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Issue 4, 2024
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Extracts from the Internet


Test of the equivalence principle

Since it was formulated, the General Relativity Theory (GRT) has undergone several successful tests, including the recent detection of gravitational waves and observation of the black hole shadow. But GRT is possibly not ultimate but is an approximation to a more complete theory allowing for quantum effects and additional interactions. It is not excluded that these new interactions carried, for example, by dilaton-like particles can at a certain small level violate the weak equivalence principle (independence of the free fall acceleration of bodies of their mass and composition). Paper [1] presents the final results of the MICROSCOPE experiment carried out to test the equivalence principle in the Earth’s orbit under weak noise. The device contains two concentric cylinders of different composition (titanium and platinum alloys) soaring in weightlessness, whose mutual accelerations are measured by high-precision electrostatic accelerometers. A reference device also exists with cylinders of the same composition to control systematic uncertainties. Over the entire observation time, the Eötvös parameter was found to be η=[-1.5±2.3(stat.)±1.5(syst.)]×10−15, which is 4.6 times better than the previous value. For the currently accessible accuracy, this result confirms validity of the equivalence principle, which suggests constraints on new interactions. [1] Touboul P et al. Phys. Rev. Lett. 129 121102 (2022)

The search for the neutrinoless double beta decay

The hypothetic neutrinoless double beta decay, forbidden by the Standard Model of elementary particles, is being searched for in some experiments [2]. Its discovery would mean that neutrino is a Majorana particle (its own antiparticle). Such kind of decay has not yet been revealed, but some lower bounds have been obtained on the lifetime of different nuclei with respect to the neutrinoless double beta decay. In its conception, the CUPID-0 experiment, now being performed in the Gran Sasso National Laboratory (Italy), is a development of the CUORE experiment, which has recently obtained the best restrictions on the decay of 130Te nuclei. An additional peak corresponding to a neutrinoless channel is being sought in the spectrum of 82Se nuclei decay. The main novelty of CUPID-0 compared to CUORE is the use of scintillation bolometers. For 82Se decay to the ground state of 82Kr nucleus, the limit on the half-life was obtained to be T1/2<4.6×1024  years (20 times better than the previous limits) [3]. From this, the limit on the effective mass of Majorana neutrino is mββ<(263-545) MeV. Also obtained were the most stringent limits on the 82Se decay to excited 82Kr states, whose signature is emission of additional gamma-ray photons under deexcitation. [2] Simkovic F ; Phys. Usp. 64 1238 (2021); UFN 191 1307 (2021) [3] Azzolini O et al. Phys. Rev. Lett. 129 111801 (2022)

Intrinsic c-quark in the proton

The proton consists of two u-quarks, one d-quark, and gluons. A large number of quark-antiquark pairs of all other types (a quark sea) must also appear and disappear inside the proton. Quarks with masses exceeding the mass of the proton itself are called intrinsic quarks. Of particular interest are intrinsic c-quarks, since their mass (1.25 GeV) is a little larger than the proton mass,and according to the estimates they can carry over ≈ 0.5-2 % of proton momentum. However, several previous attempts to detect an intrinsic charm quark gave ambiguous and mutually exclusive results, and this fundamental question has remained open for nearly 40 years. The NNPDF collaboration has undertaken a new analysis of the data on pp collisions at the Large Hadron Collider and the data of some other experiments to confirm the existence of the intrinsic c-quark in protons at 3 σ level [4]. The data were processed applying, in particular, “machine learning” technologies. [4] Ball R D et al. Nature 608 483 (2022)

Electron-photon pairs

A. Feist (Max Planck Institute for Multidisciplinary Sciences and Göttingen University, Germany) and their co-authors developed a new method for obtaining quantum-entangled electron-photon pairs [5] using a circular optical high-quality-photon-chip-based microresonator generating an evanescent electromagnetic field around. While passing near the cavity the electrons from the electron microscope interacted with the evanescent field inducing generation of photons, which left the microresonator through optical fiber. The electron-photon pairs thus produced were described by a unified wavefunction (were quantum entangled). The obtained pairs possessed a high quantum fidelity. Quantum entangled electron-photon pairs are used in quantum optics for quantum-enhanced visualization and in various fields of quantum communication. [5] Feist A et al. Science 377 777 (2022)

Interference of nonidentical single particles

Quantum interference most often occurs between identical particles. However, it is also possible between different systems [6]. Interference has already been observed between a quasiparticle - magnon (spin excitation quantum) and a collective photon state, and also between a photon and a coherent NOON state. K. Su (South ChinaNormal University) and their co-authors were the first to demonstrate quantum interference between nonidentical single particles (a photon and a magnon) in a Hong-Ou-Mandel interferometer and examined their quantum statistics [7]. In the interferometer, a hybrid splitter was exploited, in which conversion between the indicated particles could take place through the dark polariton statein an ensemble of atoms, and the splitter could be transferred from the Hermitian to the non-Hermitian regime. This allowed the crossover from the bosonic to fermionic quantum statistics, although both the input states were bosonic. Moreover, three fermion states were simulated by three input single photons. [6] Yuboshitz V L, Podgoretskii M I JETP 28 469 (1969), ``Interference of nonidentical particles''; ZhETF 55 904 (1968) [7] Su K et al. Phys. Rev. Lett. 129 093604 (2022)

