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| Issue 12, 2024 |
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Extracts from the Internet | ||
Interaction between muons and protons1 February 2013The MuCap experiment being conducted at the Paul Scherrer Institute (Switzerland) measured the rate of capture of muons by protons μ- + p → n + νμ, as a beam of μ- passed through a chamber filled with gaseous hydrogen; one of the predictions of quantum chromodynamics was confirmed. In ≈ 97% of cases, the intermediate state was the muonic hydrogen — a hydrogen atom with a muon replacing the electron — in singlet state. The rate of capture depends on formfactors of hadronic weak currents, among which only the value of the pseudoscalar formfactor of axial currents gP remained uncertain. The muons μ- that failed to interact then decayed in flight, μ- → e- + νμ + anti-νe, and the ejected electrons were recorded. The capture rate was measured by comparing electron fluxes in the cases of presence and absence of hydrogen in the chamber. The value of gP (for which earlier experiments gave a broad range of values from 2 to 15) was then calculated from the capture rate. The MuCap experiment yielded the value gP = 8.06 ± 0.55 which agrees well with the value gP = 8.26 ± 0.23 calculated using the chiral perturbations theory. The team on the MuCap collaboration includes some Russian researchers from the V.P. Konstantinov St. Petersburg Institute of Nuclear Physics. Source: Phys. Rev. Lett. 110 012504 (2013) Anderson localization for photons1 February 2013In 1958 Ph. Anderson predicted that waves in nonuniform media should stop propagating in diffuse manner and as the concentration of defects (scattering centers) increases above a certain threshold, they assume a localized configuration caused by multiple scattering and interference. This effect, known as the Anderson localization, was initially discussed for electron waves. The experiment by Ò. Sperling (Constanza University, Germany) and his colleagues was the first direct observation of the Anderson localization for the propagation of light. For the media Sperling et al chose titanium oxide powder TiO2 with particle sizes from 170 to 540 nm. A “photon cloud” produced in the depth of the specimen by a flash of focused laser beam expanded through the powder. The evolution of the cloud was monitored using high-speed photocamera with time resolution of less than a nanosecond. The size of the photon cloud first grew following the diffusion law ∝ (Dt)1/2 where D is the diffusion coefficient for photons. As theoretically predicted, the cloud stopped expanding as its size grew to more than the Anderson localization length given by the Mott – Ioffe – Regel limit. Absorption of photons in the medium did not affect the cloud diameter but only resulted in general drop in light intensity. The fact of Anderson localization was additionally confirmed by varying light wavelength and specimen thickness. Source: Nature Photonics 7 48 (2013) Quantum spin liquid1 February 2013Y.S. Lee (MIT, USA) and colleagues experimentally demonstrated for the first time the existence in a 3D system of quantum spin liquid which was theoretically predicted by Ph. Anderson in 1973. A quantum spin liquid is a medium consisting of magnetic excitations; by its degree of disorder, it reminds a liquid and imparts to matter unusual magnetic properties. An important property of the quantum spin liquid is that it supports quasiparticles with fractional quantum numbers. A strongly correlated quantum spin liquid was discovered in the mineral herbertsmithite — an antiferromagnetic compound ZnCu3(OD)6Cl2 — by using neutron diffraction which revealed continuum in the excitation spectrum. Measurements were performed with the neutron spectrometer of the National Institute of Standards and Technologies (NIST). The sample was a very pure and hard crystal which grew for 10 months. According to some models, the quantum spin liquid may play an important role in the mechanism of high-temperature superconductivity. Source: Nature 492 406 (2012) Negative temperatures in a gas1 February 2013The concept of negative temperature for the absolute temperature scale was introduced to describe the case of inverse population of discrete quantum levels when upper energy levels have more particles than lower levels. Then the parameter T in Boltzmann’s formula Pi ∝ exp(-Ei/kBT) is negative, T < 0. Negative temperatures have already been implemented in systems with localized spins. Researchers at the Ludwig – Maximilian Munich University and the Max Planck Institute of Quantum Optics (Germany) were the first to obtain negative temperatures for the degrees of freedom of translational motion of 39K atoms in the Bose – Einstein condensate; the condensate was placed in a dipole trap and in the optical lattice, that is, in the periodical potential created by laser beams. The atoms could tunnel between the cells of the lattice and in the vicinity of the Feshbach resonance the type of pairwise interaction among atoms depended on the magnetic field. By reducing the potential of the optical lattice and transforming the mode of interatomic interaction to mutual attraction, it was possible to calculate their energy distribution, similar to the Bose – Einstein distribution with negative temperature. This distribution was determined by measuring the absorption at the stage of free outward flight of atoms after turning off the trapping potential. Since the lattice limited the kinetic energies of particles, the system remained stable for about 600 µs. In the thermodynamic sense, a system with negative temperature is “hotter” than a system with positive temperature since on contact the heat flows from the former system to the latter. A stable negative-temperature system should be at negative pressure, by analogy to the cosmological dark energy. Source: Science 339 52 (2013) New WMAP results1 February 2013The results are given of processing the data accumulated over the nine years of observation of the anisotropy of the microwave background radiation using the WMAP satellite probe. These results are fully compatible with the standard ACDM cosmological model. The accuracy of calculation of cosmological parameters was improved by combining the WMAP data with the data of other telescopes, as well as by refining the methods of analysis. WMAP observations taken together with the data on Ia supernova, acoustic oscillations and measurements of the Hubble constant (using the value H0 = 78.3 ± 2.4 km s-1 Mpc-1) give for the cosmological parameter of density of baryonic matter the value Ωb h2 = 0.02223 ± 0.00033 (here h = H0 /(100 km s-1 Mpc-1)), for the dark matter they give Ωc h2 = 0.1153 ± 0.0019, for the dark energy — ΩΛ h2 = 0.7135( + 0.0095 - 0.0096) and for space curvature — Ωk = -0.0027( + 0.0039 - 0.0038). The power exponent of the spectrum of cosmological perturbations is ns = 0.9608 ± 0.0080, and the scenario of flat spectrum with ns = 1 is excluded at the 5 σ level. The total normalization of spectrum is 109Δ2 R = 2.464 ± 0.072, and the contribution of the tensor mode of perturbations was r < 0.13. It was also shown that the mass of the three flavors of neutrino is < 0.44 eV, while the effective number of relativistic degrees of freedom during the photon decoupling epoch at z ≈ 1090 reaches 3.26 ± 0.35, which agrees with the value 3.04 predicted by the Standard model of elementary particles. As for the parameter of state of dark energy, the following interval of values is obtained: w = p/ρ = -1.037(+0.071-0.070). Source: arXiv:1212.5226 [astro-ph.CO] |
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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