 |
RSS feeds |
|
|||||
| Issue 11, 2024 |
|
||||||
|
|
|
||||||
Extracts from the Internet | ||
Lifetime of Λ0b and anti-B01 October 2013The collaboration LHCB in which researchers from Russian institutes play an important active role measured the ratio of lifetimes of the Λ0b and anti-B0 mesons born of pp-collisions at the center-of-mass energy 7 TeV. The particles Λ0b were first studied on the basis of their decay chains Λ0b → J/ψπ+K-. The measured ratio of the life-times was 0.976 ± 0.012(stat.)±0.006(syst.). Taking into account the lifetime of anti-B0 known from other experiments, the lifetime of Λ0b is found to be 1.482 ± 0.018(stat.) ± 0.012(syst.) ps. According to theoretical calculations carried out by “heavy quark expansion” technique, the nearness of lifetimes of the two particles finds its explanation in that both incorporate a b-quark whose properties to a large extent dictate the properties and lifetime of their decay. Source: Phys. Rev. Lett. 111 102003 (2013) Relaxation in an isolated quantum system1 October 2013T. Langen (Vienna institute of technology, Austria) et al. traced the transition of an isolated quantum system which initially was placed in a defined quantum state, to a classical equilibrium state. An elongated cloud of several thousand Bose atoms 87Rb placed on an “atomic chip” was split into two parts placed in phase-coherent states. The interaction between the parts lead to the chaotization of the quantum phases and to relaxation to a classical state even without any external effect on the quantum system. The origin of relaxation gradually spread from the point of contact of the two parts to the entire system. Phase correlation at different points of the cloud was measured from the observations of wave interference of the atoms at this stage of cloud expansion once the trap potential has been turned off. The maximum velocity of relaxation spreading came to 1.2 ± 0.1 mm s-1. An alternative version of cloud evolution could be not the expansion of relaxation region but a simultaneous transition of the entire system to a new state; however, the experiment rejected this version. The mechanism of transition of the insolated quantum system to a classical state has already been discussed in theoretical publications and the limiting maximum velocity given above has already been predicted. Source: Nature Physics 9 640 (2013) Structure of metallic glasses1 October 2013A. Hirata and his colleagues at the Tohoku university, Japan, and Joint Institute of Hgh Temperatures of the Russian Academy of Sciences, Russia have for the first time resolved individual elemental icosahedra (regular twenty-facet forms)in the atomic sctucture of metallic glasses, i.e. solid metallic substances that are ordered on small scale but amorphous on a large scale. F.C. Frank theoretically suggested in 1952 that atoms in metallic glasses join into icosahedral structures. Manifestations of icosahedral structures of metallic glasses have indeed been observed in numerous x-ray and neutron diffrraction experiments though only on average. The new experiment succeeded in observing individual elementary icosahedra. To achieve this, a very narrow electron beam (only 0.36 nm wide) was passed through a thin layer of metallic glass Zr80Pt20. If the beam passed through an icosahedron, the observed picture was characteristically an electron defraction pattern depending on the orientation of the icosahedron. In this manner, symmetry axes of the second, third and fifth orders were observed, as derived from the positions of defraction spots. It was found that the icosahedra in the metallic glass are somewhat distorted. The reason for this is that space cannot fully fill the space by regular icosahedra and therefore they are distorted in the metal glass as a result of competition with face-centered cubic structure. Source: Science 341 376 (2013) Neutrons produced in artificial electric discharges1 October 2013Neuton fluxes at energies from ≈ 10-2 eV to tens of MeV, generated during natural atmospheric discharges have been recorded on numerous occasions (see Phys. Usp. 55 532 (2012)). A.V. Agafonov (P.N. Lebedev Institute of Physics of The Russian Academy of Sciences) and his colleagues were the first to record neutron pulses generated by artificial high-voltage discharges set up in lab environment. In order to generate electric field intensities on the order of ≈1 MV m-1, the Marks generator was applied and discharge current reached 10 to 15 kA. Neutrons were recorded both by calorimetric method using track detectors and in real time, using plastic scyntillation detectors. Track detectors fixed alpha particles from the reaction 10Be + n → 7Li + 4He occurring due to low-energy (thermal) neutrons and due to the reaction 12C + n → 3α + n' initiated by neutrons with energies ≥ 10 MeV. The experiment was unique in that, first, the track detectors were for the first time placed directly in the discharge zone. The confidence in observing triplets of alpha particles produced in disintegration of the nuclei 12C, is higher than 10 σ. Neutron pulses were observed in 25-30% of cases from the total number of all discharges when the neutron flux was several times higher than the background level caused by cosmic rays. Neutron pulses were born at the very initial stage of the electric discharge and correlated with x-ray pulses. The neutron flux varied with distance slower than according to the inverse squares law and therefore, the region of generation of neutrons was likely not limited to a sigle point of the discharge zone but was extended. So far the mechanism of generation of neutrons due to electric discharges remains unclear. Source: Phys. Rev. Lett. 111 115003 (2013) New PAMELA results1 October 2013The PAMELA detector on board the Russian satellite “Resource DK1” received new data on the content of positrons e+ as a component of cosmic rays. Earlier PAMELA measurements established that the relative content of e+(with respect to the sum of e+ and e- fluxes) begins to increase in the 10 GeV energy range, even though according to standard computations, this ratio should be expected to decrease with energy. This result was later confirmed by the data of the Enrico Fermi Space Observatory and the AMS-2 experiment on the International Space Station. The new PAMELA results were obtained during the period of minimum solar activity in 2006-2009 and are characterized with very good accuracy. The growth in relative increase of the e+ flux was confirmed up to energies of about 300 GeV, with the GeV spectrum e+ up to 200 GeV being measured while that in the range 200-300 GeV being bounded from below. It is important that in contrast to earlier PAMELA measurements, not only the relative but the absolute value of the flux e+ was measured. The cause of the growth in the relative flux e+ has not been clarified so far. Annihilation of dark matter, acceleration of positrons in pulsars or in stellar flashes were suggested as possible explanations, as well as additional injection and acceleration of particles under supernova explosions. Russian scientists from National Research Nuclear University “MEPhI”, P.N. Lebedev Physical institute, and A.F. Joffe institute of the Russian Academy of Sciences took part in the international PAMELA experiment. Source: Phys. Rev. Lett. 111 081102 (2013) |
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. | |
|
© 1918–2024 Uspekhi Fizicheskikh Nauk Email: ufn@ufn.ru Editorial office contacts About the journal Terms and conditions |