1 September 2017
The photon-photon scattering γγ → γγ was first evidenced directly in the CERN LHC experiment with the ATLAS detector. This process is impossible in the framework of classical electrodynamics because of linearity of Maxwell’s equations, but in quantum electrodynamics the photon interaction is due to production of pairs of virtual charged particles in intermediate states as was predicted by W. Heisenberg and H. Euler in 1936. The γγ → γγ scattering has already been observed but only indirectly in measurements of the anomalous magnetic moment of leptons and in some other processes. The ATLAS collaboration investigated “ultra-peripheral” lead nuclear collisions when the impact parameter of nuclei moving towards each other exceeds the diameter of the nucleus. The nuclei fly by each other transferring to the excited state without breaking. Near a moving nucleus a strong electromagnetic field exists which under ultrarelativistic velocities of the nuclei is a cloud of “quasi-real” photons located almost on the mass surface. After their interaction these photons are detected finally as real photons. 13 candidate events of the γγ → γγ process were registered, whereas the predicted value was 7.3 and the expected background value was 2.6 ± 0.7, i.e., the statistical significance of the result was 4.4σ. The measured lead ion interaction cross section is well consistent with the calculations within the Standard Model.
Source: Nature Physics 13 852 (2017)
The search for “sub-gravitational” forces
1 September 2017
M. Jaffe (University of California, Berkeley, USA) with colleagues measured the gravitational attraction of caesium atoms to a small-sized (several cm) and small-mass (190 g) tungsten cylinder, whereas in the previous experiments the source masses amounted to tens and hundreds of kg. The gas was cooled in a trap (a light-induced lattice) to a temperature of ≈ 300 nK and was jolted by a shift of the lattice. Laser pulses were used to transfer the atoms to the state of motion along two vertical trajectories, one lagging relative to the other. These trajectories are the interferometer arms. Attraction to the cylinder induces an additional atomic phase difference on the trajectories, which was measured by the interference pattern observed using the fluorescent radiation of atoms. The cylinder-induced gravitational acceleration calculated within the Newton theory is 65 ± 5 nm s-2, and the experimentally obtained limit on the possible additional acceleration was < 49 nm s-2. Thus the experiment with a small-mass source allowed a search for “sub-gravitational” forces which are weaker than the gravitational forces. The existence of such forces on small scales had been predicted by some models of cosmological dark energy. The limits on the parameters of the theory of a “chameleon field” obtained in this experiment improved the previous limits by two orders of magnitude and only left a small region of admissible parameters. With improved measuring accuracy it will be possible either to confirm or to completely “close down” this theory in the nearest future. The limits on the parameter of the scalar field self-action in the “symmetron” theory were also improved by two orders of magnitude. In future it is planned to examine the gravitational Aharonov – Bohm effect in a similar experiment and to perform precise measurements of the gravitational constant G.
Source: Nature Physics, online publication of July 3, 2017
Quantum radio-frequency magnetometer
1 September 2017
The use of quantum effects in measuring devices improves substantially their capability for quantum coherence is fairly sensitive to the external action. F.M. Ciurana (The Barcelona Institute of Science and Technology, Spain) and his colleagues demonstrated a new method of measuring the forms of superweak radio-frequency pulses by combining the stroboscopic method and quantum measurements. An ensemble of 1.5 × 106 87Rb atoms was used at a temperature of 16 µK in an optical trap in a constant magnetic field. The examined signal, i.e., a weak variable magnetic field of radio-frequency range was applied in the direction perpendicular to the constant magnetic field. Faraday rotation of the polarization plane of laser light passing through a cloud of atoms was registered. These were so-called “quantum nondemolition measurements” which do not violate the quantum coherence of the investigated system. At the initial moment atomic spins were directed along the constant magnetic field and precessed coherently between measurements. A series of successive measurements was conducted by the stroboscope method, which allowed an examination of the waveform. This method was used to examine sinusoidal signals and signals with a linearly heightening frequency. The atoms being in a quantum-entangled state the noise was suppressed by 25 % and the sensitivity was attained comparable with that of the best magnetometers of the same frequency range but operating according to other principles.
Source: Phys. Rev. Lett. 119 043603 (2017)
Polarizer of terahertz radiation
1 September 2017
Radiation of THz range positioned between microwave and IR ranges has a number of useful applications but needs special equipment. R. Mendis (Brown University, USA) with colleagues fabricated a new material which enables singling out linearly polarized components from a beam of terahertz radiation. The material consists of a stack of parallel 30 µm-thick metal plates located at a distance of 300 µm from one another. The array of plates is 20 × 22 mm2 in size, 2 mm thick and from the electromagnetic point of view is equivalent to a lattice of parallel-plate waveguides. A beam of radiation is directed at an angle of 45° to the stack plane. If the vector of the electric field of the wave is parallel to the metal plates, the radiation is reflected almost completely and if perpendicular, the radiation passes almost freely. With the help of two such polarizers an isolator of terahertz radiation (material with a very low back reflection) was produced with efficiency exceeding that of ferrite isolators.
Source: Scientific Reports 7 5909 (2017)
Two-step magnetic reconnection in a solar flare
1 September 2017
T. Gou (University of Science and Technology of China and University of Graz, Austria) with colleagues studied the effect of two-stage reconnection of magnetic lines of force during the solar flare of May 13, 2013. The observations were carried out at the Solar Dynamics Observatory (SDO). The flare had two separate episodes of energy release. The first episode refers to typical events and is characterized by the eruption of a magnetic flux rope. The second episode is on the contrary rather unusual. It was stronger than the first one and exhibited a heightened X-ray and even gamma-ray emission. During the second episode the long magnetic-loop leg began accelerating sharply to a speed of 130 km s-1 and in some time disappeared, which was accompanied by a diffuse plasma eruption in the perpendicular direction. A possible interpretation of these processes is magnetic reconnection of the loop after its eruption.
Source: Astrophys. J. Lett. 845 L1 (2017)
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.