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| Issue 12, 2024 |
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Extracts from the Internet | ||
Detection of CNO neutrinos from the Sun1 January 2021The star energy comes from nuclear fusion reactions that also produce neutrinos. In the Sun, 99 % of energy is released in thermonuclear pp cycle producing neutrinos that were registered as far back as the early 1970s. Neutrinos born in another thermonuclear cycle in the Sun, the so-called CNO (carbon-nitrogen-oxygen) cycle only yielding 1 % of the energy, was first recorded in the Borexino experiment carried out at the Gran Sasso National Laboratory in Italy [1]. These neutrinos had not been observed before because of the low interaction rate and the presence of backgrounds. In the CNO cycle, C, N, and O nuclei are catalyzers, which was shown by Bethe and Weizsackerin the 1930s. The Borexino detector is located at the low-background mountain tunnel. It contains 280 tons of scintillator observed by 2212 photomultipliers. Registered are flashes of light from neutrino scattering by electrons. The main source of the background in the CNO neutrino energy range is the decay of 11C and 210Bi nuclei. The thermal stabilization of the detector performed in 2016 made it possible to decrease convection and to estimate more accurately the signal from 210Bi. After that, it became possible to identify with ≈ 5 σ significance the signal from neutrinos produced in the CNO cycle. The results of measurements agree perfectly with the standard solar model and with the Mikheyev – Smirnov – Wolfenstein mechanism of neutrino oscillations. Thus, the neutrino spectroscopy of the Sun completely ended with registration of CNO neutrinos. Helioseismology and measurements of solar matter opacity show somewhat different data on metallicity (the content of elements heavier than helium), which is known as the “solar metallicity problem”. The CNO neutrino data are as yet consistent with both low- and high-metallicity solar models, but further observations are needed to solve the problem. For the experiments with solar neutrinos see [2]. [1] The Borexino Collaboration, Agostini M et al. Nature 587 577 (2020) [2] Derbin A V Phys. Usp. 57 512 (2014); UFN 184 555 (2014) Diffusion in an ultracold Fermi gas1 January 2021In ultracold Fermi gases, the free path l of atoms is determined by the distance between them and the particle velocities v≈ h/(2πml) are restricted by the uncertainty principle, and therefore at low temperatures the diffusion coefficient must reach the value D≈ h/(2πm). Such universal behavior of diffusion has already been observed earlier in 4He, but for atomic Fermi gases the experimental situation remained ambiguous. P.B. Patel (Massachusetts Institute of Technology, USA) with co-authors investigates diffusion in the gas of 6Li atoms and confirmed universality of D at low temperatures [3]. The optical trap was a hollow light tube boundedon both sides by slab laser beams. In this trap, the gas was highly uniform. Modulating the intensity of one of the slab beams by the sinusoidal law, one could generate sound waves in the gas that were observed directly by the absorption method. The character of wave attenuation determined the diffusion coefficient at different sound frequencies. At high temperatures, D showed the Boltzmann temperature dependence T3/2, while at temperatures below the superfluid transition temperature the universal quantum limit D≈ h/(2πm) was attained. The present study can clarify both a number of processes in superconductors and the properties of matter in merging neutron stars. [3] Patel P B et al. Science 370 1222 (2020) Efimov effect near the Feshbach resonance1 January 2021In the 1970s, V.N. Efimov predicted theoretically [4] the presence of an infinite number of bound states of three Boson particles. The existence of these “Efimov states” has already been experimentally confirmed. X. Xie (the University of Colorado, USA) with co-authors investigated inelastic scattering of ultracold gas of 39K atoms near a Feshbach resonance with an intermediate atomic coupling strength [5]. The experimental conditions allowed the Efimov effect to be well isolated from the Van der Waals interaction effects. The character of scattering depends on the relation between the effective scale of a two-particle resonance and the particle scattering length and also on the gas temperature. The examination of the parameter space showed four distinct features, i.e., maxima in the three-body recombination coefficient. The positions of three of these features correspond to within an accuracy of 10 % to the universal ratios predicted on the basis of Efimov theory, while the fourth feature does not satisfy the universal dependence. [4] Efimov V, Physics Letters B 33, 563 (1970). [5] Xie X Phys. Rev. Lett. 125 243401 (2020) Direct visualization of dark excitons1 January 2021Excitons are bound states of electrons and holes kept together by Coulomb forces. If an electron and a hole belong to the same valley of the conduction band, then the excitons are called bright, and if they belong to different valleys, they are called dark. Dark excitons cannot absorb light themselves, since the electrons and holes in them have different momenta. J. Madeo (Okinawa Institute of Science and Technology, Japan) with co-authors investigated dark excitons in a two-dimensional film of tungsten diselenide semiconductor one molecule thick using photoelectron spectroscopy with angular resolution [6]. The events of electron escape from a WSe2 monolayer under the action of a linearly polarized extreme ultraviolet radiation beam were resolved in time (this was necessary as the excitons are short-lived), momentum and energy on a unified experimental platform. Both dark excitons themselves and their interaction with bright excitons at different energies and momenta were observed. It was established that the amount of dark excitons exceeds that of bright excitons. Dark excitons and the “exciton liquid” they form may possibly help in creating information and energy transfer devices on microscales. For excitons in semiconductors see [7-9]. [6] Madeo J Science 370 1199 (2020) [7] Glazov M M, Suris R A Phys. Usp. 63 (11) (2020); UFN 190 1121 (2020) [8] Durnev M V, Glazov M M Phys. Usp. 61 825 (2018); UFN 188 913 (2018) [9] Lozovik Yu E Phys. Usp. 61 1094 (2018); UFN 188 1203(2018) The search for an additional parity violation in relic radiation1 January 2021The search for effects beyond the Standard Model of elementary particles in the data of cosmological observations is of great interest. An additional parity violation might affect the character of relic radiation polarization. The polarization rotation has already been notices by Planck collaboration, but the result had a serious uncertainty. Y. Minami (High Energy Accelerator Research Organization KEK, Japan) and E. Komatsu (Max Planck Institute for Astrophysics, Germany and the University of Tokyo, Japan) applied a new method of data processing proposed earlier by Y. Minami, E. Komatsu and their colleagues, which provided a factor-of-2 smaller uncertainty [10]. The method makes use of microwave galactic emission. The telescope calibration error α is contained in the data on both relic radiation and galactic emission. And the additional polarization rotation angle β can only belong to relic radiation. The decomposition in multipoles and the allowance for the difference in the frequency spectra allows distinguishing between two contributions. It was obtained to good accuracy that α is close to zero, and for β the value of 0.35 ± 0.14 deg was found. Thus, the polarization plane rotation, revealed with statistical significance of 2.4 σ, may testify to parity violation beyond the Standard Model, i.e., to a new physics in the early Universe. The rotation may be caused, for example, by an axion-like field associated with dark matter or dark energy. However, further investigations are needed to confirm the obtained result. For the prospects of the new physics beyond the Standard Model see [11,12]. [10] Minami Y, Komatsu E Phys. Rev. Lett. 125 221301 (2020) [11] Troitsky S V Phys. Usp. 55 72 (2012); UFN 182 77 (2012) [12] Kazakov D I Phys. Usp. 62 364 (2019); UFN 189 387 (2019) |
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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