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A novel phenomenological approach to total cross-section measurements at the LHC
No full textWe propose a novel, data-driven method for determining total charm cross sections in proton-proton collisions by extrapolating measured fiducial cross sections without assuming any particular fragmentation model. In this way, the method accounts for the recently observed charm fragmentation non-universality at the LHC through a data-driven extrapolation function called ddFONLL. Applied to production at 5 and 13 TeV, this approach yields total charm cross sections that fully incorporate fragmentation non-universality. The results, which differ significantly from previous universality-based estimates, are consistent with NNLO QCD predictions and enable direct comparisons free from fragmentation assumptions. We use this to evaluate the sensitivity of total cross-section measurements to parton distribution functions and the charm-quark mass. An outlook is given on the potential of further expanding the use of the ddFONLL method
Cation Vacancies in Ti‐Deficient TiO Nanosheets Enable Highly Stable Trapping of Pt Single Atoms for Persistent Photocatalytic Hydrogen Evolution
No full textThe stabilization of single-atom catalysts on semiconductor substrates is pivotal for advancing photocatalysis. TiO, a widely employed photocatalyst, typically stabilizes single atoms at oxygen vacancies—sites that are accessible but prone to agglomeration under illumination. Here, we demonstrate that cation vacancies in Ti-deficient TiO nanosheets provide highly stable anchoring sites for Pt single atoms, enabling persistent photocatalytic hydrogen evolution. Ultrathin TiO nanosheets with intrinsic Ti vacancies are synthesized via lepidocrocite-type titanate delamination and Pt single atoms are selectively trapped within these vacancies through a simple immersion process. The resulting Pt-decorated nanosheets exhibit superior photocatalytic hydrogen evolution performance, outperforming both Pt nanoparticle-loaded nanosheets and benchmarked Pt single-atom catalysts on P25. Crucially, Pt atoms anchored at Ti vacancies display remarkable resistance to light-induced agglomeration, a key limitation of conventional single-atom photocatalysts. Density functional theory calculations reveal that Pt incorporation into Ti vacancies is highly thermodynamically favorable and optimizes hydrogen adsorption energetics for enhanced catalytic activity. This work highlights the critical role of cation defect engineering in stabilizing single-atom co-catalysts and advancing the efficiency and durability of photocatalytic hydrogen evolution
Compact electro-optical bunch length detector: from an expert device to an operator tool
No full textLaser-based electro-optic detection (EOD) has been a valuable tool to measure the longitudinal electron bunch shape with sub-ps resolution for almost a decade, but it has always been a tool for expert use. Recently, the server and the user interface has been updated allow automated laser locking, time calibration and measurements to prepare for general operator use at the EuXFEL. It is currently prepared for EO Spectral Decoding measurements, but the implementation of advanced reconstruction algorithms (Diversity Enhanced EO Spectral Decoding, DEOS[*]) is ongoing. The paper presents details of the setup and the user interface as well as recent measurements
Engineering High-Order Harmonic Generation through Gas Confinement at Sub-Millimeter Lengths
No full textAttosecond light sources based on high-order harmonic generation (HHG) constitute to date the only table-top solution for producing coherent broadband radiation covering the spectral range from the extreme ultraviolet to the soft X-rays. The so-called emission cutoff can be extended towards higher photon energies by increasing the driving wavelength at the expense of conversion efficiency. An alternative route is to overdrive the process by using higher laser intensities, with the challenging requirement of interacting with higher plasma densities over short propagation distances. Here, we address this challenge by using a differentially pumped glass chip designed for optimal gas confinement over sub-mm lengths. By driving HHG with multicycle pulses at either 800 nm or 1500 nm, we demonstrate a cutoff extension by a factor of two compared to conventional phase matching approaches and surpassing the present record using multicycle fields. Our three-dimensional propagation simulations, in excellent agreement with the experiment, confirm that gas confinement is crucial since efficient phase matching of cutoff harmonics occurs only for short propagation lengths. Additionally, we show that the high photon energy component is not only temporally confined to the leading edge of the driving pulse, but also spatially confined in the near-field to an off-axis contribution due to reshaping of the driving field along propagation inside the medium. Our findings contribute to the fundamental understanding of HHG across different regimes
