Total Annihilation Rate Research Articles

Longitudinal-optical-phonon resonant thermal emission efficiency and spectrum control of metal-GaAs surface micro-stripe structures

Phonon-based tera-Hertz or mid-infrared emission has been investigated because it can be applied to high-efficiency light sources for optical communication and material diagnostics, where thermal energy can be utilized as an energy source of emission. Particularly, nano- and micro-surface structures have been investigated for versatile emission controllability. We present the polarized longitudinal optical (LO) phonon resonant thermal emission of radiation (LORE) peaked at approximately 8.5 THz in a range of 450–650 K from simple surface Au–GaAs–Au micro-stripe structures on undoped (u-) GaAs wafers, where the metal component, which has been avoided for surface phonon polariton (SPhP) emission, is introduced. This LORE is based on electric dipoles formed by coherently oscillating polarization charges on pairs of interfaces and has a feature of structure size or emission direction-independent frequency, which is quite different from the properties of SPhPs. The dependence of emission properties on structural factors and temperature reveals the approximately evenly matched radiative and nonradiative LO-phonon annihilation rates in the local surface field and shows the controllability of emission and absorption balance based on the surface structures. Higher emission efficiency in the higher temperature region is found for structures with narrow emission windows of less than 2 μm. The decrease in the volume occupied by electric dipoles reduces the reabsorption probability of LORE, which shortens the effective radiative lifetime and, resultantly, increases the fraction of the radiative rate in the total annihilation rate. By comparing the emission from a pure graphite wafer, a short LORE mean time interval of approximately 2 ps is found. The background emission subject to Planck’s law is significantly reduced particularly in the region of 250–300 cm−1 using emission window widths narrower than 2 μm because of the sub-diffraction phenomenon.

Nucleosynthesis constraints through γ-ray line measurements from classical novae

Context. Classical novae belong to the most frequent transient events in the Milky Way and are key agents of ongoing nucleosynthesis. Despite their large numbers, they have never been observed in soft γ-ray emission. Measurements of their γ-ray signatures would provide insights into explosion mechanism and nucleosynthesis products. Aims. Our goal is to constrain the ejecta masses of 7Be and 22Na from classical novae through their γ-ray line emissions at 478 and 1275 keV. Methods. We extracted posterior distributions on the line fluxes from archival data of the INTEGRAL/SPI spectrometer telescope. We then used a Bayesian hierarchical model to link individual objects and diffuse emission, and to infer ejecta masses from the whole population of classical novae in the Galaxy. Results. Individual novae are too dim to be detectable in soft γ-rays, and the upper bounds on their flux and ejecta mass uncertainties cover several orders of magnitude. Within the framework of our hierarchical model, we can nevertheless infer tight upper bounds on the 22Na ejecta masses, given all uncertainties from individual objects as well as diffuse emission, of < 2.0 × 10−7 M⊙ (99.85th percentile). Conclusions. In the context of ONe nucleosynthesis, the 22Na bounds are consistent with theoretical expectations and exclude that most ONe novae occur on white dwarfs with masses of about 1.35 M⊙. The upper bounds from 7Be are uninformative. From the combined ejecta mass estimate of 22Na and its β+ decay, we infer a positron production rate of < 5.5 × 1042 e+ s−1, which would mean 10% at most of the total annihilation rate in the Milky Way.

Effect of energetic disorder on triplet-triplet annihilation in organic semiconductors

We present a model for the triplet-triplet annihilation in organic phosphorescent host-guest systems that considers the effect of the energetic disorder on the kinetics of the annihilation. Nonequilibrium (time-dependent) transport of the triplets that arises from the progressive relaxation of triplets in the energy landscape is taken into account in the model. Triplet excitons are considered to move between the guest sites (dye molecules) via the thermally activated tunneling mechanism; the annihilation step, however, takes place through the F\"orster mechanism. Based on the model developed, and by calculating the time evolution of the triplet concentration, we investigate the effect of the various parameters on the effective annihilation coefficient. At low dye concentrations, it is shown that relaxation of the triplets is not completed during their lifetimes, and therefore, the annihilation process occurs entirely in the nonequilibrium. We also address the competition between two pathways for the annihilation process: single-step, long-range annihilation, and diffusion-assisted annihilation. By a quantitative comparison with experimental data reported in the literature, we demonstrate certain conditions in which there is a considerable contribution from the diffusion to the total annihilation rate. The model presented in this work can also be used for the excitons that diffuse by the F\"orster mechanism.

