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Issue 4, 2024
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Extracts from the Internet


The search for sterile neutrinos

The BEST (Baksan Experiment on Sterile Transitions) experiment carried out at the Baksan neutrino observatory of INR RAS provided new data confirming the presence of so-called “gallium anomaly” (lack of electron neutrinos), which may testify to the existence of sterile neutrino νs [1]. The gallium anomaly has been discovered in SAGE and GALLEX experiments on solar neutrino detection on the basis of the reaction 71Ga(νe,e−) 71Ge. During detector calibration by radioactive sources, the count rate turned out to be lower than expected. A possible explanation may be transformation (oscillations) of part of electron neutrinos into sterile neutrinos νe→νs that do not participate directly in weak interactions. Hypothetic νs have not yet been reliably detected, but some indications of their existence have been obtained in oscillation experiments. Rather massive νs could also constitute “warm” dark matter in the Universe. In the new gallium experiment BEST, V.N. Gavrin (Institute for Nuclear Research, RAS) and his colleagues registered neutrinos from the radioactive source 51Cr with the well-known activity, prepared on the Dmitrovgrad reactor. In the BEST detector, two gallium volumes were applied at different average distances to the source. The existence of two oscillation lengths decreases theoretical uncertainties. In interpretation of the results, an improved reaction cross section was used, which provides a higher confidence that all the nuclear-physical processes are taken into account correctly. In the new measurements, the deficit of νe amounted to 20-24 %, and the general fidelity of the existence of gallium anomaly increased from 2-3 σ to over 5 σ [2]. Thereby, it heightens the probability of νs existence, provided that no other reason for the divergence is found. The most probable values of the mass parameter and the angle of mixing νs are Δ m2≈1.25 eV2 and sin2θ≈0.34 (considering the SAGE and GALLEX data). Another BeEST (Beryllium Electron capture in Superconducting Tunnel junctions) experiment gave the first constraint on νs obtained using beta decays 7Be→7Li [3]. 7Be nuclei were implanted into a superconducting tunnel junction quantum sensor. Decays with capture of one of the electrons from the electron shell into the atomic nucleus were observed. Then, a neutrino (not registered in the experiment) will escape and the nucleus will get a recoil. The recoil energy excites electrons and thus causes tunnel current steps. In the recoil energy spectrum, four peaks were observed corresponding to the ground and excited states of 7Li nuclei. In the presence of νs, the spectrum must have been distorted. This effect has not been revealed with the present-day accuracy. The obtained constraint on the matrix element of νe and νs mixing in the mass interval between 100 and 850 keV is better by an order of magnitude than those obtained in other experiments. νs is also being searched for in NOvA experiment conducted with a participation of scientists from INR RAS, JINR, and LPI RAS [4]. Measurements are being taken at Fermilab by two detectors at distances of 1 and 810 km from the antineutrino acceleration source, but no oscillations into νs have yet been found. The absence of a notable νs contribution allowed obtaining additional constraints on the mixing angles and mass parameters. For reliable conclusions concerning the existence of νs, further studies are needed. [1] Barinov V V et al., arXiv:2109.11482 [nucl-ex] [2] Barinov V, Gorbunov D, arXiv:2109.14654 [hep-ph] [3] Friedrich S et al. Phys. Rev. Lett. 126 021803 (2021) [4] Acero M A et al. Phys. Rev. Lett. 127 201801 (2021)

φ-meson-proton interaction

A shift of the hadron mass in a nuclear medium through a partial restoration of chiral symmetry is predicted in the framework of quantum chromodynamics. Some evidence of this effect was obtained in experiments with φ mesons. It is however difficult to interpret the obtained data because of theoretical and experimental uncertainties in the mechanism of #-meson-nucleon interaction. Investigated in the ALICE experiment at the Large Hadron Collider were pp collisions with an energy of 13 TeV in the center-of-mass system, and interactions between produced φ-mesons and protons were observed [5]. The φp interaction was established to have the character of attraction with the main contribution to it made by elastic processes, while inelastic only amount to <0,17 %. The interaction constant gN-φ=0.14±0.03(stat.)±0.02(syst.) was found from the measurements of two-particle correlation functions. These new data may possibly clarify different effects related to φ-meson-nucleon interaction. Russian researchers from BINP SB RAS, INR RAS, JINR, MIPT, NRC “Kurchatov institute” (including ITEP, SRC IHEP, and PINP), MEPhI, RFNC-VNIIEF, and SPbU are taking part in the ALICE experiment. [5] Acharya S et al. Phys. Rev. Lett. 127 172301 (2021)

