4,608 research outputs found
Standard Model Higgs Search Strategy at LEP
The Standard Model Higgs boson has been searched for by the four LEP
experiments in the last twelve years. The data collected at LEP in the year
2000 suggest the first observation of a Higgs boson. In this letter, I describe
the basic concepts of the Higgs search at LEP, with emphasis in the statistical
method used to combine the results from the LEP experiments.Comment: 4 pages, 8 figures, submitted to Moriond QCD 200
Spin Analysis of the Process e+e- --> tau+tau- at LEP
Using the data collected by the four experiments at LEP during 1990-1994, a
precise measurement of the tau longitudinal polarisation (P_tau) has been
performed, as well as the measurement of the transverse-transverse and
transverse-normal tau spin correlations. From the P_tau measurement, assuming
lepton universality of the neutral currents, the effective weak mixing angle
has been determined to be sin(theta_W) = 0.2325 +- 0.0006. The Standard Model
predictions are consistent with the measured results.Comment: Invited talk to San Miniato 1997, 6 pages, 10 figures, Latex, macro
espcrc2.sty include
Measurement of the Higgs Cross Section and Mass with Linear Colliders
We report on the accuracy of the measurement of the Higgs boson mass and the
total cross section of the process e+e- -> ZH that would be achieved in a
linear collider operating at a centre-of-mass energy of 350 GeV, assuming an
integrated luminosity of 500 (1/fb). For that we have exploited the recoil mass
off the Z using its leptonic decays into electron and muon pairs. The Higgs
mass is determined with 150 MeV accuracy, the recoil mass resolution is about
1.5 GeV and the cross section is obtained with a statistical error of 3%.Comment: 5 pages, 4 figures, to be published in the Proceedings of the LCWS99
(Sitges, Spain) by World Scientific Publishing Company (Singapore
Fluorine in AGB Carbon Stars Revisited
A reanalysis of the fluorine abundance in three Galactic AGB carbon stars (TX
Psc, AQ Sgr and R Scl) has been performed from the molecular HF (1-0) R9 line
at 2.3358 m. High-resolution (R) and high signal to noise
spectra obtained with the CRIRES spectrograph and the VLT telescope or from the
NOAO archive (for TX Psc) have been used. Our abundance analysis uses the
latest generation of MARCS model atmospheres for cool carbon rich stars. Using
spectral synthesis in LTE we derive for these stars fluorine abundances that
are systematically lower by dex in average with respect to the sole
previous estimates by Jorissen, Smith & Lambert (1992). The possible reasons of
this discrepancy are explored. We conclude that the difference may rely on the
blending with C-bearing molecules (CN and C) that were not properly taken
into account in the former study. The new F abundances are in better agreement
with the prediction of full network stellar models of low mass AGB stars. These
models also reproduce the -process elements distribution in the sampled
stars. This result, if confirmed in a larger sample of AGB stars, might
alleviate the current difficulty to explain the largest [F/O] ratios found by
Jorissen et al. In particular, it may not be necessary to search for
alternative nuclear chains affecting the production of F in AGB stars.Comment: 25 pages, 3 figures. to be appear in The Astrophysical Journal (Jan
2009 issue
High resolution optical spectroscopy of IRAS 09425-6040 (=GLMP 260)
We present high resolution optical spectroscopic observations of IRAS
09425-6040, a peculiar, extremely red, C-rich AGB star showing prominent O-rich
dust features in its ISO infrared spectrum attributed to crystalline silicates.
Our analysis shows that IRAS 09425-6040 is indeed a C-rich star slightly
enriched in lithium (log (Li/H) + 12 ~ 0.7) with a low 12C/13C = 15+-6 ratio.
We also found some evidence that it may be enriched in s-elements. Combining
our results with other observational data taken from the literature we conclude
that the star is possibly an intermediate-mass TP-AGB star (M > 3 M_sun) close
to the end of its AGB evolution which may have only very recently experienced a
radical change in its chemistry, turning into a carbon-rich AGB star.Comment: 5 pages, 2 figures, accepted for publication in A&
Determination of the Higgs boson spin with a linear e+e- collider
The energy dependence of the production cross section of a light Higgs boson
is studied at threshold and compared to the expectations of several spin
assumptions. Cross section measurements at three centre-of-mass energies with
an integrated luminosity of 20 fb-1 allow the confirmation of the scalar nature
of the Higgs Boson.Comment: 4 pages (Latex), 4 figures (Postscript
The puzzle of the CNO isotope ratios in AGB carbon stars
Previous determinations of the oxygen isotopic ratios in AGB carbon stars
were at odds with the existing theoretical predictions. We aim to redetermine
the oxygen ratios in these stars using new spectral analysis tools and further
develop discussions on the carbon and nitrogen isotopic ratios in order to
elucidate this problem. Oxygen isotopic ratios were derived from spectra in the
K-band in a sample of galactic AGB carbon stars of different spectral types and
near solar metallicity. Synthetic spectra calculated in LTE with spherical
carbon-rich atmosphere models and updated molecular line lists were used. The
CNO isotope ratios derived in a homogeneous way, were compared with theoretical
predictions for low-mass (1.5-3 M_o) AGB stars computed with the FUNS code
assuming extra mixing both during the RGB and AGB phases. For most of the stars
the 16O/17O/18O ratios derived are in good agreement with theoretical
predictions confirming that, for AGB stars, are established using the values
reached after the FDU according to the initial stellar mass. This fact, as far
as the oxygen isotopic ratios are concerned, leaves little space for the
operation of any extra mixing mechanism during the AGB phase. Nevertheless, for
a few stars with large 16O/17O/18O, the operation of such a mechanism might be
required, although their observed 12C/13C and 14N/15N ratios would be difficult
to reconcile within this scenario. Furthermore, J-type stars tend to have lower
16O/17O ratios than the normal carbon stars, as already indicated in previous
studies. Excluding these peculiar stars, AGB carbon stars occupy the same
region as pre-solar type I oxide grains in a 17O/16O vs. 18O/16O diagram,
showing little spread. This reinforces the idea that these grains were probably
formed in low-mass stars during the previous O-rich phases.Comment: Accepted fo publication in A&
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