26,170 research outputs found
The use of the tau in new particle searches at DELPHI
Several new particle searches have been performed in the DELPHI experiment
involving tau leptons in the resulting final state. The topology and special
characteristics of the tau leptons have been used to discriminate the signal
from the Standard Model background. Limits on new particles have been set,
playing an important role the channels with tau leptons.Comment: Invited talk at the Seventh International Workshop on Tau Lepton
Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 10 pages, LaTeX, 9 eps
figure
The diameter of type D associahedra and the non-leaving-face property
Generalized associahedra were introduced by S. Fomin and A. Zelevinsky in
connection to finite type cluster algebras. Following recent work of L. Pournin
in types and , this paper focuses on geodesic properties of generalized
associahedra. We prove that the graph diameter of the -dimensional
associahedron of type is precisely for all greater than .
Furthermore, we show that all type associahedra have the non-leaving-face
property, that is, any geodesic connecting two vertices in the graph of the
polytope stays in the minimal face containing both. This property was already
proven by D. Sleator, R. Tarjan and W. Thurston for associahedra of type .
In contrast, we present relevant examples related to the associahedron that do
not always satisfy this property.Comment: 18 pages, 14 figures. Version 3: improved presentation,
simplification of Section 4.1. Final versio
Cluster algebras of type D: pseudotriangulations approach
We present a combinatorial model for cluster algebras of type in terms
of centrally symmetric pseudotriangulations of a regular -gon with a small
disk in the centre. This model provides convenient and uniform interpretations
for clusters, cluster variables and their exchange relations, as well as for
quivers and their mutations. We also present a new combinatorial interpretation
of cluster variables in terms of perfect matchings of a graph after deleting
two of its vertices. This interpretation differs from known interpretations in
the literature. Its main feature, in contrast with other interpretations, is
that for a fixed initial cluster seed, one or two graphs serve for the
computation of all cluster variables. Finally, we discuss applications of our
model to polytopal realizations of type associahedra and connections to
subword complexes and -cluster complexes.Comment: 21 pages, 21 figure
On the Influence of X-Ray Galaxy Clusters in the Fluctuations of the Cosmic Microwave Background
The negative evolution found in X--ray clusters of galaxies limits the amount
of available hot gas for the inverse Compton scattering of the Cosmic Microwave
Background (the Sunyaev--Zel'dovich effect). Using a parametrisation of the
X-ray luminosity function and its evolution in terms of a coalescence model (as
presented in the analysis of a flux limited X-ray cluster sample by Edge et al.
1990), as well as a simple virialised structure for the clusters (which
requires a gas to total mass fraction \approxgt 0.1 in order to reproduce
observed properties of nearby clusters) we show that the Compton distortion
parameter is about two orders of magnitude below the current FIRAS upper
limits. Concerning the anisotropies imprinted on arcmin scales they are
dominated by the hottest undetected objects. We show that they are negligible
({\Delta T\over T}\approxlt 10^{-7}) at wavelengths \lambda\approxgt 1~mm.
At shorter wavelengths they become more important ( at ~mm), but in fact most clusters will produce an
isolated and detectable feature in sky maps. After removal of these signals,
the fluctuations imprinted by the remaining clusters on the residual radiation
are still much smaller. The conclusion is that X-ray clusters can be ignored as
sources of Cosmic Microwave Background fluctuations.Comment: 20 pages Plain Tex, 7 figures available upon request,UCAST-94-0
B Mixing and Lifetimes at the Tevatron
The Tevatron collider at Fermilab provides a very rich environment for the
study of b-hadrons. Both the D0 and CDF experiments have collected a sample of
about 1 fb^{-1}. We report results on three topics: b-hadron lifetimes,
polarization amplitudes and the decay width difference in Bs to Jpsi Phi, and
Bs mixing.Comment: Flavor Physics & CP Violation Conference, Vancouver, 200
Soft versus Hard X-ray emission in AGN: partial covering and warm plus cold absorber models
We analyse the ROSAT PSPC hardness ratio and the 0.5-2 keV to 2-10 keV flux
ratio of 65 Active Galactic Nuclei (AGN) for which there are both ROSAT
archival observations available and 2-10 keV fluxes, mostly from the HEAO-1
MC-LASS survey. We conclude that the simplest spectral model for the AGN that
can accommodate the variety of X-ray colours obtained is a standard power law
(with energy spectral index ) plus a keV black body
both partially absorbed. In our sample, type 1 AGN require an absorbing column
around with covering fractions between 20 and 100\%,
while type 2 AGN display larger columns and coverage. This simple
model also provides a good link between soft and hard AGN X-ray luminosity
functions and source counts. We also consider a warm absorber as an alternative
model to partial covering and find that the the presence of gas in two phases
(ionized and neutral) is required.Comment: 10 pages, Latex (mn.sty), 1 table, 5 figures included (epsf),
postscript version also available via anonymous ftp at
ftp://astsun1.unican.es/pub/ceballos/ . Accepted for publication in MNRA
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