Fakultät für Physik
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Full NLO corrections to 3jet production and R32 at the LHC
The European Physical Journal C 2019; 79(4): Art. 321We present the evaluation of the complete set of NLO corrections to threejet production at the LHC. To this end we consider all contributions of O(αnsαm) with n+m=3 and n+m=4. This includes in particular also subleading Born contributions of electroweak origin, as well as electroweak virtual and QED realradiative corrections. As an application we present results for the three over twojet ratio R32. While the impact of nonQCD corrections on the total cross section is rather small, they can exceed −10% for high jet transverse momenta. The R32 observable turns out to be very stable against electroweak corrections, receiving absolute corrections below 5% even in the highpT region. 
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Extreme velocity gradients in turbulent flows
New Journal of Physics 2019; 21(4): Art. 043004Fully turbulent flows are characterized by intermittent formation of very localized and intense velocity gradients. These gradients can be orders of magnitude larger than their typical value and lead to many unique properties of turbulence. Using direct numerical simulations of the Navier–Stokes equations with unprecedented smallscale resolution, we characterize such extreme events over a significant range of turbulence intensities, parameterized by the Taylorscale Reynolds number (Rl). Remarkably, we find the strongest velocity gradients to empirically scale as t l  Rb K 1 , with b » 0.775 0.025,where tK is theKolmogorov time scale (with its inverse, tK1, being the rms of velocity gradient fluctuations). Additionally, we observe velocity increments across very small distances r h,where η is theKolmogorov length scale, to be as large as the rms of the velocity fluctuations. Both observations suggest that the smallest length scale in the flow behaves as h l Ra,with a = b  1 2 , which is at odds with predictions from existing phenomenological theories.Wefind that extreme gradients are arranged in vortex tubes, such that strain conditioned on vorticity grows on average slower than vorticity, approximately as a power law with an exponent g < 1, which weakly increaseswith Rl.Using scaling arguments,we get b = (2  g)1,which suggests that βwould also slowly increasewith Rl.We conjecture that approaching themathematical limit of infinite Rl, strain and vorticity would scale similarly resulting in g = 1and hence extreme events occurring at a scale h l R1/2 corresponding to b = 1. 
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Eigenstate thermalization and quantum chaos in the Holstein polaron model
Physical Review B 2019; 99(15): Art. 155130The eigenstate thermalization hypothesis (ETH) is a successful theory that provides sufficient criteria for ergodicity in quantum manybody systems. Most studies were carried out for Hamiltonians relevant for ultracold quantum gases and singlecomponent systems of spins, fermions, or bosons. The paradigmatic example for thermalization in solidstate physics are phonons serving as a bath for electrons. This situation is often viewed from an openquantumsystem perspective. Here, we ask whether a minimal microscopic model for electronphonon coupling is quantum chaotic and whether it obeys ETH, if viewed as a closed quantum system. Using exact diagonalization, we address this question in the framework of the Holstein polaron model. Even though the model describes only a single itinerant electron, whose coupling to dispersionless phonons is the only integrabilitybreaking term, we find that the spectral statistics and the structure of Hamiltonian eigenstates exhibit essential properties of the corresponding randommatrix ensemble. Moreover, we verify the ETH ansatz both for diagonal and offdiagonal matrix elements of typical phonon and electron observables, and show that the ratio of their variances equals the value predicted from randommatrix theory. 
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A Theoretical Framework to Derive Simple, FiringRateDependent Mathematical Models of Synaptic Plasticity
Frontiers in Computational Neuroscience 2019; 13: Art. 26Synaptic plasticity serves as an essential mechanism underlying cognitive processes as learning and memory. For a better understanding detailed theoretical models combine experimental underpinnings of synaptic plasticity and match experimental results. However, these models are mathematically complex impeding the comprehensive investigation of their link to cognitive processes generally executed on the neuronal network level. Here, we derive a mathematical framework enabling the simplification of such detailed models of synaptic plasticity facilitating further mathematical analyses. By this framework we obtain a compact, firingratedependent mathematical formulation, which includes the essential dynamics of the detailed model and, thus, of experimentally verified properties of synaptic plasticity. Amongst others, by testing our framework by abstracting the dynamics of two wellestablished calciumdependent synaptic plasticity models, we derived that the synaptic changes depend on the square of the presynaptic firing rate, which is in contrast to previous assumptions. Thus, the herepresented framework enables the derivation of biologically plausible but simple mathematical models of synaptic plasticity allowing to analyze the underlying dependencies of synaptic dynamics from neuronal properties such as the firing rate and to investigate their implications in complex neuronal networks. 
