t-quark production in proton-nucleus collisions
1 February 2018
Production of t-quarks, the heaviest particles in the Standard Model, were earlier observed on Tevatronin pp collisions only, where t-quark had been first discovered, and also in pp collisions on the Large Hadron Collider. In the CMS experiment carried out on LHC, the t-quark production was first registered in the collision of protons with lead nuclei with the center-of-mass energy of s1/2=8.16 TeV. Earlier in analogous experiments, s1/2 and the intensity of ion beams were insufficient for t-quark recording. In the CMS experiment, t-anti-t pairs were most often produced in the interaction of two gluons gg → t-àíòè-t+..., and then t-quarks decayed rapidly into W-bosons. The events were chosen in which one of the W-bosons decayed along the lepton channel and the other along the hadron channel, which led to the formation of four jets and a high-energy unit lepton. 710 events satisfying the criteria of t-quark production were registered, and the statistical significance of the result was 5σ. The measured t-quark production cross section σtt=45 ± 8 nb is well consistent with the theoretical calculations.
Source: Phys. Rev. Lett. 119 242001 (2017)
Verification of the Jarzynski relation for a single ion
1 February 2018
Much attention is being paid now to the study of thermodynamic relations at the quantum level. As distinct from most relations, which in the case of nonequilibrium processes have the form of inequalities, the Jarzynski relation is written down as an equality relating the mean work done under an arbitrary nonequilibrium change in the state of the system to the difference of equilibrium free energies in its final states. Similar relations were earlier derived by G.N. Bochkov and Yu.E. Kuzovlev (for the history of the question see
Phys.-Usp. 54 625 (2011) è Phys.-Usp. 56 590 (2013)). In their experiment, T.P. Xiong (Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences) et al. confirmed the Jarzynski relation for a single 40Ca+ ion in the Paul trap. The ion spin states prepared and measured using laser pulses coded the quantum bit. Examined was the relation between the nonequilibrium processes and the information transferred during measurements, and the Jarzynski relation was confirmed successfully for the evolution of the system between two measurements.
Source: Phys. Rev. Lett. 120 010601 (2018)
The properties of supercooled water
1 February 2018
Water has some interesting physical properties that distinguish it from the majority of other liquids. The growth of thermodynamic functions of water, such as isothermal compressibility, under deep cooling shows that two phases of liquid supercooled water with different densities may existand fluctuate into each other, and at a high pressure a critical point must exist where these phases coexist. It is however difficult to analyze this region of phase diagram directly because water crystallization is spontaneous in this region even without participation of impurities. A. Nilsson (Stockholm University, Swiss) with colleagues used the method of X-ray pulse scattering by drops to investigate the properties of supercooled water at a temperature of ≈ 228 K and at an almost zero pressure. Drops ≈ 14 µm in diameter were ejected to vacuum and underwent evaporative cooling. Their temperature was determined by the duration of cooling at different distances from the nozzle. The isothermal compressibility, as well as the correlation length found from the character of X-ray diffraction on liquid drops that did not have enough time to crystallize turned out to have maxima at 227.2 ± 1 K for ordinary water Í2Î and at 233.0 ± 1 K for heavy water D2O. These measurements confirm the hypothesis about the existence of two phases of supercooled water and testify indirectly to the presence of a critical point at a high pressure.
Source: Science 358 1589 (2017)
1 February 2018
Ordinary radio wavesused in radio communication attenuate rapidly in dense media, but very low-frequency (VLF) waves with f=3-30 kHz can penetrate through very large depths. The development of atomic magnetometers capable of measuring superlow magnetic fields paves the way for application of VLF waves for the purpose of communication and positioning in buildings, tunnels, underground and in water although the transmission capacity of the communication channel is rather poor because of the low carrier frequency. The researchers from the National Institute of Standards and Technologies (NIST, USA) V. Gerginov, F.C.S. da Silva and D. Howe used the atomic magnetometer to record modulated magnetic signals of binary phase manipulation. An inductance coil served as a transmitter. In the magnetometer, rubidium 87Rb atoms were used in a glass vessel in the magnetic field. The magnetic signal was registered by its effect on the atomic precession frequency and, thus, on the polarization of the laser beam transmitted through the vessel. The use of phase manipulation allowed singling out this signal over the external noise level and, first of all, over the background of alternating currentof 60 Hz and its harmonics, and the geomagnetic background. Magnetic signals of ≈ 10−12 T were registered at frequencies of less than 1 kHz from the distance of dozens of meters.
Source: Review of Scientific Instruments 88 125005 (2017)
Gravitational burst GW 170817 and β-decays of nuclei
1 February 2018
In a laboratory experiment carried out by a group of researchers from several American universities, an unexpected signal was registered coincident in time with the gravitational wave burst GW 170817 generated by the coalescence of two neutron stars. Measured was the rate of beta decay of Si-32 and Cl-36 nuclei whose samples were every half an hour put in succession into one and the same plastic scintillation detector. Before the burst GW 170817 and some time after it the rates of nuclear decay randomly fluctuated by the usual law N1/2 without mutual correlation. But the decays turned out to be strongly correlated (at the level of 95 %) during the five-hour interval following the burst. Namely, the rates of Si-32 and Cl-36 decays during the five successive series of half-hour measurements simultaneously decreased and then again increased, the probability of accidental coincidence being no higher than ≈ 6.7×10−3. If these correlations are interpreted as the effect of neutrinos or other particles emitted under neutron star merging, then from the time delay one can restrict the masses of these particles mx≤16 eV at the energy of 10 MeV. This interval includes admissible neutrino masses mν≤2 eV. The correlations of radioactive decays with the season or with the level of solar activity have already been observed in some experiments, however, at the current level of knowledge a process is unknown which might affect the decay of nuclei because the neutrino fluxes that reach the Earth are too small for this purpose. F. Fischbach et al. suggest seeking such correlations in the data of other experiments carried out during the registration of GW 170817. See also the discussion in Phys.-Usp. 41 1025 (1998),
Phys.-Usp. 43 203 (2000), and
Phys.-Usp. 43 205 (2000).
Source: arXiv:1801.03585 [astro-ph.HE]
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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