Dark matter self-interactions from a general spin-0 mediator

Dark matter particles interacting via the exchange of very light spin-0 mediators can have large self-interaction rates and obtain their relic abundance from thermal freeze-out. At the same time, these models face strong bounds from direct and indirect probes of dark matter as well as a number of constraints on the properties of the mediator. We investigate whether these constraints can be consistent with having observable effects from dark matter self-interactions in astrophysical systems. For the case of a mediator with purely scalar couplings we point out the highly relevant impact of low-threshold direct detection experiments like CRESST-II, which essentially rule out the simplest realization of this model. These constraints can be significantly relaxed if the mediator has CP-violating couplings, but then the model faces strong constraints from CMB measurements, which can only be avoided in special regions of parameter space.Comment: 20 pages, 5 figures + appendices. v2: Matches published version. v3: fixed typographical mistake in eq. (2.8). v4: fixed typographical mistake in eq. (C.4

Deterministic and stochastic dynamics in bacterial systems

Microorganisms form an essential part of our biosphere and represent roughly 14 percent of the biomass on earth. In spite of this abundance, the majority of chemical and physical processes governing the live of microorganisms remain poorly understood. In this work, we focus on three different phenomena from the realm of microorganisms and aim to explain the physical processes behind them. We examine how the bacterium Shewanella Putrefaciens exploits a mechanical instability to wrap its flagellum around its cell body, effectively forming a screw that allows the bacterium to escape from traps. Based on a numerical model we study the onset of screw formation in dependence of the flagellar geometry and the existence of multiple equilibrium configurations of the flagellum. Furthermore, we study the effects of actively swimming microorganisms on the diffusion of passive tracer particles. By means of a numerical simulation we examine a single swimmer-tracer interaction and use the results to develop a model based on continuous time random walks that captures a series of swimmer-tracer interactions. We derive an analytical expression for the one dimensional probability density function of the tracer displacements and use numerical simulations to approximate the two- and three-dimensional distributions. We then extend the model to include periods of free tracer diffusion between the tracerswimmer interactions and fit this extended model to a number of experimentally observed tracer distributions. In the third part of this work we examine how the cylindrical shape of a bacterium affects the isotropic trajectories of membrane proteins when observed with a microscope. We derive an analytical expression for the anisotropic distribution of the particle displacements when projected in the observation plane and use this result to calculate the mean squared displacement curves. Finally, we use numerical simulations to study the effects of a limited focus depth and to understand the resulting challenges for the estimation of the diffusion coefficients

On the interpretation of dark matter self-interactions in Abell 3827

Self-interactions of dark matter particles can potentially lead to an observable separation between the dark matter halo and the stars of a galaxy moving through a region of large dark matter density. Such a separation has recently been observed in a galaxy falling into the core of the galaxy cluster Abell 3827. We estimated the DM self-interaction cross section needed to reproduce the observed effects and find that the sensitivity of Abell 3827 has been significantly overestimated in a previous study. Our corrected estimate is $\tilde{\sigma}/m_\text{DM} \sim 3\:\text{cm}^2\:\text{g}^{-1}$ when self-interactions result in an effective drag force and $\sigma/m_\text{DM} \sim 1.5\:\text{cm}^2\:\text{g}^{-1}$ for the case of contact interactions, in some tension with previous upper bounds.Comment: 5 pages, 3 figures; v2: matches published versio

Objektivierung von funktionellen und strukturellen Beeinträchtigungen sensorischer Afferenzen bei neuroimmunologischen Krankheitsbildern