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Timeresolved xray phasecontrast tomography of sedimenting microspheres
New Journal of Physics 2019; 21(4): Art. 043017We have implemented a timedependent (dynamic) xray tomography of sedimenting microspheres suspended in water. To achieve phase contrast at high magnification we use the divergent and highly coherent beam emitted from an xray waveguide. Holograms are recorded with 5 ms acquisition time while the sample is rotated at 1 Hz, over a run of 40 s. We show that under these conditions, more than 20 000 individual particle trajectories can be tracked. The analysis of the trajectories shows apparent superdiffusive behavior due to collective flow patterns, as also further evidenced by plotting the temporal averaged spatial distribution of particle densities and velocities. 
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MagneticFieldInduced Suppression of JahnTeller Phonon Bands in (La0.6Pr0.4)0.7Ca0.3MnO3: the Mechanism of Colossal Magnetoresistance shown by Raman Spectroscopy
Scientific Reports 2019; 9(1): Art. 2387A longstanding issue in the physics of the colossal magnetoresistance is the role of electronphonon coupling, which manifests itself as JahnTeller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magneticfielddependent Raman spectroscopy on thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 in a range of H = 050 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, TC = 197 K, the intensity of the JahnTeller stretching mode at 614 cm1 and of the bending mode at 443 cm1 was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La0.6Pr0.4)0.7Ca0.3MnO3. Our work provides direct evidence that the reduction of the amount of JahnTeller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance. 
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Monomerization of the photoconvertible fluorescent protein SAASoti by rational mutagenesis of single amino acids
Scientific Reports 2018; 8(1): Art. 15542Photoconvertible fluorescent proteins (PCFPs) are widely used as markers for the visualization of intracellular processes and for subdiffraction singlemolecule localization microscopy. Although wild type of a new photoconvertible fluorescent protein SAASoti tends to aggregate, we succeeded, via rational mutagenesis, to obtain variants that formed either tetramers or monomers. We compare two approaches: one is based on the structural similarity between SAASoti and Kaede, which helped us to identify a single point mutation (V127T) at the protein's hydrophobic interface that leads to monomerization. The other is based on a chemical modification of amino groups of SAASoti with succinic anhydride, which converts the protein aggregates into monomers. Massspectrometric analysis helped us to identify that the modification of a single εamino group of lysine K145 in the strongly charged interface AB was sufficient to convert the protein into its tetrameric form. Furthermore, sitedirected mutagenesis was used to generate mutants that proved to be either monomeric or tetrameric, both capable of rapid greentored photoconversion. This allows SAASoti to be used as a photoconvertible fluorescent marker for in vivo cell studies. 
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NearEdge Soft Xray Absorption Mass Spectrometry of Protonated Melittin
Journal of The American Society for Mass Spectrometry 2018; 29(11) p.21382151We have investigated the photoionization and photofragmentation yields of gasphase multiply protonated melittin cations for photon energies at the Kshell absorption edges of carbon, nitrogen, and oxygen. Two similar experimental approaches were employed. In both experiments, mass selected [melittin+qH]q+ (q=24) ions were accumulated in radiofrequency ion traps. The trap content was exposed to intense beams of monochromatic soft Xray photons from synchrotron beamlines and photoproducts were analyzed by means of timeofflight mass spectrometry. Mass spectra were recorded for fixed photon energies, and partial ion yield spectra were recorded as a function of photon energy. The combination of mass spectrometry and soft Xray spectroscopy allows for a direct correlation of protein electronic structure with various photoionization channels. Nondissociative single and double ionization are used as a reference. The contribution of both channels to various backbone scission channels is quantified and related to activation energies and protonation sites. Soft Xray absorption mass spectrometry combines fast energy deposition with single and double ionization and could complement established activation techniques. Graphical Abstractᅟ. 
