The $N{\pi }^0{\pi }^0$ decays of positive-parity $N^{*}$ and ${\mathrm{\Delta }}^{*}$ resonances at about 2 GeV are studied at ELSA by photoproduction of two neutral pions off protons. The data reveal clear evidence for several intermediate resonances: $\mathrm{\Delta }(1232)$, $N(1520)3/2^{-}$, and $N(1680)5/2^{+}$, with spin parities $J^P=3/2^{+}$, $3/2^{-}$, and $5/2^{+}$. The partial wave analysis (within the Bonn-Gatchina approach) identifies $N(1440)1/2^{+}$ and the $N(\pi \pi {)}_{S\text{wave}}$ (abbreviated as $N\sigma$ here) as further isobars and assigns the final states to the formation of nucleon and $\mathrm{\Delta }$ resonances and to nonresonant contributions. We observe the known $\mathrm{\Delta }(1232)\pi$ decays of $\mathrm{\Delta }(1910)1/2^{+}$, $\mathrm{\Delta }(1920)3/2^{+}$, $\mathrm{\Delta }(1905)5/2^{+}$, $\mathrm{\Delta }(1950)7/2^{+}$, and of the corresponding spin-parity series in the nucleon sector, $N(1880)1/2^{+}$, $N(1900)3/2^{+}$, $N(2000)5/2^{+}$, and $N(1990)7/2^{+}$. For the nucleon resonances, these decay modes are reported here for the first time. Further new decay modes proceed via $N(1440)1/2^{+}\pi$, $N(1520)3/2^{-}\pi$, $N(1680)5/2^{+}\pi$, and $N\sigma$. The latter decay modes are observed in the decay of $N^{*}$ resonances and at most weakly in ${\mathrm{\Delta }}^{*}$ decays. It is argued that these decay modes provide evidence for a 3-quark nature of $N^{*}$ resonances rather than a quark-diquark structure.

In this article, the concept of a recently funded R&D project for the installation of a proof-of-concept cavity-based x-ray free electron laser (CBXFEL) demonstrator experiment at the European XFEL facility is presented, with the first results expected in 2024. It is composed of an x-ray cavity design in backscattering geometry with a 133 m round trip length using cryogenically cooled diamond crystals. It employs the concept of retroreflection to reduce the sensitivity to vibrations. The FEL radiation is produced in four undulator segments of 20 m total length. Simulations at 16 GeV beam energy and 250 pC bunch charge show that the expected x-ray pulses in saturation surpass state-of-the-art x-ray sources considering spectral flux and three-dimensional coherence. However, the stability of the proof of concept setup is severely challenged by the finite thermal transport in the diamond crystals. Therefore, suitable measures such as cooling the crystals to 70 K are explained in this paper and additional ones will have to be developed in the course of this project.

In recent years, superconducting radio-frequency (SRF) cavities have been considered as candidates for qubits in quantum computing, showing longer photon lifetimes and, therefore, longer decoherence times of a cavity stored qubit compared to many other realizations. In modern particle accelerators, SRF cavities are the workhorse. Continuous research and development efforts are being undertaken to improve their properties, i.e., to increase the accelerating field and lower the surface resistance, which in turn increase the energy reach and duty cycle of accelerators. While some experimental milestones have been achieved, the mechanisms behind the still observed losses remain not fully understood. In this contribution we are going to show that a recently reported temperature treatment of Nb SRF cavities in the temperature range of 573–673 K, which reduces the residual surface resistance to unprecedented values, is linked to a reorganization of the niobium oxide and near-surface vacancy structure and that this reorganization can explain the observed improved performance in both applications, quantum computing and SRF cavities.

ALD of superconducting films for various high performance applications.

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $\gamma p\to p{\pi }^0$ for photon energies between 0.85 and 2.50 GeV. The ${\pi }^0$ mesons are observed in their dominant neutral decay mode: ${\pi }^0\to \gamma \gamma$. For the first time, the differential cross sections cover the very forward region, ${\theta }_{\mathrm{c}.\mathrm{m}.}<{60}^{\circ }$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).

For more than a decade free-electron lasers (FELs) have been in operation, providing scientists from many disciplines with the benefits of ultrashort, nearly transversely coherent radiation pulses with wavelengths down to the Ångstrom range. If no further techniques are applied, the FEL will only amplify radiation from the stochastic distributed electron density in the electron bunch. Contemporary developments aim at producing stable and single-mode radiation by preparing an electron bunch with favorable longitudinal electron density distributions using magnets and conventional laser pulses (seed), hence the name “seeding.” In recent years, short wavelength FELs at high electron beam energies and high repetition rates were proposed and built. At those repetition rates, an external seed with sufficient power to manipulate the electron beam cannot be provided by present state-of-the-art laser systems, thus no external seeding scheme could be applied yet. In this paper, we present ways to seed FELs to generate short wavelength radiation at high repetition rates, making use of tested electron beam manipulation schemes. For our parameter study, we used the parameters of FLASH, the free-electron laser in Hamburg. First simulations are presented, showing the feasibility of the method proposed.