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.

Electron-Positron Annihilation in Ultra-Strong Magnetic Fields. Comparison of One- and Two-Photon Annihilation at Middly Relativistic Regime

We consider the oneand two-photon e+e− annihilation processes in an ultra-strong magnetic eld, at middly relativistic regime. Such conditions are reached in neutron stars and especially in magnetars, where electrons and positrons ow within the magnetar corona with average momenta of the order of p ∼ m0c, where m0 electron mass, c light speed. We pay special attention to the ratio of the total annihilation rates of both processes. The well-known result is that in the limit p → 0 and for magnetic induction above the critical Schwinger value B0 = 4.41×10 T, one-photon annihilation dominates over the two-photon process. Results presented in this article verify the current knowledge about this ratio in magnetars; the calculations indicate that for particles moving with middly relativistic momenta both processes can be equally important.

Galactic-centre gamma rays in CMSSM dark matter scenarios

We study the production of γ rays via LSP annihilations in the core of the Galaxy as a possible experimental signature of the constrained minimal supersymmetric extension of the Standard Model (CMSSM), in which supersymmetry-breaking parameters are assumed to be universal at the GUT scale, assuming also that the LSP is the lightest neutralino χ. The part of the CMSSM parameter space that is compatible with the measured astrophysical density of cold dark matter is known to include a {τ̃1}−χ coannihilation strip, a focus-point strip where χ has an enhanced Higgsino component, and a funnel at large tan β where the annihilation rate is enhanced by the poles of nearby heavy MSSM Higgs bosons, A/H. We calculate the total annihilation rates, the fractions of annihilations into different Standard Model final states and the resulting fluxes of γ rays for CMSSM scenarios along these strips. We observe that typical annihilation rates are much smaller in the coannihilation strip for tan β = 10 than along the focus-point strip or for tan β = 55, and that the annihilation branching ratios differ greatly between the different dark matter strips. Whereas the current Fermi-LAT data are not sensitive to any of the CMSSM scenarios studied, and the calculated γ-ray fluxes are probably unobservably low along the coannihilation strip for tan β = 10, we find that substantial portions of the focus-point strips and rapid-annihilation funnel regions could be pressured by several more years of Fermi-LAT data, if understanding of the astrophysical background and/or systematic uncertainties can be improved in parallel.

General relativistic ray-tracing algorithm for the determination of the electron-positron energy deposition rate from neutrino pair annihilation around rotating neutron and quark stars

We present a full general relativistic numerical code for estimating the energy–momentum deposition rate (EMDR) from neutrino pair annihilation (⁠⁠). The source of the neutrinos is assumed to be a neutrino-cooled accretion disc around neutron and quark stars. We calculate the neutrino trajectories by using a ray-tracing algorithm with the general relativistic Hamilton's equations for neutrinos and derive the spatial distribution of the EMDR due to the annihilations of neutrinos and antineutrinos around rotating neutron and quark stars. We obtain the EMDR for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology (MIT) bag model equation of state and in the colour–flavour-locked (CFL) phase. The distribution of the total annihilation rate of the neutrino–antineutrino pairs around rotating neutron and quark stars is studied for isothermal discs and accretion discs in thermodynamical equilibrium. We demonstrate both the differences in the equations of state for neutron and quark matter and rotation with the general relativistic effects significantly modify the EMDR of the electrons and positrons generated by the neutrino–antineutrino pair annihilation around compact stellar objects, as measured at infinity.

