167 research outputs found
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
Measurement of the D+ and Ds+ decays into K+K-K+
We present the first clear observation of the doubly Cabibbo suppressed decay
D+ --> K-K+K+ and the first observation of the singly Cabibbo suppressed decay
Ds+ --> K-K+K+. These signals have been obtained by analyzing the high
statistics sample of photoproduced charm particles of the FOCUS(E831)
experiment at Fermilab. We measure the following relative branching ratios:
Gamma(D+ --> K-K+K+)/Gamma(D+ --> K-pi+pi+) = (9.49 +/- 2.17(statistical) +/-
0.22(systematic))x10^-4 and Gamma(Ds+ --> K-K+K+)/Gamma(Ds+ --> K-K+pi+) =
(8.95 +/- 2.12(statistical) +2.24(syst.) -2.31(syst.))x10^-3.Comment: 10 pages, 8 figure
Study of Hadronic Five-Body Decays of Charmed Mesons
We study the decay of D+ and Ds+ mesons into charged five body final states,
and report the discovery of the decay mode D+ -> K+K-Pi+Pi+Pi-, as well as
measurements of the decay modes D+ -> K-Pi+Pi+Pi+Pi-, Ds+ -> K+K-Pi+Pi+Pi-, Ds+
-> PhiPi+Pi+Pi- and D+/Ds+ -> Pi+Pi+Pi+Pi-Pi-. An analysis of the resonant
substructure is also included, with evidence suggesting that both decays
proceed primarily through an a1 vector resonance.Comment: 11 pages, 3 figure
New Measurements of the D+ to K* mu nu Form Factor Ratios
Using a large sample of D+ to K- pi+ mu+ nu decays collected by the FOCUS
photoproduction experiment at Fermilab, we present new measurements of two
semileptonic form factor ratios: rv and r2. We find rv = 1.504 \pm 0.057 \pm
0.039 and r2 = 0.875 \pm 0.049 \pm 0.064. Our form factor results include the
effects of the s-wave interference discussed in a previous paper.Comment: 12 pages, 5 figure
The Target Silicon Detector for the FOCUS Spectrometer
We describe a silicon microstrip detector interleaved with segments of a
beryllium oxide target which was used in the FOCUS photoproduction experiment
at Fermilab. The detector was designed to improve the vertex resolution and to
enhance the reconstruction efficiency of short-lived charm particles.Comment: 18 pages, 14 figure
Measurement of the relative branching ratio BR(\Xi_c^+ \to p^+ K^-\pi^+)\BR(\Xi_c^+ \to \Xi^- \pi^+ \pi^+)
We report the observation of the Cabibbo suppressed decay \Xi_c^+ \to p
K^-\pi^+ using data collected with the FOCUS spectrometer during the 1996--97
Fermilab fixed target run. We find a \Xi_c^+ signal peak of 202\pm35 events. We
have measured the relative branching ratios BR(\Xi^+_c\to p
K^-\pi^+)/BR(\Xi^+_c\to\Xi^-\pi^+\pi^+)= 0.234 \pm 0.047 \pm 0.022 and
BR(\Xi^+_c\to p \bar{K}^*(892)^0)/BR(\Xi^+_c\to p K^-\pi^+)= 0.54 \pm 0.09 \pm
0.05 .Comment: 9 pages, 4 figure
Observation of a 1750 MeV/c^2 Enhancement in the Diffractive Photoproduction of K^+K^-
Using the FOCUS spectrometer with photon beam energies between 20 and 160
\gev, we confirm the existence of a diffractively photoproduced enhancement in
at 1750 \mevcc with nearly 100 times the statistics of previous
experiments. Assuming this enhancement to be a single resonance with a
Breit-Wigner mass shape, we determine its mass to be
\mevcc and its width to be \mevcc. We find no
corresponding enhancement at 1750 \mevcc in , and again neglecting any
possible interference effects we place limits on the ratio . Our results are consistent with previous
photoproduction experiments, but, because of the much greater statistics,
challenge the common interpretation of this enhancement as the
seen in annihilation experiments.Comment: 10 pages, 5 figure
Charm System Tests of CPT and Lorentz Invariance with FOCUS
We have performed a search for CPT violation in neutral charm meson
oscillations. While flavor mixing in the charm sector is predicted to be small
by the Standard Model, it is still possible to investigate CPT violation
through a study of the proper time dependence of a CPT asymmetry in right-sign
decay rates for and \d0b\to K^+\pi^-. This asymmetry is
related to the CPT violating complex parameter and the mixing parameters
and : . Our 95% confidence
level limit is . Within the
framework of the Standard Model Extension incorporating general CPT violation,
we also find 95% confidence level limits for the expressions involving
coefficients of Lorentz violation of GeV, GeV, and GeV,
where is the factor which incorporates mixing parameters ,
and the doubly Cabibbo suppressed to Cabibbo favored relative strong phase
.Comment: 12 pages 5 figure
Study of the Cabibbo-suppressed decay modes D0-->Pi-pi+ and D0-->K-K+
Using data from the FOCUS (E831) experiment at Fermilab, we present a new
measurement for the branching ratios of the Cabibbo-suppressed decay modes
D0-->Pi-Pi+ and D0-->K-K+. We measured:
Gamma(D0-->K-K+)/Gamma(D0-->Pi-Pi+) = 2.81 +/- 0.10(stat) +/- 0.06(syst),
Gamma(D0-->K-K+)/Gamma(D0-->K-Pi+) = 0.0993 +/- 0.0014(stat) +/- 0.0014(syst),
and Gamma(D0-->Pi-Pi+)/Gamma(D0-->K-Pi+) = 0.0353 +/- 0.0012 (stat) +/-
0.0006(syst).
These values have been combined with other experimental data to extract the
ratios of isospin amplitudes and the phase shifts for the D-->KK and D-->PiPi
decay channels.Comment: 12 pages, 1 Figure, accepted for publication in Phys.Lett.
Quantum Dimensional Zeeman Effect in the Magneto-optical Absorption Spectrum for Quantum Dot - Impurity Center Systems
Magneto-optical properties of the quantum dot - impurity center (QD-IC)
systems synthesized in a transparent dielectric matrix are considered. For the
QD one-electron state description the parabolic model of the confinement
potential is used. Within the framework of zero-range potential model and the
effective mass approach, the light impurity absorption coefficient for the case
of transversal polarization with respect to the applied magnetic field
direction, with consideration of the QD size dispersion, has been analytically
calculated. It is shown that for the case of transversal polarization the light
impurity absorption spectrum is characterized by the quantum dimensional Zeeman
effect.Comment: 18 pages, 1 figure, PDF fil
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