Extracts from the Internet

CP asymmetry in D0 → K-K+ and D0 → π-π+ decays

The difference of CP asymmetry ΔACP in D0 → K-K+ and D0 → π-π+ decays was measured in the LHCb experiment. The asymmetry ACP in each of the two above-mentioned channels is determined as a relative difference of the decay rates for particles and the corresponding antiparticles. In the Standard Model, ACP is expressed through elements of the Kabibbo-Kobayashi-Maskawa matrix. The theoretically expected value is ΔACP<1%, but an exact calculation is hampered by uncertainties in the parameters of the theory. The CP symmetry violation was reliably measured earlier for K and B mesons, whereas for charmed particles the result remains ambiguous. In the LHCb experiment, pp collisions at center-of-mass energies of 7 TeV and 8 TeV were examined. For D0 → K-K+ and D0 → π-π+ decays the value ΔACP=[-0.10±0.08(stat)±0.03(syst)]% was obtained to an accuracy best for today, that is, at the attained precision level the difference in ACP values was not found. Source: Phys. Rev. Lett. 116 191601 (2016)

One-way quantum steering

Two independent groups of researchers headed by C.-F. Li (University of Science and Technology of China) and G. Pryde (Griffith University, Australia) demonstrated the effect of one-way quantum Einstein-Podolsky-Rosen (EPR) steering in qubit experiments on the basis of photon pairs. The conception of steering was introduced by E. Schrodinger as applied to the reduction of the wave function of a distant system. The one-way steering effect occurs when measurements of the state of one of the two quantum-entangled particles suggest a conclusion about the state of the second particle whereas the inverse is impossible, that is, the measured state of the second particle does not help in finding the state of the first one. The one-way steering effect, which was first demonstrated by D.J. Saunders et al. in 2010, had been observed before only in the case of a limited class of Gaussian states. However, the Gaussian states alone are insufficient for practical application, and in the two new experiments a wider class of states including the Werner states of photons was examined. The violation of “steering inequalities” proving asymmetry was tested. The asymmetric nonlocality can find application in quantum communication network when verification is only possible for one data transmission direction. Sources: Phys. Rev. Lett. 116 160403 (2016), Phys. Rev. Lett. 116 160404 (2016)

Design of superconductors with defined properties

High-temperature superconductors are capable of transmitting high current without thermal loss, but the maximum (critical) loss-free current is limited because of the motion of superconducting vortices and is practically completely determined by the defect-induced pinning of the superconducting vortices. The defect distribution in samples can in many cases be purposefully varied, which opens up the prospects of creating superconductors with prescribed characteristics. However, the complicated character of vortex-defect interaction that can hardly be predicted in analytical calculations presents a problem. I.A. Sadovskii (Argonne National Laboratory, USA) with colleagues worked out a new approach to the design of superconductors with prescribed properties based on computer simulation of vortex-defect interaction with allowance for experimentally known properties of superconductors. Modeled was the nonstationary dynamics of vortices upon their scattering by defects within the framework of Ginzburg – Landau theory. The results of calculations carried out on graphical processors were compared with the experimental angular dependence of critical current in REBa2Cu3O7-δ samples (where RE is a rare-earth element) with two sets of defects. In the first case the sample was used with a chemically synthesized massif of nanorods of barium zirconid. In the second case the first sample was additionally exposed to heavy lead ions leaving through tracks at an angle to the existing defects. The results of calculations showed good quantitative agreement with experimental data, which became a successful verification of the new approach. In particular the conclusion was confirmed that the contribution of defects is not additive. In future such computations will be used to calculate an optimal concentration and types of defects to maximize the critical current in a superconductor. After that, samples with prescribed properties will be synthesized on the basis of given calculations. Source: Advanced Materials, online publication of March 31, 2016

Half-integer total angular momentum of a photon

K.E. Ballantine, J.F. Donegan and P.R. Eastham (Trinity College, Dublin, Ireland) conducted an experiment demonstrating that the total angular momentum of a photon (a mixture of spin and orbital angular momenta) can in some cases assume half-integer values in units of the Planck constant h/(2π) for the reason that the character of electromagnetic field quantization may change with decreasing system’s dimension. A similar effect was observed earlier in application to electrons in quasi-two-dimensional systems. The described experiment was aimed at examining photons in a beam of light in which the polarization vector was bounded by the plane of the beam cross section. The beam underwent certain transformations of phase and polarization with the help of biaxial crystals and prisms. The beam was transmitted through a Mach – Zehnder interferometer for singling out states with certain angular momenta, and at the output the photons were registered by a photodiode. The presence of photons with half-integer angular momenta was established by the characteristic spectrum of shot noise in the photodiode current which was consistent to high accuracy with the theoretical prediction. Source: Science Advances 2 e1501748 (2016)

History of Universe reionization

Using the Planck Telescope data on anisotropy of cosmic microwave background radiation it became possible to specify the time dependence of the Universe reionization. Nearly 13 billion years ago a repeated hydrogen ionization of the Universe took place. A possible reason for this process was radiation of the first stars. The Thompson optical depth τ=0.058±0.012 — an important parameter integrally characterizing reionization — was found by the Planck Telescope data. To investigate the reionization history, additional data were used on the kinematic Sunyaev – Zeldovich effect obtained on Atacama Cosmology Telescope and South Pole Telescope. The use of these data made it possible to remove the existing degeneracy in the parameters of the cosmological model. The average redshift at which reionization occurred is found to lie between z=7.8 and z=8.8, and the reionization period duration is Δz<2.8. The reconstruction of the time dependence of reionization is likely to provide better insight into the origin of its sources. Source: arXiv:1605.03507 [astro-ph.CO]

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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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