Photonuclear reaction γn→K0Σ0

The cross section of the reaction γn→K0Σ0 near the threshold K* was measured in the BGOOD experiment being performed in the University of Bonn with a participation of Russian scientists from PINP and INP RAS [6]. A peak was revealed at an energy of ≈2050 MeV, which may correspond to the multiquark vector meson-baryon resonance. This model was proposed by A. Ramos and E. Oset in 2013 for the uds-quark sector. An analogous model in the form of a pentaquark configuration in the sector of charmed heavy quarks was applied earlier to explain # states observed in the LHCb experiment at the Large Hadron Collider. In the BGOOD experiment, liquid-hydrogen and deuterium targets were exposed to gamma-ray quanta of bremsstrahlung generated by an electron beam. The reaction products were observed using magnetic spectrometers and track and plastic scintillation detectors. Candidate particles K# were selected by recording photon pairs from decays K0S→π0π0→(γγ)(γγ). Other criteria of selection were also employed. In the framework of statistical uncertainties, the peak at 2050 MeV corresponds to the model of A. Ramos and E. Oset. Thus, the multiquark state in the light-quark sector was revealed, perhaps, for the first time. However, since alternative interpretations are not excluded, additional statistical data and new experiments are needed. For nuclear photonics, see [7, 8]. [6] Kohl K et al., arXiv:2108.13319 [nucl-ex] [7] Nedorezov V G, Turinge A A, Shatunov Yu M Phys. Usp. 47 341 (2004); UFN 174 353 (2004) [8] Nedorezov V G, Rykovanov S G, Savel’ev A B Phys. Usp. 64(12) (2021); UFN 191 1281 (2021)

Diffraction method of measuring the Casimir-Polder force

The Casimir-Polder force appears between a separate atom and a polarized surface owing to quantum fluctuations of an electromagnetic field. Measuring this force is of interest from the fundamental viewpoint for the search for new interactions on nanometer scales and may appear to be important for microelectromechanical systems. C. Garcion (Sorbonne University Paris North, France) and their co-authors demonstrated in their experiment a new technique of measuring Casimir-Polder force on scales of ≈115-51 nm in the course of interference of an atomic beam on a flat diffraction grating [9]. The idea of their method implied that when the atom approaches the grating, it is affected by the Casimir-Polder force, which results in a decrease of the grating gap effective width, which affects, in turn, the interference pattern. The grating was made by the electron lithography method in a 100-nm Si3N4 plate on a 1×1 mm2 plate. A beam of metastable argon atoms was sent to the grating at a velocity of 20 m s−1. The interference pattern recorded by the detector behind the plate corresponded to the main contribution from the Casimir-Polder potential with noticed nearly 15-% deviations associated with the contribution of van der Waals potential. Thus, the experiment demonstrated a high sensitivity of the new diffraction method. [9] Garcion C et al., Phys. Rev. Lett. 127 170402 (2021)

Low-energy excitations in Cu3Zn(OH)6FBr

Y. Wei (Beijing National Laboratory for Condensed Matter Physics and the Institute of Physics, Chinese Academy of Sciences) and their co-authors investigated specific heat conductivity of a low-temperature compound Cu3Zn(OH)6FBr [10]. The state of quantum spin liquid is assumed to take place in this polycrystal, but more detailed studies are needed for a reliable conclusion. In particular, the picture can be clarified trough an analysis of elementary excitations in low-temperature Cu3Zn(OH)6FBr. The researchers have found that as the temperature lowers, the graph of thermal conductivity shows an arm at ≈4 K, while with a further cooling the thermal conductivity takes on a power-law form ∝T1.7. This behavior is best of all explained by the interaction of vortex excitations – visons with magnetic impurities. Such an effect resembles a coherence length variation in the Pippard model for superconductors and may testify to a distant quantum entanglement of excitations. [10] Wei Y et al., Chinese Physics Letters 38 097501 (2021)

