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| Issue 12, 2024 |
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Extracts from the Internet | ||
Elementary particle physics1 June 2007
Single t-quarks
The D0 Collaboration at the Fermilab Tevatron
proton--antiproton collider presented first evidence for the production of
single (non-free) top quarks among the products of the reactions.
Earlier experiments recorded only events in which the t-quarks were created in
pairs with the antiquark. These processes of creation
of single t-quarks follow the electroweak mechanism and their total cross
section was found to be 4.9+-1.4pb. Also for the first time, the element
Vtb of the Cabibbo-Kobayashi-Maskawa matrix was found without the a priori assumption of its unitarity. The measured value is reported as
0.68< Vtb< 1 at 95% C.L.
Sources: Phys. Rev. Lett. 98 181802 (2007) Fluctuations in antiferromagnets1 June 2007Fluctuations due to thermal vibrations of domain walls in ferromagnetic materials, causing measurable variations of magnetic field, attracted researchers for nearly a century now. Conducting such measurements in antiferromagnets is very difficult due to the small spatial scale of fluctuations. A new method, developed at the Argonne National Laboratory, made it possible for the first time to measure fluctuations in antiferromagnets. The method consisted in studying the diffraction of a coherent x-ray beam on a chromium crystal. The magnetism in this material is caused by spin density waves of conduction electrons. The corresponding electric charge density waves scatter x-ray photons. Fluctuations result in slow restructuring of domain walls (of spin wave configuration) which manifests itself in changes of the diffraction pattern with characteristic timescale on the order of one hour. Fluctuations were recorded on a scale on the order of 1µm, and were observable even at very low temperatures, as low as 4K. In this case the fluctuation mechanism was quantum mechanical tunneling. Fluctuations characterize the stability of magnetic properties of matter at the microscopic level and therefore the data obtained may prove useful for developing nanometer-scale antiferromagnetic structures. Sources: Nature 447 68 (2007) Sudden death of entanglement of quantum states1 June 2007Luiz Davidovich and colleagues at the Federal University of Rio de Janeiro (Brazil) have for the first time experimentally observed entanglement sudden death (ESD) of quantum states of two quantum bits (qubits). If the coherence of individual quantum states decays not faster than asymptotically on interaction with certain local environments, then - as predicted theoretically - the entanglement of two qubits may suddenly be destroyed over a finite time by the same interactions. The experiment was carried out with entangled pairs of photons in different polarization states. Photons were sent through interferometer legs comprising an optical medium in which decoherence and entanglement death could occur. The degree of entanglement was measured at the output of the instrument using interference filters. As predicted by the theory, events of sudden loss of coherence by qubits were indeed recorded. The ESD effect may create certain difficulties for building the quantum computer since error-correction schemes that have been proposed for the mode of slow loss of coherence cannot work if entanglement is suddenly destroyed. Therefore, a computation in a quantum computer has to be completed before an ESD occurs. Sources: Science 316 579 (2007) A ring of dark matter1 June 2007A ring-shaped structure in the distribution of dark matter was detected in the galaxy cluster Ñ10024+17 using the Hubble space telescope. Luminous galaxies and the gas of the cluster are embedded in a massive dark matter halo. Light emitted by galaxies propagates through the halo and is deflected due to gravitational lensing. The distribution of mass in the cluster was reconstructed from the statistics of shape distortion of 1300 galaxies. Photometric measurements on the cluster made it possible to measure the red shifts of the galaxies. Earlier conclusion on bimodal distribution of galaxies over red shift has been confirmed. This indicates that the cluster Ñ10024+17 in fact consists of two individual clusters with red shifts z=0.381 and z=0.395, projected onto one line of sight. The density of dark matter in the cluster smoothly decreases away from its center but a small local maximum, ring-shaped when projected onto the celestial sphere, is found at the radius of about 1.3x106 light years. The following hypothesis is suggested for the formation of this ring. The two clusters forming the Ñ10024+17 suffered a nearly head-on collision and now move away along the line of sight. As the clusters were flying through each other, the layers of dark matter in each cluster were subjected to additional gravitational attraction by the other cluster, forcing both clusters to contract; later they expanded again, tending to equilibrium state. These radial oscillations of dark matter resulted in the formation of some excess density - a ring - at a certain radius. The authors carried out numerical simulation of the head-on collision of two clusters which also resulted in a ring-like feature supporting the suggested hypothesis of ring formation. Sources: http://arxiv.org/abs/0705.2171 |
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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