Annihilation Model Research Articles

Relieving Tensions Related to the Dark Matter Interpretation of the Fermi-LAT Data

Recently, many studies indicate that the GeV gamma ray excess signal from the central Milky Way can be best explained by ∼40–50 GeV dark matter annihilating via the b b ¯ channel. However, this model appears to be disfavored by the recent Fermi-LAT data for dwarf spheroidal galaxies and the constraint from synchrotron radiation. In this article, we describe a consistent picture to relieve the tensions between the dark matter annihilation model and the observations. We show that a baryonic feedback process is the key to alleviate the tensions and the ∼40–50 GeV dark matter model is still the best one to account for the GeV gamma ray excess in the Milky Way.

Trapped-Hole Diffusion in Photoexcited CdSe Nanorods

Surface charge-carrier traps are ubiquitous in colloidal semiconductor nanocrystals and fundamentally impact excited-state relaxation, making it critical to understand both their nature and their dynamics. Here, using photoluminescence upconversion and transient absorption spectroscopy, we study hole trapping and the dissociation between electrons and trapped holes in nonuniform CdSe nanorods and monitor their subsequent recombination dynamics. These recombination dynamics are described well with a diffusion–annihilation model wherein the trapped hole undergoes a random walk on the nanocrystal surface until it encounters the electron. This model fits the nonexponential excited-state decay over more than 7 orders of magnitude in time with a single adjustable parameter. The characterization of the spatial dynamics of trapped holes in CdSe nanostructures extends our fundamental understanding of excited-state dynamics in this important class of materials. The surface motion of trapped holes may have important...

Diffusion-reaction model for positron trapping and annihilation at spherical extended defects and in precipitate-matrix composites

The exact solution of a diffusion$-$reaction model for the trapping and annihilation of positrons in small extended spherical defects (clusters, voids, small precipitates) with competitive rate-limited trapping in vacancy-type point defects is presented. Closed-form expressions are obtained for the mean positron lifetime and for the intensities of the two positron lifetime components associated with trapping at defects. The exact solutions can be conveniently applied for the analysis of experimental data and allow an assessment in how far the usual approach, which takes diffusion limitation into account by means of effective diffusion-trapping rates, is appropriate. The model is further extended for application to larger precipitates where diffusion- and reaction limited trapping is not only considered for the trapping from the matrix into the precipitate$-$matrix interface but also for the trapping from inside the precipitates into the interfaces. This makes the model applicable to all type of composite structures where spherical objects are embedded in a matrix irrespective of their size and their number density.

Antiproton tagging and vertex fitting in a Timepix3 detector

Studies of antimatter are important for understanding our universe at a fundamental level. There are still unsolved problems, such as the matter-antimatter asymmetry in the universe. The AEgIS experiment at CERN aims at measuring the gravitational fall of antihydrogen in order to determine the gravitational force on antimatter. The proposed method will make use of a position-sensitive detector to measure the annihilation point of antihydrogen. Such a detector must be able to tag the antiproton, measure its time of arrival and reconstruct its annihilation point with high precision in the vertical direction. This work explores a new method for tagging antiprotons and reconstructing their annihilation point. Antiprotons from the Antiproton Decelerator at CERN were used to obtain data on direct annihilations on the surface of a silicon pixel sensor with Timepix3 readout. These data were used to develop and verify a detector response model for annihilation of antiprotons in this detector. Using this model and the antiproton data it is shown that a tagging efficiency of 50± 10% and a vertical position resolution of 22 ± 0.5 μm can be obtained.

