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Issue 4, 2024
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Extracts from the Internet


Anomalous magnetic moment of the muon

Since 2006, elementary particle physics has faced the problem of anomalous muon magnetic moment: the g-2 value measured at the Brookhaven Laboratory differed markedly from the theoretically calculated one [1]. This difference, if it really exists, may indicate the contribution of new physics beyond the Standard Model. The Muon g-2 Collaboration, conducting an experiment at Fermilab (USA), presented the results of new g-2 measurements, which showed a factor of two improvement in precision compared to the results of 2021 [2]. The difference between the cyclotron frequency and the precession frequency of antimuon spins in the magnetic field in an accumulative superconducting ring was measured. The accuracy has improved mainly owing to reduction of the systematic errors and to the increase in the data volume. The new result of g-2 measurement differs from the predictions of the Standard Model at a level of 5σ. In the coming years, an even more significant increase in the accuracy of the experiment and the completion of new theoretical calculations are expected, which may clarify the problem. [1] Logashenko I B, Eidel’man S I Phys. Usp. 61 480 (2018); UFN 188 540 (2018) [2] Aguillard D P et al., Fermilab

Tetraquarks

Tetraquarks are particles – hadrons consisting of four quarks. The previous reports on recording tetraquarks, whose quarks have different flavors, were incomplete (an isospin partner was not found) or not yet confirmed in independent experiments. The LHCb Collaboration reported on a sufficiently reliable recording of two new tetraquarks with composition (c anti-s} u anti-d) and (c anti-s anti-u} d) [3] and masses of near 2.9 GeV. This doubly charged open-charm tetraquark anti-s and its neutral partner obviously belong to one and the same isospin triplet. Proton-proton collisions at energies of 7.8 and 13 TeV were investigated. Tetraquarks were identified as intermediate resonances in the decays B0→anti-D0D+sπ− and B+→anti-D−D+sπ+ related by isospin symmetry. [3] Aaij R et al. Phys. Rev. Lett. 131 041902 (2023)

New isomeric states in 136Cs nuclei

Xenon-based detectors are used to search for dark matter particles, neutrinoless double beta decay, and low-energy neutrinos. Neutrinos can interact with 136Xe nuclei through charged currents, and registration of photons emitted by excited Cs(*) nuclei allows the neutrino energy to be measured. S J Haselschwardt (Lawrence Berkeley National Laboratory, USA) and their co-authors discovered new low-lying isomeric states in 136Cs nuclei [4]. Recorded in the experiment was gamma-ray emission in the 136Xe(p,n)136Cs reaction. From these data, a large number of new nuclear transitions and two levels of 136Cs nuclei with the lifetime of the order of 100 ns were found. The existence of such states provides unique possibilities for recording rare events on the basis of a delayed-coincidence tag suppressing background. [4] Haselschwardt S J et al., Phys. Rev. Lett. 131 052502 (2023)

Rare 40K nucleus decay

When decaying, 40K isotope nuclei are transformed into 4040Ar nuclei. Predicted was the existence of another rare channel for the decay of 40K into the ground state of 40Ar nucleus with capture of an electron, but this channel has not been observed earlier. As distinct from the decay into excited states, the decay into the ground state is not accompanied by a further emission of γ-photons. The theory does not yet allow reliably calculating the probability of such a decay, and as a rough estimate, the probability was most often assumed to be 2 %. The KDK collaboration has performed a new experiment and was the first to reliably register 40K decays into the ground state [5]. Highly sensitive gamma-ray and X-ray photon detectors were employed operating by the coincidence and anticoincidence methods. As a result, the probability of 40K decays into the ground state was measured to be twice as low as typically expected, and the hypothesis of their absence was excluded at the level of 4σ. The background from the decay of 40K nuclei is an important factor in many experiments on searching for dark matter particles and neutrinoless double beta decay, and an unknown probability introduced some uncertainty to the experimental results. Moreover, the above-mentioned double error had led to several-ten million years errors in the geological dating by the potassium-argon method. [5] Stukel M et al. Phys. Rev. Lett. 131 052503 (2023)

Direct observation of nonlocal fermion pairing

The Fermi-Hubbard model describes successfully a crossover type transition from the local fermion pairing in Bose-Einstein condensates to the nonlocal (spatially separated) pairing in the Bardeen-Cooper-Schrieffer mechanism. The most interesting is the “pseudo-gap” near the crossover, where even at temperatures above the critical superfluidity temperature, pairs are formed. T Hartke (Research Laboratory of Electronics, Massachusetts Institute of Technology, USA) and co-authors were the first to directly observe the process of nonlocal fermion pairing in the “pseudo-gap” regime [6]. They placed interacting fermionic potassium atoms at a temperature of the order of nK to an optical lattice and used a quantum microscope to obtain several thousand images showing spin directions at the lattice sites. The correlation analysis revealed the formation and spatial ordering of nonlocal fermion pairs in the case of two-dimensional attracting Hubbard gas. [6] Hartke T et al. Science 381 82 (2023)

Kapitza trap

The Kapitza pendulum is a mechanical pendulum whose oscillations are dynamically stabilized through rapid vibrations of the suspension point [7]. This stabilization effect is widely used in various fields of physics. The Kapitza pendulum effect has already been used in the Paul trap of charged particles and for light confinement in a dielectric structure. On the basis of this effect, J Jiang (Kaiserslautern University of Technology, Germany) and co-authors have created for the first time a trap for ultracold atoms and called it the “Kapitza trap” [8]. The trap is based on two laser beams focused through the lens, which have special frequency shifts and are modulated by periodic signals. The resultant Gaussian potential contained the Bose-Einstein condensate of 87Rb atoms. An absorption image of a cloud of atoms was obtained through the same lens, and a characteristic configuration was seen, theoretically predicted on the basis of the Gross-Pitaevskii theory for the case of Kapitza trap. Observed was not only a spherical annular layer, but also its asymmetry at the level of 1 % also predicted by the theory. The created Kapitza trap can be used to study quantum tunneling effects, chaos phenomena in the classical and quantum regimes, as well as other fundamental processes. [7] Kapitza P L UFN 44 7 (1951) [8] Jiang J et al. Phys. Rev. Lett. 131 033401 (2023)

