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Issue 11, 2024
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Extracts from the Internet


Oscillations of reactor antineutrinos

The Daya Bay collaboration, in which Russian researchers from JINR (Dubna) participate, presented the most precise new data on oscillations of electron antineutrino born in atomic reactors [1]. A decrease in the number of antineutrinos as they move away from the reactor provides information about the mixing angle θ13, important for solving the mass hierarchy problem and searching for CP invariance violation in neutrino oscillations. In the Daya Bay experiment, conducted in China, an inverse beta decay anti-νe+p→ e+n is observed through recording a positron and a neutron captured by gadolinium atoms in eight detectors at a distance of 500 and 1650 m from the reactors. The gadolinium mass makes up 0.1 % in a liquid scintillator scanned by photomultipliers. The data for 5.55×106 events gave sin22θ13=0.0851 ± 0.0024. For the direct and inverse mass ordering, this gives Δm232=(2.466 ± 0.060) × 10−3 eV2 and Δm232=-(2.571 ± 0.060)×10−3 eV2, respectively. The neutrino oscillation hypothesis was first suggested by B Pontekorvo in 1957 [2] (see also [3 – 5]). [1] An F P et al. Phys. Rev. Lett. 130 161802 (2023) [2] Pontecorvo B Sov. Phys. JETP 7 172 (1958); ZhETF 34 247 (1958) [3] Bednyakov V A, Naumov D V, Smirnov O Yu Phys. Usp. 59 225 (2016); UFN 186 233 (2016) [4] Kudenko Yu G Phys. Usp. 61 739 (2018); UFN 188 821 (2018) [5] Kolupaeva L D et al. Phys. Usp. 66 647 (2023); UFN 193 801 (2023)

The data of Borexino experiment

Although the neutrino-recording Borexino experiment in the underground Gran Sasso Laboratory (Italy) ended in October of 2021, the processing of its data presents new interesting results. The Borexino collaboration has sought coincidences between neutrino events and bursts of gravitational waves observed by LIGO/Virgo interferometers [6]. Neutrinos might be born together with gravitational waves upon merging of black holes and neutron stars. Although no coincidences have been found, their absence gave new limits on the neutrino emission mechanisms. The Borexino phase III analysis, carried out recently by the Borexino collaboration itself, allowed refinement of the magnitude of the flux and the spectrum of solar neutrinos from the CNO cycle of nuclear reactions in the solar core [7]. The hypothesis on the absence of CNO neutrino recording is excluded with significance of 7 σ, and the above-mentioned analysis of neutrino spectra gives preference to solar models with high metallicity. Interestingly, an alternative processing of Borexino data, performed by researchers L B Bezrukov, I S Karpikov, and V V Sinev from INR RAS and MEPhI, including neutrinos from the decay of 40K nuclei in the Earth, shows a better agreement with the measured neutrino spectra than the model without 40K [8, 9]. The low-metallicity Sun model becomes preferential. The presence in the Earth of a large amount of 40K must also result in a heightened release of radiogenic heat from the Earth interior. [6] D. Basilico et al. Eur. Phys. J. C 83 538 (2023) [7] Appel S et al. Physical Review Letters 129 252701 (2022) [8] Bezrukov L B, Karpikov I S, Sinev V V arXiv:2304.02747 [hep-ex] [9] Bezrukov L B et al. Izvestiia RAN. Seriia fizicheskaia 87 1042 (2023)

Galactic neutrinos

Our Galaxy, especially the region of its stellar disc, is visible in the entire electromagnetic range from radio waves to gamma rays. The IceCube collaboration, which registers neutrinos with the detector in Antarctica, presented the first neutrino image of the Galaxy [10]. Gamma photons are produced in the interaction of cosmic rays with interstellar gas in the Galaxy, and F W Stecker predicted in 1979 that neutrinos must be born in the same processes. A flux of these galactic neutrinos was later calculated in detail by V S Berezinskii jointly with T Gaisser, F Halzen, and T Stanev. When examining the southern part of the sky, where the Galactic center lies, IceCube identified cascade events associated with the neutrino interaction within the detector itself. Although the accuracy of determining the direction from cascade events is lower than from track events, it is easier to filter out the strong background of atmospheric muons in cascade events. Thanks to the use of modern methods of computer data processing, known as “machine learning”, galactic neutrinos were singled out at the confidence level of 4,5 σ. The signal has the form of diffuse inhomogeneous emission concentrated near the Galactic disc. The existence of a galactic-neutrino component in the IceCube data was earlier revealed by Yu Yu Kovalev, A V Plavin, and S V Troitskii on the basis of the analysis of track events [11]. For the role of neutrinos in astrophysics, see [12]. [10] Abbasi R et al. Science 380 1338 (2023) [11] Kovalev Y, Plavin A V, Troitsky S V The Astrophysical Journal Letters 940 L41 (2022) [12] Pontekorvo B M Sov. Phys. Usp. 6 1 (1963); UFN 79 3 (1963)

Registration of gravitational waves in the NANOGrav project

In 1978, M V Sazhin (P K Sternberg State Astronomical Institute of MSU, Moscow) proposed the way of recording gravitational waves from massive binaries on the basis of pulsar observations [13]. Gravitational waves passing between pulsars and an observer on the Earth must perturb space-time and thus shift signal phases. The NANOGrav collaboration has already had evidence for the existence of nHz-scale gravitational-wave background, but the statistical significance of the result was low. In June of 2023, NANOGrav presented data for 15 years [14] with significance of (3.5-4) σ. 67 pulsars were observed and correlated phase shifts of their pulses corresponding to gravitational-wave passage were recorded with radio telescopes. The quadrupole character of correlations has made it possible to exclude other reasons for phase shifts, including the motion of the Solar System through the inhomogeneous medium affecting the radio wave propagation. The NANOGrav observations do not identify individual sources, but only register the general stochastic background of gravitational waves. Its spectrum is close to the power law spectrum with exponent γ=3.2 ± 0.6. A similar background with γ=13/3 and with the same amplitude could make pairs of supermassive black holes with masses (108-1010)M☉ located in the centers of galaxies and approach following their merging. The extragalactic neutrinos recorded by IceCube were probably born near supermassive black holes [15]. It should not however be excluded that the gravitational-wave background has other, more exotic sources, for example, phase transitions or collapses of domain walls in the early Universe. The registration of gravitational nHz-range background was simultaneously reported by another three collaborations. Higher-frequency gravitational waves from merging of black hole of stellar masses and neutron stars have already been observed since 2016 by LIGO/Virgo/KAGRA interferometers. Earlier, the emission of gravitational waves was indirectly revealed by the change in the binary pulsar orbit. [13] Sazhin M V Soviet Astronomy 22, 36 (1978) [14] Agazie G et al. The Astrophysical Journal Letters 951 L8 (2023) [15] Plavin A V, Kovalev Y Y, Kovalev Y A, Troitsky S V The Astrophysical Journal 894 101 (2020)

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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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