Showing papers by "Frankfurt Institute for Advanced Studies published in 2014"

Spike avalanches in vivo suggest a driven, slightly subcritical brain state

Abstract: In self-organized critical (SOC) systems avalanche size distributions follow power-laws. Power-laws have also been observed for neural activity, and so it has been proposed that SOC underlies brain organization as well. Surprisingly, for spiking activity in vivo, evidence for SOC is still lacking. Therefore we analyzed highly parallel spike recordings from awake rats and monkeys, anaesthetized cats, and also local field potentials from humans. We compared these to spiking activity from two established critical models: the Bak-Tang-Wiesenfeld model, and a stochastic branching model. We found fundamental differences between the neural and the model activity. These differences could be overcome for both models through a combination of three modifications: (1) subsampling, (2) increasing the input to the model (this way eliminating the separation of time scales, which is fundamental to SOC and its avalanche definition), and (3) making the model slightly sub-critical. The match between the neural activity and the modified models held not only for the classical avalanche size distributions and estimated branching parameters, but also for two novel measures (mean avalanche size, and frequency of single spikes), and for the dependence of all these measures on the temporal bin size. Our results suggest that neural activity in vivo shows a melange of avalanches, and not temporally separated ones, and that their global activity propagation can be approximated by the principle that one spike on average triggers a little less than one spike in the next step. This implies that neural activity does not reflect a SOC state but a slightly sub-critical regime without a separation of time scales. Potential advantages of this regime may be faster information processing, and a safety margin from super-criticality, which has been linked to epilepsy.

Multiple reentrant phase transitions and triple points in Lovelock thermodynamics

Abstract: We investigate the effects of higher curvature corrections from Lovelock gravity on the phase structure of asymptotically AdS black holes, treating the cosmological constant as a thermodynamic pressure. We examine how various thermodynamic phenomena, such as Van der Waals behaviour, reentrant phase transitions (RPT), and tricritical points are manifest for U(1) charged black holes in Gauss-Bonnet and 3rd-order Lovelock gravities. We furthermore observe a new phenomenon of ‘multiple RPT’ behaviour, in which for fixed pressure the small/large/small/large black hole phase transition occurs as the temperature of the system increases. We also find that when the higher-order Lovelock couplings are related in a particular way, a peculiar isolated critical point emerges for hyperbolic black holes and is characterized by non-standard critical exponents.

Constraining Neutron Star Matter with Quantum Chromodynamics

Abstract: In recent years, there have been several successful attempts to constrain the equation of state of neutron star matter using input from low-energy nuclear physics and observational data. We demonstrate that significant further restrictions can be placed by additionally requiring the pressure to approach that of deconfined quark matter at high densities. Remarkably, the new constraints turn out to be highly insensitive to the amount—or even presence—of quark matter inside the stars.

Confronting LHC data with the statistical hadronization model

Abstract: The most recent data from the CERN LHC are compared with calculations within the statistical hadronization model. The parameters temperature und baryon chemical potential are fitted to the data. The best fit yields a temperature of 156 MeV, slightly below the expectation from RHIC data. Proton yields are nearly three standard deviations below this fit and possible reasons are discussed.

Interacting quark matter equation of state for compact stars

Abstract: Lattice quantum chromodynamics (QCD) studies of the thermodynamics of hot quark-gluon plasma demonstrate the importance of accounting for the interactions of quarks and gluons if one wants to investigate the phase structure of strongly interacting matter. Motivated by this observation and using state-of-the-art results from perturbative QCD, we construct a simple, effective equation of state (EOS) for cold quark matter that consistently incorporates the effects of interactions and furthermore includes a built-in estimate of the inherent systematic uncertainties. This goes beyond the MIT bag model description in a crucial way, yet leads to an EOS that is equally straightforward to use. We also demonstrate that, at moderate densities, our EOS can be made to smoothly connect to hadronic EOSs, with the two exhibiting very similar behavior near the matching region. The resulting hybrid stars are seen to have masses similar to those predicted by the purely nucleonic EOSs.

