6,615 research outputs found
Top quark forward-backward asymmetry from the model
The forward-backward asymmetry in top quark pair production,
measured at the Tevatron, is probably related to the contribution of new
particles. The Tevatron result is more than a deviation from the
standard model prediction and motivates the application of alternative models
introducing new states.
However, as the standard model predictions for the total cross section
and invariant mass distribution for this process are in
good agreement with experiments, any alternative model must reproduce these
predictions. These models can be placed into two categories: One introduces the
s-channel exchange of new vector bosons with chiral couplings to the light
quarks and to the top quark and another relies on the t-channel exchange of
particles with large flavor-violating couplings in the quark sector. In this
work we employ a model which introduces both s- and t-channel nonstandard
contributions for the top quark pair production in proton antiproton
collisions. We use the minimal version of the model (3-3-1 model) that predicts the existence of a new neutral gauge
boson, called . This gauge boson has both flavor-changing couplings
to up and top quarks and chiral coupling to the light quarks and to the top
quark. This very peculiar model coupling can correct the for top quark
pair production for two ranges of mass while leading to cross
section and invariant mass distribution quite similar to the standard model
ones. This result reinforces the role of the 3-3-1 model for any new physics
effect.Comment: 12 pages, 4 figures, 2 table
U B V R I Photometry of Stellar Structures throughout the Disk of the Barred Galaxy NGC 3367
We report new detailed surface U, B, V, R, and I photometry of 81 stellar
structures in the disk of the barred galaxy NGC 3367. The images show many
different structures indicating that star formation is going on in the most
part of the disk. NGC 3367 is known to have a very high concentration of
molecular gas distribution in the central regions of the galaxy and bipolar
synchrotron emission from the nucleus with two lobes (at 6 kpc) forming a
triple structure similar to a radio galaxy. We have determined the U, B, V, R,
and I magnitudes and U - B, B - V, U - V, and V - I colors for the central
region (nucleus), a region which includes supernovae 2003 AA, and 79 star
associations throughout NGC 3367. Estimation of ages of star associations is
very difficult due to several factors, among them: filling factor, metallicity,
spatial distribution of each structure and the fact that we estimated the
magnitudes with a circular aperture of 16 pixels in diameter, equivalent to
kpc. However, if the colors derived for NGC 3367 were similar to
the colors expected of star clusters with theoretical evolutionary star tracks
developed for the LMC and had a similar metallicity, NGC 3367 show 51 percent
of the observed structures with age type SWB I (few tens of Myrs), with seven
sources outside the bright surface brightness visible disk of NGC 3367.Comment: Accepted for publication (abr 2007) in The Astronomical Journal (July
2007 issue
The Post-Quasistatic Approximation as a test bed for Numerical Relativity
It is shown that observers in the standard ADM 3+1 treatment of matter are
the same as the observers used in the matter treatment of Bondi: they are
comoving and local Minkowskian. Bondi's observers are the basis of the
post--quasitatic approximation (PQSA) to study a contracting distribution of
matter. This correspondence suggests the possibility of using the PQSA as a
test bed for Numerical Relativity. The treatment of matter by the PQSA and its
connection with the ADM 3+1 treatment are presented, for its practical use as a
calibration tool and as a test bed for numerical relativistic hydrodynamic
codes.Comment: 4 pages; to appear as a Brief Report in Physical Review
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