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| Issue 4, 2024 |
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Extracts from the Internet | ||
Search for the Higgs boson on the Large Hadron Collider1 January 2012The ATLAS and CMS experiments at the Large Hadron Collider are registering particles to which could decay, according to calculations, Higgs bosons created in pp collisions. By the ATLAS data accumulated during 2011, the mass of the Higgs — assuming it exists — may lie at 95 % probability in the range 116-130 GeV, while CMS data limit the Higgs mass to the interval 115-127 GeV. Both experiments revealed a small event excess which may constitute an indication of creation of the H. ATLAS points to an excess in decay channels H → γγ, H → ZZ and H → WW. With the most conservative interpretation, the global significance of detection of the Higgs with the mass ≈ 126 GeV over these channels is 2.3 σ. In the CMS experiment, a similar excess is observed at ≈ 123 GeV at a 2 σ level in five independent decay channels simultaneously. It is necessary to additionally verify that this excess does correspond to the Higgs boson of the Standard Model, not to some other particle. So far low statistical significance does not permit drawing a reliable conclusion on detection or absence of the Higgs but it is expected that the statistics sufficient for this may be accumulated some time in 2012. Source: CERN Press Release Quantum correlations in diamonds1 January 2012I.A. Walmsley (University of Oxford, United Kingdom) et al. succeeded in creating entangled quantum state for phonons, ie. vibrations of crystal lattice excited by laser pulses in two diamonds in the course of Raman scattering at room temperature. Diamond crystals, 3 mm in size, were separated by about 15 cm. Photons from the pump laser passed through a polarizing beamsplitter and entered two crystals following different optical paths which combined into one after the passage through the crystals. Signals of the single-photon detector at the output could not reveal through which of the two crystals the photon has passed and consequently phonons excited in one of the crystals were quantum-correlated with with phonons in the second crystal. The area covered by each of the phonons was approximately 0.05×0.25 mm; on this area, approximately 1016 atoms vibrated coherently at a frequency of about 40 THz. Phonons were recorded as they scattered pulses of the second laser sent after a fixed time of 350 fs after the main pumping pulses, ie. during the time of decoherence of phonons (≈ 7 ps). Quantum entanglement was confirmed by the correlations of the polarization states of Raman-scattered Stokes and anti-Stokes photons. Source: Science 334 1253 (2011) The homodyne detector for atoms1 January 2012Ě. Oberthaler (University of Heidelberg, Germany) and his colleagues have implemented the method of homodyne detection of quantum correlations in a Bose – Einstein condensate of rubidium atoms at a temperature 10-7 Ę. This experiment revealed, for the first time in the case of atoms, quantum correlations of continuous variables: oscillation phases. The atoms were trapped into one-dimensional optical lattice at several hundred atoms per cell. Initially the condensate was prepared in the state of hyperfine splitting (F,mF) = (2,0) which was an analogue of the state of photons in the pumping beam in optical experiments. Spin-changing collisions between atoms created a nonlinear binding of the Zeeman states mF = ± 1. The authors studied fluctuations involving at most several atoms of the condensate. According to the measurements, fluctuations in the number of atoms in the states mF = +1 and mF = -1 were certainly correlated. The quantum nature of correlations was confirmed by measurements of the phase of oscillations of the the number of atoms relative to the phase of the local oscillator (atoms prepared in the state (1,±1)). Source: Nature 480 219 (2011) Friction at the nanoscale1 January 2012R.W. Carpick (University of Pennsylvania, USA) and his colleagues explored the ageing of static friction on the nanometer scale using the atomic force microscope. Friction may be enhanced through gradual increase in contact area of the samples, or by formation of new connections between them. In this experiment, one of the surfaces was the quartz tip of the atomic force microscope. Its intermittent slipping along the quartz specimen surface was recorded in reflected light. The measurements were performed over time up to ≈ 100 s. As in experiments with macroscopic samples, it was found in this particular case that static friction force increases approximately as a logarithm of time. It is most probable that Si-OH silanol groups entered the chemical reaction, producing Si-O-Si siloxane bonds between two surfaces. The chemical nature of strengthening of friction force was confirmed by the fact that in the case of the second surface made of diamong or graphite, no strengthening was observed as chemical interaction was too weak. This study may also have shed light on the mechanism of earthquakes, owing to certain common features of behavior. It is assumed that intermittent sliding of rocks does occur in seismically active zones and friction force increases with time, thereby enhancing earthquakes at subsequent slips. Source: Nature 480 233 (2011) Indirect search for dark matter particles1 January 2012Dwarf spheroidal galaxies — satellites of our Galaxy — are promising objects for searching for annihilation of particles of dark matter as these dwarfs are characterized by low gas content and low rate of star formation; hence, the level of gamma-rays background generated by cosmic rays must be low too. In principle, this should facilitate detection of the gamma-ray signal of annihilation. A search for gamma emission from 10 dwarf galaxies was conducted using the Enrico Fermi Gamma Ray Observatory; new constraints, the strongest at the moment, on annihilation cross section of weakly interacting particles of dark matter were obtained. These constraints came very closely to the quantity ⟨σ v⟩=3×10-26 cm3 s-1 at which particles are born in the early Universe, in the amounts exactly right for explaining the dark matter. Researchers in the collaboration Fermi-LAT applied the condition of ⟨σ v⟩ being less than 3×10-26 cm3 s-1and found the minimum possible mass of the particles to be ≈ 27 GeV in the hadronic channel of annihilation and ≈ 37 GeV in the leptonic channel. A. Geringer-Sameth and S.M. Koushiappas of the Brown University (USA) concluded, using an alternative analysis of the signals and backgrounds, that data on dwarf galaxies obtained by the Enrico Fermi telescope exclude, at 95 % probability, particles of dark matter with masses lower than 40 GeV in the hadronic channel of annihilation. Sources: Phys. Rev. Lett. 107 241302 (2011), Phys. Rev. Lett. 107 241303 (2011) |
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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