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| Issue 11, 2024 |
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Extracts from the Internet | ||
Long-range particle correlations in pp collisions at the Large Hadron Collider1 October 2010A new type of two-particle angular correlations of particles created in collisions was observed in the experiment carried out by the CMS collaboration at the Large Hadron Collider to study pp collisions. The distribution of azimuthal angles φ of particles' emission from the point of reactiond and pseudorapidities η(θ) (function of polar angle θ relative to the beam axis) and correlation functions R( Δη, Δφ ) were constructed for pairs of particles. A ridge-shaped elevation of the two-dimensional surface R( Δη, Δφ ) at energies of about 7 TeV was observed for 2.0 < |Δη| < 4.8 in the case of events with high multiplicity, with a maximum near the point Δφ ≈ 0. This means there is a kind of correlation between the particles emitted at similar azimuthal angles φ despite a large difference in the value of η. Previously, no such angular correlations of particles in pp and p-anti-p collisions were observed. So far mathematical modeling of the interaction between particles cannot reproduce this feature of the correlation function: the difficulty lies in that according to the theory particles with large Δη should be independent of each other. It is possible that the detected long-range correlations are the result of collective interactions between particles at very high densities of matter. Source: arXiv:1009.4122v1 [hep-ex] Relativistic time dilation measured in the laboratory1 October 2010Predictions of the theory of relativity on the slowing down of clocks in a gravitational field and in the case of motion with respect to the inertial reference frame have been confirmed at the NIST (the National Institute of Standards and Technology, USA). The atomic clocks worked on individual trapped ions of 27Al+ interacting with auxiliary “logic” ions 9Be+ or 25Mg+ used to record changes in the internal state of 27Al+ in transitions 1S0-3P0. High Q of the system, f0/Δf = 1.4 × 1017 allowed measuring relative frequency shifts of about ≈10-16. Two copies of the clocks were placed 75 m apart and were connected by optical fibers to compare how they kept time. In the first experiment, the ion in a trap executed harmonic oscillations at a typical speed of ≈ 10 m s-1 relative to the stationary ion in the second trap. The measured relativistic dilation of time in the first clock as a function of ion velocity exactly coincided with the theoretical predictions. In the second experiment, one of the clocks was lifted by 33 cm, which led to acceleration of its “ticking” due to reduction in gravitational field. The measured relative frequency shift (4.1 ± 1.6) × 10-17 was also in good agreement with the calculated value. These effects of time dilation have previously been measured in a number of experiments at high speeds and large altitude differences; the resulting corrections are taken into account even in satellite navigation systems. The method of measuring small time dilations using a pair of atomic clocks may find practical application in geodesy and high-precision experiments searching for changes of fundamental physical constants with time. Source: Science 329 1630 (2010) New measurement of the gravitational constant1 October 2010Laboratory experiments with a torsion balance have earlier achieved relative accuracy of ≈10-5 in measurements of the gravitational constant G. H.V. Parks (University of Colorado and NIST, USA) and J.E. Faller (Sandia National Laboratories, USA) improved an alternative interferometric technique of G measurements using pendulums, making it possible to achieve comparable accuracy. Laser interferometric measurements detected a change in the distance between pendulums suspended from strings and oscillating relative to four tungsten cylinders — sources of the gravitational field — with masses 120 kg each. The second arm of the interferometer, which served as the standard of distance, was fixed between the pivots of the pendulums. The value obtained in measuring G (G = (6.67234 ± 0.00014) × 10-8 sm3 g-1 s-2) is three standard deviations below the value of G recommended in 2008 by The Committee on Data for Science and Technology (CODATA) but is consistent with the earlier value supported by CODATA in 1986. The diference with the results of the currently best-accuracy experiment with torsion balance reaches 10σ. The revision of the magnitude of G that occurred between 1986 and 2008 was caused by the study of inelasticity in suspension strings in torsion balances. The causes of the observed discrepancies between the results of a new experiment of H.V. Parks and J.E. Faller and those of previous measurements are not yet clear. Source: Phys. Rev . Lett. 105 110801 (2010) Quantum random number generator1 October 2010Ñ. Gabriel (Max Planck Institute for the Physics of Light and Institute of Optics, Information and Photonics of the University of Erlangen-Nuremberg, Germany) and his colleagues from Germany and Denmark created a random number generator which operates by using the random nature of the zero-point vacuum fluctuations of the electromagnetic field. In contrast to classical algorithms of generation of random numbers, in the quantum case it is in principle impossible to predict a sequence of numbers. The new generator operates on laser splitters with two optical inputs and synchronous photon detectors at the outputs. Even in the absence of a signal in one of the inputs, the total signal contains a random contribution from quantum fluctuations, and this property is used to generate random numbers. The calculated probability distribution function for the number of responses was divided into segments with equal cumulative probability, corresponding to different random numbers. Random numbers are generated at a rate of 12 Mbps but certain improvements can greatly increase the speed. The new quantum random number generator is of fairly simple design and can be used, for example, in cryptography and for numerical Monte Carlo simulation. Source: Nature Photonics 4 711 (2010) Sources of ultra-high energy cosmic rays1 October 2010Three years ago, correlation of directions of arrival of ultra-high energy cosmic rays with directions to the active galactic nuclei was found by processing the data of the Pierre Auger detector consisting of an array of surface and Cerenkov detectors (see Phys. Usp. 50 1289 (2007)). The galaxies were not farther then 75 Mps, which is admissible as distance to sources in terms of the Greisen--Zatsepin--Kuzmin effect. The coincidence of directions is defined as the direction of the axis of an extensive air shower (cascade of particles) pointing to an area of radius 3.1° around the active nucleus. According to information obtained prior to August 31, 2007 this correlation was obeyed by 69(+11-13)% of the reported events. The volume of statistical data more than doubled between 31 August 2007 and 31 December 2009; according to revised data, the fraction of events whose directions correlates with the positions of active galactic nuclei is 38(+7-6)%, while the fraction of random coincidences in the case of isotropic distribution of sources is evaluated at the level of 21%. The strongest concentration of events recorded by Pierre Auger was observed in the direction of the nearest radio galaxy Centaur A (NGC 5128), and no correlations were found for the nearest large cluster of Virgo galaxies and the giant radio galaxy M87. Even though ultrahigh-energy cosmic rays (E ≈ 1017-1020 eV) have been recorded for several decades now, the mechanism of their origin and the nature of the sources are not yet understood. It proved possible to establish from the behavior of the events observed in the detectors that the primary particles of highest-energy cosmic rays are protons or atomic nuclei, and that photons can cause only a small fraction of events. Source: arXiv:1009.1855v2 [astro-ph.HE] |
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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