arXiv subjects

Torsten Bringmann

Publications and source records attributed to Torsten Bringmann.

At least 55 records · Page 3Linked to original sources

arXiv2018

Constraining self-interacting dark matter with scaling laws of observed halo surface densities

The observed surface densities of dark matter halos are known to follow a simple scaling law, ranging from dwarf galaxies to galaxy clusters, with a weak dependence on their virial mass. Here we point out that this can not only be used to provide a method to determine the standard relation between halo mass and concentration, but also to use large samples of objects in order to place constraints on dark matter self-interactions that can be more robust than constraints derived from individual objects. We demonstrate our method by considering a sample of about 50 objects distributed across the whole halo mass range, and by modelling the effect of self-interactions in a way similar to what has been previously done in the literature. Using additional input from simulations then results in a constraint on the self-interaction cross section per unit dark matter mass of about $σ/m_χ\lesssim 0.3$ cm$^2$/g. We expect that these constraints can be significantly improved in the future, and made more robust, by i) an improved modelling of the effect of self-interactions, both theoretical and by comparison with simulations, ii) taking into account a larger sample of objects and iii) by reducing the currently still relatively large uncertainties that we conservatively assign to the surface densities of individual objects. The latter can be achieved in particular by using kinematic observations to directly constrain the average halo mass inside a given radius, rather than fitting the data to a pre-selected profile and then reconstruct the mass. For a velocity-independent cross-section, our current result is formally already somewhat smaller than the range $0.5-5$ cm$^2$/g that has been invoked to explain potential inconsistencies between small-scale observations and expectations in the standard collisionless cold dark matter paradigm.

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arXiv2018

DarkBit: A GAMBIT module for computing dark matter observables and likelihoods

We introduce DarkBit, an advanced software code for computing dark matter constraints on various extensions to the Standard Model of particle physics, comprising both new native code and interfaces to external packages. This release includes a dedicated signal yield calculator for gamma-ray observations, which significantly extends current tools by implementing a cascade decay Monte Carlo, as well as a dedicated likelihood calculator for current and future experiments (gamlike). This provides a general solution for studying complex particle physics models that predict dark matter annihilation to a multitude of final states. We also supply a direct detection package that models a large range of direct detection experiments (DDcalc), and provides the corresponding likelihoods for arbitrary combinations of spin-independent and spin-dependent scattering processes. Finally, we provide custom relic density routines along with interfaces to DarkSUSY, micrOMEGAs, and the neutrino telescope likelihood package nuLike. DarkBit is written in the framework of the Global And Modular Beyond the Standard Model Inference Tool (GAMBIT), providing seamless integration into a comprehensive statistical fitting framework that allows users to explore new models with both particle and astrophysics constraints, and a consistent treatment of systematic uncertainties. In this paper we describe its main functionality, provide a guide to getting started quickly, and show illustrative examples for results obtained with DarkBit (both as a standalone tool and as a GAMBIT module). This includes a quantitative comparison between two of the main dark matter codes (DarkSUSY and micrOMEGAs), and application of DarkBit's advanced direct and indirect detection routines to a simple effective dark matter model.

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arXiv2017

GAMBIT: The Global and Modular Beyond-the-Standard-Model Inference Tool

We describe the open-source global fitting package GAMBIT: the Global And Modular Beyond-the-Standard-Model Inference Tool. GAMBIT combines extensive calculations of observables and likelihoods in particle and astroparticle physics with a hierarchical model database, advanced tools for automatically building analyses of essentially any model, a flexible and powerful system for interfacing to external codes, a suite of different statistical methods and parameter scanning algorithms, and a host of other utilities designed to make scans faster, safer and more easily-extendible than in the past. Here we give a detailed description of the framework, its design and motivation, and the current models and other specific components presently implemented in GAMBIT. Accompanying papers deal with individual modules and present first GAMBIT results. GAMBIT can be downloaded from gambit.hepforge.org.

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arXiv2017

Electroweak and Higgs Boson Internal Bremsstrahlung: General considerations for Majorana dark matter annihilation and application to MSSM neutralinos

