1 May 2018
In accelerator neutrino experiments, the initial neutrino energies are as a rule unknown and can only be found through the study of reaction products. Meanwhile, the presence of a monoenergetic neutrino source would be extremely useful, for example, in the study of nuclei and in the search for sterile neutrinos. One of such sources is the decay of kaons K+ at rest. In the MiniBooNE experiment, being carried out in Fermilab (USA), neutrinos with a fixed energy of 236 MeV from K+ → μ+νμ decays were obtained and registered for the first time. K+ mesons were born in collisions of a proton beam with a target, and the scattering of K+ in a metal absorber led to their deceleration to almost zero velocity. The monoenergetic νμ arrived at the detector from the absorber ≈ 200 ns later than the background of nonmonoenergetic νμ born in other parts of the setup (in the target and in the decay pipe), which allowed distinguishing monoenergetic νμ in time. The statistical significance of monoenergetic νμ recording was 3.9σ.
Source: Phys. Rev. Lett. 120 141802 (2018)
The test of materials for thermonuclear reactors
1 May 2018
A unique experimental bench BETA (Beam of Electrons for materials Test Applications) on which one can test the effect of extreme thermal loads on construction materials to be used in the International Experimental Thermonuclear Reactor ITER was created at G.I. Budker Institute of Nuclear Physics of Siberian Branch Russian Academy of Sciences. The inner elements of ITER diverter will undergo frequent sharp heating (thermal shocks) for which reason it is of importance to investigate the stability of their surface. As distinct from the majority of earlier studies, BETA allows examination of a fast surface heating not only after its termination by examining the samples taken off the setup, but also in real time directly during the action and immediately after it. The application of an electron beam instead of laser pulses or accelerated plasma streams makes it possible to get rid of interfering light and to make extensive use of different optical methods of surface diagnostics. BETA experiments are being carried out by the joint group of workers and postgraduates of G.I. Budker Institute of Nuclear Physics of Siberian Branch Russian Academy of Sciences, Novosibirsk State University and Novosibirsk State Technical University and are supported by the Russian Science Foundation (project № 17-79-20203). BETA is based on the original source of a high-power electron beam with an arc plasma emitter. Its pulses with duration of 100-300 µs are capable of reproducing a heat shock with a power density up to 15 GW/m2 on a tungsten-target surface area of ≈ 1 cm2. The same target is exposed to a laser beam and CCD cameras are used to observe the reflected light; the straight beam is shaded and observed is only the surrounding diffuse halo due to laser radiation scattering from irregularities and defects. At first, the effect of pulses was examined that heated tungsten to a temperature not higher than the melting temperature. The effect of an electron pulse caused two successive scattering-factor heightenings. The first of them took place simultaneously with the electron pulse when the sharp temperature rise induced surface bulging with characteristic sizes of the order of grain size in the tungsten structure. However, owing to cooling and inverse elastic deformation the irregularities were smoothed out and the scattering factor decreased again. Unexpected was the fact that about a second after a pulse the scattering factor again rose sharply and further on remained constant. The second rise is explained by the occurrence of irreversible crackson the surface, but the reason for such a large time lag is unknown. Then experimentsw ere performed with much more powerful pulses that heated tungsten to temperatures much higher than the melting temperature. Then an intense flow of 2-7 µm tungsten microdrops from the surface was observed. 3D trajectories of ejected microdrops were registered by CCD cameras performing observations at three angles. The emergence of these microdrops is due to the surface melting and to the extension of heating deep into the material. In the evaporation-induced cooling the temperature is higher under than on the surface. This leads to boiling of the overheated liquid with ejection of microdrops. The experiment allows estimation of resistance of the walls of future thermonuclear reactors to fast plasma ejections (ELM events) and other like effects.
