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| Issue 11, 2024 |
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Extracts from the Internet | ||
Search for the Higgs boson at the Tevatron1 April 2011The total array of data currently available on the CDF è D0 experiments at the Tevatron accelerator of the Fermilab indicates that the Higgs boson cannot have a mass in the range from 156 to 183 GeV. Most of this interval is excluded at confidence level 95%, although the level achieved in some sub-intervals is only 90%. Higgs bosons are particles predicted by the Standard Model and are expected to be produced at the Tevatron in p-anti-p collisions. So far the search for various channels of their decay yielded negative results. In view of the excluded area, the Higgs boson mass can lie either in a narrow range of 183-185 GeV or in the lower-mass range of 114-156 GeV. In the latter case the Higgs boson must decay not into W±- and Z0 bosons but into heavy quarks that significantly complicate the task of identifying it. The Tevatron accelerator is to accumulate more statistics until the final closing of the accelerator planned for September 2011. The search for the Higgs boson also started in the ATLAS and CMS experiments at the Large Hadron Collider. Source: arXiv:1103.3233v2 [hep-ex] Experimental verification of the no-hiding theorem1 April 2011Indian researchers J.R. Samal, A.K. Pati and A. Kumar demonstrated in their experiment the validity of the no-hiding theorem formulated by S.L. Braunstein è A.K. Pati in 2007. According to the no-hiding-theorem, the quantum information that is bleached from a system cannot be hidden in the quantum correlations between this system and its environment but is transferred to objects in the environment. In this experiment the transfer of information in a system of three quantum qubits, prepared as nuclei of the atoms of hydrogen, fluorine and carbon in a single molecule of 13CHFBr2 was studied by an NMR technique. Application of a series of electromagnetic pulses placed the first qubit in a particular state, which was followed by randomization and a transfer of information from the first to the other two qubits. Measurement of the qubit state showed that eventually the entire quantum information has been transferred to the third qubit, and that correlations between the qubits did not contain additional information. It was also shown that the information which was on the first qubit from the start could be restored from the states of the second and third qubits up to a unitary transformation. Source: Phys. Rev. Lett. 106 080401 (2011) Spin-orbit coupling for atoms1 April 2011Y.-J. Tin, Ê. Jimenez-Garcia and L.B. Spielman of the Joint Quantum Institute (JQI, U.S.A.) demonstrated the effect of spin-orbit coupling for atomic 87Rb in Bose – Einstein condensate. Owing to the contribution of internal orbital motions to the total angular momentum, the spin-orbit coupling has a somewhat different mechanism for atoms than for electrons. In the experiment described, sublevels mF = -1 and mF = 0 of the ground state 5S1/2, F = 1 of 87Rb were selected as the two spin states of the atoms. These sub-levels were linked to the state of the atom by a pair of laser beams whose frequencies were slightly offset from the Raman resonance using acousto-optic modulation. The beams intersected at the right angles at the center of the optical dipole trap containing 1.8 × 105 atoms of the Bose – Einstein condensate. The properties of the condensate were found from the way the cloud expanded after the trap potential had been switched off. The spin-orbit coupling in the condensate is effectively given as the sum of equal Rashba and Dresselhaus contributions, i.å. the corresponding interaction terms in the Hamiltonian contained only the product kxσy of the wave number and the Pauli matrices, assuming the magnetic field to be directed along the z axis. The strength of the spin-orbit interaction could be controlled by an external magnetic field. When it increased beyond a certain critical value, condensate atoms with mF = -1 and mF = 0 spatially separated into two clouds, thus demonstrating the effect of spin-orbit coupling for the atoms of 87Rb. Source: Nature 471 83 (2011) Spontaneous quantum transitions in superconducting structure1 April 2011R. Vijay and his colleagues at the University of California observed for the first time spontaneous quantum transitions in so-called “artificial atoms”, that is, superconducting transmon qubit that has discrete energy levels. The qubit was an aluminum ring cooled down to 30 mK and was capacitively linked to a microcavity. Microwave photons are reflected in the cavity receiving a phase shift of magnitude that depends on the qubit state. This shift was measured using a low-noise parametric amplifier based on comparator circuit; the reverse effect of measurements on the qubit was found to be minimal and not perturbing its state. This last feature is a consequence of the fact that the system was in the quantum eigenstate of the measurand. The states of the qubit were monitored every 10 ns, which allowed to establish the excited state of the qubit in real time (the average lifetime is 320 ns) and record its quantum jump to the ground state; the distribution of lifetimes decreased exponentially as is expected for spontaneous decay. In the future this technique may find practical applications for correcting errors in quantum computations. Source: Phys. Rev. Lett. 106 110502 (2011) The origin of giant gamma-ray bubbles above the galactic disk1 April 2011In 2010 the Fermi Gamma-ray Space Telescope observed gigantic volumes of space on both sides of the galactic disk that emit radiation in the gamma range at energies of 1-100 GeV and power of Fγ ≈ 4 × 1037 erg s-1. These structures, with diameters of the order of 10 kpc, are also visible on X-ray images obtained with the satellite ROSAT and in radio observations of the WMAP detector. It has been suggested earlier that these gamma bubbles have been generated as a result of energy release at the galactic center. The model in which energy is released by a large number of supernovae can not explain these observations, since no multiple residues of such explosions are found. For this reason the central black hole and events of accretion onto it were proposed as another plausible energy source. K.S. Cheng (University of Hong Kong), D.O. Chernyshov and V.A. Dogel (P.N. Lebedev Institute of Physics, RAS, Moscow), Ñ.-Ì. Ko and W.-H. Ip (Institute of Astronomy, Taiwan) offered a theoretical explanation positing that energy is pumped into gamma-ray bubbles due to the destruction of stars passing in the proximity of the supermassive black hole. Approximately one half of the material of a captured star is injected into the halo at a velocity of ≈ 109-1010 sm s-1, heats up the plasma in it and causes its luminosity in the x-ray range. The resulting shock waves accelerate electrons to energies on the order of TeV. Moving in magnetic fields, these electrons emit synchrotron radio emission, and inverse Compton scattering of background photons generates gamma radiation. The rate of destruction of stars required in this model is asymp; 3 × 10-5 yr-1. This model faithfully reproduces both the observed shape of the giant gamma-bubbles, and the spectra of their radiation in a number of spectral ranges. Source: Astrophysical Journal Letters (accepted) |
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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