The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S–2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering1, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle2–5, in line with recent determinations of the proton charge radius6–9, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.

Laser wakefield acceleration (LWFA) and its particle-driven counterpart, particle or plasma wakefield acceleration (PWFA), are commonly treated as separate, though related, branches of high-gradient plasmabased acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we present a comprehensive study of this regime, which we may term laser-plasma wakefields. Using datasets of hundreds of shots, we demonstrate the influence of beam loading on the spectral shape of electron bunches. Similar results are obtained using both 100-TW-class and few-cycle lasers, highlighting the scale invariance of the involved physical processes. Furthermore, we probe the interplay of dual electron bunches in the same or in two subsequent plasma periods under the influence of beam loading. We show that, with decreasing laser intensity, beam loading transitions to a beam-dominated regime, where the first bunch acts as the main driver of the wakefield. This transition is evidenced experimentally by a varying acceleration of a lowenergy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. Our results also present an important step in the development of LWFA using controlled injection in a shock front. The electron beams in this study reach record performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10 pC MeV −1), and angular charge density (beyond 300 pC μsr −1 at 220 MeV). We provide an experimental scaling for the accelerated charge per terawatt (TW) of laser power, which approaches 2 nC at 300 TW. With the expanding availability of petawatt-class (PW) lasers, these beam parameters will become widely accessible. Thus, the physics of laser-plasma wakefields is expected to become increasingly relevant, as it provides new paths toward low-emittance beam generation for future plasma-based colliders or light sources.

The shape of a wave carries all information about the spatial and temporal structure of its source, given that the medium and its properties are known. Most modern imaging methods seek to utilize this nature of waves originating from Huygens’ principle. We discuss the retrieval of the complete kinetic energy distribution from the acoustic trace that is recorded when a short ion bunch deposits its energy in water. This novel method, which we refer to as Ion-Bunch Energy Acoustic Tracing (I-BEAT), is a refinement of the ionoacoustic approach. With its capability of completely monitoring a single, focused proton bunch with prompt readout and high repetition rate, I-BEAT is a promising approach to meet future requirements of experiments and applications in the field of laser-based ion acceleration. We demonstrate its functionality at two laser-driven ion sources for quantitative online determination of the kinetic energy distribution in the focus of single proton bunches.

Laser-driven X-ray sources are an emerging alternative to conventional X-ray tubes and synchrotron sources. We present results on microtomographic X-ray imaging of a cancellous human bone sample using synchrotron-like betatron radiation. The source is driven by a 100-TW-class titaniumsapphire laser system and delivers over 10 8 X-ray photons per second. Compared to earlier studies, the acquisition time for an entire tomographic dataset has been reduced by more than an order of magnitude. Additionally, the reconstruction quality benefits from the use of statistical iterative reconstruction techniques. Depending on the desired resolution, tomographies are thereby acquired within minutes, which is an important milestone towards real-life applications of laser-plasma X-ray sources.

