Electron-positron Pair Annihilation Research Articles

First-principles calculations for intrinsic defects in MgO: Positron lifetime and momentum distribution of electron–positron pairs

We presented first-principles calculations of the formation energies for various charge states of intrinsic defects in magnesium oxide, including VO, VMg, VMg+O, VO+Mg+O, and VMg+O+Mg. These results can be used to predict their thermodynamic stability and detectability of these defects via positron annihilation spectroscopy. We employed two-component density functional theory to compute the positron annihilation properties, incorporating the effects of temperature and spin polarization. We then performed calculations of the momentum distributions of annihilating electron–positron pairs in these intrinsic defects in MgO. Using one-dimensional momentum distributions (Doppler-broadening spectra), we calculated the line-shape parameters Srel and Wrel. Our positron lifetime calculations were compared with experimental data. These research provides insight into the positron annihilation characteristics of defects in MgO.

A mega-electron volt emission line in the spectrum of a gamma-ray burst.

A long gamma-ray burst (GRB) is observed when the collapse of a massive star produces an ultrarelativistic outflow pointed toward Earth. Gamma-ray spectra of long GRBs are smooth, typically modeled by joint power-law segments describing a continuum, with no detected spectral lines. We report a significant (>6σ) narrow emission feature at ~10 mega-electron volts (MeV) in the spectrum of the bright GRB 221009A. Over 80 seconds, it evolves in energy (~12 to ~6 MeV) and in luminosity (~1.1 to <0.43 × 1050 erg second-1) but has a constant width of ~1 MeV. We interpret this feature as a blueshifted spectral line produced by the annihilation of electron-positron pairs, potentially in the same location responsible for emitting the brightest GRB pulses.

Neutrinos from pre-supernova in the framework of TQRPA method

Abstract We propose a new method for calculating spectra and luminosities for (anti)neutrinos produced in the pre-supernova environment by weak processes with hot nuclei. It is based on the thermal quasiparticle random phase approximation (TQRPA), that allows microscopic thermodynamically consistent calculations of the weak-interaction response of nuclei at finite temperatures. For realistic representative pre-supernova conditions from the stellar evolution code MESA, we compute (anti)neutrino luminosities and spectra arising from neutral- and charged-current weak reactions with hot 56Fe and compare them with the contribution of thermal processes. We find that the TQRPA approach produces not only a higher total luminosity of electron neutrinos (mainly born in the electron capture reaction), compared to the standard technique based on the large-scale shell model (LSSM) weak-interaction rates, but also a harder neutrino spectrum. Besides, applying the TQRPA and LSSM, we find that in the context of electron antineutrino generation, the neutral-current nuclear de-excitation (ND) process via neutrino-antineutrino pair emission is at least as important as the electron-positron pair annihilation process. We also show that flavor oscillations enhance the high-energy contribution of the ND process to the electron antineutrino flux. This could potentially be important for pre-supernova antineutrino registration by the Earth’s detectors.

Updated constraints on primordial black hole evaporation

The Hawking evaporation process, leading to the production of detectable particle species, constrains the abundance of light black holes, presumably of primordial origin. Here, we reconsider and correct constraints from soft gamma-ray observations, including of the gamma-ray line, at 511 keV, produced by electron-positron pair-annihilation, where positrons originate from black hole evaporation. First, we point out that the INTEGRAL detection of the Large Magellanic Cloud provides one of the strongest bounds attainable with present observations; and that future MeV gamma-ray telescopes, such as GECCO, will greatly enhance such constraints. Second, we discuss issues with previous limits from the isotropic flux at 511 keV and we provide updated, robust constraints from recent measurements of the diffuse Galactic soft gamma-ray emission and from the isotropic soft gamma-ray background.