Source: Physica Scripta 93 035602 (2018), G.I. Budker Institute of Nuclear Physics of Siberian Branch Russian Academy of Sciences
Measurement of modular variables
1 May 2018
The so-called modular quantum variables connected with the periodic functions were first discussed by Y. Aharonov, H. Pendleton, and A. Petersen in 1969 in the context of Aharonov – Bohm effect. These variables are distinguished from conventional variables by the operation analogous to taking residue of division and the corresponding operators can commute even if the initial operators were noncommuting. C. Fluhmann (Swiss Federal Institute of Technology Zurich) and his colleagues implemented measurements of the modular coordinate and modular momentum of an entrapped 40Ca+ ion. The ion spin levels formed a qubit whose quantum state was involved in the measurement process. The ion was periodically affected by laser pulses and the fluorescent radiation from transitions between the levels of the ion oscillating in the trap was registered. The measurement of temporal correlators for modular variables showed violation of the Leggett – Garg inequalities and the influence of the previous measurements of the series on the results of the consequent measurements was demonstrated.
Source: Phys. Rev. X 8 021001 (2018)
Condensation of surface plasmon modes
1 May 2018
Researchers from Helsinki Polytechnic Institute (Finland) obtained Bose – Einstein condensate of surface plasmon modes which are a combination of photons and plasma oscillations generated in a two-dimensional array of gold nanometer rods at room temperature. The array was formed by T.K. Hakala with colleagues using electron-beam lithography on the glass plate surface and was immersed in a solution of organic molecules which provided weak coupling between surface excitations. The array edge was exposed to laser pumping pulses, and CCD cameras were employed to observe the excitation spectrum along the surface of the entire array. The condensate only exists during several ps but manages to thermalize through the interactions with the solution and emits short light pulses. The experiment demonstrated the transition (crossover) from the radiation of Bose – Einstein condensate to the usual laser generation.
Source: Nature Physics, online publication of April 16, 2018
The effect of neutrino on the spectrum of baryon acoustic oscillations
1 May 2018
A phase shift caused by the effect of relic neutrino background on acoustic waves that existed in the early Universe has recently been revealed in the microwave background radiation spectrum. The additional phase shift affected the position of acoustic peaks in the spectrum. The relic neutrinos made up about 41 % of the energy density in the early Universe and, like relic radiation, they are the remnants of the hot stage of its evolution. The distribution of galaxies on a large scale was also modulated by acoustic waves, and therefore the spectrum of the galactic waves must also contain an analogous phase shift. D. Baumann (the University of Amsterdam, Netherlands) et al. reported on the first measurement of neutrino-induced phase shift in the distribution of galaxies according to the BOSS DR12 data which include 1198006 galaxies on red shifts z=0.2-0.75. A nonzero phase shift was revealed with a confidence exceeding 95 %. Thus, this is another confirmation of the Standard Cosmological ΛCDM model. Source: arXiv:1803.10741 [astro-ph.CO] 100601 (2018)
Protons from Jupiter magnetosphere
1 May 2018
The PAMELA detector working on board the Russian satellite Resurs-DK was used to study variations of the proton flux from the composition of cosmic rays. Along with periodicity associated with the 11-year cycle of solar activity, an unexpected and distinct periodicity with a quasi-period of ≈ 450 days was observed for geomagnetic cutoff rigidity below 15 GV. In the researchers’ opinion, this periodicity may have been due to Jupiter in whose high-power magnetosphere protons are accelerated. These protons reach the Earth and make a small contribution to the general flux of cosmic rays. Earlier observations have already testified to the fact that in the Jupiter magnetosphere electrons are accelerated (they are registered in the minima of the 11-year cycles) and fluxes of protons and helium nuclei are possibly generated in the bursts. In view of measurement errors a real period may prove to be shorter than ≈ 450 days and may correspond to the Jupiter orbital period of ≈ 400 days. Other explanations are nevertheless not yet excluded.
Source: Èñòî÷íèê: Astrophys. J. Lett. 852 L28 (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.
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