Mobius strip microlaser

Y. Song (Paris-Saclay University, France and Lanzhou University, China) and their co-authors examined a microlaser whose resonator was a polymer ring in the form of a Mobius strip nearly ≈50 µm in radius fabricated of a photoresist IP-G780 by the method of direct lithography [11]. The photoresist was doped over the volume by dye particles (active medium), and pumping was realized transversely by an additional laser. The set of lasing generation modes measured by spectrometer and an SSD camera differed from the set of circular modes in conventional ring microlasers (“whispering gallery” modes) and corresponded to periodic geodesics (the shortest trajectories) of light on a one-sided Mobius strip. [11] Song Y et al., Phys. Rev. Lett. 127 203901 (2021)

Proton therapy

In INR RAS, as in some other Russian research centers, nuclear physics medicine has been developed for many years already. One of the directions is proton beam therapy – the use of proton beams for treatment of oncological diseases. By varying the accelerator energy, proton beams allow finding the place where protons with a maximum absorbed dose stop and minimizing irradiation of the surrounding nonmalignant tissues. A group of authors from INR RAS worked out a conception of compact linear accelerators for proton therapy. The aim is to obtain a pulsed proton beam with a maximum energy up to 230 MeV and a cross section of the order of mm. They performed numerical simulation, using Geant4, of the effect of monoenergetic proton beam on tissues [12]. These calculations estimated the characteristics of irradiation dose distribution. An optimum version for dose field formation in the region of tumor is a magnetic scanning system. For nuclear medicine, see [13-18]. [12] Ovchinnikova L et al., “Effect of a Proton Beam from a Linear Accelerator for Radiation Therapy”, in Proc. RuPAC'21 (2021) pp. 182-185 [13] Kravchuk L V Phys. Usp. 53 635 (2010); UFN 180 665 (2010) [14] Kravchuk L V Phys. Usp. 64(12) (2021); UFN 191 1249 (2021) [15] Akuliniche S V Phys. Usp. 57 1239 (2014); UFN 184 1363 (2014) [16] Zhuikov B L Phys. Usp. 59 481 (2016); UFN 186 544 (2016) [17] Klenov G I, Khoroshkov V S Phys. Usp. 59 807 (2016); UFN 186 891 (2016) [18] Zhuikov B L, Ermolaev S V Phys. Usp. 64(12) (2021); UFN 191 1387 (2021)

Lunar neutrinos

Charged particle (cosmic ray) fluxes incident on the Earth’s atmosphere cause the birth of “atmospheric” neutrinos. Similarly, collision of cosmic rays with lunar soil leads to hadron cascade and “lunar” νgeneration. The birth of ν with energies over 10 GeV in this process has already been considered, in particular, by researchers from INR RAS G.T. Zatsepin and L.V. Volkova who showed that the flux of such ν from the Moon is smaller by 2 - 4 orders of magnitude than the flux of atmospheric ν. Researchers from INR RAS and MIPT S.V Demidov and D.S. Gorbunov calculated theoretically the ν flux from the Moon in the region of lower energies of 10 MeV - 10 GeV [19]. Pions and kaons produced in hadron cascades decelerate in the lunar regolith. As a result, in the decay of these particles a noticeable monochromatic component appears in the low-energy spectrum of lunar ν. It has turned out that at an energy of <53 MeV a flux of lunar ν may exceed that of atmospheric ν by an order of magnitude, but in the direction towards the Moon only. It may be the next generation of neutrino telescopes that will be able to register ν from the Moon. The direction of ν arrival, 12-% flux variations during the Moon motion along an elliptic orbit around the Earth, as well as the shape of the spectrum may help distinguish lunar ν above the background. [19] Demidov S, Gorbunov D, Phys. Rev. D 104 043023 (2021)

Horizons of black hole merger

In recent years, gravitational-wave LIGO/VIRGO detectors have found events of black hole (BH) merger. Researchers from Germany and Canada D. Pook-Kolb, R.A. Hennigar, and I. Booth developed a new method of identifying trapped surfaces of light in numerical calculations and, using it, traced the behavior of horizons in merging BHs with different masses [20]. It has turned out that after collision of BHs their apparent horizons interpenetrate, their common outer apparent horizon appears, tending with time to event horizon, while the inner apparent horizon moves towards the center until it is annihilated by trapped surfaces. In a sense, the smaller BH continues its existence inside the larger one. In nonstationary geometry, a large number of such intersecting trapped surfaces appear, and their number tends to infinity. The authors investigated trapped surface stability to discover that only three of them, coinciding with horizons, remain stable. [20] Pook-Kolb D, Hennigar R A, Booth I, Phys. Rev. Lett. 127 181101 (2021)

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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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