohl.pdf

11:00 Coffee break

4 Nuclear polarization, nuclear radii and Vud

Speaker: Mikhail Gorshteyn (JGU Mainz)

Gorshteyn Trento July 28.pdf

5 MMCs for high resolution x-ray spectroscopy

Speaker: Andreas Fleischmann (KIP, Heidelberg University)

MMC_at_ECT_2025_Fleischmann.pptx

12:50 Lunch break

6 MMC Array to Study X-ray Transitions in Muonic Atoms

The QUARTET collaboration aims to improve the accuracy of absolute nuclear charge radii of light atoms from Li to Ne up to one order of magnitude through high resolution x-ray spectroscopy of muonic atoms. Metallic Magnetic Calorimeters (MMCs) operated at mK have shown to be ideal detectors in a test experimentat PSI. MMCs are characterized by a high resolving power of several thousand, a stable calibration function and a high quantum efficiency in the energy range of interest. The performance obtained by the MMC array developed for the test experiment is presented along with the effect of Michel electrons interacting and analysis methods developed to identify and eliminate them. Based on those results, a new MMC array has been developed. This new array has been optimized to detect x-rays up to 120 keV and to reduce the effect of Michel electrons. The results obtained in a first experiment will be discussed as well as the impact of the performance on the spectroscopy of muonic B and C.

Speaker: Daniel Kreuzberger (Kirchhoff-Institut für Physik, Universität Heidelberg)

Trento_quartet_MMC_talk_Kreuzberger.pdf

7 Isotope-shift Measurements in Muonic 10B and 11B 2p-1s Transitions with a Metallic Magnetic Calorimeter

Speaker: César Godinho (LIBPhys-UNL)

Isotope-Shift measurements CG.pptx

16:20 Coffee break

8 PAX (antiProtonic Atom X-ray spectroscopy)

The PAX experiment is a new effort to improve the study of x ray transitions in antiprotonic atoms for testing Bound State QED (BSQED). By selecting transitions between circular Rydberg states, where the bound antiproton resides orders of magnitude closer to the nucleus than an electron, while avoiding any nuclear overlap with its wavefunction, the dominant uncertainties that limit the accuracy of measurements in HCI are neutralized. Employing novel microcalorimeter detector technologies, namely Transition Edge Sensors (TES), PAX aims at testing BSQED by measuring these transitions at levels of accuracy up to two orders of magnitude greater than previous efforts with Germanium detectors. We present preliminary results from PAX's May 2025 test beam at CERN, including TES compatibility with an accelerator environment, and first measurements of select antiprotonic atoms. We also discuss signal treatment, spectral reconstruction, charged particle background subtraction, and next steps.

Speaker: Gonçalo Baptista (Laboratoire Kastler Brossel)

ECT_workshop_Goncalo.pptx

9 Charge radius determination via bound electron g-factors (REMOTE)

The gyromagnetic g-factor of bound electrons in highly charged ions is ideal for testing quantum electrodynamics (QED) in the strongest electric fields. Additionally, the bound electron g factor is significantly influenced by the nuclear properties due to the close vicinity of the electrons to the nucleus. This allows the extraction of high precision nuclear charge radii. The ALPHATRAP experiment is a dedicated cryogenic Penning-trap setup to measure these bound electron g-factor of single HCIs. By co-trapping two hydrogenlike neon ions (20Ne9+ and 22Ne9+) we have determined their isotope g-factor shift with 13 digits precision in respect of g. This allows to test the QED recoil contribution to highest precision and to improve the isotopic mean square nuclear charge radius difference by a factor of eight compared to the literature value. Furthermore, we set limits on hypothetical new physics beyond the standard model. I will present recent studies and future prospects.

Speaker: Fabian Heiße

10 Discussion

20:00 Social Pizza-Dinner

At the pizzeria-restaurant Antico Pozzo

Tuesday 29 July

11 New perspectives in the charge radii determination for light nuclei

The QUARTET experiment aims to improve the radii of light nuclei by an order of magnitude. To do so we employ a novel quantum sensing technology for photon energies—metallic magnetic calorimeters. We have taken data with enriched targets of 6Li, 7Li, 9Be, 10B and 11B with enough statistical accuracy to significantly improve their radii. In this talk I will show preliminary results from the ongoing analysis and discuss the needs from atomic and nuclear theory.

Speaker: Ben Ohayon (Technion IIT)

ECT Ben.pdf

12 New physics bounds from the spectroscopy of muonic atoms

Experimental setups involving laser spectroscopy of muonic systems have undergone a revolution during the past two decades, providing data with unprecedented precision. The combined use of effective field theories, precision computations, and highly accurate experimental data allows for the spectroscopy of muonic atoms to be a competent and reliable testing ground for new physics in the keV-GeV range as well as for precision measurements within the Standard Model. We will present the EFT framework to carry out such study at next-to-leading order and analyze the applications in two main classes of systems: purely leptonic, such as muonium, and semileptonic systems, where the newly attained proton radius has pushed the theoretical precision of hydrogen and muonic hydrogen spectroscopy.

