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Issue 4, 2024
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Extracts from the Internet


Search for sterile neutrinos

In the international Daya Bay experiment carried out in China, light sterile neutrinos have been sought in the assumption that they correspond to the fourth mass state of ordinary neutrinos. Sterile neutrinos, if they do exist, do not participate in weak interactions of the Standard model and are inaccessible for recording by ordinary detectors. They nevertheless might be revealed indirectly by the disappearance of ordinary neutrinos upon their transformations (oscillations) to sterile neutrinos. In Daya Bay experiment, the antineutrinos anti-νe are born in six nuclear reactors and are recorded by detectors at different distances. The effect of neutrino oscillations is investigated by the decrease with distance of the flux of anti-νe and modification of their energy spectrum. The data of the first 217 days (more than 300 000 events of neutrino interactions were observed within this period) are consistent with the model of oscillations of three known neutrino flavors, and no additional oscillations to sterile neutrinos has as yet been detected. The fourth mass state of neutrinos was sought in the almost unexplored mass range of 10-3 eV2 ≤ |Δm241| ≤ 0.1 eV2, and new restrictions were obtained on the mixing angle θ14 depending on |Δm241|. The study of sterile neutrinos is interesting, in particular, in that by exerting influence on the Universe evolution, these particles can contribute to the mean Universe density. Source: Phys. Rev. Lett. 113 141802 (2014)

β-delayed decays of 56Zn nuclei

In β-decays of most nuclei possessing proton radioactivity, a proton is the first to escape from the nucleus, and only after that does the excited nucleus emit a γ-photon. S.E.A. Orrigo (The University of Valencia, Spain) with colleagues observed a rare mode of β-decay of 56Z nuclei where a proton leaves the nucleus after a γ-photon. This is due to the presence of additional symmetries in the nucleus that affect the probability of the processes. Earlier such decays were only observed in the case of light 32Ar nuclei greatly differing from 56Zn in structure. In the experiment performed in GANIL laboratory (France), charged fragments of decaying nuclei and gamma rays were registered. Altogether three evens of the indicated rare decays were recorded. The experimental data specified the magnitudes of the interactions corresponding to Fermi and Gamov – Teller transitions. The results obtained are of importance for understanding the structure of nuclei and the mechanism of their decay. Source: Phys. Rev. Lett. 112 222501 (2014)

Majorana fermions in a chain of atoms

A group of research workers from Princeton University and the University of Texas atAustin (USA) performed an experiment in which quasi-particles with the properties of Majorana fermions at the edge of an iron atomic chain were observed with a higher confidence than in previous experiments. A ferromagnetic atomic chain on the surface of a superconducting lead crystal was investigated at a temperature of 1.4 K. The chain became a so-called topological superconductor because of the spin-orbit iron-lead interatomic interactions. A scanning tunnel microscope was used to measure the energy spectrum depending on the spatial position. A spectral peak corresponding to Majorana fermions was observed at the ends of the chain. The experiment was repeated in a weak magnetic field violating lead superconductivity. The indicated peak disappeared, which excluded the magnetic resonance hypothesis (the Kondo effect) as a possible reason for peak occurrence. The Majorana fermions can find application in quantum calculations. It is possible that decoherence-resistant topological qubits can be created from these fermions. Source: Science 346 602 (2014)

Photoconductivity in a thin MoS2 layer

As a rule, the conductivity of semiconductors exposed to light is enhanced by the occurrence of additional charge carriers, i.e., electrons and holes. However, C.H. Lui (Massachusetts Technological Institute, USA) with colleagues have found that in a thin (only three atoms thick ) layer of MoS2 semiconductor the inverse phenomenon takes place, namely, when exposed to laser pulses it exhibits conductivity by approximately 70 % lower at temperatures from 4 to 350 K. This effect is due to the heightening of quasi-particle binding energy in the two-dimensional case compared to bulksemiconductors. In a thin illuminated MoS2 layer, bound electron-hole pairs — excitons appear which interact with the free electrons already existing in MoS2 to form quasi-particles called trions. The latter are bound systems of one hole and two electrons. They possess charge of a single electron, but their effective mass is thrice as large. Thus, the charge carrier concentration does not increase under the action of light, while their mobility and, therefore, the sample conductivity diminishes. To measure the conductivity, terahertz radio pulses synchronized with light pulses were transmitted through the sample, and the decrease of radio pulse damping testified to conductivity lowering. Earlier, short-lived trions were observed at low temperatures only, but in this experiment they also appeared at room temperature. Source: Phys. Rev. Lett. 113 166801 (2014)

Positrons in cosmic rays

By AMS detector installed on board the International space station it was detected for the first time that at an energy above ≈ 200 GeV the relative fraction of positrons e+in the composition of cosmic rays stops increasing with heightening energy. The enhancement of the relative fraction of e+ in the resultant flux of e+ and e- beginning with the energy of ≈ 30 GeV was revealed earlier by PAMELA, Fermi, and other detectors. According to the recent AMS data the e+ spectrum becomes harder, whereas the e- spectrum changes little. This contradicts the predictions of conventional models of e+ generation in the interaction of cosmic rays with interstellar gas and radiation. The reason for e+ excess now remains unknown. Hypotheses have been put forward that e+ is generated upon dark matter particle annihilation, are born near pulsars or are additionally accelerated in cosmic ray sources. Large statistics for e+ energies from 0.5 GeV to 500 GeV has been accumulated for 30 months of AMS detector operation. It turns out that for ≈ 200 GeV the relative e+ fraction stops growing. Given this, the e+ flux remains isotropicat the achieved ≈ 3 % level of precision. If the excess e+ are due to dark matter particle annihilation, the termination of e+ fraction growth corresponds to the dark matter particle mass of ≈ 1 TeV. Source: Phys. Rev. Lett. 113 121101 (2014)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

It is compiled from a multitude of Internet sources.
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