2,499 research outputs found
Electroweak and QCD corrections to top-pair hadroproduction in association with heavy bosons
We compute the contribution of order to the cross
section of a top-antitop pair in association with at least one heavy Standard
Model boson -- , , and Higgs -- by including all effects of QCD, QED,
and weak origin and by working in the automated MadGraph5_aMC@NLO framework.
This next-to-leading order contribution is then combined with that of order
, and with the two dominant lowest-order ones,
and , to obtain phenomenological results
relevant to a 8, 13, and 100~TeV collider.Comment: 27 pages, 8 figure
Weak corrections to Higgs hadroproduction in association with a top-quark pair
We present the calculation of the next-to-leading contribution of order
to the production of a Standard Model Higgs boson in
association with a top-quark pair at hadron colliders. All effects of weak and
QCD origin are included, whereas those of QED origin are ignored. We work in
the MadGraph5_aMC@NLO framework, and discuss sample phenomenological
applications at a 8, 13, and 100 TeV collider, including the effects of
the dominant next-to-leading QCD corrections of order .Comment: 29 pages, 38 figure
The automation of next-to-leading order electroweak calculations
We present the key features relevant to the automated computation of all the
leading- and next-to-leading order contributions to short-distance cross
sections in a mixed-coupling expansion, with special emphasis on the first
subleading NLO term in the QCD+EW scenario, commonly referred to as NLO EW
corrections. We discuss, in particular, the FKS subtraction in the context of a
mixed-coupling expansion; the extension of the FKS subtraction to processes
that include final-state tagged particles, defined by means of fragmentation
functions; and some properties of the complex mass scheme. We combine the
present paper with the release of a new version of MadGraph5_aMC@NLO, capable
of dealing with mixed-coupling expansions. We use the code to obtain
illustrative inclusive and differential results for the 13-TeV LHC.Comment: 121 pages, 16 figure
Boron depletion in 9 to 15 M(circle dot) stars with rotation
The treatment of mixing is still one of the major uncertainties in stellar evolution models. One open question is how well the prescriptions for rotational mixing describe the real effects. We tested the mixing prescriptions included in the Geneva stellar evolution code (GENEC) by following the evolution of surface abundances of light isotopes in massive stars, such as boron and nitrogen. We followed 9, 12 and 15 M(O) models with rotation from the zero age main sequence up to the end of He burning. The calculations show the expected behaviour with faster depletion of boton for faster rotating stars and more massive stars. The mixing at the surface is more efficient, than predicted by prescriptions used in other codes and reproduces the majority of observations very well However two observed stars with strong boron depletion but, no nitrogen enhancement still can not be explained and let the question open whether additional mixing processes are acting in these massive star
The impact of stellar rotation on the CNO abundance patterns in the Milky Way at low metallicities
We investigate the effect of new stellar models, which take rotation into
account, computed for very low metallicities on the chemical evolution of the
earliest phases of the Milky Way. We check the impact of these new stellar
yields on a model for the halo of the Milky Way that can reproduce the observed
halo metallicity distribution. In this way we try to better constrain the ISM
enrichment timescale, which was not done in our previous work. The stellar
models adopted in this work were computed under the assumption that the ratio
of the initial rotation velocity to the critical velocity of stars is roughly
constant with metallicity. This naturally leads to faster rotation at lower
metallicity, as metal poor stars are more compact than metal rich ones. We find
that the new Z = 10-8 stellar yields computed for large rotational velocities
have a tremendous impact on the interstellar medium nitrogen enrichment for
log(O/H)+12 < 7 (or [Fe/H]< -3). We show that upon the inclusion of the new
stellar calculations in a chemical evolution model for the galactic halo with
infall and outflow, both high N/O and C/O ratios are obtained in the very-metal
poor metallicity range in agreement with observations. Our results give further
support to the idea that stars at very low metallicities could have initial
rotational velocities of the order of 600-800kms-1. An important contribution
to N from AGB stars is still needed in order to explain the observations at
intermediate metallicities. One possibility is that AGB stars at very low
metallicities also rotate fast. This could be tested in the future, once
stellar evolution models for fast rotating AGB stars will be available.Comment: Contribution to Nuclei in the Cosmos IX (Proceedings of Science - 9
pages, 4 figs., accepted) - Version 2: one reference added in the caption of
Fig.
Higgs production in association with bottom quarks
We study the production of a Higgs boson in association with bottom quarks in
hadronic collisions, and present phenomenological predictions relevant to the
13 TeV LHC. Our results are accurate to the next-to-leading order in QCD, and
matched to parton showers through the MC@NLO method; thus, they are fully
differential and based on unweighted events, which we shower by using both
Herwig++ and Pythia8. We perform the computation in both the four-flavour and
the five-flavour schemes, whose results we compare extensively at the level of
exclusive observables. In the case of the Higgs transverse momentum, we also
consider the analytically-resummed cross section up to the NNLO+NNLL accuracy.
In addition, we analyse at the effects of the
interference between the and gluon-fusion production modes.Comment: 33 pages, 17 figure
SPINSTARS at low metallicities
The main effect of axial rotation on the evolution of massive PopIII stars is
to trigger internal mixing processes which allow stars to produce significant
amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars
produce much more primary nitrogen than PopIII stars for a given initial mass
and rotation velocity. The very metal poor stars undergo strong mass loss
induced by rotation. One can distinguish two types of rotationnaly enhanced
stellar winds: 1) Rotationally mechanical winds occurs when the surface
velocity reaches the critical velocity at the equator, {\it i.e.} the velocity
at which the centrifugal acceleration is equal to the gravity; 2) Rotationally
radiatively line driven winds are a consequence of strong internal mixing which
brings large amounts of CNO elements at the surface. This enhances the opacity
and may trigger strong line driven winds. These effects are important for an
initial value of of 0.54 for a 60 M at
, {\it i.e.} for initial values of
higher than the one (0.4) corresponding to observations at solar .
These two effects, strong internal mixing leading to the synthesis of large
amounts of primary nitrogen and important mass losses induced by rotation,
occur for between about 10 and 0.001. For metallicities above 0.001
and for reasonable choice of the rotation velocities, internal mixing is no
longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of
First Stars III, Santa Fe, 200
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