In dieser kumulativen Habilitationsschrift werden eigene Arbeiten zusammengefasst, die sich thematisch mit der Objektivierung sensorischer Afferenzstörungen bei neuroimmunologischen Krankheitsbildern befassen. B-mode Ultraschall, eine in der klinischen Routine weit verbreitete Methode, wurde als objektives Untersuchungsverfahren zur Beurteilung der Pupillenfunktion etabliert, Normwerte von Pupillendurchmessern und Pupillenkonstriktionszeiten wurden für verschiedene Altersgruppen publiziert. Das frühzeitige Erkennen und Behandeln einer ON ist entscheidend, um irreversible Schäden des N. opticus zu verhindern. Eine Funktionsstörung des afferenten visuellen Systems bei Patienten mit ON ließ sich gut mittels B-mode Ultraschall objektivieren und quantifizieren. B-mode Ultraschall könnte als bildgebender Biomarker für ein RAPD und als Surrogat-Parameter für eine Läsion des N. opticus in klinischen Studien und in der Routinediagnostik Anwendung finden. In einer weiteren Studie wurde die visuelle Lebensqualität bei NMOSD und MS Patienten mit Zustand nach ON untersucht. Die Einschränkung an visueller Lebensqualität korrelierte mit dem Ausmaß an funktioneller und struktureller Schädigung des afferenten visuellen Systems, gemessen mittels OCT und Visus. Die Integrität des afferenten olfaktorischen Systems wurde bei der NMOSD, PPMS und AE in eigenen Arbeiten erstmals untersucht. Zusammenfassend erwies sich das Riechvermögen bei diesen seltenen neuroimmunoloigschen Krankheitsbildern als deutlich eingeschränkt, wobei wir dabei von unterschiedlichen Pathomechanismen ausgehen. Riechstörungen sind möglicherweise ein weiteres Symptom dieser phänotypisch sehr heterogenen Krankheitsbilder. Eine niederschwellige Riechtestung im Rahmen der differentialdiagnostischen Zuordnung sollte bei den untersuchten Krankheitsbildern in Erwägung gezogen werden

Colliding clusters and dark matter self-interactions

When a dark matter halo moves through a background of dark matter particles, self-interactions can lead to both deceleration and evaporation of the halo and thus shift its centroid relative to the collisionless stars and galaxies. We study the magnitude and time evolution of this shift for two classes of dark matter self-interactions, viz. frequent self-interactions with small momentum transfer (e.g. due to long-range interactions) and rare self-interactions with large momentum transfer (e.g. contact interactions), and find important differences between the two cases. We find that neither effect can be strong enough to completely separate the dark matter halo from the galaxies, if we impose conservative bounds on the self-interaction cross-section. The majority of both populations remain bound to the same gravitational potential and the peaks of their distributions are therefore always coincident. Consequently any apparent separation is mainly due to particles which are leaving the gravitational potential, so will be largest shortly after the collision but not observable in evolved systems. Nevertheless the fraction of collisions with large momentum transfer is an important characteristic of self-interactions, which can potentially be extracted from observational data and provide an important clue as to the nature of dark matter.Comment: 13 pages + appendices, 8 figures, v2: minor corrections, references added - matches published versio

Interference in vehicle-to-vehicle communication networks - analysis, modeling, simulation and assessment

In wireless vehicular communication networks the periodic transmission of status updates by all vehicles represents a basic service primitive, in particular for safety related applications. Due to the limited communication resources the question raises how much data each node may provide such that the quality of service required by applications can still be guaranteed under realistic interference conditions. Local broadcasts capacity is introduced and analyzed to tackle this open question

Parametric Representation of Tactile Numerosity in Working Memory

Estimated numerosity perception is processed in an approximate number system (ANS) that resembles the perception of a continuous magnitude. The ANS consists of a right lateralized frontoparietal network comprising the lateral prefrontal cortex (LPFC) and the intraparietal sulcus. Although the ANS has been extensively investigated, only a few studies have focused on the mental representation of retained numerosity estimates. Specifically, the underlying mechanisms of estimated numerosity working memory (WM) is unclear. Besides numerosities, as another form of abstract quantity, vibrotactile WM studies provide initial evidence that the right LPFC takes a central role in maintaining magnitudes. In the present fMRI multivariate pattern analysis study, we designed a delayed match-to-numerosity paradigm to test what brain regions retain approximate numerosity memoranda. In line with parametric WM results, our study found numerosity-specific WM representations in the right LPFC as well as in the supplementary motor area and the left premotor cortex extending into the superior frontal gyrus, thus bridging the gap in abstract quantity WM literature