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| Issue 12, 2025 |
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Extracts from the Internet | ||
Ultraperipheral nuclear collisions at the Large Hadron Collider (LHC)1 June 2025If atomic nuclei do not overlap geometrically during a close flyby of two relativistic ions, then strong interactions of nucleons are impossible, but the nuclei can nevertheless be excited by the mutual electromagnetic field and undergo dissociation. Ultraperipheral collisions of 208Pb – 208Pb nuclei at a center-of-mass energy of 5.02 TeV were investigated in the ALICE experiment at LHC, and the copper-to-gold nuclear conversion in photonuclear reactions were observed for the first time [1]. Hadron calorimeters were used to measure cross sections of reactions with up to three protons and three neutrons emitting from a 208Pb nucleus. In such collisions, different isotopes of thallium, mercury, gold, and copper are formed. The theoretical predictions of the RELDIS model have has turned out to differ sometimes from the results of the experiment. For instance, for processes with emission of one and two protons, the model reaction cross section is smaller by ≈ 17-25 %. The obtained new data may possibly help to improve the theoretical description of nuclear collisions. For the fundamental problems of nuclear physics, see [2]. [1] Acharya S et al. Phys. Rev. C 111 054906 (2025) [2] Matveev V A UFN 194 1250 (2024); Phys. Usp. 67 1180 (2024) A search for dark matter (DM) particles1 June 2025Although the mean DM (hidden mass of the Universe) density is 5.6 times higher than the mean density of ordinary baryon matter, the composition of DM is still unknown. Probable candidates are new elementary particles beyond the Standard Model, and in some experiments the effects of interaction between DM particles and detector substance are sought. Presented are the first results of the search for light (≤ 1 GeV) DM particles in the underground XENONnT (Gran Sasso, Italy) experiment, where the detector working medium is liquid xenon [3]. Since for the case of light DM particles the signal of direct scintillation photons is small, the search was concentrated at observation of the secondary scintillation emission of electrons, which may have been emitted in the interaction with DM particles. Several DM models were considered: particles with masses from 10 to 106 keV, interacting with electrons via mediator particles, and bosons (axionlike particles or dark photons). DM particles have not yet been registered, but for the above-mentioned models, constraints on the interaction cross section and on the coupling constant with electrons have been obtained. These constraints confirm, and in some parameter ranges improve the constraints found in previous experiments. For another principle of detecting axionlike DM, see [4]. [3] Aprile E et al. Phys. Rev. Lett. 134 161004 (2025) [4] Vergeles S N et al. Phys. Usp. 66 109 (2023); UFN 193 113 (2023) Gauge gravitation theory1 June 2025The construction of the quantum gravitation theory was started as far back as 1935 by M P Bronshtein [5]. Since then, several approaches were proposed to quantization of gravitational field and to its unification with fields of the Standard Model (SM) of elementary particles, but this problem has not yet been solved completely. M Partanen and J Tulkki (Aalto University, Finland) developed an extension of TEGR (teleparallel equivalent of the General Relativity Theory) theory, allowing gravitation to be considered as a gauge field and involving all the SM fields [6]. In their work they used compact finite-dimensional gauge symmetry groups and eight-spinor formalism. Feynman diagrams are presented for graviton interaction with ordinary particles, resembling particle interactions in the SM and renormalizability of the formulated theory is shown in one-loop approximation. If renormalizability survives in all perturbation theory orders (this has not yet been proved), the new theory will be able to describe high-energy processes. [5] Gorelik G E Phys. Usp. 48 1039 (2005); UFN 175 1093 (2005) [6] Partanen M, Tulkki J Reports on Progress in Physics 88 057802 (2025) High-pressure hydrates1 June 2025Water and hydrogen are known to form various stoichiometric compounds at high pressures. The host molecules of H2O make up a sublattice resembling pure ice, whereas H2 molecules occupy intermediate positions or substitute for H2O. A transition between the C2 ((H2O)H2) and C3 ((H2O)(H2)2) phases has already been recorded in the range of 44 - 60 GPa in previous experiments with laser heating, but the structural information on C3 remained limited. A F Goncharov (Carnegie Institute, USA) and his co-authors performed a new experiment under conditions of excess hydrogen, in which a transformation from C2 to C3 was observed at room temperature in the pressure range of 47-103 GPa, and after decompression C3 remained metastable up to 40 GPa [7]. X-ray diffraction and Raman spectroscopy measurements were performed in laser-heated diamond cells. It has turned out that at a pressure of 69 GPa the structure of the phase C3 is cubic with the space group Fd3m. The authors’ “first-principles” calculations predict pressures at which C2 and C3 remain stable or metastable. [7] Goncharov A F et al., arXiv:2505.07091 [cond-mat.mtrl-sci] The 21-cm neutral hydrogen line in the epoch of reionization1 June 2025The hydrogen reionization in the Universe, that took place at redshifts z ≈ 6.4 – 15 was caused by the radiation of first stars and black holes, but the details of these processes are not yet completely clear, especially in connection with J Webb observations of an unexpectedly large number of early galaxies (see review [8]). One of the methods of studying the epoch of reionization is to measure the relic radiation absorption in the 21-cm neutral hydrogen line. This absorption cannot be now reliably detected because of strong backgrounds, but some constraints on its value were obtained. New constraints of the kind are presented in two observation problems. Using LOFAR antenna grating, an upper limit on the absorption power of (146.61 mK)2 was obtained for the first time at z = 9.16 and the wave number k = 0.05 Mpc−1 from the direction to the source 3C196, where the background radiation shows a cold spot [9]. And using the Murchison Widefield Array (MWA) the strongest value was obtained in all directions – (30 – 40 mK)2 at z = 6 – 7 and k ≈ 0.13 Mpc−1 [10]. The presented data indicate the presence of a heated intergalactic medium at redshifts z = 6.5 – 7.0, which limits the proposed “cold reionization” models. [8] Sil’chenko O K Phys. Usp. 68 177 (2025); UFN 195 188 (2025) [9] Ceccotti E et al., arXiv:2504.18534 [astro-ph.CO] |
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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