Ultrafast pump-probe experiments open the possibility to track fundamental material behaviour like changes in its electronic configuration in real time. To date, most of these experiments are performed using an electron or a high-energy photon beam, which is synchronized to an infrared laser pulse. Entirely new opportunities can be explored if not only a single, but multiple synchronized, ultra-short, high-energy beams are used. However, this requires advanced radiation sources that are capable of producing dual-energy electron beams, for example. Here, we demonstrate simultaneous generation of twin-electron beams from a single compact laser wakefield accelerator. The energy of each beam can be individually adjusted over a wide range and our analysis shows that the bunch lengths and their delay inherently amount to femtoseconds. Our proof-of-concept results demonstrate an elegant way to perform multi-beam experiments in future on a laboratory scale.Understanding the dynamics of materials on the time scale of electronic, atomic and molecular motion is one of the grand challenges of contemporary physics, chemistry and biology. 1 A particularly useful tool to study these phenomena are pump-probe experiments, where a process is triggered using a pump pulse and its temporal evolution is subsequently examined using a probe pulse. Importantly, the properties of the radiation pulses dictate which type of systems can be studied with this method. Commonly available short-pulse infrared lasers are mostly used as pump to excite or manipulate weakly-bound electronic 2 and magnetic 3 states, to ionize 4 or to heat a target 5 . The induced dynamics of the system are probed with a short electron or photon beam. For instance, electron 6 or X-ray 7 diffraction are sensitive to atomic arrangement, while X-ray absorption spectroscopy 8 is a particularly useful tool to study complex systems, because materials exhibit welldistinguishable transitions in the X-ray regime, i.e. element selectivity 9 .In the case of processes governed by atomic motion, the vibrational period (∼ 100 fs) needs to be resolved 10 . So far, the required femtosecond X-ray pulses for pump-probe experiments could only be provided at accelerator-based lightsources 11 , using either femtoslicing beamlines 12 or free-electron lasers (FELs) 13,14 . In the near future, laser-driven accelerators 15 can serve as complementary or alternative femtosecond radiation sources. Their ultrashort 16 MeV-to-GeV-scale 17,18 electron beams are already being used to provide femtosecond photon beams in the THz 19 , ultraviolet 20 , Xray 21,22 and γ-ray 23 regimes. While laser-driven Xray sources were initially limited to performing basic radiography 24,25 , recent experiments have started to take advantage of the sources' temporal resolution in pump-probe studies of warm dense matter 26 and laser-driven shock waves 27 .An entirely new class of experiments becomes available when short-wavelength pulses are used for both pumping and probing [28][29][30] . In this dual-color operation i...

Abstract.We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy. The proton charge radius puzzleThe historical route to the proton charge radius (r p ) is from elastic electron-proton scattering. In a completely complementary fashion, it has been obtained also from "high-precision" laser spectroscopy of hydrogen (H). Since a few years, "high-sensitivity" laser spectroscopy of muonic hydrogen (μp) offers a third way. The value extracted from μp with a relative accuracy of 5×10 −4 is an order of magnitude more accurate than obtained from the other methods. Yet the value is 4% smaller than derived from electron-proton scattering and H spectroscopy with a disagreement at the 7σ level [1][2][3][4][5].In the last five years as summarized in [6,7] various cross checks and refinements of boundstate QED calculations needed for the extraction of r p from μp have been performed, together with investigations of the proton structure. Several suggestions in the field of "beyond standard model" BSM physics have been articulated, re-analysis of scattering data have been carried out and new experiments have been initiated. Despite this, presently the discrepancy still persists and the resolution a

Plasma wakefield accelerators are capable of sustaining gigavolt-per-centimeter accelerating fields, surpassing the electric breakdown threshold in state-of-the-art accelerator modules by 3-4 orders of magnitude. Beam-driven wakefields offer particularly attractive conditions for the generation and acceleration of high-quality beams. However, this scheme relies on kilometer-scale accelerators. Here, we report on the demonstration of a millimeter-scale plasma accelerator powered by laser-accelerated electron beams. We showcase the acceleration of electron beams to 128 MeV, consistent with simulations exhibiting accelerating gradients exceeding 100 GV m−1. This miniaturized accelerator is further explored by employing a controlled pair of drive and witness electron bunches, where a fraction of the driver energy is transferred to the accelerated witness through the plasma. Such a hybrid approach allows fundamental studies of beam-driven plasma accelerator concepts at widely accessible high-power laser facilities. It is anticipated to provide compact sources of energetic high-brightness electron beams for quality-demanding applications such as free-electron lasers.

Laser wakefield acceleration of electrons represents a basis for several types of novel X-ray sources based on Thomson scattering or betatron radiation. The latter provides a high photon flux and a small source size, both being prerequisites for high-quality X-ray imaging. Furthermore, proofof-principle experiments have demonstrated its application for tomographic imaging. So far this required several hours of acquisition time for a complete tomographic dataset. Based on improvements to the laser system, detectors and reconstruction algorithms, we were able to reduce this time for a full tomographic scan to 3 minutes. In this paper, we discuss these results and give a prospect to future imaging systems. *