Latest achievements in femtosecond synchronization of large scale facilities
No full textThe laser-based synchronisation systems for the European XFEL and FLASH provide femtosecond-stable timing references for tens of clients along the accelerator and the experiment halls over many kilometres of optical fibre. Recently, benchmarking experiments revealed a point-to-point timing stability with sub-femtosecond rms timing jitter. At the same time geophysical effects like ocean waves and earthquakes do not only affect the performance of the system, but their impact can clearly be identified. To improve the temporal resolution in X-ray/optical pump-probe experiments, additional arrival time monitors for both the electrons and the optical laser pulses are currently being installed, allowing for a posteriori data sorting and eventually active feedbacks. Further, the optical reference oscillators and advanced synchronisation schemes are being developed, resulting in timing jitter on the sub-hundred attoseconds level
Modelling of Longitudinal-Longitudinal polarisation of the ZZ final state in the four-lepton decay channel at the ATLAS experiment
No full textThis note summarises the state-of-the-art modelling of the polarised ZZ production used in the ATLAS Run2 ZZ polarisation measurement. Monte Carlo samples for the main production modes, quark-induced q¯q→ZZ process, the electroweak (EW) production of ZZ in association with two jets, qq→ZZjj, and the loop-induced gg→ZZ were simulated for either the polarised or unpolarised ZZ final state in the four-charged-lepton decay channel. The simulated polarisation fractions of q¯q→ZZ events were reweighted to account for the next-to-leading order QCD and EW corrections, calculated at fixed-order with the MoCaNLO program. For the loop-induced gg→ZZ process, the simulated unpolarised events were reweighted to obtain polarisation templates by taking the predicted polarisation fraction calculated by MoCaNLO. No reweighting is applied to the EW qq→ZZjj process and the original MC simulation of polarisation templates is used. Kinematic distributions at the truth level are presented, allowing comparisons with current and future theoretical predictions
Directed Self-Assembly of the Organic Semiconductor C8-BTBT-C8 in Anodic Aluminum Oxide Nanopores
No full textControlling the self-assembly of organic semiconductors at the nanoscale is critical for advancing high-performance electronic and photonic devices, yet remains challenging due to their intrinsic anisotropic crystallization and sensitivity to processing conditions. Here, we demonstrate that cylindrical nanoconfinement within anodic aluminum oxide membranes provides a versatile platform to precisely tune the molecular orientation and phase behavior of the prototypical organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). Combining temperature-dependent high-resolution synchrotron X-ray scattering with optical birefringence measurements, we uncover that confinement geometries (pore diameters 25–180 nm) and surface chemistry govern the emergence of distinct smectic A textures, featuring molecular layers either parallel or perpendicular to the pore axis. The competition between axial and radial smectic layering is modulated by pore size, surface hydrophilicity, and thermal history, enabling reversible control over domain orientations and transitions between liquid crystalline and crystalline states. Notably, nanoconfinement stabilizes the smectic phase over an expanded temperature range compared to bulk, while inducing complex multi-domain configurations owing to geometric constraints and anchoring conditions. Our results elucidate fundamental mechanisms by which anisotropic nanoscale confinement directs the self-organization of highly conjugated organic molecules, with implications for optimizing directional charge transport and anisotropic optical responses in organic–inorganic hybrid nanoarchitectures. This study establishes nanoconfinement as a powerful strategy to engineer morphology and functional properties in organic semiconducting materials with nanoscale precision
Characterization and correction of multilayer X-ray optics
