2,526 research outputs found
The diverse origins of neutron-capture elements in the metal-poor star HD 94028 : possible detection of products of i-process nucleosynthesis
We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = â1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = â0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy
Consistency and lattice renormalization of the effective theory for heavy quarks
The effective theory describing infinite mass particles with a given
velocity, has a great interest in heavy flavor physics. It has the unpleasant
characteristic that the energy spectrum is unbounded from below; this fact is
the source of the problems in the formulation of the euclidean theory. In this
paper we present an analysis of the euclidean effective theory, that is rather
complete and has positive conclusions. A proof of the consistency of the
euclidean theory is presented and a technique for the evaluation of the
amplitudes in perturbation theory is described. We compute also the one-loop
renormalization constants of the lattice effective theory and of the
heavy-heavy current that is needed for the determination of the Isgur-Wise
function. A variety of effects related to the explicit breaking of the Lorentz
symmetry of lattice regularization is demonstrated. The most peculiar
phenomenon is that the heavy quark velocity receives a finite renormalization.
Finally, we compute the lattice-continuum renormalization constant of the
Isgur-Wise current. It is needed for the conversion of the values of the matrix
elements computed with the lattice effective theory, to the values in the full
theory.Comment: 28 pages, Latex version 2.09, SISSA 56/93/E
Hadron collider limits on anomalous couplings
A next-to-leading log calculation of the reactions and
is presented including a tri-boson
gauge coupling from non-Standard Model contributions. Two approaches are made
for comparison. The first approach considers the tri-boson coupling
as being uniquely fixed by tree level unitarity at high energies to its
Standard Model form and, consequently, suppresses the non-Standard Model
contributions with form factors. The second approach is to ignore such
considerations and calculate the contributions to non-Standard Model tri-boson
gauge couplings without such suppressions. It is found that at Tevatron
energies, the two approaches do not differ much in quantitative results, while
at Large Hadron Collider (LHC) energies the two approaches give significantly
different predictions for production rates. At the Tevatron and LHC, however,
the sensitivity limits on the anomalous coupling of are too weak to
usefully constrain parameters in effective Lagrangian models.Comment: Revtex 23 pages + 8 figures, UIOWA-94-1
Radiatively corrected shape function for inclusive heavy hadron decays
We discuss the non-perturbative and the radiative corrections to inclusive B
decays from the point of view known from QED corrections to high energy e^+ e^-
processes. Here the leading contributions can be implemented through the so
called ``radiator function'' which corresponds to the shape function known in
heavy hadron decays. In this way some new insight into the origin of the shape
function is obtained. As a byproduct, a parameterization of the radiatively
corrected shape function is suggested which can be implemented in Monte Carlo
studies of inclusive heavy hadron decays.Comment: LaTeX, uses a4, graphicx and psfrag, 10 pages. The complete paper is
also available at http://www-ttp.physik.uni-karlsruhe.de/Preprints
Resummed B -> X_u l nu Decay Distributions to Next-to-Leading Order
We perform factorization of the most general distribution in semileptonic B
-> X_u decays and we resum the threshold logarithms to next-to-leading order.
From this (triple-differential) distribution, any other distribution is
obtained by integration. As an application of our method, we derive simple
analytical expressions for a few distributions, resummed to leading
approximation. It is shown that the shape function can be directly determined
by measuring the distribution in m_X^2/E_X^2, not in m_X^2/m_B^2. We compute
the resummed hadron energy spectrum, which has a ``Sudakov shoulder'', and we
show how the distribution in the singular region is related to the shape
function. We also present an improved formula for the photon spectrum in B->X_s
gamma which includes soft-gluon resummation and non-leading operators in the
effective hamiltonian. We explicitly show that the same non-perturbative
function - namely the shape function - controls the non-perturbative effects in
all the distributions in the semi-leptonic and in the rare decay.Comment: LaTex file, 19 pages, 3 postscript figures; minor changes, some typos
correcte
Granular Flows in a Rotating Drum: the Scaling Law between Velocity and Thickness of the Flow
The flow of dry granular material in a half-filled rotating drum is studied.
The thickness of the flowing zone is measured for several rotation speeds, drum
sizes and beads sizes (size ratio between drum and beads ranging from 47 to
7400). Varying the rotation speed, a scaling law linking mean velocity vs
thickness of the flow, , is deduced for each couple (beads, drum).
The obtained exponent is not always equal to 1, value previously reported
in a drum, but varies with the geometry of the system. For small size ratios,
exponents higher than 1 are obtained due to a saturation of the flowing zone
thickness. The exponent of the power law decreases with the size ratio, leading
to exponents lower than 1 for high size ratios. These exponents imply that the
velocity gradient of a dry granular flow in a rotating drum is not constant.
More fundamentally, these results show that the flow of a granular material in
a rotating drum is very sensible to the geometry, and that the deduction of the
``rheology'' of a granular medium flowing in such a geometry is not obvious
A new formulation of the effective theory for heavy particles
We derive the effective theories for heavy particles with a functional
integral approach by integrating away the states with high velocity and with
high virtuality. This formulation is non-perturbative and has a close
connection with the Wilson renormalization group transformation. The fixed
point hamiltonian of our transformation coincides with the static hamiltonian
and irrelevant operators can be identified with the usual corrections to
the static theory. No matching condition has to be imposed between the full and
the static theory operators with our approach. The values of the matching
constants come out as a dynamical effect of the renormalization group flow.Comment: 26 pages, plain Latex + 4 postscript figures (appended at the end),
Preprint Roma1 993-94 (some missing lines in a few formulas have been
restored; minor changes
Strong Decays of Strange Charmed P-Wave Mesons
Goldstone boson decays of P-wave mesons are studied within the
framework of Heavy Hadron Chiral Perturbation Theory. We first analyze the
simplest single kaon decays of these strange charmed mesons. We derive a model
independent prediction for the width of and use experimental
information on to constrain the S-wave contribution to decay.
Single and double pion decay modes are then discussed and shown to be
significantly restricted by isospin conservation. We conclude that the pion
channels may offer the best hope for detecting one strange member of an
otherwise invisible P-wave flavor multiplet.Comment: 16 pages, 2 updated figures not included but available upon request,
CALT-68-1902. (Revised estimates for error on width and for isospin
violating neutral pion decay of .
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