Electron-positron energy deposition rate from neutrino pair annihilation on the rotation axis of neutron and quark stars

We investigate the deposition of energy due to the annihilations of neutrinos and antineutrinos on the rotation axis of rotating neutron and quark stars, respectively. The source of the neutrinos is assumed to be a neutrino-cooled accretion disk around the compact object. Under the assumption of the separability of the neutrino null geodesic equation of motion we obtain the general relativistic expression of the energy deposition rate for arbitrary stationary and axisymmetric space-times. The neutrino trajectories are obtained by using a ray tracing algorithm, based on numerically solving the Hamilton-Jacobi equation for neutrinos by reversing the proper time evolution. We obtain the energy deposition rates for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. The electron-positron energy deposition rate on the rotation axis of rotating neutron and quark stars is studied for two accretion disk models (isothermal disk and accretion disk in thermodynamical equilibrium). Rotation and general relativistic effects modify the total annihilation rate of the neutrino-antineutrino pairs on the rotation axis of compact stellar, as measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino-antineutrino annihilation.

Delayed emission of cold positronium from mesoporous materials

It is well known that ortho-positronium (ortho-Ps) atoms are emitted with high efficiency from various porous materials following the implantation of positrons. Since the ortho-Ps lifetime in a mesoporous material may be a substantial fraction of the ortho-Ps vacuum lifetime ($142$ ns), the time dependence of Ps emission may have to be considered when conducting certain types of experiments, such as time of flight measurements or pulsed ortho-Ps-laser interactions, when using this kind of target as a positronium source. By taking into account the positron implantation profile and subsequent Ps diffusion and decay in a mesoporous film we calculate the time dependent ortho-Ps emission rate $\ensuremath{\Gamma}$($t$), which in turn allows us to establish the total annihilation rate, arising from the decay of ortho-Ps both inside and outside the sample. Using time-delayed laser spectroscopy and single-shot lifetime measurements we have directly probed the rate at which Ps is emitted into vacuum from a target with $~$3-nm diameter pores and have observed delayed ortho-Ps emission that is consistent with our model. From the ortho-Ps decay spectrum we find that, whereas a simple two-component lifetime fit gives a short lifetime of $25.3$$\ifmmode\pm\else\textpm\fi{}$$0.3$ ns, an analysis that properly takes into account the emission rate yields an ortho-Ps lifetime inside the porous material of $32.3$$\ifmmode\pm\else\textpm\fi{}$$1.2$ ns, demonstrating that the ortho-Ps escape rate into vacuum can significantly modify the apparent lifetime of ortho-Ps inside a mesoporous material. Our measurements yield a Ps diffusion coefficient $D$ $=$ $0.07$ $\ifmmode\pm\else\textpm\fi{}$ $0.01$ cm${}^{2}$ ${\mathrm{s}}^{\ensuremath{-}1}$, which is consistent with a tunneling limited diffusion process.

Calculation of density matrices in the formation of positronium

We study single collisions between spin-polarized positron beams and targets that result in the production of positronium and target ions. We consider unpolarized as well as polarized targets with $0,1$, and $2$ unpaired electrons. First, we use angular momentum coupling to calculate the spin scattering matrices in all three cases. Then, for some targets, we define an angle $\ensuremath{\beta}$ between the polarization vectors of the positron beam and the target, and calculate density matrices whose elements are functions of $\ensuremath{\beta}$. Finally, from the density matrices, we obtain the probabilities of forming para- and ortho-positronium atoms for specific spins of the target ions. We present ratios of para- to ortho-positronium yields as well as total positronium annihilation rates.

Electron-positron energy deposition rate from neutrino pair annihilation in the equatorial plane of rapidly rotating neutron and quark stars

The neutrino–antineutrino annihilation into electron–positron pairs near the surface of compact general relativistic stars could play an important role in supernova explosions, neutron star collapse, or for close neutron star binaries near their last stable orbit. General relativistic effects increase the energy deposition rates due to the annihilation process. We investigate the deposition of energy and momentum due to the annihilations of neutrinos and antineutrinos in the equatorial plane of the rapidly rotating neutron and quark stars, respectively. We analyse the influence of general relativistic effects, and obtain the general relativistic corrections to the energy and momentum deposition rates for arbitrary stationary and axisymmetric space–times. We obtain the energy and momentum deposition rates for several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology bag model equation of state and in the colour-flavour-locked phase, respectively. Compared to the Newtonian calculations, rotation and general relativistic effects increase the total annihilation rate measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino–antineutrino annihilation.

