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A fixed-target platform for serial femtosecond crystallography in a hydrated environment
No full textFor serial femtosecond crystallography at X-ray free-electron lasers, which entails collection of single-pulse diffraction patterns from a constantly refreshed supply of microcrystalline sample, delivery of the sample into the X-ray beam path while maintaining low background remains a technical challenge for some experiments, especially where this methodology is applied to relatively low-ordered samples or those difficult to purify and crystallize in large quantities. This work demonstrates a scheme to encapsulate biological samples using polymer thin films and graphene to maintain sample hydration in vacuum conditions. The encapsulated sample is delivered into the X-ray beam on fixed targets for rapid scanning using the Roadrunner fixed-target system towards a long-term goal of low-background measurements on weakly diffracting samples. As a proof of principle, we used microcrystals of the 24 kDa rapid encystment protein (REP24) to provide a benchmark for polymer/graphene sandwich performance. The REP24 microcrystal unit cell obtained from our sandwiched in-vacuum sample was consistent with previously established unit-cell parameters and with those measured by us without encapsulation in humidified helium, indicating that the platform is robust against evaporative losses. While significant scattering from water was observed because of the sample-deposition method, the polymer/graphene sandwich itself was shown to contribute minimally to background scattering
Laser-induced dynamics of molecules with strong nuclear quadrupole coupling
No full textWe present a general variational approach for computing the laser-induced rovibrational dynamics of molecules taking into account the hyperfine effects of the nuclear quadrupole coupling. The method combines the general variational approach TROVE, which provides accurate rovibrational hyperfine energies and wave functions for arbitrary molecules, with the variational method RichMol, designed for generalized simulations of the rovibrational dynamics in the presence of external electric fields. We investigate the effect of the nuclear quadrupole coupling on the short-pulse laser alignment of a prototypical molecule CFClBrI, which contains nuclei with large quadrupole constants. The influence of the nuclear quadrupole interactions on the post-pulse molecular dynamics is negligible at early times, first several revivals, however at longer timescales the effect is entirely detrimental and strongly depends on the laser intensity. This effect can be explained by dephasing in the laser-excited rotational wavepacket due to irregular spacings between the hyperfine-split nuclear spin states across different rotational hyperfine bands
Baryogenesis from a dark first-order phase transition
No full textWe present a very minimal model for baryogenesis by a dark first-order phase transition. It employs a new dark SU(2) gauge group with two doublet Higgs bosons, two lepton doublets, and two singlets. The singlets act as a neutrino portal that transfer the generated asymmetry to the Standard Model. The model predicts ∆N = 0.09–0.13 detectable by future experiments as well as possible signals from exotic decays of the Higgs and Z bosons and stochastic gravitational waves
Characterization of carbides in Q&P steels using a combination of high-resolution methods
No full textCurrent strategies for the 3rd generation of advanced high strength steels focus on the creation of a multiphase microstructure containing substantial amounts of retained austenite to enhance the mechanical properties. In the case of quenching and partitioning (Q&P) steels, the stabilization of austenite is achieved by carbon diffusion from the supersaturated martensite into the austenite, but carbon partitioning is often jeopardized by competing reactions such as carbide formation. In the present study, in-situ high-energy X-ray diffraction (HEXRD) was used to study the transformation kinetics during Q&P processing, and especially carbide formation at higher partitioning temperatures.It was found that this carbide precipitation resulted mainly from martensite tempering and partially also from a decomposition of the austenite. The detection of minor diffraction peaks that appeared during the partitioning step was assigned to θ- and χ-carbide formation, which was supported by correlative atom probe tomography (APT) and transmission electron microscopy (TEM). Additionally, NbC was detected by APT, but the amount was obviously too low to be detected by HEXRD. The applied methods are finally compared with regard to their applicability for carbide identification in Q&P steels
Catalytic cleavage of HEAT and subsequent covalent binding of the tetralone moiety by the SARS-CoV-2 main protease