Initial state fluctuations and final state correlations in relativistic heavy-ion collisions

Abstract: We review the phenomenology and theory of bulk observables in ultra-relativistic heavy-ion collisions, focusing on recent developments involving event-by-event fluctuations in the initial stages of a heavy-ion collision, and how they manifest in observed correlations. We first define the relevant observables and show how each measurement is related to underlying theoretical quantities. Then we review the prevailing picture of the various stages of a collision, including the state-of-the-art modeling of the initial stages of a collision and subsequent hydrodynamic evolution, as well as hadronic scattering and freeze-out in the later stages. We then discuss the recent results that have shaped our current understanding and identify the challenges that remain. Finally, we point out open issues and the potential for progress in the field.

Transport Coefficients of Bulk Viscous Pressure in the 14-moment approximation

Abstract: We compute the transport coefficients that appear in the fluid-dynamical equations for the bulk viscous pressure and shear-stress tensor using the 14-moment approximation in the limit of small, but finite, masses. In this limit, we are able to express all these coefficients in terms of known thermodynamic quantities, such as the thermodynamic pressure, energy density, and the velocity of sound. We explicitly demonstrate that the ratio of bulk viscosity to bulk relaxation time behaves very differently, as a function of temperature, than the ratio of shear viscosity to shear relaxation time. We further explicitly compute, for the first time, the transport coefficients that couple the bulk viscous pressure to the shear-stress tensor and vice versa. The coefficient that couples bulk viscous pressure to shear-stress tensor is found to be orders of magnitude larger than the bulk viscosity itself, suggesting that bulk viscous pressure production owes more to this coupling than to the expansion rate of the system.

Efficient transfer entropy analysis of non-stationary neural time series.

Abstract: Information theory allows us to investigate information processing in neural systems in terms of information transfer, storage and modification. Especially the measure of information transfer, transfer entropy, has seen a dramatic surge of interest in neuroscience. Estimating transfer entropy from two processes requires the observation of multiple realizations of these processes to estimate associated probability density functions. To obtain these necessary observations, available estimators typically assume stationarity of processes to allow pooling of observations over time. This assumption however, is a major obstacle to the application of these estimators in neuroscience as observed processes are often non-stationary. As a solution, Gomez-Herrero and colleagues theoretically showed that the stationarity assumption may be avoided by estimating transfer entropy from an ensemble of realizations. Such an ensemble of realizations is often readily available in neuroscience experiments in the form of experimental trials. Thus, in this work we combine the ensemble method with a recently proposed transfer entropy estimator to make transfer entropy estimation applicable to non-stationary time series. We present an efficient implementation of the approach that is suitable for the increased computational demand of the ensemble method's practical application. In particular, we use a massively parallel implementation for a graphics processing unit to handle the computationally most heavy aspects of the ensemble method for transfer entropy estimation. We test the performance and robustness of our implementation on data from numerical simulations of stochastic processes. We also demonstrate the applicability of the ensemble method to magnetoencephalographic data. While we mainly evaluate the proposed method for neuroscience data, we expect it to be applicable in a variety of fields that are concerned with the analysis of information transfer in complex biological, social, and artificial systems.

Influence of hadronic bound states above T c on heavy-quark observables in Pb + Pb collisions at the CERN Large Hadron Collider

Abstract: We investigate how the possible existence of hadronic bound states above the deconfinement transition temperature ${T}_{c}$ affects heavy-quark observables, such as the nuclear modification factor, the elliptic flow, and azimuthal correlations. Lattice QCD calculations suggest that above ${T}_{c}$ the effective degrees of freedom might not be exclusively partonic but that a certain fraction of hadronic degrees of freedom might already form at higher temperatures. This is an interesting question by itself but also has a strong influence on other probes of the strongly interacting matter produced in ultrarelativistic heavy-ion collisions. A substantial fraction of hadronic bound states above ${T}_{c}$ reduces on average the interaction of the heavy quarks with colored constituents of the medium. We find that all studied observables are highly sensitive to the active degrees of freedom in the quark-gluon plasma.

Azimuthal correlations of heavy quarks in Pb + Pb collisions at √s =2.76 TeV at the CERN Large Hadron Collider

Abstract: In this paper we study the azimuthal correlations of heavy quarks in $\mathrm{Pb}+\mathrm{Pb}$ collisions with $\sqrt{s}=2.76$ TeV at the Large Hadron Collider. Due to the interaction with the medium, heavy quarks and antiquarks are deflected from their original direction and the initial correlation of the pair is broadened. We investigate this effect for different transverse momentum classes. Low-momentum heavy-quark pairs lose their leading-order back-to-back initial correlation, while a significant residual correlation survives at large momenta. Due to the larger acquired average deflection from their original directions the azimuthal correlations of heavy-quark pairs are broadened more efficiently in a purely collisional energy loss mechanism compared to that including radiative corrections. This discriminatory feature survives when next-to-leading-order production processes are included.

