12,535 research outputs found
ASCA Observation of the Lyman-limit Quasar PKS2145+067
X-ray observation of a famous Lyman-limit quasar PKS2145+067at z_em= 0.990
was carried with ASCA. The source showed a 2-10 keV flux of 1.3 E-11 erg cm^-2
sec^-1 (L_X = 2.5 E46 erg sec^-1 for H_0=50 km s^-1 Mpc^-1) described by a
power-law spectrum with a photon index Gamma=1.63 +/- 0.04 . In the ASCA energy
band, no excess absorption was detected implying the absorption column density
at z_ ab=0.791 was less than 1.6 E21 cm^-2 if absorbing medium had a metal
abundance of 0.5 solar. Comparison with previous Einstein and ROSAT
observations shows that PKS2145+067 has increased its luminosity by a factor of
2--3 between 1991 and 1998.Comment: 15 pages, 3figures, Latex(PASJadd,sty, PASJ95.sty) accepted in PAS
Four-quark final state in W-pair production: Case of signal and background
We discuss theoretical predictions for W-pair production and decay at LEP2
and higher energies in a form suitable for comparison with raw data. We present
a practical framework for calculating uncertainties of predictions given by the
KORALW and grc4f Monte Carlo programs. As an example we use observables in the
decay channel: the total four-quark (four-jet) cross section
and two-quark/jet invariant-mass distribution and cross section, in the case
when the other two may escape detection. Effects of QED bremsstrahlung,
effective couplings, running W and Z widths, Coulomb interaction and the
complete tree level set of diagrams are discussed. We also revisit the question
of technical precision of the new version 1.21 of the KORALW Monte Carlo code
as well as of version 1.2(26) of the grc4f one.
Finally we find predictions of the two programs to have an overall physical
uncertainty of 2%.
As a side result we show, on the example of an invariant mass
distribution, the strong interplay of spin correlations and detector cut-offs
in the case of four-fermion final states.Comment: 26 pages, LaTe
Non-trivial Center Dominance in High Temperature QCD
We investigate the properties of quarks and gluons above the chiral phase
transition temperature using the RG improved gauge action and the Wilson
quark action with two degenerate quarks mainly on a lattice. In
the one-loop perturbation theory, the thermal ensemble is dominated by the
gauge configurations with effectively center twisted boundary
conditions, making the thermal expectation value of the spatial Polyakov loop
take a non-trivial center. This is in agreement with our lattice
simulation of high temperature QCD. We further observe that the temporal
propagator of massless quarks at extremely high temperature remarkably agrees with the temporal propagator of free
quarks with the twisted boundary condition for , but
differs from that with the trivial boundary condition. As we increase
the mass of quarks , we find that the thermal ensemble continues to be
dominated by the twisted gauge field configurations as long as and above that the trivial configurations come in. The
transition is essentially identical to what we found in the departure from the
conformal region in the zero-temperature many-flavor conformal QCD on a finite
lattice by increasing the mass of quarks. We argue that the behavior is
consistent with the renormalization group analysis at finite temperature.Comment: 16 pages, 9 figures; 4 tables, an appendix adde
QED Radiative Correction for the Single-W Production using a Parton Shower Method
A parton shower method for the photonic radiative correction is applied to
the single W-boson production processes. The energy scale for the evolution of
the parton shower is determined so that the correct soft-photon emission is
reproduced. Photon spectra radiated from the partons are compared with those
from the exact matrix elements, and show a good agreement. Possible errors due
to a inappropriate energy-scale selection or due to the ambiguity of energy
scale determination are also discussed, particularly for the measurements on
triple gauge-couplings.Comment: 17 pages, 6 Postscript figure
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