Excess of events in XENON1T
1 August 2020
In the XENON1T experiment carried out in Gran Sasso National Laboratory (Italy) , an excess of recoil electrons was observed , which can be explained by the scattering of new particles by electrons not described by the Standard Model. The XENON1T detector contains 2 tons of liquid xenons visible through photomultipliers. The low background at the Laboratory allows search for rare particles and interactions. The electron scatterings registered at the energy of 1 to 7 keV exceeded the expected value of 232±15 background events by 53. The excess is considered to be possibly due to #-decays of tritium nuclei in the xenon composition present as an impurity. This explanation does not imply going beyond the Standard Model, but the tritium content in the detector has not yet been measured. The observed excess is best of all (with statistical significance of 3.5σ) explained by the scattering of axions, i.e., hypothetic particles born inside the Sun in various processes. The axions proposed initially for the solution of the problem of CP invariance in strong interactions are regarded as one of the main candidates for the role of dark matter particles in the Universe. With a lower significance of 3.2 σ the excess can be explained by the scattering of neutrinos from the Sun under the codition that neutrinos have the magnetic moment μν=(1.4-2.9)×10−11 of Bohr magnetons. This value is close to the upper limit o btained by the direct method in Borexino experiment but contradicts the indirect astrophysical restrictions. The event excess is not excluded to be due to other particles, for instance, “dark photons”. More reliable conclusions will require a further gain in statistics.
 Bettini A Phys. Usp. 44 931 (2001); UFN 171 977 (2001)
 Aprile E et al., arXiv:2006.09721 [hep-ex]
A branched flow of light
1 August 2020
In a disordered scattering medium, waves may propagate along individual channels divergent and branching like tree branches due to diffraction and caustic formation. In optics, a branched flow was discovered and investigated in 2002 in P.N. Lebedev Physical Institute of the Russian Academy of Sciences (FIAN) and reported by A.V. Startsev and Yu.Yu. Stoilov [3, 4] (for a more detailed description of this phenomenon see ). A thin soap film where laser radiation of optical and IR ranges was introduced was observed through a microscope (see [3 – 6]). The film showed natural thickness fluctuations that caused fluctuations of the effective refractive index. Transverse light scattering allowed observation of light intensity distribution in the film. Contrary to expectations, some light threads did not spread chaotically, but remained collimated along large paths to branch then into smaller fibers. In a new akin experiment, A. Patsyk (Israel Institute of Technology - Technion) with co-authors observed branched laser light in a soap film . The film thickness was equal to 1-2 light wavelength and the light was directed to it through optical fiber. A branched propagation similar to that reported earlier by A.V. Startsev and Yu.Yu. Stoilov was observed and its statistical characteristics were found. They are of a universal form and only depend on the correlation length of inhomogeneities and on the average variation of the refractive index. The distance at which the flow begins branching is also described by a simple universal dependence. A branched flow was observed earlier for electron waves in semiconductors. It is predicted that in a three-dimensional case waves can propagate along branched two-dimensional surfaces.
 Startsev A V, Stoilov Yu Yu, Quantum Electron. 33 380 (2003)
 Startsev A V, Stoilov Yu Yu Quantum Electron. 34 569 (2004)
 Stoilov Yu Yu Phys. Usp. 47 1261 (2004); UFN 174 1359 (2004)
 Startsev A V, Stoilov Yu Yu Quantum Electron. 42 750 (2012)
 Patsyk A et al. Nature 583 60 (2020)
Cascade of phase transitions in graphene
1 August 2020
The study of bilayer graphene whose layers are turned with respect to each other by the so-called “magic angle” ≈ 1,1° testified to the fact that cooling must induce phase transitions due to electron band occupation. U. Zondiner (Weizmann Institute, Israel) with colleagues revealed  a cascade of such phase transitions. Graphene on substrate was investigated at a temperature above the superconducting transition temperature. The band occupation was examined by measuring the electron compressibility using a nanotube-based one-electron transistor. Characteristic jumps due to band occupation were revealed, after which a Dirac type dispersion relation occurred. Some quantum degrees of freedom disappear under cooling, but new collective degrees of freedom appear. The results of measurements are interpreted as retraction of electrons from partially occupied former bands by the new band. The properties of high-temperature graphene may inherit a number of low-temperature effects, which may provide insight into the mechanisms of superconductivity. For graphene and its properties see [9-11].