Emergence in a complex network with two types of directed edges – A numerical investigation

I have considered the “electron-positron pair annihilation” and have come to the conclusion that while the current models and associated mathematics are very useful, some essential questions remain unanswered.To give direction to an alternative, I pose the following question and answer it by way of example.What may a model look like containing objects with the following properties:1. The objects consist of components.2. The environment comprises the same components.3. There are conditions under which objects are formed and then remain stable. Accordingly, the objects have at least one property that, with time, does not change or changes only insignificantly.4. There are conditions under which the objects are destroyed or dissolved.5. The objects are dynamic, i.e., at any time, they are formed by different components.To answer the above question, I create a network of nodes and two types of directed edges. The edges give properties to the nodes and, at the same time, want to move to nodes with certain properties. Thus, the edges have a target. Since the targets of the edge types contradict each other, the edges always keep moving. What one type of edges creates is destroyed by the other type of edges and vice versa. The network thus has an internal dynamic.If the average node degree exceeds the threshold of approx. 11, simple objects are formed. The objects are the more stable,•the higher the average node degree.•the more nodes and edges the network has.If edges are now randomly deleted from the network so that the average node degree falls below 11, the objects disappear again. If someone looks at the objects at different times, they are formed by different edges again and again.The article presents a simple and well-defined model for emergent behavior.

New physics searches at the ILC positron and electron beam dumps

We study capability of the ILC beam dump experiment to search for new physics, comparing the performance of the electron and positron beam dumps. The dark photon, axion-like particles, and light scalar bosons are considered as new physics scenarios, where all the important production mechanisms are included: electron-positron pair-annihilation, Primakoff process, and bremsstrahlung productions.We find that the ILC beam dump experiment has higher sensitivity than past beam dump experiments, with the positron beam dump having slightly better performance for new physics particles which are produced by the electron-positron pair-annihilation.

Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Summary Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI3) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects ( I i ) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI3-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I i and the more effective passivation of iodine vacancies ( V I ) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.

Comment on “Nondispersive analytical solutions to the Dirac equation”

In our Comment we question the validity of the claim made by Campos and Cabrera [Phys. Rev. Res. 2, 013051 (2020)] that their solutions of the Dirac equation in an external time-dependent electromagnetic field describe beams of electrons. In every time-dependent field, no matter how weak, which has an infinite time duration, there is continuous electron-positron pair creation and annihilation. Without the proper accounting for these processes, the mathematical solutions of the Dirac equation are not directly applicable to realistic physical situations. In particular, the time evolution of the average values $\ensuremath{\langle}x\ensuremath{\rangle}$ and $\ensuremath{\langle}y\ensuremath{\rangle}$ does not describe the electron trajectory but the motion of some combination of the electron and positron charge distributions with pathological properties (zitterbewegung).

Gamma-ray spectra in the positron-annihilation process of molecules at room temperature

In this study, a fully self-consistent method was developed to obtain the wave functions of the positron and electrons in molecules simultaneously. The wave function of a positron at room temperature, with a characteristic energy of approximately 0.04 eV [1], was used to analyse the experimental results of its annihilation in helium, neon, hydrogen, and methane molecules. The interactions between the positron and molecule provide a significant correction in the gamma-ray spectra of the annihilating electron–positron pairs. It was also observed that high-order correlations offered almost no correction in the spectra, as the interaction between the low-energy positron and electrons cannot drive the electrons into excited electronic states. More accurate studies, which consider the coupling of the positron–electron pair states and vibration states of nuclei, must be undertaken.

Resonant annihilation and production of high-energy electron-positron pairs in an external electromagnetic field

A resonant process of annihilation and production of high-energy electron-positron pairs in an external electromagnetic field is studied theoretically. This process is the annihilation channel of an electron-positron scattering. It is shown that the resonance in an external electromagnetic field is possible only when the certain combination of electron and positron initial energies is more than threshold energy. Also, the angle between initial electron and initial positron momenta directions must be small and satisfy the resonant conditions. This angle is determined by the high-energy of the initial pair and the threshold energy. An emerging electron-positron pair also flies out in a narrow cone along the direction of the initial pair and must be ultrarelativistic. For each fixed angle, energies of the final electron and positron can take from one to two values. It is shown that the resonant differential cross section can significantly exceed the corresponding Bhabha cross section without an external field.

One-photon pair annihilation in pulsed plane-wave backgrounds

We study the $2\rightarrow1$ process of electron-positron pair annihilation to a single photon in a plane-wave background. The probability of the process in a pulsed plane wave is presented, and a locally constant field approximation is derived and benchmarked against exact results. The stricter kinematics of annihilation (compared to the $1\rightarrow2$ processes usually studied) leads to a stronger dependence on the incoming particle states. We demonstrate this by studying the effect that initial state wavepackets have on the annihilation probability. The effect of annihilation in a distribution of particles is studied by incorporating the process into Monte Carlo simulations.

