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| Issue 12, 2024 |
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Extracts from the Internet | ||
New baryons are discovered1 January 2015Resonance particles Ξb'- and Ξb*- with masses of respectively MeV/c2 and 5955 MeV/c2 consisting of b-, d-, and s-quarks were detected for the first time on Large Hadron Collider by LHCb collaboration. The existence of these particles in the Ξb isodoublet composition had been predicted theoretically in the quark model. In the Ξb'- baryon with spin parity JP=(1/2)+, the d-quark spin is opposite to the s-quark spin, while the Ξb*- baryon has JP=(3/2)+ and the spins of the indicated quarks are homomallous. New particles were born in pp collisions and were registered as peaks in the spectrum of their decay into Ξb0 and π-. The masses of the new baryons exceed only slightly the sum of Ξb0 and π- masses and are near the kinematic threshold of the decay; it had been assumed before that the Ξb'- mass can be below the threshold. The LHCb experiment is being conducted with participation of Russian researchers from seven research institutes. Source: arXiv:1411.4849 [hep-ex] Quantum interference on the molecule1 January 2015In the well-known two-slit quantum experiment, the interference pattern disappears if it is known through which slit particles go. This information can be obtained, for example, by measuring the slit recoil momentum. X.-J. Liu (the Cyclic electron accelerator SOLEIL, France) with colleagues realized the version of this experiment making use of the effect of photoemission on O2 molecules. The role of two slits was played by two atoms of the molecule, and the interference pattern was observed in the angular distribution of Auger electrons escaping after irradiation of molecules by photons from a synchrotron X-ray source. If photons were scattered by the O2 molecule without its dissociation then it received recoil as a whole. It was unknown in this case which of the two atoms of the molecule were responsible for the scattering, and the interference pattern was observed. If the molecules dissociated into atoms, then the O+ ion and Auger-electron momenta were recorded. This made it possible to identify the scatterer atom, and as had been predicted by quantum mechanics, the Auger-electron distribution showed no interference. Source: Nature Photonics, on-line publication of 1.12.2014 Structure of trimers 4He3 and 3He4He21 January 2015The space structure of the 4He3 molecule has been discussed for already more than 20 years. This molecule has very low binding energy because of the weak 4He atom polarizability. According to some papers, 4He atoms in the 4He3 molecule are arranged as a regular triangle, while the calculations of other authors suggest that it should be a linear molecule. The new experiment performed by R. Dorner (Nuclear Physics Institute, Goethe University, Germany) with colleagues showed that the 4He3 molecule has actually no definite structure, but instead the 4He atoms form a chaotic cloud (are distributed with approximately equal probability inside a spherical volume). The experiment was performed on the COLTRIMS setup, where the 4He3 and 3He4He2 molecules were formed when gaseous helium escaped the nozzle and having passed through the diffraction grating were mass separated. Then under the influence of femtosecond laser pulses the atoms in the molecules were ionized, scattered because of Coulomb repulsion and were registered in detectors by coincidence method. This method allowed measurement of atomic momenta and reconstruction of the form of initial 4He3 molecules, i.e., determination of the distances and angles between atomic bonds. The wide statistical distance and angular distribution (without pronounced peaks) suggested the conclusion that these molecules have no ordered structure. The same experiment investigated the structure of 3He4He2 molecules and showed that they have the shape of a “halo” when the 3He atom rotates far from a more compact 4He2 pair. Source: Nature Communications 5 5765 (2014) Electron dynamics in silicon1 January 2015M. Schultze (University of California, Berkley, USA) with colleagues observed the real-time dynamics of the energy gap variation in silicon upon electron conduction-to-valence-band transition. With the help of laser pulses the electrons in the sample were excited and synchronous measurements were taken of sample absorption spectra upon exposure to the pulses of extreme UV range several tens of attoseconds long. The electrons tunneled over to the conduction band because the Keldysh adiabaticity parameter made up γ ≈ 0.5. After the electron excitation the electron gap narrowed rapidly within the time of ≈ 450 as. This narrowing is a purely electronic effect and is due to the scattering of electrons by each other in the conduction band. Then, after 60 ± 10 fs, a strong absorption spectrum variation was again detected as a result of crystal lattice vibration with a period of 64 fs under the effect of optical phonons born upon electron excitation. The results of the experiment may appear to be useful in ultrafast microelectronics. Source: Science 346 1348 (2014) Pulsar timing and the structure of neutron stars1 January 2015The researchers from the Washington University in St. Luis (USA) M.G. Alford and K. Schwenzer received new data on the composition of neutron stars from stability of their rotation periods. The question now remains open of whether the so-called quark matter is contained in the depth of a neutron star or whether neutron stars contain purely neutron matter. M.G. Alford and K. Schwenzer calculated the amplitude of the r-mode of global oscillations of the neutron star shape depending on the star matter viscosity, i.e., on the star composition. In the presence of quark matter the r-mode damping is stronger and the quadrupole moment is smaller. Hence, the star must radiate less energy in the form of gravitational waves and decelerate weaker its rotation. The quantitative verification of this effect made use of the observational data of small-mass X-ray binary stars (systems consisting of a neutron and an ordinary star) and the data of radio observations of millisecond pulsars. The millisecond pulsar temperature is unknown, but M.G. Alford and K. Schwenzer elaborated the method independent of the temperature data. The researchers concluded that the pulsar timing data agree better with the presence of quark matter inside a neutron star than with a purely neutron composition. In the case of a purely neutron composition, an additional not yet considered source of the oscillation r-mode damping is obviously needed. Source: Phys. Rev. Lett. 113 251102 (2014) |
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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