Turbulence statistics

In 1941, Kolmogorov put forward a hypothesis that turbulence in the so-called inertial region at large Reynolds numbers Re has a universal form, when characteristic velocities depend in a power-law manner on the scale of turbulent pulsations. Turbulence originates on the largest scales, then the energy is transferred to medium scales, and finally dissipates on small scales due to viscosity. C Kuchler, G P Bewley, and E Bodenschatz performed at Max Planck Institute for Dynamics and Self-Organization (Germany) a new large-scale experiment to study turbulence in the air [9]. In a tunnel with a volume of 88 m3, turbulence with different initial scale distributions was excited using individual controlled blades, and Re was varied over a wide range by changing the air pressure. The evolution of turbulent air movements was examined by the effect of air flows on nanofilaments. The 2nd order structure functions turned out to have an effective exponent differing from the Kolmogorov law 2/3 and depending on the scale even in the inertial regime, but independent of Re themselves. This result has not yet been completely explained theoretically. For incorrect premises in the derivation of the power-law form of the structure functions and for the conception of stretching vortex filaments, see [10]. The results obtained show that the turbulence physics in the inertial range is more sophisticated than expected. For atmospheric turbulence, see [11]. [9] Kuchler C, Bewley G P, Bodenschatz E Phys. Rev. Lett. 131 024001 (2023) [10] Zybin K P, Sirota V A Phys. Usp. 58 556 (2015); UFN 185 593 (2015) [11] Lukin V P Phys. Usp. 64 280 (2021); UFN 191 292 (2021)

A new test of the equivalence principle

V V Singh (G W Leibniz University of Hannover, Germany) and co-authors carried out a new test of the equivalence principle underlying the General Relativity Theory, with the use of the Moon laser location [12]. In this method, the distance from the Moon to the Earth was measured with high precision using Moon-based mirrors placed in 1969. The difference of the passive-to-gravitational mass ratio for iron and aluminum would show up in different tidal effect of the Earth to the Moon mantle, consisting of basalts with excessive iron, and the core consisting of excessive aluminum (the Moon internal structure has been studied by seismic data). This would cause an additional correction to the Moon orbital velocity and to the distance to it. V V Singh at al. managed to lower (till 3.9×10−14), compared to the previous result of 1986, the upper limit on the combination of quantities, playing the role of Etvesh parameter, by two orders of magnitude. According to the data of space experiment MICROSCOPE, which was carried out on microsatellites, the accuracy of the recent test of equality of inert and passive gravitational masses under other conditions reaches 10−15. [12] Singh V V et al. Phys. Rev. Lett. 131 021401 (2023)

A dark-matter deficient galaxy

Almost all galaxies contain invisible dark matter (DM) with mass several times exceeding the mass of an ordinary baryon matter (stars, gas, and dust). However, some galaxies have an anomalous visible-invisible component ratio. S Comeron (University of La Laguna and Canary Islands Institute of Astrophysics, Spain) and their co-authors investigated the galaxy NGC 1277 using the data from the 2.7-m telescope of the McDonald Observatory and performed numerical simulation of its dynamics within the anisotropic Jeans model [13]. The massive galaxy NGC 1277 belongs to the early Hubble type and is probably a so-called relic galaxy that avoided merging with other large galaxies. An unexpected result was that within a radius of 6 kpc, the DM mass in NGC 1277 is <5 % of the total mass, whereas in the standard cosmological model it must be 60 %. The reason for an almost complete lack of DM in the galaxy has not yet been exactly clarified, and the mechanism of the birth of this galaxy is unknown. According to one of the hypotheses, the galaxy might have lost DM in collision with another galaxy (as in the case of the “Bullet” cluster). [13] Comeron S et al. Astron. Astrophys. 675 A143 (2023)

A search for cosmic neutrino sources

In recent years, the IceCube and Baikal-GVD detectors have registered high-energy astrophysical neutrino. The IceCube data have recently identified the galactic component of neutrino flux, but most neutrinos are of extragalactic origin. Some neutrino sources have not yet been identified, although indications exist that neutrinos might have been produced in active blazar galaxies. In particular, this is evidenced by the observation of correlations of neutrinos with radio emission from blazars [14]. The Baikal-GVD collaboration performed a new search for correlations between neutrinos and known astrophysical objects [15]. The use of Baikal water as the working substance of the detector in Baikal-GVD allows a more accurate measurement of the neutrino arrival direction compared to ice-based experiments. The new studies concerned cascade events of interaction of neutrinos with energies exceeding 100 TeV. At the moment, the volume of statistical data does not allow reliable conclusions, but it was a coincidence of cascades with several bright and flaring blazars (2023 + 335, 2021 + 317, 2050 + 364, 0529 + 075, TXS 0506 + 056, 0258-184, 1935-179) observed by VLBI telescopes and with galactic sources LSI + 61 303 and Swift J0243.6 + 6124. [14] Plavin A, Kovalev Y Y, Kovalev Y A, Troitsky S Astrophys. J. 894 101 (2020) [15] Allakhverdyan V A et al. arXiv:2307.07327 [astro-ph.HE]

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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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