Rate of dark photon emission from electron positron annihilation in massive stars

We calculate the rate of production of dark photons from electron-positron pair annihilation in hot and dense matter characteristic of supernova progenitors. Given the non-linear dependence of the emission rate on the dark photon mass and current astrophysical constraints on the dark photon parameter space, we focus on the mass range of 1--10 MeV. For the conditions under consideration both mixing with the in-medium photon and plasma effects on the electron dispersion relation are non-negligible and are explored in detail. We perform our calculations to the leading order in the fine-structure constant. Transverse and longitudinal photon modes are treated separately given their different dispersion relations. We consider the implications for the evolution of massive stars when dark photons decay either into particles of the standard model or of the dark sector.

Neutrino quantum kinetics in compact objects

Neutrinos play a critical role of transporting energy and changing the lepton density within core-collapse supernovae and neutron star mergers. The quantum kinetic equations (QKEs) combine the effects of neutrino-matter interactions treated in classical Boltzmann transport with the neutrino flavor-changing effects treated in neutrino oscillation calculations. We present a method for extending existing neutrino interaction rates to full QKE source terms for use in numerical calculations. We demonstrate the effects of absorption and emission by nucleons and nuclei, electron scattering, electron-positron pair annihilation, nucleon-nucleon bremsstrahlung, neutrino-neutrino scattering. For the first time, we include all these collision terms self-consistently in a simulation of the full isotropic QKEs in conditions relevant to core-collapse supernovae and neutron star mergers. For our choice of parameters, the long-term evolution of the neutrino distribution function proceeds similarly with and without the oscillation term, though with measurable differences. We demonstrate that electron scattering, nucleon-nucleon bremsstrahlung processes, and four-neutrino processes dominate flavor decoherence in the protoneutron star (PNS), absorption dominates near the shock, and all of the considered processes except elastic nucleon scattering are relevant in the decoupling region. Finally, we propose an effective decoherence opacity that at most energies predicts decoherence rates to within a factor of 10 in our model PNS and within 20% outside of the PNS.

Non-perturbative quantum chromodynamics and short strings in electron-positron pair annihilation to hadrons

We develop a new mathematical method to study the non-perturbative quantum chromodynamics effects in low-energy processes using ordinary and analytic perturbation theory (PT and APT). We apply this tool to extract the lowest dimensional condensates from the high-precision fits of the data on e+e–-annihilation to pion channels: &nbsp;e+e− → π+π−, e+e− → 2π+2π−, e+e− → π+π−2π0, e+e− → 3π+3π− and e+e− → 2π+2π−2π0. The special attention is devoted to the existence of the contribution of dimension 2 operator. Our consideration is based on the dependence of Adler function corresponding to e+e−-annihilation into pions on both Q and Borel transform parameter M2. It is shown that within the framework of the proposed method the C2 coefficient of the dimension 2 operator is negative and is closer to zero for analytic perturbation theory, which may therefore be partially responsible for the effect of short strings.

Electron-positron pairs in hot plasma of accretion column in bright X-ray pulsars

Abstract The luminosity of X-ray pulsars powered by accretion on to magnetized neutron stars covers a wide range over a few orders of magnitude. The brightest X-ray pulsars recently discovered as pulsating ultraluminous X-ray sources reach accretion luminosity above $10^{40}\, {\rm erg\ \rm s^{-1}}$ which exceeds the Eddington value more than by a factor of 10. Most of the energy is released within small regions in the vicinity of magnetic poles of accreting neutron star - in accretion columns. Because of the extreme energy release within small volume accretion columns of bright X-ray pulsars are one of the hottest places in the Universe, where the internal temperature can exceed 100 keV. Under these conditions, the processes of creation and annihilation of electron-positron pairs can be influential but have been largely neglected in theoretical models of accretion columns. In this Letter, we investigate properties of a gas of electron--positron pairs under physical conditions typical for accretion columns. We argue that the process of pair creation can crucially influence both the dynamics of the accretion process and internal structure of accretion column limiting its internal temperature, dropping the local Eddington flux and increasing the gas pressure.