Speaker: Clara Peset (University of Madrid Complutense)

SpectroscopyEFT_Peset.pdf

11:00 Coffee break

13 Precision Physics with Few-Electron Ions: Testing the Standard Model and Beyond

High-precision spectroscopy of one- and few-electron ions provides stringent tests of the Standard Model in regimes of strong nuclear fields. Inner-shell electrons experience extreme binding, leading to relativistic dynamics and significant quantum electrodynamic contributions. Our theoretical framework includes rigorous treatments of these effects to enable accurate predictions of energy levels and other atomic properties. Precise measurements and calculations allow the determination of masses and electromagnetic properties of the particles which make up the ion. Moreover, few-electron ions serve as sensitive probes for physics beyond the Standard Model. By modeling hypothetical interactions, such as additional short-range forces, and quantifying their effects on observables, we derive competitive constraints on new physics parameters.

Speaker: Zoltan Harman (Max Planck Institute for Nuclear Physics)

Harman-ECT-talk.pdf

14 Charge radii determined by laser spectroscopy of He-like ions

The atomic structure of few-electron systems is well understood and allows for accurate ab initio calculations of mass-shift and field-shift factors in non-relativistic quantum electrodynamics calculations (NR-QED) to extract precise nuclear charge radii. We have started to determine absolute and differential charge radii of the light elements from Be to N using collinear laser spectroscopy. Helium-like ions of these species provide laser-accessible atomic transitions that can be calculated with the required accuracy in the NR-QED approach. As a first step, the 1s2s 3S1 → 1s2p 3P transitions in 12−14C4+ were determined using the Collinear Apparatus for Laser Spectroscopy and Applied Science (COALA) at the Technical University of Darmstadt. Moreover several charge states of B have been addressed.

Speaker: Wilfried Nörtershäuser (TU Darmstadt)

Nortershauser_Trento_public.pdf

12:50 Lunch break

15 Precision spectroscopy of helium-like uranium

Speaker: Günter Weber (Helmholtz Institute Jena)

16 TBA

Speaker: Shikha Rathi (The Helen Diller Quantum Center, Department of Physics, Technion-Israel Institute of Technology, Haifa, 3200003, Israel)

ECT_shikha.pdf

16:20 Coffee break

17 Reference Radii for odd-Z and heavy elements

Odd-Z and heavy elements do not possess enough stable isotopes to allow common approaches for the determination of absolute charge radii. Novel techniques are required, and in particular the production of the target material is a particular challenge. Key cases arising from the ERC NSHAPE programme will be brought forward.

Speaker: Thomas Elias COCOLIOS (KU Leuven)

20250728-Cocolios-web.pdf

18 Muonic x-ray spectroscopy on Si

The nuclear charge radii of silicon isotopes provide valuable input for searches for physics beyond the Standard model and for constraining the neutron equation of state. Therefore, laser spectroscopy on {28-32}Si has recently been performed and is planned for other isotopes in the near future. However, for extraction of the nuclear charge radii from laser spectroscopy, the mass and field shift parameters are needed, and currently, they are poorly known, and different models provide inconsistent trends. Hence, these parameters need to be re-evaluated, which can be achieved by using the King plot method combined with the determination of the nuclear charge radius of three silicon isotopes ({28, 29, 30}Si) by using muonic x-ray spectroscopy. In this contribution, we report on preliminary results from the muonic x-ray spectroscopy campaign performed in 2024 at the Paul Scherrer Institute. The experiment employed the GIANT HPGe detector array, enabling extraction of the x-ray transitions.

Speaker: marie deseyn (KU Leuven)

ECTstar_webversion_DESEYNMARIE.pdf

19 Discussion

Wednesday 30 July

20 Nuclear structure effects in light muonic atoms

I will present an overview of our calculations of nuclear structure effects in light muonic atoms. I will present our published work for s-shell nuclei, and discuss strategies to tackle p-shell nuclei

Speaker: Sonia Bacca

bacca_ECT_muonic_2025.pdf

21 Ab initio nuclear corrections to muonic lithium atoms

Speaker: Mehdi Drissi (TRIUMF)

Trento Muonic Atoms Talk Mehdi Drissi.pdf

11:00 Coffee break

22 Electromagnetic radii of light nuclei from variational Monte Carlo calculations

Speaker: Saori Pastore (Washington University & Los Alamos National Laboratory)

ECT2025_Pastore.pdf

23 Nuclear radii from muonic atoms spectroscopy

In my talk I will present the latest improvements in the theory for heavy muonic atoms and discuss the exsisting limitations for further improvemets.