What the Milky Way’s dwarfs tell us about the Galactic Center extended gamma-ray excess

The Milky Way's Galactic Center harbors a gamma-ray excess that is a candidate signal of annihilating dark matter. Dwarf galaxies remain predominantly dark in their expected commensurate emission. In this work we quantify the degree of consistency between these two observations through a joint likelihood analysis. In doing so we incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center extended gamma-ray excess (GCE) detected by the Fermi Gamma-Ray Space Telescope. The preferred range of annihilation rates and masses expands when including these unknowns. Even so, using two recent determinations of the Milky Way halo's local density leaves the GCE preferred region of single-channel dark matter annihilation models to be in strong tension with annihilation searches in combined dwarf galaxy analyses. A third, higher Milky Way density determination, alleviates this tension. Our joint likelihood analysis allows us to quantify this inconsistency. We provide a set of tools for testing dark matter annihilation models' consistency within this combined data set. As an example, we test a representative inverse Compton sourced self-interacting dark matter model, which is consistent with both the GCE and dwarfs.

Excitonic Emission of Monolayer Semiconductors Near-Field Coupled to High-Q Microresonators.

We present quantum yield measurements of single layer WSe2 (1L-WSe2) integrated with high-Q (Q > 106) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe2 in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor (Q > 106) below the bandgap of 1L-WSe2. The cavity quantum yield is QYc ≈ 10–3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe2 is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.

Baryon-antibaryon annihilation and reproduction in relativistic heavy-ion collisions

The quark rearrangement model for baryon-antibaryon annihilation and reproduction ($B\bar B\leftrightarrow 3M$) - incorporated in the Parton-Hadron-String Dynamics (PHSD) transport approach - is extended to the strangeness sector. A derivation of the transition probabilities for the three-body processes is presented and a strangeness suppression factor for the invariant matrix element squared is introduced to account for the higher mass of the strange quark compared to the light up and down quarks. In simulations of the baryon-antibaryon annihilation and reformation in a box with periodic boundary conditions we demonstrate that our numerical implementation fulfills detailed balance on a channel-by-channel basis for more than 2000 individual $2 \leftrightarrow 3$ channels. Furthermore, we study central Pb+Pb collisions within PHSD from 11.7$A$ GeV to 158$A$ GeV and investigate the impact of the additionally implemented reaction channels in the strangeness sector. We find that the new reaction channels have a visible impact essentially only on the rapidity spectra of antibaryons. The spectra with the additional channels in the strangeness sector are closer to the experimental data than without for all antihyperons. Due to the chemical redistribution between baryons/antibaryons and mesons we find a slightly larger production of antiprotons thus moderately overestimating the available experimental data. We additionally address the question if the antibaryon spectra (with strangeness) from central heavy-ion reactions at these energies provide further information on the issue of chiral symmetry restoration and deconfinement. However, by comparing transport results with/without partonic phase as well as including/excluding effects from chiral symmetry restoration we find no convincing signals in the strange antibaryon sector for either transition due to the strong final-state interactions.

A Vacancy Model of Pore Annihilation During Hot Isostatic Pressing of Single Crystals of Nickel-Base Superalloys

An improved diffusion model of pore annihilation during hot isostatic pressing of single crystals of nickel-base superalloys is proposed. The model considers dissolution of pores by emission of vacancies and their diffusion sink to low-angle boundaries. The calculation, which takes into account pore size distribution, predicts the kinetics of pore annihilation similar to experimental one.

Steady state of isolated systems versus microcanonical ensemble in cell model of particle creation and annihilation

A simple model of particle creation and annihilation in an isolated assembly of particles with conserved energy and fixed volume, the Cell Model (CM), is formulated. With increasing time, particle number distribution, obtained by averaging over many systems, approaches a time-independent, steady state (SS) distribution. Dependence of the SS distribution on creation and annihilation conditional reaction probabilities is studied. The results obtained for the SS are compared with predictions of statistical mechanics within the microcanonical ensemble (MCE). In general, the predictions of both models are different. They agree only if the creation and annihilation conditional reaction probabilities are equal. This condition also results in the detailed balance in the SS.

Possible Dark Matter Annihilation Signal in the AMS-02 Antiproton Data.

