275 research outputs found
Hard Thermal Loops, Gauged WZNW Action and the Energy of Hot Quark-Gluon Plasma
The generating functional for hard thermal loops in QCD is rewritten in terms
of a gauged WZNW action by introducing an auxiliary field. This shows in a
simple way that the contribution of hard thermal loops to the energy of the
quark-gluon plasma is positive.Comment: 9 pages, CU-TP 60
3-quasi-Sasakian manifolds
In the present paper we carry on a systematic study of 3-quasi-Sasakian
manifolds. In particular we prove that the three Reeb vector fields generate an
involutive distribution determining a canonical totally geodesic and Riemannian
foliation. Locally, the leaves of this foliation turn out to be Lie groups:
either the orthogonal group or an abelian one. We show that 3-quasi-Sasakian
manifolds have a well-defined rank, obtaining a rank-based classification.
Furthermore, we prove a splitting theorem for these manifolds assuming the
integrability of one of the almost product structures. Finally, we show that
the vertical distribution is a minimum of the corrected energy.Comment: 17 pages, minor modifications, references update
Model Independent Higgs Boson Mass Limits at LEP
We derive model-independent constraints on Higgs mass and couplings from
associated signals for higher masses, accessible at LEP2. This work is
motivated by the fact that, in many extensions of the standard model, the Higgs
boson can have substantial "invisible" decay modes, for example, into light or
massless weakly interacting Goldstone bosons associated to the spontaneous
violation of lepton number below the weak scale.Comment: FTUV/93-19, 13 pag + 2 figures(not included but available upon
request), Late
Joint resummation in electroweak boson production
We present a phenomenological application of the joint resummation formalism
to electroweak annihilation processes at measured boson momentum Q_T. This
formalism simultaneously resums at next-to-leading logarithmic accuracy large
threshold and recoil corrections to partonic scattering. We invert the impact
parameter transform using a previously described analytic continuation
procedure. This leads to a well-defined, resummed perturbative cross section
for all nonzero Q_T, which can be compared to resummation carried out directly
in Q_T space. From the structure of the resummed expressions, we also determine
the form of nonperturbative corrections to the cross section and implement
these into our analysis. We obtain a good description of the transverse
momentum distribution of Z bosons produced at the Tevatron collider.Comment: 27 pages, LaTeX, 8 figures as eps files. Some additions to earlier
version, this version as published in Phys. Rev. D66 (2002) 01401
The 2009 World Average of
Measurements of , the coupling strength of the Strong Interaction
between quarks and gluons, are summarised and an updated value of the world
average of is derived. Building up on previous reviews,
special emphasis is laid on the most recent determinations of . These
are obtained from -decays, from global fits of electroweak precision data
and from measurements of the proton structure function \F_2, which are based
on perturbative QCD calculations up to ; from hadronic event
shapes and jet production in \epem annihilation, based on
QCD; from jet production in deep inelastic scattering and from
decays, based on QCD; and from heavy quarkonia based on
unquenched QCD lattice calculations. Applying pragmatic methods to deal with
possibly underestimated errors and/or unknown correlations, the world average
value of results in . The
measured values of , covering energy scales from Q \equiv \mtau
= 1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD
and therefore significantly test the concept af Asymptotic Freedom.Comment: 14 pages, 7 figure
Seasonal drought limits tree species across the Neotropics
Within the tropics, the species richness of tree communities is strongly and positively associated with precipitation. Previous research has suggested that this macroecological pattern is driven by the negative effect of water-stress on the physiological processes of most tree species. This process implies that the range limits of taxa are defined by their ability to occur under dry conditions, and thus in terms of species distributions it predicts a nested pattern of taxa distribution from wet to dry areas. However, this âdry-toleranceâ hypothesis has yet to be adequately tested at large spatial and taxonomic scales. Here, using a dataset of 531 inventory plots of closed canopy forest distributed across the Western Neotropics we investigated how precipitation, evaluated both as mean annual precipitation and as the maximum climatological water deficit, influences the distribution of tropical tree species, genera and families. We find that the distributions of tree taxa are indeed nested along precipitation gradients in the western Neotropics. Taxa tolerant to seasonal drought are disproportionally widespread across the precipitation gradient, with most reaching even the wettest climates sampled; however, most taxa analysed are restricted to wet areas. Our results suggest that the âdry toleranceâ hypothesis has broad applicability in the world's most species-rich forests. In addition, the large number of species restricted to wetter conditions strongly indicates that an increased frequency of drought could severely threaten biodiversity in this region. Overall, this study establishes a baseline for exploring how tropical forest tree composition may change in response to current and future environmental changes in this region
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