 Zondiner U et al. Nature 582 203 (2020)
 Morozov S V, Novoselov K S, Geim A K Phys. Usp. 51 776 (2008); UFN 178 776 (2008)
 Lozovik Yu E, Merkulova S P, Sokolik A A Phys. Usp. 51 727 (2008); UFN 178 757 (2008)
 Ratnikov P V, Silin A P Phys. Usp. 61 1139 (2018); UFN 188 1249 (2018)
Superradiation in acoustics
1 August 2020
Ya.B. Zel’dovich, A.V. Rozhanskii and A.A. Starobinskii [12 – 14] predicted theoretically the possibility of electromagnetic wave amplification under their scattering by a rapidly rotating metallic cylinder and also wave amplification and production of particles of a rotating black hole. This process was called “superradiation”. On its basis, S. Hawking predicted the effect of black hole quantum evaporation. The condition of amplification has the form ω < lΩ, where ω is the incident wave frequency, Ω is the rotation frequency and l is the order of the angular mode. Electromagnetic wave superradiation has not been observed in experiments because of the necessity of a very fast rotation. The condition for amplification is simpler attained for sound waves, and M. Cromb (the University of Glasgow, Great Britain) with co-authors observed it for the first time in their acoustic experiment . Instead of scattering by cylinders, the sound was transmitted between two discs through an absorbing medium. 16 loudspeakers were placed along the circumference of a motionless disc. Each of them sounded with a phase shift so that the sound front had a spiral structure with different l. Two microphones were placed on the rotating disc. Between the two discs was a thin layer of absorbing foam, and the sound could only reach the microphones after passing through the foam. The Zel’dovich condition was met beginning with the rotation frequency of 15 Hz. A 30 % amplification weas observed above 25 Hz, which confirmed the prediction of Ya.B. Zel’dovich and his colleagues.
 Zel’dovich Ya B JETP Lett 14 180 (1971)
 Zel’dovich Ya B J. Exp. Theor. Phys. 35 1085 (1972)
 Zel’dovich Ya B, Rozhanskii L V, Starobinskii A A Radiophys. Quantum
Electron. 29 761 (1986)
 Cromb M et al. Nature Physics, online-publication of 22 June 2020
Gravitational-wave burst GW190814
1 August 2020
The gravitational wave detectors LOGO/Virgo registered the event GW190814 corresponding to the merging of compact objects with a record large mass ratio . A more massive object of the pair with mass 23.2+1.1−1.0M☉ is obviously a black hole, while the origin of the other object of mass 2.59+0.08−0.09M☉ is not yet clear. It may appear to be either the most massive neutron star or the lightest black hole of those observed in binary systems. The above-mentioned mass of the light object is near the upper boundary of admissible neutron star masses or even exceeds it. At the same time, the known astrophysical black holes have masses of ≥5M☉. Thus, the mass of the light object is typical neither of neutron stars nor of black holes. This may be a black hole formed by the merging of two neutron stars. Such a binary system could have been formed dynamically in a young star cluster. The burst source is at a distance of ≈ 240 Mpc, and no associated electromagnetic radiation from it has been registered. Owing to the large mass ratio this event made it possible to confirm the predictions of the General Relativity (GR) in the earlier unexplored region where high multipoles are excited upon object merging. For the discovery of gravitational waves see  and for GR effects see .
 Abbott R et al. The Astrophysical Journal Letters 896 L44 (2020)
 Reitze D H Phys. Usp. 60 823 (2017); UFN 187 884 (2017)
 Scheel M A, Thorne K S Phys. Usp. 57 342 (2014); UFN 184 367 (2014)
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.
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