HYPERACCRETING NEUTRON STAR DISKS AND NEUTRINO ANNIHILATION

Newborn neutron stars surrounded by hyperaccreting and neutrino-cooled disks may exist in some gamma-ray bursts and/or supernovae. In this paper, we further study the structure of such a neutron star disk based on the two-region (i.e., inner and outer) disk scenario following our previous work, and calculate the neutrino annihilation luminosity from the disk in various cases. We investigate the effects of the viscosity parameter {alpha}, energy parameter {epsilon} (measuring the neutrino cooling efficiency of the inner disk), and outflow strength on the structure of the entire disk as well as the effect of emission from the neutron star surface boundary emission on the total neutrino annihilation rate. The inner disk satisfies the entropy-conservation self-similar structure for the energy parameter {epsilon} {approx_equal} 1 and the advection-dominated structure for {epsilon} < 1. An outflow from the disk decreases the density and pressure but increases the thickness of the disk. Moreover, compared with the black hole disk, the neutrino annihilation luminosity above the neutron star disk is higher, and the neutrino emission from the boundary layer could increase the neutrino annihilation luminosity by about one order of magnitude higher than the disk without boundary emission. The neutron star disk with the advection-dominatedmore » inner disk could produce the highest neutrino luminosity while the disk with an outflow has the lowest. Although a heavily mass-loaded outflow from the neutron star surface at early times of neutron star formation prevents the outflow material from being accelerated to a high bulk Lorentz factor, an energetic ultrarelativistic jet via neutrino annihilation can be produced above the stellar polar region at late times if the disk accretion rate and the neutrino emission luminosity from the surface boundary layer are sufficiently high.« less

WIMPonium and boost factors for indirect dark matter detection

We argue that WIMP dark matter can annihilate via long-lived “WIMPonium” bound states in reasonable particle physics models of dark matter (DM). WIMPonium bound states can occur at or near threshold leading to substantial enhancements in the DM annihilation rate, closely related to the Sommerfeld effect. Large “boost factor” amplifications in the annihilation rate can thus occur without large density enhancements, possibly preferring colder less dense objects such as dwarf galaxies as locations for indirect DM searches. The radiative capture to and transitions among the WIMPonium states generically lead to a rich energy spectrum of annihilation products, with many distinct lines possible in the case of 2-body decays to γγ or γZ final states. The existence of multiple radiative capture modes further enhances the total annihilation rate, and the detection of the lines would give direct over-determined information on the nature and self-interactions of the DM particles.

Pressure dependence of positron annihilation in boron phosphide

We have obtained the valence electron and positron charge densities in boron phosphide from wavefunctions derived in a model pseudopotential band structure calculation. The charge densities are calculated at pressures ranging from 0 to 90 GPa. We find that the positron density is maximum in the open interstices and is excluded not only from the ion cores, but also to a considerable degree from the valence bonds. Electron momentum densities are calculated in the (001–110) plane. The observation that the total positron annihilation rate increases with pressure has been explained in terms of increased positron penetration into the ion cores.

POSITRON STATES IN Si1-xGex ALLOYS: DEVIATION FROM VEGARD'S LAW

Electron-positron momentum densities along different crystallographic directions and positron bulk lifetime in Si 1-x Ge x alloys have been investigated within the pseudopotential formalism employing the independent particle model. Special attention has been given to the effect of the deviation of the alloy lattice parameters from Vegard's rule on the studied quantities. It is found that using Vegard's law leads to an underestimation of the total positron annihilation rate indicating therefore an overestimation of the positron bulk lifetime. This result could not be checked using the Siethoff relation (H. Siethoff, Phys. Stat. Sol.B205, R3 (1998)). Moreover, this relation predicts a monotonic dependence of the positron bulk lifetime on the alloy composition which disagrees with the positron lifetime measurement.