Watch the hands: infants can learn to follow gaze by seeing adults manipulate objects

Abstract: Infants gradually learn to share attention, but it is unknown how they acquire skills such as gaze-following. Deak and Triesch (2006) suggest that gaze-following could be acquired if infants learn that adults’ gaze direction is likely to be aligned with interesting sights. This hypothesis stipulates that adults tend to look at things that infants find interesting, and that infants could learn by noticing this tendency. We tested the plausibility of this hypothesis through video-based micro-behavioral analysis of naturalistic parent–infant play. The results revealed that 3- to 11-month-old infants strongly preferred watching caregivers handle objects. In addition, when caregivers looked away from their infant they tended to look at their own object-handling. Finally, when infants looked toward the caregiver while she was looking at her own hands, the infant’s next eye movement was often toward the caregiver’s object-handling. In this way infants receive adequate naturalistic input to learn associations between their parent’s gaze direction and the locations of interesting sights.

Simulations of dose enhancement for heavy atom nanoparticles irradiated by protons.

Abstract: A possible dose enhancement effect by proton or electron irradiation in the vicinity of nanoparticles consisting of different high Z atomic materials has been investigated using the track structure Monte Carlo code TRAX. In the simulations, Fe, Ag, Gd, Pt and Au nanoparticles (r = 22 and 2 nm) were irradiated with monoenergetic proton beams at energies of therapeutic interest (2, 80 and 300 MeV) and 44 keV electrons. Due to the large number of electrons in atoms with high atomic numbers, many electrons can be released in Auger cascades in addition to the primary ionization process. The potential additional nanoscopic radial dose contributions in the presence of metallic nanoparticles are assessed by comparison with liquid water and water simulated with the same density as the metallic materials. We find a noticeable impact of Auger electrons emitted from the nanoparticles. Special focus has been given to the assessment of complete sets of low-energy electron cross sections for the nanoparticle materials.

Centrality dependence of hadronization and chemical freeze-out conditions in heavy ion collisions at \sqrt s_{NN} = 2.76 TeV

Abstract: We present an analysis of hadronic multiplicities measured in Pb-Pb collisions at ${\sqrt{s}}_{NN}=2.76$ TeV as a function of the collision centrality within the statistical hadronization model. Evidence is found of a dependence of the chemical freeze-out temperature as a function of centrality, with a slow rise from central to peripheral collisions, which we interpret as an effect of posthadronization inelastic scatterings. Using correction factors calculated by means of a simulation based on the urqmd model, we are able to obtain a significant improvement in the statistical model fit quality and to reconstruct the primordial chemical equilibrium configuration. This is characterized by a nearly constant temperature of about 164 MeV, which we interpret as the actual hadronization temperature.

Astrophysical weak-interaction rates for selected A=20 and A=24 nuclei

Abstract: We have evaluated the electron capture rates on $^{20}\mathrm{Ne}$, $^{20}\mathrm{F}$, $^{24}\mathrm{Mg}$, and $^{24}\mathrm{Na}$ and the $\ensuremath{\beta}$ decay rates for $^{20}\mathrm{F}$ and $^{24}\mathrm{Na}$ at temperature and density conditions relevant for the late-evolution stages of stars with $M=8{M}_{\ensuremath{\bigodot}}$--12${M}_{\ensuremath{\bigodot}}$. The rates are based on recent experimental data and large-scale shell-model calculations. We show that the electron capture rates on $^{20}\mathrm{Ne}$ and $^{24}\mathrm{Mg}$ and the $^{20}\mathrm{F}$ and $^{24}\mathrm{Na}$ $\ensuremath{\beta}$-decay rates are based on data in this astrophysical range, except for the capture rate on $^{20}\mathrm{Ne}$, which we predict to have a dominating contribution from the second-forbidden transition between the $^{20}\mathrm{Ne}$ and $^{20}\mathrm{F}$ ground states in the density range ${log}_{10}\ensuremath{\rho}{Y}_{e}(\mathrm{g}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})=9.3$--9.6. The dominance of a few individual transitions allows us to present the various rates by analytical expressions at the relevant astrophysical conditions. We also derive the screening corrections to the rates.

