On previous and present investigations of resonance gamma-ray interaction with nuclei at the Institute of Theoretical and Experimental Physics (ITEP, Moscow)

A brief survey of theoretical and experimental work that is devoted to studying the resonance absorption and scattering of gamma rays by nuclei and which was initiated at the Institute of Theoretical and Experimental Physics (ITEP, Moscow) in the 1950s and has been continued to date is given. Investigations of various versions of interaction in beta decay, magnetic-field-perturbed angular distributions of resonantly scattered gamma rays, the problem of the Mössbauer gamma resonance of long-lived isomeric states of nuclei, and the resonance scattering of annihilation photons by nuclei are described.     

The ideal resolution of a liquid xenon calorimeter

The energy and angular resolution of a liquid xenon calorimeter for gamma rays in the GeV region is studied by a Monte-Carlo simulation. For a size of (70 cm)³, the energy resolution becomes as good as 0.2 % (r.m.s) at 10 GeV. The angular resolution is order of 1° (median angle) at several GeV without using the pair converter separated from the main body of the calorimeter. An effective SΩ of 1 m²sr is attainable. These factors meet the conditions required for observation of line gamma rays emerging from the annihilation of a sort of hypothetical dark matter.     

Temperature effects on positron annihilation in metallic single crystals

A Ge(Li) spectrometer was used to measure the temperature-dependence of the intensity of gamma rays resulting from positron annihilation in single crystals of Pb, Sn, Ge and Cu. In contrast to recent angular distribution results, no significant temperature effects were observed.     

Positron-electron annihilation in the field of a fixed electric dipole

Positron annihilation from a dipole Ps state caused by simultaneous localization of the positron and electron in the field of a fixed electrical dipole is considered. The angular correlation curve of annihilation -quanta is calculated. For a fixed value of dipole moment the half width of the correlation curve decreases with decrease in dipole charge. With increase in dipole moment the correlation curve narrows. In a number of cases the given annihilation mechanism may be responsible for the appearance of the characteristic narrow component in the angular correlation spectrum. This component is usually ascribed to decay of a positronium atom. It is stressed that in interpreting the narrow component one must be careful in choosing the corresponding annihilation mechanism. Experimental results are analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 74–78, August, 1989.     

Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

Cosmic ray constraints on the annihilations of relic particles in the galactic halo

We re-evaluate the fluxes of cosmic ray antiprotons, positrons and gamma rays to be expected from the annihilations of relic particles in the galactic halo. We stress the importance of observational constraints on the possible halo density of relic particles, and specify their annihilation cross sections by the requirement that their cosmological density close the Universe. We use a Monte Carlo programme adapted to fit e⁺e⁻ data to calculate the p̄, e⁺ and γ spectra for some supersymmetric relic candidates. We find significantly smaller p̄ fluxes than previously estimated, and conclude that present upper limits on cosmic ray p̄ and e⁺ do not exclude any range of sparticle masses. We discuss the prospects for possible future constraints on sparticles from cosmic γ rays.     

Dark matter profile in the galactic center

We describe a quasiequilibrium profile of dark matter particles in the inner parsec of the Galaxy, rhodm proportional to r(-3/2). This "minicusp" profile is caused by scattering with the dense stellar cluster around the supermassive black hole in Sgr A* and is independent of the initial conditions. The implications for detection of gamma rays from annihilation of weakly interacting massive dark matter particle in the Galactic center are a mild enhancement of the flux and a characteristic central feature in the angular distribution which could be detectable by high-resolution atmospheric Cherenkov telescopes.     

Investigations of positronium formation and destruction using 3gamma/2gammaannihilation-ratio measurements

Positronium (Ps) produced by 4 to 40 eV positrons colliding with Ne, Ar, Kr, CO2, and O2 is investigated by measuring the ratio of signals of two gamma rays in coincidence resulting from (a) three gamma annihilation of ortho-Ps and (b) two annihilation gamma rays due to para-Ps decay and destruction of ortho-Ps at an aluminum scattering cell surface. These ratios provide evidence that relates to the kinetic energy dependence of ortho-Ps interactions with an aluminum surface, the Ps formation potential at this surface, and the fact that Ps is being formed with inner orbital electrons for CO2 and O2.     

Measurement of Angular Distributions of Gamma Rays from the Inelastic Scattering of 14.1-MeV Neutrons by Carbon and Oxygen Nuclei

The results obtained by measuring the angular and energy distributions of gamma rays originating from the inelastic scattering of 14.1-MeV neutrons by carbon and oxygen nuclei are presented. The measurements in question were performed by the tagged-neutron method in a beam of an ING-27 standard portable neutron generator. The angular distributions of gamma rays emitted by the 2⁺ state of ¹²С at 4.43 MeVand the 3^? state of ¹⁶O at 6.13 MeV were obtained.     

Investigation of multipole mixtures in gamma transitions by means of the reaction 232Th(n, n′γ)

The spectrumof gamma rays from the reaction ²³²Th(n, n′γ) induced by a beam of fast reactor neutrons and the angular distribution of the gamma rays in question with respect to this beam were measured. The multipole-mixture parameter δ was found for many gamma transitions.     