Massless Composite Bosons Formed by the Coupled Electron-Positron Pairs and Two-Photon Angular Correlations in the Colliding Beam Reaction e-e+→Bγγ with Emission of the Massless Boson

The approach in which the electron and positron are treated as ordinary, different particles, each being characterized by the complete set of the Dirac plane waves, is examined. This completely symmetric representation that is beyond the standard QED makes it necessary to choose another solution of the Dirac equation for the free particle propagator as compared to that used currently. The Bethe-Salpeter equation with these particle propagators is solved in the ladder approximation. A new solution has been found represented by the massless composite bosons formed by the coupled electron-positron pairs with the coupling equal to the fine structure constant. It has been demonstrated that (1) the massless boson states have normalizable complex wave functions which are transversely compressed plane waves; (2) the transverse radius of the wave functions diverges as the boson energy goes to zero; that is, the composite bosons cannot be at rest; (3) increasing the boson energy results in an extension of the transverse wave function in the momentum space and a corresponding contraction of the real space coordinate wave function. The new reaction e-e+→Bγγ is investigated with the products composed of the massless composite boson and two photons. The cross-section of this reaction is derived for nonrelativistic colliding beams of spin-polarized electrons and positrons. In this case the 2γ angular correlation spectrum is characterized by a narrow peak with the full-width-at-half-maximum not exceeding 0.2 mrad. It is shown that in order to distinguish between the conventional annihilation of the singlet electron-positron pair with the two-photon emission and the new examined reaction yielding the three particles, experiments are proposed with the extremely nonrelativistic colliding beams.

Defect Characterization in Thermoelectric Zn-Sb Systems by Positron Annihilation

Positron lifetimes and momentum distributions of annihilating electron-positron pairs have been calculated for vacancies in ZnSb and Zn4Sb3. The calculated positron lifetimes for bulk ZnSb and Zn4Sb3 are 203 ps and 208 ps, and for VZn in ZnSb and Zn4Sb3 are 249 ps and 237 ps respectively. The calculated momentum distribution results indicate the VZn in both ZnSb and Zn4Sb3 has less characterization from elemental Zn. Using coincidence Doppler broadening spectra combined with lifetime measurements can reveal the vacancy structure in ZnSb and Zn4Sb3.

First-principles calculations of momentum distributions of annihilating electron–positron pairs in defects in UO2

We performed first-principles calculations of the momentum distributions of annihilating electron–positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO2 using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO2 and of mixed actinide oxides.

Orientational dependence of the creation and annihilation of electron–positron pairs in a single-crystal at high energies

The process of electron—positron pair annihilation with the creation of a single photon and the opposite process of lepton-pair creation by a high-energy photon without the participation of a third body (in vacuum) are forbidden in accordance with the energy and momentum conservation laws. These processes can occur in materials, but their probability decreases on account of the participation of a third body. However, in oriented single crystals, where relativistic charged particles can exist in states bound to crystal atomic planes or axes (the channeling mode), single-vertex processes (in perturbation theory) can occur. In this paper, the probability of such processes and the threshold conditions for their implementation are estimated.

Fully self-consistent calculations of momentum distributions of annihilating electron-positron pairs in SiC

We performed calculations of momentum distributions of annihilating electron-positron pairs in various fully relaxed vacancy defects in SiC. We used self-consistent two-component density functional theory schemes to find the electronic and positronic densities and wave functions in the considered systems. Using the one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) we calculated the line-shape parameters $S$ and $W$. We emphasize the effect of the experimental resolution and the choice of the integration ranges for the $S$ and $W$ parameters on the distributions of the points corresponding to different defects in the $S(W)$ plot. We performed calculation for two polytypes of SiC, $3C$, and $6H$ and showed that for silicon vacancies and clusters containing this defect there were no significant differences between the Doppler spectra. The results of the Doppler spectra calculations were compared with experimental data obtained for $n$-type $6H$-SiC samples irradiated with 4-MeV Au ions. We observed a good general agreement between the measured and calculated points.