4,623 research outputs found
Maximizing phonon thermal conductance for ballistic membranes
At low temperatures, phonon scattering can become so weak that phonon
transport becomes ballistic. We calculate the ballistic phonon conductance G
for membranes using elasticity theory, considering the transition from three to
two dimensions. We discuss the temperature and thickness dependence and
especially concentrate on the issue of material parameters. For all membrane
thicknesses, the best conductors have, counter-intuitively, the lowest speed of
sound.Comment: 4 pages, 4 figures, proceedings to phonons 2007 conferenc
Bottom Quark Mass Determination from low-n Sum Rules
We study the uncertainties in the MSbar bottom quark mass determination using
relativistic sum rules to O(alpha_S^2). We include charm mass effects and
secondary b bbar production and treat the experimental continuum region more
conservatively than previous analyses. The PDG treatment of the region between
the resonances Upsilon (4S) and Upsilon (5S) is reconsidered. Our final result
reads: m_b(m_b)=(4.20 \pm 0.09) GeV.Comment: 4 pages, Talk given at QCD 03, Montpellier, France, 2-9 July 200
Gluon Fragmentation into Heavy Quarkonium
The dominant production mechanism for heavy quark-antiquark bound states in
very high energy processes is fragmentation, the splitting of a high energy
parton into a quarkonium state and other partons. We show that the
fragmentation functions describing these processes can be calculated
using perturbative QCD. We calculate the fragmentation functions for a gluon to
split into S-wave quarkonium states to leading order in the QCD coupling
constant. The leading logarithms of , where is the factorization
scale and is the heavy quark mass, are summed up using Altarelli-Parisi
evolution equations.Comment: LateX 11 pages (3 figures available upon request). NUHEP-TH-92-2
Five-loop fermion anomalous dimension for a general gauge group from four-loop massless propagators
We extend the result of the analytic calculation of
the quark mass anomalous dimension in pQCD [1] to the case of a generic gauge
group. We present explicit formulas which express the relevant renormalization
constants in terms of four-loop massless propagators. We also use our result to
shed new light on the old puzzle of the absence of even zetas in results of
perturbative calculations for a class of physical observables.Comment: An important reference [47] is correcte
Scalar Correlator at O(alpha_s^4), Higgs Decay into b-quarks and Bounds on the Light Quark Masses
We compute, for the first time, the absorptive part of the massless
correlator of two quark scalar currents in five loops. As physical applications
we consider the O(alpha_s^4) corrections to the decay rate of the Standard
Model Higgs boson into quarks, as well as the constraints on the strange quark
mass following from QCD sum rules.Comment: 5 pages, REVTeX4. Few references updated. The final version, accepted
for publication. The complete paper is also available via the www at
http://www-ttp.physik.uni-karlsruhe.de/Preprints
Quark Mass and Field Anomalous Dimensions to
We present the results of the first complete analytic calculation of the
quark mass and field anomalous dimensions to in QCD
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