730 research outputs found
Reannihilation of self-interacting dark matter
We explore the phenomenology of having a second epoch of dark matter
annihilation into dark radiation long after the standard thermal freeze-out.
Such a hidden reannihilation process could affect visible sectors only
gravitationally. As a concrete realization we consider self-interacting dark
matter (SIDM) with a light force mediator coupled to dark radiation. We
demonstrate how resonantly Sommerfeld enhanced cross sections emerge to induce
the reannihilation epoch. The effect is a temporally local modification of the
Hubble expansion rate and we show that the Cosmic Microwave Background (CMB)
measurements -- as well as other observations -- have a high sensitivity to
observe this phenomenon. Special attention is given to the model region where
late kinetic decoupling and strong self-interactions can alleviate several
small-scale problems in the cold dark matter paradigm at the same time.
Interestingly, we find that reannihilation might here also simultaneously lower
the tension between CMB and low-redshift astronomical observations of
and . Moreover, we identify reannihilation as a clear signature to
discriminate between the phenomenologically otherwise almost identical vector
and scalar mediator realizations of SIDM.Comment: Version submitted to journal. Extended with improved estimates and
updated plots. 25 pages, 9 figures and 2 table
Dark Matter Sommerfeld-enhanced annihilation and Bound-state decay at finite temperature
Traditional computations of the dark matter (DM) relic abundance, for models
where attractive self-interactions are mediated by light force-carriers and
bound states exist, rely on the solution of a coupled system of classical
on-shell Boltzmann equations. This idealized description misses important
thermal effects caused by the tight coupling among force-carriers and other
charged relativistic species. We develop for the first time a comprehensive
ab-initio derivation for the description of DM long-range interactions in the
presence of a hot and dense plasma background directly from non-equilibrium
quantum field theory. Most importantly, the scattering and bound states get
strongly mixed in the thermal plasma environment, representing a characteristic
difference from a pure vacuum theory computation. The main result of this work
is a novel differential equation for the DM number density, written down in a
form which is manifestly independent under the choice of what one would
interpret as a bound or a scattering state at finite temperature. The collision
term, unifying the description of annihilation and bound state decay, turns out
to have in general a non-quadratic dependence on the DM number density. This
generalizes the form of the conventional Lee-Weinberg equation which is
typically adopted to describe the freeze-out process. We prove that our general
number density equation is consistent with previous literature results under
certain limits.Comment: 46 pages, 10 figure
Functional MRI for the early detection of Parkinsonâs disease: ROI analysis of the putamen in a population at risk of developing Parkinsonâs disease
Background: While the concept of prodromal Parkinsonâs disease (PD) is well established, reliable markers for the diagnosis of this disease stage are still lacking. We investigated the functional connectivity of the putamina in a resting-state functional MRI analysis in persons with at least two prodromal factors for PD, which is considered a high risk for PD (HRPD) group, in comparison to PD patients and controls.
Methods: We included 16 PD patients, 20 healthy controls and 20 HRPD subjects. Resting state echo planar images and anatomical T1-weighted images were acquired with a Siemens Prisma 3 Tesla scanner. The computation of correlation maps of the left and the right puta-men to the rest of the brain was done in a voxel-wise approach using the REST toolbox. Finally, group differences in the correlation maps were compared on voxel-level and summarized in cluster z-statistics.
Results: Compared to both PD patients and healthy controls, the HRPD group showed higher functional connectivity of both putamina to brain regions involved in execution of motion and coordination (cerebellum, vermis, pre- and postcentral gyrus, supplementary motor area) as well as the planning of movement (precuneus, cuneus, superior medial frontal lobe).
Conclusions: Higher functional connectivity of the putamina of HRPD subjects to other brain regions involved in motor execution and planning may indicate a compensatory mechanism. Follow-up evaluation and independent longitudinal studies should test whether our results re-flect a dynamic process associated with a prodromal PD state
Measurements of grain boundary networks in deep polar ice cores - A digital image processing approach
Ice covers a significant part of the Earthâs surface and is one key component of the global climate system. A thorough understanding of ice flow is crucial for modeling the response of ice sheets to past and upcoming climate changes. Glenâs law, a experimentally derived exponential relationship between stress and strain rate, is usually applied. However, it does not adequately capture microstructural changes observed for high total shear strains and long time scales which cannot be reproduced by laboratory experiments. Deep ice cores provide insights into the natural evolution of microstructure (grain boundary networks). Large sets of sublimation groove images, mapping grain boundaries in high resolution, are available along the EDML (East Antarctica) and NEEM (Greenland) ice cores. A digital image processing approach has been developed to derive grain size, grain shape, shape of grain boundaries, and density of sub-grain boundaries in a consistent way. An automatic assignment to c-axes orientation measurements allows estimating the error of calculated parameters. Depth profiles along both ice cores are presented and interpreted based on variations in impurity content, temperature differences, and involved time scales. Furthermore, the presence of effective negative pressures caused by air bubbles and clathrate hydrates is taken into account
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