Diode effect in superconductivity

In some nonconventional superconductors, the external magnetic field can coexist with superconductivity. This underlay the recently discovered effect of “superconducting diode”, when the critical current of superconductor depends on direction. However, this phenomenon was only observed in the presence of an external magnetic field. J.-X. Lin (Brown University, USA) and their co-authors demonstrated for the first time a superconducting diode without an external magnetic field in twisted trilayer graphene [8]. The direction of the diode effect can be changed by an out-of-plane magnetic field (the “material training” method) and also by changing the carrier density and the layer twist angle. The superconducting diode effect in graphene can evidently be explained by an imbalance in the valley occupation, which perhaps leads to finite-momentum Cooper paring. This study presents a direct proof of coexistence between superconductivity and time-reversal symmetry breaking at the atomic level. [8] Lin J.-X. et al. Nature Physics 122 153601 (2022)

“Antilaser”

Coherent radiation absorption (“antilaser”) has already been realized in experiment through creation of resonance structures [9], but for only one or several modes with different incidence angles. Y. Slobodkin (The Hebrew University of Jerusalem, Israel) and his co-authors proposed theoretically and demonstrated experimentally a method for creating coherent absorption, where the absorber is self-tuned for each possible mode as to reach its complete absorption [10]. That is, a signal with any wavefront is perfectly absorbed. The method is based on a formal accordance of such coherent absorption with a time-reversed degenerate-cavity laser radiation. For the laser effect reversal, it was proposed to place two lenses and a thin layer of absorbing medium in the cavity. Owing to mode degeneracy and interference the light remains in the cavity and passes many times through the absorbing medium. This provides an ideal coherent absorption of any combination of modes irrespective of their relative phases and directions. The experiment performed by the described method confirmedthe conception efficiency– the 94-% efficiency of absorption of a compound signal consisting of over 1000 modes. [9] Rosanov N N Phys. Usp. 60 818 (2017); UFN 187 879 (2017) [10] Slobodkin Y et al. Science 377 995 (2022)

The local breakdown of Ohm’s law in graphene

The Ohm’s law (j=σE) takes place on scales exceeding the average electron free path in a medium, when electricity transport is a diffusive process. It has been predicted, however, that the transport in Fermi liquids can be determined by electron-electron collisions.This leads to viscous transport, where a flow of current resembles a flowof liquid. A. Jenkins (University of California, Santa Barbara, USA) and their colleagues observed a change from one flow pattern to another and a local breakdown of Ohm’s law in graphene monolayer near the etched narrow neck [11]. The magnetic field was registered by a scanning nitrogen-vacancy (NV center)-based magnetometer to find the profile of the field-generating electric current. In the ohmic regime, the current is concentrated near the narrowing boundaries, and at a temperature of < 200 Ê – it is closer to the center. This is consistent with a crossover from diffusive to viscous electron transport. It was also shown that for a liquid type behavior of the current the Landauer-Sharvin conductance limit corresponding to the ballistic regime of electrons is overcome. [11] Jenkins A et al. Phys. Rev. Lett. 129 087701 (2022)

Laboratory modeling of astrophysical discs

In gas disks around black holes, a high accretion rate must be accompanied by angular momentum transport from the central disk region outwards. Exact mechanisms responsible for the angular momentum transport are not yet clear. This may be magnetorotational instability or a purely hydrodynamic transport without a magnetic field. Along with analytical and numerical calculations, one of the research methods is astrophysical disc modeling in laboratory experiments. M. Vernet, S. Fauve, and C. Gissinger from the Paris University (Sorbonne) designed a new experimental device [12] to investigate the angular momentum transfer by a turbulent flow of an electrically conducting fluid confined in a thin disk. Two contributions to the local angular momentum transport are identified: one from the poloidal recirculation induced by the presence of boundaries and the other from turbulent fluctuations in the bulk. The latter provides an efficient angular momentum transport irrespective of molecular viscosity of the fluid and leads to Kraichnan’s prediction for the Nusselt number. In this so-called ultimate regime, the experiment, therefore, provides a configuration analogous to astrophysical accretion disks. This testifies, first, to a correct theoretical description of the transportand, second, allows prediction of the accretion rates. In 1973, the theory of astrophysical discs was considerably developed in the study of N.I. Shakura and R.A. Syunyaev. For disc accretion, see also [13-15]. [12] Vernet M, Fauve S, Gissinger C Phys. Rev. Lett. 129 074501 (2022) [13] Beskin V S Phys. Usp. 46 1209 (2003); UFN 173 1247 (2003) [14] Kurbatov E P, Bisikalo D V, Kaygorodov P V Phys. Usp. 57 787 (2014); UFN 184 851 (2014) [15] Shakura N I et al. Phys. Usp. 62 1126 (2019); UFN 189 1202 (2019)

Sources of extragalactic neutrinos

The neutrino telescope IceCube in Antarctica registers 100-TeV to 10-PeV cosmic neutrinos of yet unknown origin [16]. Several studies showed that the sources of these neutrinos correlate with position of some blazers, i.e., active galactic nuclei with jets pointed towards the Earth. The important evidence of the kind has recently been obtained by Yu.Yu. Kovalev and his colleagues [17], who considered correlation with blazar radio emission. S. Buson (University of Wurzburg#, Germany) and her co-authors have carried out a search for blazars and neutrinos in IceCube over 7 years of observations by new methods – without additional assumptions and with rather uniform samples of events [18]. In such an approach it has been obtained that 10 of 19 regions with the largest number of neutrino events (hot spots) coincide with blazar positions. This confirmed the conclusions suggesting relation between the neutrino origin and blazars. The probability of chance association was estimated as 6 × 10−7. [16] Troitsky S V Phys. Usp. 64 1261 (2021); UFN 191 1333 (2021) [17] A. Plavin, Yu.Y. Kovalev, Y.A. Kovalev, and S. Troitsky, The Astrophysical Journal 894 101 (2020) [18] Buson S et al. The Astrophysical Journal Letters 933 L43 (2022)

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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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