Using the latest AMS-02 cosmic-ray antiproton flux data, we search for a potential dark matter annihilation signal. The background parameters about the propagation, source injection, and solar modulation are not assumed apriori but based on the results inferred from the recent B/C ratio and proton data measurements instead. The possible dark matter signal is incorporated into the model self-consistently under a Bayesian framework. Compared with the astrophysical background-only hypothesis, we find that a dark matter signal is favored. The rest mass of the dark matter particles is ∼20-80 GeV, and the velocity-averaged hadronic annihilation cross section is about (0.2-5)×10^{-26} cm^{3} s^{-1}, in agreement with that needed to account for the Galactic center GeV excess and/or the weak GeV emission from dwarf spheroidal galaxies Reticulum 2 and Tucana III. Tight constraints on the dark matter annihilation models are also set in a wide mass region.

Synchrotron Emission from Dark Matter Annihilation: Predictions for Constraints from Non-detections of Galaxy Clusters with New Radio Surveys

Abstract The annihilation of dark matter particles is expected to yield a broad radiation spectrum via the production of Standard Model particles in astrophysical environments. In particular, electrons and positrons from dark matter annihilation produce synchrotron radiation in the presence of magnetic fields. Galaxy clusters are the most massive collapsed structures in the universe, and are known to host ∼μG-scale magnetic fields. They are therefore ideal targets to search for, or to constrain the synchrotron signal from dark matter annihilation. In this work, we use the expected sensitivities of several planned surveys from the next generation of radio telescopes to predict the constraints on dark matter annihilation models which will be achieved in the case of non-detections of diffuse radio emission from galaxy clusters. Specifically, we consider the Tier 1 survey planned for the Low Frequency Array (LOFAR) at 120 MHz, the Evolutionary Map of the Universe (EMU) survey planned for the Australian Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, and planned surveys for Aperture Tile in Focus (APERTIF) at 1.4 GHz. We find that, for massive clusters and dark matter masses , the predicted limits on the annihilation cross section would rule out vanilla thermal relic models for even the shallow LOFAR Tier 1, ASKAP, and APERTIF surveys.

Coupled relaxation channels of excitons in monolayer MoSe2.

Using ultrafast degenerate pump-probe spectroscopy, we have investigated the ultrafast exciton dynamics of monolayer MoSe2 at different pump fluences. The exciton-exciton annihilation, typically occurring tens of picoseconds after pump excitation, has been found to have a substantial correlation with the initial relaxation process dominated by the defect trapping of excitons. A new exciton-exciton annihilation model has been proposed by introducing a coupling term that accounts for the initial relaxation contribution. This coupling term can be tuned by varying the pump excitation intensity and at a high intensity it vanishes due to the full occupation of the defect states. At the same time, the final electron-hole recombination is found to be affected by the heat accumulation effect originating from the high intensity pump pulses.

Solutions of the generalized kinetic model of annihilation for a mixture of particles of two types

The evolution of the concentrations of particles of two types that annihilate at collision is considered. The kinetic model describing the dynamics of the mixture is represented by a system of two first-order nonlinear partial differential equations. It is shown that the solutions of this model are related to the solutions of the inhomogeneous transport equations by the Backlund transform. Analytic solutions of the problem about penetration of particles of the first type from the left half-plane into the right half-plane occupied by the particles of the second type (the two-dimensional penetration problem or molecular beam problem) and of the problem of outflow of the particles of the first type from a circular source into a domain occupied by the particles of the second type are obtained. Possible generalizations of the model are discussed.