No full textHere we present the crystal structure of SARS-CoV-2 main protease (Mpro) covalently bound to 2-methyl-1-tetralone. This complex was obtained by co-crystallization of Mpro with HEAT (2-(((4-hydroxyphenethyl)amino)methyl)-3,4-dihydronaphthalen-1(2H)-one) in the framework of a large X-ray crystallographic screening project of Mpro against a drug repurposing library, consisting of 5632 approved drugs or compounds in clinical phase trials. Further investigations showed that HEAT is cleaved by Mpro in an E1cB-like reaction mechanism into 2-methylene-1-tetralone and tyramine. The catalytic Cys145 subsequently binds covalently in a Michael addition to the methylene carbon atom of 2-methylene-1-tetralone. According to this postulated model HEAT is acting in a pro-drug-like fashion. It is metabolized by Mpro, followed by covalent binding of one metabolite to the active site. The structure of the covalent adduct elucidated in this study opens up a new path for developing non-peptidic inhibitors
Emittance Measurements at Laser-Wakefield Accelerators
No full textLaser-wakefield accelerators enable the generation of electron beams with initially nm-small emittances and GeV-level beam energies within cm-scale distances and are thereforepromissing candidates for drivers of future experiments. However, the percent-level energy-spreads and shot-to-shot fluctuations in beam quality can pose a limit to the transportability of plasma-generated beams, and further impete the measurement of the beam emittance using conventional methods.In the framework of this thesis, two energy-resolved phase-space diagnostics, a single-shot and a multi-shot method, have been implemented at the LUX laser-plasma accelerator.Electron beams from ionization injection are imaged by a compact quadrupole doublet from a virtual source into a dispersive electron spectrometer, which allows to measurethe beam emittance, beam size, divergence and phase-space correlation on the singleenergy-slice level. The results from both methods agree within 3%, which verifies theapplicability of the single-shot method at our setup and prooves the reproducability ofthe generated electron beams. A complex variation of the phase-space within the broadenergy spectrum is observed.The implemented diagnostics further allow to measure the chromatic effects of the transportoptics on the beam phase-space and a first measurement of the chromatic emittanceis presented. For a 2%-energy-spread sub-interval of the spectrum the normalized beamemittance grows by 10% from (0.83 0.07) mm mrad to finally (0.93 0.07) mm mradinside the capturing optic and is conserved afterwards. With the achieved measurement precision, based on a detailed accuracy study andcalibrated with high statistic scans, a reliable phase-space diagnostic has been developedfor emittance optimization for future experiments at LUX
Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses
No full textThe recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within an XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths
Identification of heavy, energetic, hadronically decaying particles using machine-learning techniques
No full textMachine-learning (ML) techniques are explored to identify and classify hadronic decays of highly Lorentz-boosted W/Z/Higgs bosons and top quarks. Techniques without ML have also been evaluated and are included for comparison. The identification performances of a variety of algorithms are characterized in simulated events and directly compared with data. The algorithms are validated using proton-proton collision data at 13 TeV, corresponding to an integrated luminosity of 35.9 fb. Systematic uncertainties are assessed by comparing the results obtained using simulation and collision data. The new techniques studied in this paper provide significant performance improvements over non-ML techniques, reducing the background rate by up to an order of magnitude at the same signal efficiency
Generic dijet soft functions at two-loop order: uncorrelated emissions
No full textWe extend our algorithm for automating the calculation of two-loop dijet soft functions to observables that do not obey the non-Abelian exponentiation theorem, i.e. to those that require an independent calculation of the uncorrelated-emission contribution. As the singularity structure of uncorrelated double emissions differs substantially from the one for correlated emissions, we introduce a novel phase-space parametrisation that isolates the corresponding divergences. The resulting integrals are implemented in SoftSERVE 1.0, which we release alongside of this work, and which we supplement by a regulator that is consistent with the rapidity renormalisation group framework. Using our automated setup, we confirm existing results for various jet-veto observables and provide a novel prediction for the soft-drop jet-grooming algorithm
Straightening of Superconducting HERA Dipoles for the Any-Light-Particle-Search Experiment ALPS II
No full textAt DESY the ALPS II experiment is being installed in the HERA tunnel to search for axion like particles (ALPs). A laser beam will be injected into a string of superconducting HERA dipole magnets, to produce ALPs. After passing a light tight wall, the ALPs can reconvert into photons in a second string of HERA dipoles.The sensitivity of the experiment will be increased by two mode-matched optical cavities before and behind the wall. The curvature of the magnets limits the performance of the optical resonators, Therefore the aperture for the optical resonators inside the HERA dipoles was increased by straightening the curved magnet yoke. The procedure of straightening the HERA dipoles is described in this report