
Triangular singularity
1 October 2021
Quark interaction becomes stronger with decreasing energy, which hampers calculation of quark bound states. Several states unpredicted theoretically and referred to as exotic were observed in experiments [1, 2]. These states are worth studying, for it may clarify the properties of strong interaction. A resonancelike peak, called a_{1}(1420), has recently been found in the COMPASS experiment performed at an energy of 1.4 GeV in CERNin the study of collisions of πmesons with a proton target. This peak cannot be qualified as an ordinary meson resonance since it has a small width and is close to the ground state. It was assumed to be a new particle – a tetraquark or a diquarkantiquark molecule. M. Mikhasenko, B. Ketzer, and A. Sarantsev (University of Bonn, Germany and Kurchatov Institute) have hypothesized that the peak a_{1}(1420) can be explained by the socalled triangular singularity (a triangle at the diagram), when the ground state decays into K^{*} and antiK with subsequent decays K^{*}→Kπ, K antiK→f_{0}(980). But before the decay, kaons exchange quarks, which imitates the occurrence of new particles. G.D. Alekseev (JINR) and his coauthors have performed new detailed calculations [3] taking into account spin effects and higherorder scattering processes. The calculations have shown that the triangularsingularity model describes well experimental data requiring no new particles. Thus, the triangular singularity, predicted by L.D. Landau in 1959 [4], has been revealed in the lowenergy meson sector perhaps for the first time.
[1] Kalashnikova Yu S, Nefediev A V Phys. Usp. 62 568 (2019); UFN 189 603 (2019)
[2] Zhukova V I, Nefediev A V, Pakhlov P N, Eidel’man S I Phys. Usp. 64 468 (2021); UFN 191 492 (2021)
[3] Alexeev G D et al. Phys. Rev. Lett. 127 082501 (2021)
[4] Landau L D Nuclear Physics 13 181 (1959)
Ghost hyperbolic polaritons
1 October 2021
W. Ma (Huagune University of Science and Technology, China) and their coauthors predicted theoretically and demonstrated experimentally a new type of hyperbolic polaritons (with a hyperbolic dispersion law) [5]. Polaritons are quasiparticles resulting from interaction between photons and excitations of the medium [6, 7]. Known earlier were bulk and surface hyperbolic (Dyakonov) polaritons only. New polaritons, called “ghost polaritons” propagate across the surface, but have nonvanishing wave fronts in the bulk. They are a solution of the Maxwell equation in the case when the optical axis is slanted with respect to the surface and result from the interaction of IR photons with phonons in anisotropic media. The mineral calcite was employed in the experiment, in which elements of the permittivity tensor along and across the surface have different signs. Scanning optical microscopy of scattering type near field was used. Directional (raylike) diffractionless propagation of ghost hyperbolic polaritons was observed at a distance of over 20 µm  a record distance for roomtemperature polaritons. Polaritons may find practical application in subdiffraction microscopy.
[5] Ma W et al. Nature 596 362 (2021)
[6] Gavrilov S S Phys. Usp. 63 123 (2020); UFN 190 137 (2020)
[7] Gulyaev Yu V, Tarasenko S V, Shavrov V G Phys. Usp. 63 872 (2020); UFN 190 933 (2020)
Quantized vortices with violated symmetry
1 October 2021
In continuous phase transitions, a random local choice of symmetrybreaking order parameter maybe responsible for the formation of topological defects, such as quantized vortices. This process is known as the Kibble – Zurek mechanism. Researchers from Aalto University (Finland), Lancaster University (Great Britain), and L.D. Landau Institute of Theoretical Physics (Russia) examined experimentally [8] the vortex formation process with an integral and halfintegral flux value in the #He transition to the polar phase in the presence of a symmetrybreaking magnetic field. As a result of symmetry violation, the choice of the order parameter in different regions is already not quite random. It has been established that when the length scale associated with the bias field becomes smaller than the Kibble – Zurek length, the Kibble – Zurek mechanism is replaced by the adiabatic regime, and vortex formation is exponentially suppressed. (For ITP works, see [9]).
[8] Rysti J et al. Phys. Rev. Lett. 127 115702 (2021)
[9] Volovik G E Phys. Usp. 62 1031 (2029); UFN 189 1104 (2029)
Quantum complementarity of waveparticle duality
1 October 2021
The quantum complementarity principle is of special interest in application to corpuscularwave duality since it concerns the fundamental aspects of quantum mechanics. T.H. Yoon and M. Cho (Institute of Fundamental Research and University of Korean Republic) experimentally verified [10] the new complementarity relations derived by X.F. Qian and G.S. Agarwal in 2020. These relations characterize passing over from wave to corpuscular description. They used an interferometer with two nonlinear crystals, in which quantumentangled photons were born. Input signals in the arms were generated by synchronized optical lasers, and the degree of corpuscularity could be changed by changing the intensity of one of the beams. In the experiment, the complementarity relations were verified and the relation between the quantum source purity and entanglement obtained by the authors was confirmed. It has also been demonstrated that the experimental scheme with two downconversion processes is the best for investigation of the complementarity principle in the case of corpuscularwave dualism.
[10] Yoon T H, Cho M Science Advances 7 eabi9268 (2021)
Global structure of the Universe
1 October 2021
Investigation of relic radiation anisotropy provides important information on processes in the early Universe and allows drawing certain conclusions regarding its global structure. Still unclear is the question of Universe topology – of how the space goes to infinity or closes up to itself on large scales. In ordinary cosmological models with an infinite threedimensional Euclidean space, twopoint correlations of relic radiation fluctuations must be present on all scales. However, the observations of WMAP and Planck satellites have shown that these correlations are suppressed on scales separated by angles above 60°. To explain these facts, models of the Universe with finite spatial scales were considered, for example, a threedimensional torus topology model. R. Aurich (Ulm University, Germany) and his coauthors [11] proposed a new observational criterion based on statistics of the relic radiation temperature gradient, sensitive to scales of the spatial cross sections of the Universe. The application of this criterion has shown that the torus model with a circumference of about three Hubble scales is well consistent with the available observational data and solves the question of twopoint correlation suppression. However, further research is needed to draw a reliable conclusion concerning the global structure of our Universe. For observation of relic radiation, see [12].
[11] Aurich R et al. arXiv:2106.13205 [astroph.CO]
[12] Verkhodanov O V Phys. Usp. 59 3 (2016); UFN 186 3 (2016)

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.