EMMI rapid reaction task force meeting on quark matter in compact stars

Abstract: The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent observational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

Untangling perceptual memory: hysteresis and adaptation map into separate cortical networks

Abstract: Perception is an active inferential process in which prior knowledge is combined with sensory input, the result of which determines the contents of awareness. Accordingly, previous experience is known to help the brain “decide” what to perceive. However, a critical aspect that has not been addressed is that previous experience can exert 2 opposing effects on perception: An attractive effect, sensitizing the brain to perceive the same again (hysteresis), or a repulsive effect, making it more likely to perceive something else (adaptation). We used functional magnetic resonance imaging and modeling to elucidate how the brain entertains these 2 opposing processes, and what determines the direction of such experience-dependent perceptual effects. We found that although affecting our perception concurrently, hysteresis and adaptation map into distinct cortical networks: a widespread network of higher-order visual and fronto-parietal areas was involved in perceptual stabilization, while adaptation was confined to early visual areas. This areal and hierarchical segregation may explain how the brain maintains the balance between exploiting redundancies and staying sensitive to new information. We provide a Bayesian model that accounts for the coexistence of hysteresis and adaptation by separating their causes into 2 distinct terms: Hysteresis alters the prior, whereas adaptation changes the sensory evidence (the likelihood function).

Relative importance of second-order terms in relativistic dissipative fluid dynamics

Abstract: In Denicol et al. [Phys. Rev. D 85, 114047 (2012)], the equations of motion of relativistic dissipative fluid dynamics were derived from the relativistic Boltzmann equation. These equations contain a multitude of terms of second order in the Knudsen number, in the inverse Reynolds number, or their product. Terms of second order in the Knudsen number give rise to nonhyperbolic (and thus acausal) behavior and must be neglected in (numerical) solutions of relativistic dissipative fluid dynamics. The coefficients of the terms which are of the order of the product of Knudsen and inverse Reynolds numbers have been explicitly computed in the above reference, in the limit of a massless Boltzmann gas. Terms of second order in the inverse Reynolds number arise from the collision term in the Boltzmann equation, upon expansion to second order in deviations from the single-particle distribution function in local thermodynamical equilibrium. In this work, we compute these second-order terms for a massless Boltzmann gas with constant scattering cross section. Consequently, we assess their relative importance in comparison to the terms which are of the order of the product of the Knudsen and inverse Reynolds numbers.

Channeling and Radiation in Periodically Bent Crystals

Abstract: Introduction- Related Phenomena- Schemes for Periodic Bending of Crystals- Feasibility of a Positron-Based Crystalline Undulator- Positron-Based CU: Illustrative Material- CUs for Electrons and Heavy Particles- Experimental Studies of CUR - Stimulated Emission from CU- Conclusion - Motion in Periodically Bent Channel- Estimation of the Undulator Parameter due to Channeling- Poschl-Teller Potential- Interplanar Potential within the Moli'ere Approximation- References

Quantitative 4D analyses of epithelial folding during Drosophila gastrulation

Abstract: Understanding the cellular and mechanical processes that underlie the shape changes of individual cells and their collective behaviors in a tissue during dynamic and complex morphogenetic events is currently one of the major frontiers in developmental biology. The advent of high-speed time-lapse microscopy and its use in monitoring the cellular events in fluorescently labeled developing organisms demonstrate tremendous promise in establishing detailed descriptions of these events and could potentially provide a foundation for subsequent hypothesis-driven research strategies. However, obtaining quantitative measurements of dynamic shapes and behaviors of cells and tissues in a rapidly developing metazoan embryo using time-lapse 3D microscopy remains technically challenging, with the main hurdle being the shortage of robust imaging processing and analysis tools. We have developed EDGE4D, a software tool for segmenting and tracking membrane-labeled cells using multi-photon microscopy data. Our results demonstrate that EDGE4D enables quantification of the dynamics of cell shape changes, cell interfaces and neighbor relations at single-cell resolution during a complex epithelial folding event in the early Drosophila embryo. We expect this tool to be broadly useful for the analysis of epithelial cell geometries and movements in a wide variety of developmental contexts.