It is well known that the annihilation of Majorana dark matter into fermions is helicity suppressed. Here, we point out that the underlying mechanism is a subtle combination of two distinct effects, and we present a comprehensive analysis of how the suppression can be partially or fully lifted by the internal bremsstrahlung of an additional boson in the final state. As a concrete illustration, we compute analytically the full amplitudes and annihilation rates of supersymmetric neutralinos to final states that contain any combination of two standard model fermions, plus one electroweak gauge boson or one of the five physical Higgs bosons that appear in the minimal supersymmetric standard model. We classify the various ways in which these three-body rates can be large compared to the two-body rates, identifying cases that have not been pointed out before. In our analysis, we put special emphasis on how to avoid the double counting of identical kinematic situations that appear for two-body and three-body final states, in particular on how to correctly treat differential rates and the spectrum of the resulting stable particles that is relevant for indirect dark matter searches. We find that both the total annihilation rates and the yields can be significantly enhanced when taking into account the corrections computed here, in particular for models with somewhat small annihilation rates at tree-level which otherwise would not be testable with indirect dark matter searches. Even more importantly, however, we find that the resulting annihilation spectra of positrons, neutrinos, gamma-rays and antiprotons differ in general substantially from the model-independent spectra that are commonly adopted, for these final states, when constraining particle dark matter with indirect detection experiments.

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arXiv2017

ETHOS - An Effective Theory of Structure Formation: From dark particle physics to the matter distribution of the Universe

We formulate an effective theory of structure formation (ETHOS) that enables cosmological structure formation to be computed in almost any microphysical model of dark matter physics. This framework maps the detailed microphysical theories of particle dark matter interactions into the physical effective parameters that shape the linear matter power spectrum and the self-interaction transfer cross section of non-relativistic dark matter. These are the input to structure formation simulations, which follow the evolution of the cosmological and galactic dark matter distributions. Models with similar effective parameters in ETHOS but with different dark particle physics would nevertheless result in similar dark matter distributions. We present a general method to map an ultraviolet complete or effective field theory of low energy dark matter physics into parameters that affect the linear matter power spectrum and carry out this mapping for several representative particle models. We further propose a simple but useful choice for characterizing the dark matter self-interaction transfer cross section that parametrizes self-scattering in structure formation simulations. Taken together, these effective parameters in ETHOS allow the classification of dark matter theories according to their structure formation properties rather than their intrinsic particle properties, paving the way for future simulations to span the space of viable dark matter physics relevant for structure formation.

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arXiv2017

Strong constraints on self-interacting dark matter with light mediators

Coupling dark matter to light new particles is an attractive way to combine thermal production with strong velocity-dependent self-interactions. Here we point out that in such models the dark matter annihilation rate is generically enhanced by the Sommerfeld effect, and we derive the resulting constraints from the Cosmic Microwave Background and other indirect detection probes. For the frequently studied case of s-wave annihilation these constraints exclude the entire parameter space where the self-interactions are large enough to address the small-scale problems of structure formation.

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arXiv2017

Novel Spectral Features in MeV Gamma Rays from Dark Matter

Astrophysical searches for gamma rays are one of the main strategies to probe the annihilation or decay of dark matter particles. We present a new class of distinct sub-GeV spectral features that generically appear in kinematical situations where the available center-of-mass energy in such processes is just above threshold to produce excited meson states. Using a Fisher forecast with realistic astrophysical backgrounds, we demonstrate that for upcoming experiments like e-ASTROGAM and ComPair these signals can turn out to be the smoking gun in the search for particle dark matter.

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arXiv2016

Suppressing structure formation at dwarf galaxy scales and below: late kinetic decoupling as a compelling alternative to warm dark matter

Warm dark matter cosmologies have been widely studied as an alternative to the cold dark matter paradigm, the characteristic feature being a suppression of structure formation on small cosmological scales. A very similar situation occurs if standard cold dark matter particles are kept in local thermal equilibrium with a, possibly dark, relativistic species until the universe has cooled down to keV temperatures. We perform a systematic phenomenological study of this possibility, and classify all minimal models containing dark matter and an arbitrary radiation component that allow such a late kinetic decoupling. We recover explicit cases recently discussed in the literature and identify new classes of examples that are very interesting from a model-building point of view. In some of these models dark matter is inevitably self-interacting, which is remarkable in view of recent observational support for this possibility. Hence, dark matter models featuring late kinetic decoupling have the potential not only to alleviate the missing satellites problem but also to address other problems of the cosmological concordance model on small scales, in particular the cusp-core and too-big-too-fail problems, in some cases without invoking any additional input.

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arXiv2016

Ultracompact minihalos as probes of inflationary cosmology

Cosmological inflation generates primordial density perturbations on all scales, including those far too small to contribute to the cosmic microwave background. At these scales, isolated ultracompact minihalos of dark matter can form well before standard structure formation, if the perturbations have sufficient amplitude. Minihalos affect pulsar timing data and are potentially bright sources of gamma rays. The resulting constraints significantly extend the observable window of inflation in the presence of cold dark matter, coupling two of the key problems in modern cosmology.

astro-ph.CO↗

arXiv2016

ETHOS - An Effective Theory of Structure Formation: Dark matter physics as a possible explanation of the small-scale CDM problems

We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of $2.8\times 10^4\,{\rm M}_\odot$ and a softening length of $72.4\,{\rm pc}$. We demonstrate that all alternative models have only a negligible impact on large scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small scale primordial damping in the power spectrum and late time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even over-solve the too-big-to-fail problem; thus ruling them out.

