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| Issue 11, 2024 |
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Extracts from the Internet | ||
Tetraquark1 April 2016In processing the 2002-2011 data of D0 experiment carried out at the Fermilab Tevatron collider (USA), a new particle X(5568) was discovered which is most likely to be a tetraquark, i.e., the bound state of four quarks u b anti-d anti-s or d b anti-u anti-s. It can also be represented as a combination of the light and heavy mesons B0s and π±. Other type tetraquarks and a pentaquark always containing pairs of quarks of the same flavor c anti-c that possibly make up a solitary structure (core) have already been found in earlier Belle and LHCb experiments. On the contrary, all the four valence quarks in X(5568) have different flavors. One cannot now predict exactly all the characteristics of such multiquark systems because of complicacy of the calculations. The particle X(5568) is identified by the characteristic chain of its decay X(5568) → B0sπ±, B0s → J/ψφ, J/ψ → μ+μ-, φ → K+K-. The birth rate of X(5568) in p anti-p collisions is several orders of magnitude higher than expected theoretically, and this discrepancy has not yet been explained. The measured mass and width of X(5568) decay are m=5567.8 ± 2.9(stat)+0.9-1.9(syst) MeV and Γ=21.9 ± 6.4(stat)+5.0-2.5(syst) Mev, respectively and X(5568) identification significance is estimated to be 5.1σ. Source: arXiv:1602.07588 [hep-ex] Study of the mechanism of high-temperature superconductivity1 April 2016Since the discovery in 1987 of high-temperature superconductors – cuprates not described by Bardeen-Cooper-Schrieffer theory many new theoretical approaches have been proposed to explain their properties. In 2003 C.M. Varma (University of California, Riverside, USA) formulated a theory in which the interaction of electrons with fluctuations near the quantum critical point accounts for the symmetry observed in superconductors. A new experimental study of high-temperature superconductors was performed at the Institute of Physics, Chinese Academy of Sciences (Beijing, China) using a highly stable laser in angle-resolved photoemission measurements. Bi2Sr2CaCu2O8+ crystals with different degrees of doping were examined in the normal and superconducting state. The fluctuation spectrum and the binding energy of Cooper pairs were measured. The new experiment confirmed C.M. Varma’s predictions concerning the role of fluctuations. It has turned out that in the attractive d-wave channel no interaction of fermions with excitations is observed, while it is present in the s-wave channel. This is the reason why frequency–independent fluctuations are scattered predominantly at angles of ±π/2 in the attractive channel and are angle-independent in the case of repulsive interactions. Source: Science Advances 2 e1501329 (2016) Mid- IR laser1 April 2016The ordinary quartz glass cannot be used to fabricate optical-fiber lasers operating in the mid-IR range because of its poor transparency at wavelengths λ>2.8 µm. And quantum-cascade lasers are only effective beginning with λ>3.5 µm. Thus, within the wavelength range of 2.8-3.5 µm; no laser generation methods exist that would be convenient for practical application. M.R.A. Hassan (University of Bath, Great Britain) et al. demonstrated a newmid-IR laser based on ahollow-corequartz fiber filled with gaseous acetylene as theactive medium. The emission of mid-IR photons by acetylene was earlier investigated in the single-pulse regime only. In the new experiment, continuous lasing was achieved for the first time. Part of the radiation from the main optical fiber passed through the second optical fiber (a feedback loop) and was again fed to the input of the first fiber. The loop was rather long – nearly 100 m in order to provide the necessary signal delay for stable lasing. Pumping at a frequency of 1530 nm was realized using a standard diode laser. Lasing in the rage of 3.1-3.2 µm with a pulse repetition frequency of about 2.6 MHz was demonstrated. Source: Optica 3 218 (2016) Ring laser gyroscopes in geophysics1 April 2016In the Gran Sasso National Laboratory (Italy), a ring laser gyroscope GINGERino located in an underground tunnel was used to register rotational motions excited in the Earth crust by a seismic wave from an earthquake. Along with linear perturbations weaker, rotational perturbations are also present in the wave.Traditional seismographs can register them only when assembled in a curved chain, but the sensitivity of this method is low. Rotational perturbations have already been registered by ring laser gyroscopes on the surface, but the location ofGINGERino in the underground laboratory made it possible to avoid the noise sources and to perform measurements with an accuracy much higher than before. The gyroscope operation principle rests on the Sagnac effect. Two light pulses from a helium-neon laser run towards each other along a 3.6-m ring mounted on a granite block in the tunnel under a 1400-meter rock layer. Rotation results in a frequency shift and the occurrence in the resultant signal of beats with frequency proportional to the angular velocity of rotation. The sensitivity of the device constructed by G. Saccorotti (Italian National Institute of Geophysics and Volcanology) and his colleagues made up 10-11 rad s-1 at frequencies of 0.01-1 Hz.The rotational perturbations were registered in a wave from the earthquake of magnitude 7 that took place in the Atlantic Ocean. Registration of rotational motions in the Earth crust may prove to be useful for the earthquake forecast. The GINGERino experiment is a precursor of the GINGER experiment planned for the study of the General Relativity effects. Source: arXiv:1601.05960 [physics.geo-ph] Galaxy as a source of the fast radio burst1 April 2016E.F Keane (the Jodrell Bank Observatory, Great Britain) with colleagues were the first to discover the galaxy from which one of the fast radio bursts had come. The burst FRB 150418 was initially registered by the Parkes radio telescope. Then a transient radio signal coming from the same point and fading during six days was registered by the Australia Telescope Compact Array (ATCA). Such signals are rare, and therefore the probability of accidental coincidence is very low. Finally, the redshifted galaxy (#) was discovered by an optical Subaru telescope in the region of radio burst. The mechanism of generation of millisecond bursts has not yet been ascertained. The sustained signal looks like an afterglow in the radio frequency range of short gamma-ray bursts (mergers of neutron star pairs are most likely to be their sources), and the model of giant bursts from pulsars fails to reproduce such a signal. It is quite probable that several classes of fast radio bursts have different origins. This is consistent with observation of repeated bursts from the source of the other fast radio burst FRB 121102. The Arecibo radio telescope registered repeated millisecond radio bursts with sky position and dispersion measure the same as those of the first burst FRB 121102. The observations began 2.3 years after the detection of the first burst, and within three hours of observation ten additional bursts were registered with different spectra and the lack of periodicity of their arrival time. The observation of repeated bursts supports the model of bursts from neutron stars because repeated bursts are impossible upon the merging of neutron star pairs. Sources: Nature 530 453 (2016), Nature 531 202 (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. | |
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