Speaker: Natalia S. Oreshkina (MPIK (Heidelberg))

2025_08_ETC_web.pdf

12:50 Lunch break

15:00 Excursion

Thursday 31 July

24 Update on the new PRad-II experiment and plans for the deuteron radius measurement at Jefferson Laboratory

Speaker: Ashot Gasparian (NC A and T State University, NC, USA)

Gasparian_Trento_2025.pdf

25 The ground state hyperfine splitting in muonic hydrogen experiment (HyperMu) at PSI

The HyperMu experiment at PSI aims at the first measurement of the ground state hyperfine splitting in muonic hydrogen (μp) with 1 ppm precision using pulsed laser spectroscopy. This accuracy allows for a precise extraction of the proton structure contributions, including the Zemach radius and the proton polarizability. To measure the ground state hyperfine splitting in μp, we are developing a unique pulsed laser system designed to deliver 4 mJ pulses at a wavelength of 6.8 μm, randomly triggered upon muon detection. We report on the latest laser development within the experiment, the several developments of the detection system that was carried out and the optimization of the experimental parameters to obtain a successful resonance signal.

Speaker: Ahmed Ouf (Johannes gutenberg universität mainz)

11:00 Coffee break

26 Finite-Size Effects & New Physics

Atomic spectroscopy experiments at the precision frontier allow us to study low-energy nuclear structure, test bound-state QED, refine fundamental constants, and potentially find New Physics. As experimental precision is continuously improved, it is a timely task to re-examine the sensitivity of specific bound states to New Physics scenarios. Depending on their Bohr radii, hydrogen-like systems can be particularly sensitive to distinct New Physics mass ranges. In this talk, we use the example of axion-like particles to illustrate how spectroscopy experiments can be used to probe New Physics.

Speaker: Sotiris Pitelis (JGU Mainz)

27 TBA

Speaker: Vadim Lensky (JGU Mainz)

12:50 Lunch break

28 Nuclear structure effects in muonic atoms [REMOTE]

Speaker: Chen Ji (Central China Normal University)

29 Artificial Neural Networks for Nuclear Structure Corrections

We present a data‑driven analysis of dipole‑strength functions across the nuclear chart, employing an artificial neural network to model and predict nuclear dipole responses. The network is trained on experimentally measured dipole‑strength functions for 216 nuclei and tested on an additional set of 10 nuclei with available data. It not only reproduces known responses with high fidelity but also flags potential inconsistencies in certain experimental datasets, highlighting results that may warrant re‑examination. Where experimental information is sparse or absent, the model confirms existing theoretical calculations, demonstrating its broader predictive power for nuclear physics. A version of this network retrained, focused on light nuclei and rigorously physics‑informed could offer a powerful tool for delivering nuclear‑structure corrections in precision studies of muonic atoms.

Speaker: Tim Egert

Tim_Egert_AI.pdf

16:20 Coffee break

30 Discussion

20:00 Social dinner

At the Due Mori Restaurant

Friday 1 August

31 Nonlinear calcium King plot: implications for new bosons and nuclear properties [REMOTE]

At the TIQI group at ETH, we recently measured isotope shifts on the 729nm electric quadrupole transition between pairs of co-trapped calcium ions at 100mHz precision, two orders of magnitude below the previous best measurement. We combined our measurements with IS measurements made by the group of Piet Schmidt on the 570nm transition in Ca14+ and improved nuclear mass measurements made by the group of Klaus Blaum, to produce the first sub-Hz King plot. King plots in calcium had previously remained linear up to the 10Hz level -- our improved precision now reveals a large King non-linearity . Whilst the second-order mass shift is an expected SM source of nonlinearity, a decomposition analysis of the nonlinearity pattern we observe reveals evidence for at least one other contributing source. I will discuss the implications of these results, combined with input from our theory collaborators, both to our understanding of nuclear structure and to the search for new physics.

Speaker: Diana Prado Lopes Aude Craik (ETH Zürich)

32 BSM: Bounds based on muonic hydrogen, muonic deuterium and muonic helium spectroscopy [REMOTE]

Speaker: Robert Potvliege (Durham University)

Trento3.pdf

11:00 Coffee break

33 Corrections to muonic atoms from Lattice QCD [REMOTE]

I will discuss two-photon exchange (TPE) corrections to muonic atoms based on Lattice QCD, including our recent work on the subtraction function in the forward Compton amplitude.

Speaker: Xu Feng (Peking University)

34 Discussion and Closing

13:00 Lunch break