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| Issue 12, 2024 |
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Extracts from the Internet | ||
The “dead cone” effect1 July 2022The “dead cone” (DC) effect predicted in 1991 by scientists from Lund University (Sweden) and the Leningrad Institute of Nuclear Physics of the USSR Academy of Sciences (now RAS) Yu. Dokshitzer, V. Khoze, and S. Troyan [1] was for the first time directly observed in a Large Hadron Collider experiment. Previously, only indirect indications of this effect had been obtained.Collision of high-energy particlescauses a cascade production of partons (quarks and gluons) that form a parton shower, from which experimentally accessible particles appear through hadronization. The DC effect consists in suppression of gluon emission within a certain cone with axis along the quark motion initiating a particle jet.To detect the DC was difficult because of the problem with fixing the direction of the initiating quark motion and an exclusion of DC-filling background events. In the new study [2] on the ALICE detector, parton showers are reconstructed by singling out at the beginning of the jet of D0-meson containing a shower-initiating c-quark. This allowed revealing DC with statistical confidence of 7 σ, which is another successful verification of quantum chromodynamic predictions. Researchers from several Russian institutes take part in the ALICE collaboration. For multiple particle production in quantum chromodynamics, see [3]. [1] Dokshitzer Yu L, Khoze V A, Troyan S I, Journal of Physics G: Nuclear and Particle Physics 17 1602 (1991) [2] Acharya S et al. Nature 605 440 (2022) [3] Dremin I M Phys. Usp. 45 507 (2002); UFN 172 551 (2002) Anderson localization on harmonics1 July 2022A. Dikopoltsev (Israel Institute of Technology - Technion) and his co-authors performed an experiment, where they first observed the Anderson localization effect due to disorder spectral modes (Fourier transform of the two-point correlation function of the potential) going beyond the region of the wavenumbers corresponding to the disorderitself [4]. Anderson localization was predicted to be possible also on this “virtual” disorder (by analogy with atomic transitions through virtual levels). In the experiment, two optical-fiber loops several kilometers longhad path difference of 100 ns, were connected through a splitter, and had phase modulators. Light pulses generated by a laser diode were split into many other pulses which propagated randomly about the optical fiber and scattered with a phase change. In this synthetic photon medium, localization was characterized by pulse gathering into Gaussian wave packets. The maximum localization effectfell to the wave numbers twice the mean wave numbers of the disorder spectrum. [4] Dikopoltsev A et al. Science Advances 8 eabn7769 (2022) Polariton Bose-Einstein condensate1 July 2022V. Ardizzone (Institute of Nanotechnology (Nanotec), CNR, Italy) and their co-authors demonstrated experimentally Bose – Einstein condensateof polaritons corresponding to the bound state in the continuum [5]. Optically bound states in the continuum, predicted theoretically in 1929 by J. von Neumann and E. Wigner, are topological states with energy in the mode spectrum, which can propagate in the surrounding space. Investigated was a Bose-Einstein condensateof polaritons (bound systems of excitons and photons) on a heterostructure of GaAs layers. Condensation was reached under low-density conditions, which allowed examination of bound state effects in the continuum. Double peaks and the line width narrowing were observed. The vortex polarization associated with bound state charge in the continuum was also seen. The analyzed properties of polariton condensate are well consistent with the results of theoretical calculations and offer a new avenue for polariton condensate control in photon structures. [5] Ardizzone V et al. Nature 605 447 (2022) A continuous time crystal1 July 2022Time crystals are systems whose properties in the lower energy state periodically repeat in time like spatially periodic ordinary crystal lattices. The conception of time crystals was proposed by F. Vil’chek in 2012. In 2017, time crystals were first demonstrated in a nonequilibrium system, but the time crystals created since then were discrete – they were maintained by a periodically varyingpumping field (the crystal oscillations themselves in the state with the lowest energy proceeded with a different frequency). P. Kongkhambut (University of Hamburg, Germany) and their co-authors were the first to demonstrate a continuously pumped “continuous time crystal” [6]. A Bose-Einstein condensate of 87Rb atoms in an optical cavity with a continuously enhancing laser pumping was investigated. Registered were oscillations of the number of photons in the cavity following the limiting cycle stable under perturbations. This confirmed the occurrence of a continuous time crystal. Moreover, with increasing noise levelthe time crystal “melted”, i.e., the crystal fraction in the system decreased. [6] Kongkhambut P et al. Science, published online June 9, 2022 ă. An unusual neutron star1 July 2022M. Caleb (University of Manchester, Great Britain; University of Sydney and ASTRO3D-Centre, Australia) and their co-authors discovered a neutron star (NS) with unique spectro-temporal properties of radio emission [7]. Previously, radio-emitting NSs with maximum spin period of 23.5 s had been known and radio emission had been considered to cease strongly with NS spin deceleration. The discovered object PSR.J0901-4046 has a record period of 75.88 s and the dynamic age of 5.3 mln years. It is surrounded by a diffuse structure, which is probably the remnant of the supernova whose burst produced the NS. Radio emission is quasi-periodic – rather chaotic subpulses are observedwithin the profiles of principal pulses and in the intervals between them. These properties may probably help to clarify the emission mechanism and the object formation evolution. PSR.J0901-4046 may appear to be an old magnetar, but it is not yet absolutely clear how radio emission from such a slowly rotating NS is generated and what the origin of the observed quasi-periodicity is. Furthermore, X-ray emission typical of radio-emitting magnetars is not observed in PSR.J0901-4046. Since the sources like PSR.J0901-4046 are very difficult to discover, the population of these objects can be rather large. For neutron star magnetospheres, see [8]. [7] Caleb M et al. Nature Astronomy, published online May 30, 2022 ă. [8] Beskin V S Phys. Usp. 61 353 (2018); UFN 188 377 (2018) |
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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