The asymmetry in t-anti-t pairs
1 February 2011
The CDF Collaboration of the Tevatron accelerator of the E. Fermi National
Laboratory measured the spatial asymmetry of the forward-backward production of
quark-antiquark t-anti-t pairs in ð-anti-ð collisions. This asymmetry arises
because the positive charge of the proton repels the t-quark and attracts the
anti-t quark, so as a result the t-quark and the jets of particles it produces are ejected with higher probability in the direction of motion of the ð. So far 1260 events have been analyzed in which at least one of the t or anti-t-quarks has undergone a semilepton decay of the type t → lνb while the second quark decayed through the hadron channel t → qq'b. In the
rest frame of the t-anti-t pair the asymmetry depends on pseudorapidity η
and invariant mass M. For M > 450 GeV/c2 the measured asymmetry was 48 ± 11 % while taking into account the first correction in the Standard model gives 8 ± 1 %, i.e. the statistical significance of the difference is 3,4σ and asymmetry agrees with the predicted value at smaller M. These measurements are important for testing predictions and searching for new
effects, such as exotic heavy resonances which, according to some hypotheses, may
contribute to asymmetry.
Source: arXiv:1101.0034v1 [hep-ex]
Trions in carbon nanotubes
1 February 2011
Researchers at Kyoto University (Japan) R. Matsunaga, Ê. Matsuda è Y. Kanemitsu
for the first time produced and observed trion quasiparticles in carbon
nanotubes. Trions (also known as charged excitons) are three-particle bound
states consisting of one electron and two holes (electron vacancies). Trions
have been observed at room temperature by optically excitating p-doped nanotubes and
recording their absorption spectra and photoluminescence. The energy of the
spectral peak corresponding to trions is lower than that of bright excitons by
0.1-0.2 eV. The considerable difference between peak energies may be caused by
strong electron-hole interaction in carbon nanotubes. Trion have been observed
earlier in a number of different systems. The distinguishing feature of trions
is that they carry a charge and therefore can be controlled by electric fields;
consequently, this property may in the future make them useful for
nanotube-based spintronic devices.
Source: Phys. Rev. Lett. 106 037404 (2011)
The symmetry of the Möbius strip in metamaterials
1 February 2011
A group of researchers at the University of California led by X. Zhang developed
a metamaterial whose electromagnetic properties manifest a topological C3
symmetry of the Möbius strip which has no analogs in natural materials. The
metamaterial was obtained using standard electron beam lithography techniques.
It constitutes a mass of metaatoms forming trimers of composed of metallic
segments. The number of “Möbius twists” corresponds to the number of sign
changes in the interaction between a metaatom and its neighbors under 360°
rotation, which is a result of segment configuration. The spectrum of radiation
transmitted through the metamaterial was studied using infrared spectrometry.
The topological nature of the obtained resonances (i.e. dips in the spectrum) is
confirmed by the fact that the resonances predominantly depend on the number of
“twists” and are less sensitive to other characteristics of metaatoms.
Source: Phys. Rev. Lett. 105 235501 (2010)
Friction on a superconductor
1 February 2011
Ì. Kisiel and his colleagues studied the contribution of electron and phonon
excitations to the effect of contactless friction between a micromechanical probe (cantilever) and superconducting film at temperatures close to Tc. Ì. Kisiel et al studied the effect of friction on cantilever vibrations in vacuum at a distance of ≈ 0,5 nm from the niobium film. As the film cooled below Tc the friction coefficient decreased by a factor of about three. This is an indication that electron friction dominates in the metallic state and that phonon friction begins to dominate after the transition to superconducting state, because electrons form Cooper pairs and thus cease to take part in friction. The relation between friction coefficient and distance to the film and between friction coefficient and the potential difference between specimens as observed in the experiment are in good agreement with the theoretical curve.
Source: Nature Materials 10 119 (2011)
The results of the Plank telescope observations
1 February 2011
Observational data is presented, obtained by the Planck space telescope since
mid-2009. The telescope was placed at the Lagrange point L2 at a distance of
1.5 million km from the Earth. It is an array of 74 detectors operating at
frequencies from 25 to 1000 GHz, i.e. at the boundary between the IR and radio
bands. The spectrum of perturbations of the microwave radiation was measured in
the IR band for multipoles l=200-2000. The statistical characteristics of
anisotropy are identical at frequencies from at least 217 to 857 GHz. A catalog
of compact and unresolved objects was also compiled; it includes thousands of
relatively cold individual sources. The population of remote galaxies was
analyzed, in which intense star formation is proceeding at a rate exceeding the
current star formation rate in our Galaxy by a factor of 100 to 1000. The observation of these galaxies in other bands meets with difficulties because their emission is absorbed by dust clouds. In order to record the microwave background and individual objects it is of paramount importance to carefully
filter out the diffuse background radiation emitted in the Galaxy in collisions
of atoms and photons with interstellar dust; consequently, this dust also became
one of the main objects of study. By the data of the Planck telescope, dust
particles in gas-dust clouds rapidly revolve due to collisions with atoms of
gases and UV photons; their rotation generates anomalous microwave radiation in
the range ≈ 10-60 GHz. Cold gas-dust blobs in the Galaxy at temperature
7-17 Ê and mass (1-105)M☉ were studied. These clusters are distributed in the shape of filaments associated with molecular clouds; with time, new stars
will start forming in many of them. Considerable success was acieved in studying
galaxy formation by following the Syunyaev – Zeldovich effect. The
Syunyaev – Zeldovich signal was measured for 169 clusters, of which 20 were
identified for the first time. Power formula dependence was confirmed for the
Syunyaev – Zeldovich effect as a function of richness (the number of galaxies)
of clusters. The Planck data were compared with the observation of clusters in
the x-ray band on the XMM-Newton X-ray observatory and good agreement was found
with predictions based on dynamic models of clusters. The Planck and XMM-Newton
telescopes discovered a supecluster of galaxies at redshift z = 0,45 — a
rather unexpected result.
Sources: arXiv:1101.2022v1 [astro-ph.IM],
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