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Long-range particle correlations in pp collisions at the Large Hadron Collider
1 October 2010
A 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 laboratory
1 October 2010
Predictions 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 constant
1 October 2010
Laboratory 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 generator
1 October 2010
Ñ. 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 rays
1 October 2010
Three 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]
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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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