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Strong modification of the transport level alignment in organic materials after optical excitation
Nature Communications 2019; 10(1): Art. 1470Organic photovoltaic devices operate by absorbing light and generating current. These two processes are governed by the optical and transport properties of the organic semiconductor. Despite their common microscopic originthe electronic structuredisclosing their dynamical interplay is far from trivial. Here we address this issue by timeresolved photoemission to directly investigate the correlation between the optical and transport response in organic materials. We reveal that optical generation of noninteracting excitons in a fullerene film results in a substantial redistribution of all transport levels (within 0.4 eV) of the nonexcited molecules. As all observed dynamics evolve on identical timescales, we conclude that optical and transport properties are completely interlinked. This finding paves the way for developing novel concepts for transport level engineering on ultrafast time scales that could lead to novel functional optoelectronic devices. 
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Experimental Study of the Bottleneck in Fully Developed Turbulence
Journal of Statistical Physics p.123The energy spectrum of incompressible turbulence is known to reveal a pileup of energy at those high wavenumbers where viscous dissipation begins to act. It is called the bottleneck effect (Donzis and Sreenivasan in J Fluid Mech 657:171–188, 2010; Falkovich in Phys Fluids 6:1411–1414, 1994; Frisch et al. in Phys Rev Lett 101:144501, 2008; Kurien et al. in Phys Rev E 69:066313, 2004; Verma and Donzis in Phys A: Math Theor 40:4401–4412, 2007). Based on direct numerical simulations of the incompressibleNavierStokes equations, results from Donzis and Sreenivasan (657:171–188, 2010) pointed to a powerlaw decrease of the strength of the bottleneck with increasing intensity of the turbulence, measured by the Taylor microscale Reynolds number Rλ. Here we report the first experimental results on the dependence of the amplitude of the bottleneck as a function of Rλ in a windtunnel flow. We used an active grid (Griffin et al. in Control of longrange correlations in turbulence, arXiv:1809.05126, 2019) in the variable density turbulence tunnel (VDTT) (Bodenschatz et al. in Rev Sci Instrum 85:093908, 2014) to reach Rλ > 5000, which is unmatched in laboratory flows of decaying turbulence. The VDTT with the active grid permitted us to measure energy spectra from flows of different Rλ, with the smallscale features appearing always at the same frequencies. We relate those spectra recorded to a common reference spectrum, largely eliminating systematic errors which plague hotwire measurements at high frequencies. The data are consistent with a power law for the decrease of the bottleneck strength for the finite range of Rλ in the experiment. 
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Associated production of a top quark pair with a heavy electroweak gauge boson at NLO+NNLL accuracy
The European Physical Journal C 2019; 79(3): Art. 249We perform threshold resummation of soft gluon corrections to the total cross sections and the invariant mass distributions for production of a topantitop quark pair associated with a heavy electroweak boson V = W+,W− or Z in pp collisions at the Large Hadron Collider. The resummation is carried out at nexttonexttoleadinglogarithmic (NNLL) accuracy using the direct QCDMellin space technique in the threeparticle invariant mass kinematics. It is found that for the t ¯t Z process the soft gluon resummation introduces significant corrections to the nexttoleading order (NLO) results. For the central scale equal to the t ¯t Z invariant mass the corrections reach nearly 30%. For this process, the dominant theoretical uncertainty of the cross section due to the scale choice is significantly reduced at the NLO+NNLL level with respect to the NLO results. The effects of resummation are found to be less pronounced in the t ¯tW± case. The obtained results are compared to recent measurements performed by CMS and ATLAS collaborations at the LHC. 
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Evidence of a structure in K¯0Λ+c consistent with a charged Ξc(2930)+ , and updated measurement of B¯0→K¯0Λ+cΛ¯−c at Belle
The European Physical Journal C 2018; 78(11) 
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Searching for flavored gauge bosons
Journal of High Energy Physics 2019; 2019(2)Standard Model may allow an extended gauge sector with anomalyfree flavored gauge symmetries, such as Li −Lj , Bi −Lj , and B −3Li, where i, j = 1, 2, 3 are flavor indices. We investigate phenomenological implications of the new flavored gauge boson Z′ in the above three classes of gauge symmetries. Focusing on the gauge boson mass above 5 GeV, we use the lepton universality test in the Z and τ /μ decays, LEP searches, LHC searches, neutrino trident production bound, and LHC Z → 4μ searches to put constraints on the g′ −MZ′ plane. When L1 is involved, the LEP bounds on the e−e+ → ℓ−ℓ+ processes give the most stringent bounds, while the LHC bound becomes the strongest constraints in the large MZ′ region when Bi is involved. The bound from Z → 4μ productions, which is applicable for L2involved scenarios, provides stringent bounds in the small MZ′ region. One exception is the B − 3L2 scenario, in which case only a small region is favored due to the lepton universality. 