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arXiv2016

Thermal decoupling of WIMPs from first principles

Weakly interacting massive particles (WIMPs) are arguably the most natural DM candidates from a particle physics point of view. After their number density has frozen out in the early universe, determining their relic density today, WIMPs are still kept very close to thermal equilibrium by scattering events with standard model particles. The complete decoupling from the thermal bath happens as late as around 1-10 MeV, thereby setting an important cosmological scale that can directly be translated into a small-scale cutoff of the spectrum of matter density fluctuations. We present here a full analytic treatment of the kinetic decoupling process from first principles. This allows an exact determination of the decoupling scale, for an arbitrary WIMP candidate and any scattering amplitude. As an application, we consider the situation of the lightest supersymmetric particle as well as the lightest Kaluza-Klein particle that arises in theories with universal extra dimensions; furthermore, we show that our formalism can also easily be applied to, e.g., the evolution of the non-relativistic electrons into the recombination regime. Finally, we comment on the impacts for the smallest gravitationally bound structures and the prospects for the indirect detection of dark matter.

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arXiv2016

Leading QCD Corrections for Indirect Dark Matter Searches: a Fresh Look

The annihilation of non-relativistic dark matter particles at tree level can be strongly enhanced by the radiation of an additional gauge boson. This is particularly true for the helicity-suppressed annihilation of Majorana particles, like neutralinos, into fermion pairs. Surprisingly, and despite the potentially large effect due to the strong coupling, this has so far been studied in much less detail for the internal bremsstrahlung of gluons than for photons or electroweak gauge bosons. Here, we aim at bridging that gap by presenting a general analysis of neutralino annihilation into quark anti-quark pairs and a gluon, allowing e.g. for arbitrary neutralino compositions and keeping the leading quark mass dependence at all stages in the calculation. We find in some cases largely enhanced annihilation rates, especially for scenarios with squarks being close to degenerate in mass with the lightest neutralino, but also notable distortions in the associated antiproton and gamma-ray spectra. Both effects significantly impact limits from indirect searches for dark matter and are thus important to be taken into account in, e.g., global scans. For extensive scans, on the other hand, full calculations of QCD corrections are numerically typically too expensive to perform for each point in parameter space. We present here for the first time an efficient, numerically fast implementation of QCD corrections, extendable in a straight-forward way to non-supersymmetric models, which avoids computationally demanding full one-loop calculations or event generator runs and yet fully captures the leading effects relevant for indirect dark matter searches. In this context, we also present updated constraints on dark matter annihilation from cosmic-ray antiproton data. Finally, we comment on the impact of our results on relic density calculations.

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arXiv2014

Updated cosmic-ray and radio constraints on light dark matter: Implications for the GeV gamma-ray excess at the Galactic center

The apparent gamma-ray excess in the Galactic center region and inner Galaxy has attracted considerable interest, notably because both its spectrum and radial distribution are consistent with an interpretation in terms of annihilating dark matter particles with a mass of about 10-40 GeV. We confront such an interpretation with an updated compilation of various indirect dark matter detection bounds, which we adapt to the specific form required by the observed signal. We find that cosmic-ray positron data strongly rule out dark matter annihilating to light leptons, or 'democratically' to all leptons, as an explanation of the signal. Cosmic-ray antiprotons, for which we present independent and significantly improved limits with respect to previous estimates, are already in considerable tension with DM annihilation to any combination of quark final states; the first set of AMS-02 data will thus be able to rule out or confirm the DM hypothesis with high confidence. For reasonable assumptions about the magnetic field in the Galactic center region, radio observations independently put very severe constraints on a DM interpretation of the excess, in particular for all leptonic annihilation channels.

astro-ph.HE↗

arXiv2014

Enhanced Line Signals from Annihilating Kaluza-Klein Dark Matter

Monochromatic gamma ray lines have long been known to provide potential smoking gun signals for annihilating dark matter. Here, we demonstrate that the situation is particularly interesting for Kaluza-Klein dark matter because resonant annihilation is generically expected for small, but not necessarily vanishing relative velocities of the annihilating particles. We calculate the contribution from those hitherto neglected resonances and show that the annihilation rate into monochromatic photons can be significantly enhanced, in a way that is much more pronounced than for the associated production of continuum photons. For favorable astrophysical conditions, this leads to promising prospects for the detection of TeV-scale Kaluza-Klein dark matter. We also point out that the situation may be even more interesting in the vicinity of black holes, like the supermassive black hole at the center of our Galaxy, where in principle center-of-mass energies much larger than the rest mass are available. In this case, annihilating Kaluza-Klein dark matter may show the striking and unique signature of several gamma ray lines, with an equidistant spacing corresponding to twice the compactification radius of the extra dimension.