Particle annihilation in cold dark matter micropancakes

Cold primordial particle dark matter forms with a distribution in six-dimensional phase space closely approximating a three-dimensional sheet. Folds in the mapping of this sheet onto configuration space create ubiquitous sheetlike caustics (``micropancakes''). A typical weakly interacting massive particle dark matter halo has many micropancakes, each with a scale comparable to the halo itself, a width about ${10}^{\ensuremath{-}8}$ of the halo size and a typical maximum density up to about ${10}^{4}$ times the halo mean. It is suggested here that the total annihilation rate of dark matter particles is dominated by particles close to these micropancakes, so radiation is emitted predominantly from highly contorted two-dimensional surfaces rather than a filled volume. The total annihilation rate of particles is about a factor of 5 higher than predicted from N-body simulations, which cannot resolve these features. Micropancakes also produce sharp line discontinuities in the surface brightness of annihilation radiation.

First-Principles Calculations of Positron Annihilation in Solids

ABSTRACTWe present first-principles approaches based on density functional theory for calculating positron states and annihilation characteristics in condensed matter. The treatment of the electron-positron correlation effects (the enhancement of the electron density at the positron with respect to mean-field density) is shown to play a crucial role when calculating the annihilation rates. A generalized gradient approximation (GGA) takes the strong inhomogeneities of the electron density in the ion core region into account and reproduces well the experimental total annihilation rates (inverses of the positron lifetimes) by suppressing the rates given by a local density approximation (LDA). The GGA combined with an electron-state-dependent enhancement scheme gives a good description for the momentum distributions of the annihilating positron-electron pairs reproducing accurately the trends observed in the angular correlation (ACAR) or Doppler broadening measurements of the annihilation radiation. The combination of the present positron lifetime and momentum density calculations with the corresponding measurements yields a unique tool for defect identification. Especially, the investigation of various vacancy-type defects in semiconductors able to trap positrons will be an important field for these methods. We will show that the identification of vacancy-impurity complexes in highly n-Type Si and the study of the SiO2/Si interface are particularly interesting applications.

Nonlocal electron-positron correlations in solids within the weighted density approximation

Nonlocal electron-positron correlation effects in solids are studied. The weighted density approximation (WDA) is applied to calculations of the nonlocal state-selective electron-positron correlation functions. The importance of state selectivity of the electron-positron enhancement factors is discussed. The calculated WDA electron-positron enhancement factors for the core electrons are compared with those obtained within the local-density approximation. Also, differences in the electron-positron enhancement factors due to the $s,$ $p,$ $d,$ and $f$ angular momentum channels of the electron charge density are studied. The influence of the electron-positron interaction on the positron density distribution in solids is discussed. The formalism is applied to ab initio calculations of positron lifetimes in a variety of metals and silicon. The influence of various approximations to the electron-positron interaction on the positron lifetimes is also presented. Moreover, we study how the core electrons as well as different angular momentum channels of the valence electron density contribute to the total positron annihilation rates and lifetimes. The results are compared to those calculated within the local-density approximation and the generalized gradient approximation.

One‐Photon Annihilation of Thermal Positrons in the Interstellar Medium

In the interstellar medium, one-photon annihilation of thermal positrons with bound atomic electrons is investigated. The total annihilation rate of thermal positrons with an interstellar element is calculated considering the K- and higher shell (L, M, and N) contributions from abundant many-electron atoms. The results show that the main contributions to one-photon annihilation are H, He, and C atoms in hot medium (≡ kT/Ry)=1, H, Mg, and He atoms in warm medium =0.1, Mg, H, and Fe atoms in cold medium =0.01, respectively. In a hot medium, positrons are mostly annihilated with the K-shell electrons; for example, the K-shell electrons of H and He atoms. However, in a cold medium positrons are annihilated with higher shell electrons; for example, the M-shell electrons of Mg atom and the N-shell electrons for Fe atom give main contributions.

Study on momentum density in narrow-gap mixed III–V alloys by positron annihilation under pressure

The pressure dependence of the electron–positron momentum density in InAs x Sb 1− x was studied. The computational technique used here is based on the independent particle model coupled with the use of the electron pseudo-wave functions. The variation of the momentum density versus pressure is weak and shows that InAs x Sb 1− x exhibits a more metallic character. The observation that the calculated total positron annihilation rate increases with pressure has been explained in terms of increased positron penetration into the ion cores. The obtained results have an essential role in the determination of the electronic band structure of narrow gap mixed III–V alloys under pressure.