Investigating the uniformity of the excess gamma rays towards the galactic center region

We perform a composite likelihood analysis of subdivided regions within the central 26° × 20° of the Milky Way, with the aim of characterizing the spectrum of the gamma-ray galactic center excess in regions of varying galactocentric distance. Outside of the innermost few degrees, we find that the radial profile of the excess is background-model dependent and poorly constrained. The spectrum of the excess emission is observed to extend upwards of 10 GeV outside ∼5° in radius, but cuts off steeply between 10–20 GeV only in the innermost few degrees. If interpreted as a real feature of the excess, this radial variation in the spectrum has important implications for both astrophysical and dark matter interpretations of the galactic center excess. Single-component dark matter annihilation models face challenges in reproducing this variation; on the other hand, a population of unresolved millisecond pulsars contributing both prompt and secondary inverse Compton emission may be able to explain the spectrum as well as its spatial dependency. We show that the expected differences in the photon-count distributions of a smooth dark matter annihilation signal and an unresolved point source population are an order of magnitude smaller than the fluctuations in residuals after fitting the data, which implies that mismodeling is an important systematic effect in point source analyses aimed at resolving the gamma-ray excess.

Trojan War displayed as a full annihilation–diffusion–reaction model

Abstract The diffusive pair annihilation model with embedded topological domains and archaeological data is applied in an analysis of the hypothetical Trojan–Greek war during the late Bronze Age. Estimations of parameter are explicitly made for critical dynamics of the model. In particular, the 8-metre walls of Troy could be viewed as the effective shield that provided the technological difference between the two armies. Suggestively, the numbers in The Iliad are quite sound, being in accord with Lanchester’s laws of warfare.

Comparing the influence of distinct kinds of temporal disorder in a low-dimensional absorbing transition model.

Recently it was stated that temporal disorder constitutes a relevant perturbation in absorbing phase transitions for all dimensions. However, its effect on systems other than the standard contact process (CP), its competition with other ingredients (e.g., particle diffusion), and other kinds of disorder (besides the standard types) are unknown. In order to shed some light on the above-mentioned points, we investigate a variant of the usual CP, namely, the triplet annihilation model, in which the competition between triplet annihilation and single particle diffusion leads to an unusual phase diagram behavior, with reentrant shape and endless activity for sufficiently large diffusion rates. Two kinds of time-dependent disorder have been considered. In the former, it is introduced in the creation-annihilation parameters (as commonly considered in recent studies), whereas in the latter, the diffusion rate D is allowed to be time dependent. In all cases, the disorder follows a uniform distribution with fixed mean and width σ. Two values of σ have been considered in order to exemplify the regime of "weaker" and "stronger" temporal disorder strengths. Our results show that in the former approach, the disorder suppresses the reentrant phase diagram with a critical behavior deviating from the directed percolation (DP) universality class in the regime of low diffusion rates, while they strongly suggest that the DP class is recovered for larger hopping rates. An opposite scenario is found in the latter disorder approach, with a substantial increase of reentrant shape and the maximum diffusion, in which the reentrant shape also displays a critical behavior consistent with the DP universality class (in similarity with the pure model). In order to compare with very recent claims, the results from taking a bimodal distribution and critical behavior in the limit of strong disorder are presented. Also, the results derived from the mean-field theory are performed, presenting partial agreement with numerical results. Lastly, a comparison with the diffusive disordered CP is undertaken.

Modeling morphology evolution and densification during solid-state sintering via kinetic Monte Carlo simulation

Microstructure control is an important subject in solid-state sintering and plays a crucial role in determining post-sintering material properties, such as strength, toughness and density, to name but a few. The preponderance of existing numerical sintering simulations model the morphology evolution and densification process driven by surface energy minimization by either dilating the particles to be sintered or using the vacancy annihilation model. Here, we develop a novel kinetic Monte Carlo model to model morphology evolution and densification during free sintering. Specifically, we derive analytically a heterogeneous densification rate of the sintering system by considering sintering stress induced mass transport. The densification of the system is achieved by modeling the sintering stress induced mass transfer via applying effective particle displacement and grain boundary migration with an efficient two-step iterative interfacial energy minimization procedure. Coarsening is also considered in the later stages of the simulations. We show that our model can accurately capture the diffusion-induced evolution of particle morphology, including neck formation and growth, as well as realistically reproduce the overall densification of the sintered material. The computationally obtained dynamic density evolution curves for both two-particle sintering and many-particle material sintering are found to be in excellent agreement with the corresponding experimental master sintering curves. Our model can be utilized to control a variety of structural and physical properties of the sintered materials, such as the pore size and final material density.