Explosive dark matter annihilation

In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous gamma rays from the galactic center, which originate from DM annihilation.     

MeV dark matter: has it been detected?

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by INTEGRAL, is a consequence of low mass (1-100 MeV) particle dark matter annihilations. We discuss the type of halo profile favored by the observations as well as the size of the annihilation cross section needed to account for the signal. We find that such a scenario is consistent with the observed dark matter relic density and other constraints from astrophysics and particle physics.     

The supersymmetric interpretation of the EGRET excess of diffuse Galactic gamma rays

Recently it was shown that the excess of diffuse Galactic gamma rays above 1 GeV traces the Dark Matter halo, as proven by reconstructing the peculiar shape of the rotation curve of our Galaxy from the gamma ray excess. This can be interpreted as a Dark Matter annihilation signal. In this Letter we investigate if this interpretation is consistent with supersymmetry. It is found that the EGRET excess combined with all electroweak constraints is fully consistent with the minimal mSUGRA model for scalars in the TeV range and gauginos below 500 GeV.     

Characteristics of the GAMMA-400 gamma-ray telescope for searching for dark matter signatures

The GAMMA-400 gamma-ray telescope currently under development is designed to measure fluxes of gamma rays and electron-positron cosmic-ray components, which could be associated with the annihilation or decay of dark matter particles, and to survey in detail the celestial sphere in order to search for and investigate discrete gamma-ray sources; to measure the energy spectra of Galactic and extragalactic dif- fuse gamma-ray emissions; and to study gamma-ray bursts and the gamma-ray emissions of active Sun. The GAMMA-400 energy range is 100 MeV to 3000 GeV. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. The GAMMA-400 will be installed on Russia’s Navigator space platform. Observations are planned to commence in 2018.     

Gamma spectra resulting from the annihilation of positrons with electrons in a single core level

The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.     

Positron annihilation in solid and liquid metals

The angular correlation of the gamma rays resulting from the annihilation of positrons in 15 solid and liquid metals and semiconductors has been studied. Experiments have been done on each material at room temperature and at temperatures above and below the melting point.

The elements investigated fall into three categories according to the way the angular correlation distribution changes as the melting point is reached. To within the experimental angular resolution (0.5 milliradians) no change in the angular correlation distribution is observed for Li, Se, Na, and Tl upon heating from room temperature to beyond the melting point. The elements Sb, Bi, Ga, Hg, Sn and Te exhibit changes in their angular correlation distributions only upon being melted, whereas for the metals Al, Cd, In, Pb and Zn changes occur when the specimen is heated from room temperature to temperatures below the melting point.

Changes in the angular correlation distribution upon heating or melting are generally manifested as (a) a narrowing of the central part of the curve, (b) a rounding-off of the parts of the curve near the Fermi cut-off angle and (c) a change in the area of the broad background curve as compared with the area under the central peak.     

Annihilation of unthermalized positrons in a silicon single crystal at 77°K

A model is considered for the annihilation of nonrelativistic positrons from quantized states in lattice channels. Annihilation gamma rays of energy over 511 keV have been observed when the positrons from an Na²² source strike a silicon single crystal at 77°K. The experimental results agree well with the proposed model.     

Can dark matter annihilation dominate the extragalactic gamma-ray background?

Annihilating dark matter (DM) has been discussed as a possible source of gamma rays from the galactic center and as a contribution to the extragalactic gamma-ray background. Assuming universality of the density profile of DM halos, we show that it is quite unlikely that DM annihilation is a main constituent of extragalactic gamma-ray background, without exceeding the observed gamma-ray flux from the galactic center. This argument becomes stronger when we include enhancement of the density profiles by supermassive black holes or baryon cooling. The presence of a substructure may loosen the constraint, but only if a very large cross section as well as the rather flat profile are realized.     

Constraints on dark matter annihilation from M87

As the largest mass concentrations in the local Universe, nearby clusters of galaxies and their central galaxies are prime targets in searching for indirect signatures of dark matter annihilation (DMA). We seek to constrain the dark matter annihilation emission component from multi-frequency observations of the central galaxy of the Virgo cluster. The annihilation emission component is modeled by the prompt and inverse-Compton gamma rays from the hadronization of annihilation products from generic weakly interacting dark matter particles. This component is fitted to the excess of the observed data above the spectral energy distribution (SED) of the jet in M87, described with a best-fit synchrotron-self-Compton (SSC) spectrum. While this result is not sufficiently significant to claim a detection, we emphasize that a dark matter “double hump signature” can be used to unambiguously discriminate the dark matter emission component from the variable jet-related emission of M87 in future, more extended observation campaigns.     

Gamma ray line astronomy as a probe of cold dark matter

Some annihilation processes of cold dark matter particles in the galactic halo may result in monochromatic gamma rays with an astrophysically significant rate. This paper summarizes the calculation of these rates and discusses the expected gamma ray line flux in comparison with the diffuse cosmic background.