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arXiv2014

Fermi LAT Search for Internal Bremsstrahlung Signatures from Dark Matter Annihilation

A commonly encountered obstacle in indirect searches for galactic dark matter is how to disentangle possible signals from astrophysical backgrounds. Given that such signals are most likely subdominant, the search for pronounced spectral features plays a key role for indirect detection experiments; monochromatic gamma-ray lines or similar features related to internal bremsstrahlung, in particular, provide smoking gun signatures. We perform a dedicated search for the latter in the data taken by the Fermi gamma-ray space telescope during its first 43 months. To this end, we use a new adaptive procedure to select optimal target regions that takes into account both standard and contracted dark matter profiles. The behaviour of our statistical method is tested by a subsampling analysis of the full sky data and found to reproduce the theoretical expectations very well. The limits on the dark matter annihilation cross-section that we derive are stronger than what can be obtained from the observation of dwarf galaxies and, at least for the model considered here, collider searches. While these limits are still not quite strong enough to probe annihilation rates expected for thermally produced dark matter, future prospects to do so are very good. In fact, we already find a weak indication, with a significance of 3.1σ(4.3σ) when (not) taking into account the look-elsewhere effect, for an internal bremsstrahlung-like signal that would correspond to a dark matter mass of \sim150 GeV; the same signal is also well fitted by a gamma-ray line at around 130 GeV. Although this would be a fascinating possibility, we caution that a much more dedicated analysis and additional data will be necessary to rule out or confirm this option.

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arXiv2014

Tight bonds between sterile neutrinos and dark matter

Despite the astonishing success of standard $Λ$CDM cosmology, there is mounting evidence for a tension with observations at small and intermediate scales. We introduce a simple model where both cold dark matter (DM) and sterile neutrinos are charged under a new $U(1)_X$ gauge interaction. The resulting DM self-interactions resolve the tension with the observed abundances and internal density structures of dwarf galaxies. At the ame time, the sterile neutrinos can account for both the small hot DM component favored by cosmological observations and the neutrino anomalies found in short-baseline experiments.

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arXiv2014

Significant Enhancement of Neutralino Dark Matter Annihilation from Electroweak Bremsstrahlung

Indirect searches for the cosmological dark matter have become ever more competitive during the past years. Here, we report the first full calculation of leading electroweak corrections to the annihilation rate of supersymmetric neutralino dark matter. We find that these corrections can be huge, partially due to contributions that have been overlooked so far. Our results imply a significantly enhanced discovery potential of this well motivated dark matter candidate with current and upcoming cosmic ray experiments, in particular for gamma rays and models with somewhat small annihilation rates at tree level.

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arXiv2014

Constraining dark matter annihilation with the isotropic $γ$-ray background: updated limits and future potential

The nature of the Isotropic $γ$-ray Background (IGRB) measured by the Large Area Telescope (LAT) on the Fermi $γ$-ray space Telescope ({\it Fermi}) remains partially unexplained. Non-negligible contributions may originate from extragalactic populations of unresolved sources such as blazars, star-forming galaxies or galactic milli-second pulsars. A recent prediction of the diffuse $γ$-ray emission from Active Galactic Nuclei (AGN) with a large viewing angle with respect to the line-of-sight (l.o.s.) has demonstrated that this faint but numerous population is also expected to contribute significantly to the total IGRB intensity. A more exotic contribution to the IGRB invokes the pair annihilation of dark matter (DM) weakly interacting massive particles (WIMPs) into $γ$-rays. In this work, we evaluate the room left for galactic DM at high latitudes ($>10^\circ$) by including photons from both prompt emission and inverse Compton scattering, emphasizing the impact of the newly discovered contribution from misaligned AGN (MAGN) for such an analysis. Summing up all significant galactic and extragalactic components of the IGRB, we find that an improved understanding of the associated astrophysical uncertainties is still mandatory to put stringent bounds on thermally produced DM. On the other hand, we also demonstrate that the IGRB has the potential to be one of the most competitive {\it future} ways to test the DM WIMP hypothesis, once the present uncertainties are even slightly reduced. In fact, if MAGN contribute even at 90% of the maximal level consistent with our current understanding, thermally produced WIMPs would be severely constrained as DM candidates for masses up to several TeV.

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