A Multi-frequency analysis of dark matter annihilation interpretations of recent anti-particle and γ-ray excesses in cosmic structures

The Fermi-LAT observation of a γ-ray excess from the galactic-centre, as well as the PAMELA, AMS, and AMS-2 anti-particle excesses, and the recent indications of a Fermi-LAT γ-ray excess in the Reticulum II dwarf galaxy have all been variously put forward as possible indirect signatures of supersymmetric neutralino dark matter. These are of particular interest as the neutralino annihilation models which fit these observations must have observable consequences across the frequency spectrum, from radio to γ-ray emission. Moreover, since dark matter is expected to be a major constituent of cosmic structure, these multi-frequency consequences should be common to such structures across the mass spectrum, from dwarf galaxies to galaxy clusters. Thus, in this work we make predictions for the multi-frequency spectra of three well-known sources dominated by dark matter on cluster, galaxy and dwarf galaxy scales, e.g. the Coma cluster, the galaxy M81, and the Draco dwarf galaxy, using models favoured by dark matter interpretations of the aforementioned observations. We pay special attention to the consequences for these models when their cross-sections are renormalised to reproduce the recent γ-ray excess observed in the Reticulum II dwarf galaxy, as well as using cross-sections from the Fermi-LAT dwarf galaxy limits, which throw a dark matter interpretation of this excess into doubt. We find that the multi-frequency data of Coma and Draco are in conflict with the dark matter interpretation of the AMS, PAMELA and Fermi positron excess. Additionally, models derived from Fermi-LAT galactic centre observations, and AMS-2 re-analysis, present similar but less extensive conflicts. Using the sensitivity projections for the Square Kilometre Array, the Cherenkov Telescope Array, as well as the ASTROGAM and ASTRO-H satellites, we determine the detection prospects for a subset of neutralino models that remain consistent with Planck cosmological constraints. Although the SKA has the greatest sensitivity to dark matter models, we demonstrate that ASTRO-H is well positioned to probe the inverse-Compton scattering emissions from neutralino annihilation and identify characteristics of the spectra which contain information about the neutralino mass and annihilation channel. This means that, given environments with favourable X-ray backgrounds, multi-frequency observation with the next generation of experiments will allow for unprecedented sensitivity to the neutralino parameter space as well as offsetting the individual weaknesses of each observation mode. Finally we show that all of the studied models can be better tested with the SKA phase 1.

Bright gamma-ray Galactic Center excess and dark dwarfs: Strong tension for dark matter annihilation despite Milky Way halo profile and diffuse emission uncertainties

We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. The GCE and dwarf tension can be alleviated if: one, the halo is very highly concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the GC; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, increasing the local dark matter density.

Positron-Electron Annihilation Process in (2,2)-Difluoropropane Molecule**Supported by the National Natural Science Foundation of China under Grant No. 11347011 and the Natural Science Foundation Project of Shandong Province under Grant No. ZR2011AM010 and 2014 Technology Innovation Fund of Ludong University under Grant Nos. 1d151007 and ld15l016

The positron-electron annihilation process in (2,2)-difluoropropane molecule and the corresponding gamma-ray spectra are studied by quantum chemistry method. The positrophilic electrons in (2,2)-difluoropropane molecule are found for the first time. The theoretical predictions show that the outermost 2s electrons of fluoride atoms play an important role in positron-electron annihilation process of (2,2)-difiuoropropane. In the present scheme, the correlation coefficient between the theoretical gamma-ray spectra and the experiments can be 99%. The present study gives an alternative annihilation model for positron-electron pair in larger molecules.