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A case study for the formation of stanene on a metal surface
Communications Physics 2019; 2(1)The discovery and realization of graphene as an ideal twodimensional (2D) material has triggered extensive efforts to create similar 2D materials with exciting spindependent properties. Here, we report on a novel Sn 2D superstructure on Au(111) that shows similarities and differences to the expected electronic features of ideal stanene. Using spin and angleresolved photoemission spectroscopy, we find that a particular Sn/Au superstructure reveals a linearly dispersing band centered at the Γpoint and below the Fermi level with antiparallel spin polarization and a Fermi velocity of vF ≈ 1×106 m/s, the same value as for graphene. We attribute the origin of the band structure to the hybridization between the Sn and the Au orbitals at the 2D SnAu interface. Considering that freestanding stanene simply cannot exist, our investigated structure is an important step towards the search of useful stanenelike overstructures for future technological applications. 
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Interpretation of heliocentric water production rates of comets
Astronomy & Astrophysics 2019; 623: Art. A120Aims. We investigate the influence of three basic factors on water production rate as a function of heliocentric distance: nucleus shape, the spin axis orientation, and the distribution of activity on a comet’s surface. Methods. We used a basic water sublimation model driven by solar insolation to derive total production rates for different nuclei shapes and spin axis orientations using the orbital parameters of 67P/ChuryumovGerasimenko. We used known shape models derived from prior missions to the Jupiter Family and short period comets. The slopes of production rates versus heliocentric distance were calculated for the different model setups. Results. The standard (homogeneous) outgassing model confirms the wellknown result regarding the heliocentric dependence of water production rate that remains invariant for different nuclei shapes as long as the rotation axis is perpendicular to the orbital plane. When the rotation axis is not perpendicular, the nucleus shape becomes a critically important factor in determining the water production curves as the illuminated cross section of the nucleus changes with heliocentric distance. Shape and obliquity can produce changes in the illuminated cross section of up to 50% over an orbit. In addition, different spin axis orientations for a given shape can dramatically alter the pre and postperihelion production curves, as do assumptions about the activity distribution on the surface. If, however, the illuminated cross section of the nucleus is invariant, then the dependence on the above parameters is weak, as demonstrated here with the 67P/ChuryumovGerasimenko shape. The comets Hartley 2 and Wild 2 are shown to yield significantly different production curve shapes for the same orbit and orientation as 67P/CG, varying by as much as a factor of three as a result of only changing the nucleus shape. Finally, we show that varying just three basic parameters, shape, spin axis orientation, and active spots distribution on the surface can lead to arbitrary deviations from the expected inverse square law dependence of water production rates near 1 au. Conclusions. With the results obtained, we cannot avoid the conclusion that, without prior knowledge of basic parameters (shape, spin axis orientation, activity locations), it is difficult to reveal the nature of cometary outgassing from the heliocentric water production rates. Similarly, the intercomparison of water production curves of two such comets may not be meaningful. 
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Dark matter in (partially) composite Higgs models
Journal of High Energy Physics 2018; 2018(12)We construct composite and partially composite Higgs models with complex pseudoNambuGoldstone (pNGB) dark matter states from fourdimensional gaugeYukawa theories with strongly interacting fermions. The fermions are partially gauged under the electroweak symmetry, and the dynamical electroweak symmetry breaking sector is minimal. The pNGB dark matter particle is stable due to a U(1) technibaryonlike symmetry, also present in the technicolor limit of the models. However, the relic density is particle antiparticle symmetric and due to thermal freezeout as opposed to the technicolor limit where it is typically due to an asymmetry. The pNGB Higgs is composite or partially composite depending on the origin of the Standard Model fermion masses, which impacts the dark matter phenomenology. We illustrate the important features with a model example invariant under an SU(4) × SU(2) × U(1) global symmetry. 