No full textHighest resolution X-ray microscopy requires high numerical aperture (NA) optics ofexcellent quality which allow to focus X-ray beams to small focal points. MultilayerLaue lenses (MLLs) are a new type of diffractive optic with the capability to focushard X-rays with high efficiency to nanometer spots. However, they are currentlylimited by wavefront aberrations caused by layer misplacements during their fab-rication process. The determination and correction of wavefront aberration aretherefore crucial aspects for the development and improvement of MLLs designedto achieve the highest possible resolutions. Wavefront characterization for lensdevelopment cannot rely solely on access to synchrotrons, as beamtime must beapplied for and is therefore only available to a limited extent. For that reason, theaim of this thesis was to determine the requirements that have to be met in orderto enable fast and precise wavefront characterization of MLLs using a laboratory-based setup. This requires a dedicated table top X-ray system and a suitable software.To determine the optimal structure for MLLs, periodic multilayer gratings withdifferent layer thicknesses on the scale of a few nanometers were studied. It wasfound that high quality and high efficiency (> 60 % at 17.5 keV and > 80 % at60 keV) periodic multilayers can be fabricated, which are mainly limited by theinterdiffusion of the layers. The interdiffusion depth was found to be around 0.4nm for WC/SiC multilayer-based optics, the material pair our MLLs are typicallymade of. Based on these findings, high NA (>0.01) MLLs were produced for highestresolution microscopy. To determine the wavefront aberrations of the MLLs in alaboratory setup, a phase retrieval algorithm with low requirements on coherenceand monochromaticity was needed. Ptychographic X-ray speckle tracking (PXST)is an in-house developed phase gradient reconstruction algorithm that was foundto fulfill these requirements and was recently augmented to incorporate machinelearning techniques, which allow accurate phase retrieval even in noisy and lowintensity environments. It is based on the X-ray speckle tracking approach and can beunderstood as a generalized Hartmann sensor that tracks sample features betweenoverlapping images to determine local phase gradients. It was found that due tothe robustness of the algorithm, wavefront reconstructions are largely accurateregardless of the measurement conditions, as long as the lens and a suitable sampleare aligned with respect to each other. This has reduced the time required for theviicharacterization of MLLs from >10 hours to a few minutes, allowing feedback on alens’ performance on the same day it is manufactured.The wavefront error of an MLL is a map of the relative deviation from the wavefrontof an ideal lens, which can be related to the misplacements of its layers, allowingsubsequent fabrication cycles to be improved based on previous lens characteriza-tions. However, residual aberrations remain that have to be corrected externally. Asit becomes increasingly difficult to produce high NA MLLs with the required preci-sion, they tend to have larger aberrations. To correct these, a compound refractivecorrector consisting of an array of individual refractive elements had been proposed.For the first time, such a design was realized based on nano-scale 3D printing tocorrect an MLL pair for hard X-ray high-resolution imaging, resulting in a recordfocusing of 2.9 nm ×2.8 nm at 17.5 keV.The well characterized and aberration corrected MLLs were then used in a seriesof imaging schemes at the PETRA III synchrotron and the European X-ray free elec-tron laser. There, imaging techniques such as projection holography and near-fieldptychography, which benefit from the strong focusing of the lenses, were used withimproved MLLs to achieve resolutions well below 10 nm. Novel imaging techniquesthat are now possible due to high NA MLLs such as convergent beam crystallographyhave been successfully implemented. At high photon energies, where MLLs becomemore efficient in contrast to other optics, the dark-field imaging method of scanningCompton X-ray microscopy was successfully used, allowing high-resolution imagingof biological samples with minimal radiation dose. This method in particular willbenefit significantly from fourth generation synchrotrons, as their higher brightnessand larger coherence in combination with better X-ray optics allows for both higherresolution and faster data acquisition times. This should enable 3D imaging ofmicroscopic samples at high resolutions and low radiation doses
Search for the lepton-flavor-violating decays at Belle II
No full textWe present the result of a search for the charged-lepton-flavor violating decays τ→ eℓℓ, where ℓ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb recorded by the Belle II experiment at the SuperKEKB ee collider. The selection of ee→ ττ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background.Upper limits on the branching fractions between 1.3 and 2.5 × 10 are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes.[graphic not available: see fulltext