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Mutationinduced alterations of intrafilament subunit organization in vimentin filaments revealed by SAXS
Soft Matter 2019; 15(9) p.19992008Vimentin intermediate filaments constitute a distinct filament system in mesenchymal cells that is instrumental for cellular mechanics and migration. In vitro, the rodlike monomers assemble in a multistep, saltdependent manner into micrometer long biopolymers. To disclose the underlying mechanisms further, we employed small angle Xray scattering on two recombinant vimentin variants, whose assembly departs at strategic points from the normal assembly route: (i) vimentin with a tyrosine to leucine change at position 117; (ii) vimentin missing the nonαhelical carboxylterminal domain. Y117L vimentin assembles into unitlength filaments (ULFs) only, whereas ΔT vimentin assembles into filaments containing a higher number of tetramers per cross section than normal vimentin filaments. We show that the shape and inner structure of these mutant filaments is significantly altered. ULFs assembled from Y117L vimentin contain more, less tightly bundled vimentin tetramers, and ΔT vimentin filaments preserve the number density despite the higher number of tetramers per filament crosssection. 
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Human blood platelets contract in perpendicular direction to shear flow
Soft Matter 2019; 15(9) p.20092019In their physiological environment, blood platelets are permanently exposed to shear forces caused by blood flow. Within this surrounding, they generate contractile forces that eventually lead to a compaction of the blood clot. Here, we present a microfluidic chamber that combines hydrogelbased traction force microscopy with a controlled shear environment, and investigate the force fields platelets generate when exposed to shear flow in a spatiotemporally resolved manner. We find that for shear rates between 14 s1 to 33 s1, the general contraction behavior in terms of force distribution and magnitude does not differ from noflow conditions. The main direction of contraction, however, does respond to the externally applied stress. At high shear stress, we observe an angle of about 90° between flow direction and main contraction axis. We explain this observation by the distribution of the stress acting on the adherent cell: the observed angle provides the most stable situation for the cell experiencing the shear flow, as supported by a finite element method simulation of the stresses along the platelet boundary. 
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Functional Macromolecular Systems: Kinetic Pathways to Obtain Tailored Structures
Macromolecular Chemistry and Physics 2018; 220(2): Art. 1800334This article aims to stimulate research on nonequilibrium macromolecular systems, as nowadays a large toolbox to synthesize tailored macromolecules is available. A large variety of characterization methods covering a broad spectrum of length and timescales allows researchers to follow and also manipulate macromolecular systems on their paths toward equilibrium. These possibilities are paralleled by the development of new concepts of the statistical physics of nonequilibrium phenomena in macromolecular systems as well as new models and algorithms for computer simulation. 
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EnergyReduced Arrhythmia Termination Using Global Photostimulation in Optogenetic Murine Hearts
Frontiers in Physiology 2018; 9: Art. 1651Complex spatiotemporal nonlinearity as observed during cardiac arrhythmia strongly correlates with vortexlike excitation wavelengths and tissue characteristics. Therefore, the control of arrhythmic patterns requires fundamental understanding of dependencies between onset and perpetuation of arrhythmia and substrate instabilities. Available treatments, such as drug application or highenergy electrical shocks, are discussed for potential side effects resulting in prognosis worsening due to the lack of specificity and spatiotemporal precision. In contrast, cardiac optogenetics relies on light sensitive ion channels stimulated to trigger excitation of cardiomyocytes solely making use of the inner cell mechanisms. This enables lowenergy, nondamaging optical control of cardiac excitation with high resolution. Recently, the capability of optogenetic cardioversion was shown in Channelrhodopsin2 (ChR2) transgenic mice. But these studies used mainly structured and local illumination for cardiac stimulation. In addition, since optogenetic and electrical stimulus work on different principles to control the electrical activity of cardiac tissue, a better understanding of the phenomena behind optogenetic cardioversion is still needed. The present study aims to investigate global illumination with regard to parameter characterization and its potential for cardioversion. Our results show that by tuning the light intensity without exceeding 1.10 mW mm2, a single pulse in the range of 101,000 ms is sufficient to reliably reset the heart into sinus rhythm. The combination of our panoramic lowintensity photostimulation with optical mapping techniques visualized wave collision resulting in annihilation as well as propagation perturbations as mechanisms leading to optogenetic cardioversion, which seem to base on other processes than electrical defibrillation. This study contributes to the understanding of the roles played by epicardial illumination, pulse duration and light intensity in optogenetic cardioversion, which are the main variables influencing cardiac optogenetic control, highlighting the advantages and insights of global stimulation. Therefore, the presented results can be modules in the design of novel illumination technologies with specific energy requirements on the way toward tissueprotective defibrillation techniques.