5,116 research outputs found
Ultrasoft NLL Running of the Nonrelativistic O(v) QCD Quark Potential
Using the nonrelativistic effective field theory vNRQCD, we determine the
contribution to the next-to-leading logarithmic (NLL) running of the effective
quark-antiquark potential at order v (1/mk) from diagrams with one potential
and two ultrasoft loops, v being the velocity of the quarks in the c.m. frame.
The results are numerically important and complete the description of ultrasoft
next-to-next-to-leading logarithmic (NNLL) order effects in heavy quark pair
production and annihilation close to threshold.Comment: 25 pages, 7 figures, 3 tables; minor modifications, typos corrected,
references added, footnote adde
Ultrasoft Renormalization in Non-Relativistic QCD
For Non-Relativistic QCD the velocity renormalization group correlates the
renormalization scales for ultrasoft, potential and soft degrees of freedom.
Here we discuss the renormalization of operators by ultrasoft gluons. We show
that renormalization of soft vertices can induce new operators, and also
present a procedure for correctly subtracting divergences in mixed
potential-ultrasoft graphs. Our results affect the running of the
spin-independent potentials in QCD. The change for the NNLL t-tbar cross
section near threshold is very small, being at the 1% level and essentially
independent of the energy. We also discuss implications for analyzing
situations where mv^2 ~ Lambda_QCD.Comment: 31 pages, 11 fig
1S and MSbar Bottom Quark Masses from Upsilon Sum Rules
The bottom quark 1S mass, , is determined using sum rules which
relate the masses and the electronic decay widths of the mesons to
moments of the vacuum polarization function. The 1S mass is defined as half the
perturbative mass of a fictitious bottom-antibottom quark bound
state, and is free of the ambiguity of order which plagues the
pole mass definition. Compared to an earlier analysis by the same author, which
had been carried out in the pole mass scheme, the 1S mass scheme leads to a
much better behaved perturbative series of the moments, smaller uncertainties
in the mass extraction and to a reduced correlation of the mass and the strong
coupling. We arrive at GeV taking
as an input. From that we determine the
mass as GeV. The error in can be reduced if the three-loop corrections to the relation of
pole and mass are known and if the error in the strong coupling is
decreased.Comment: 20 pages, latex; numbers in Tabs. 2,3,4 corrected, a reference and a
comment on the fitting procedure added, typos in Eqs. 2 and 23 eliminate
Improved Perturbative QCD Approach to the Bottomonium Spectrum
Recently it has been shown that the gross structure of the bottomonium
spectrum is reproduced reasonably well within the non-relativistic boundstate
theory based on perturbative QCD. In that calculation, however, the fine
splittings and the S-P level splittings are predicted to be considerably
narrower than the corresponding experimental values. We investigate the
bottomonium spectrum within a specific framework based on perturbative QCD,
which incorporates all the corrections up to O(alpha_S^5 m_b) and O(alpha_S^4
m_b), respectively, in the computations of the fine splittings and the S-P
splittings. We find that the agreement with the experimental data for the fine
splittings improves drastically due to an enhancement of the wave functions
close to the origin as compared to the Coulomb wave functions. The agreement of
the S-P splittings with the experimental data also becomes better. We find that
natural scales of the fine splittings and the S-P splittings are larger than
those of the boundstates themselves. On the other hand, the predictions of the
level spacings between consecutive principal quantum numbers depend rather
strongly on the scale mu of the operator \propto C_A/(m_b r^2). The agreement
of the whole spectrum with the experimental data is much better than the
previous predictions when mu \simeq 3-4 GeV for alpha_S(M_Z)=0.1181. There
seems to be a phenomenological preference for some suppression mechanism for
the above operator.Comment: 26 pages, 16 figures. Minor changes, to be published in PR
Applications of QCD
Talk given at XIXth International Symposium on Lepton and Photon Interactions
at High Energies (LP 99), Stanford, California, 9-14 August 1999.Comment: latex, 26 page
Running of the heavy quark production current and 1/k potential in QCD
The 1/k contribution to the heavy quark potential is first generated at one
loop order in QCD. We compute the two loop anomalous dimension for this
potential, and find that the renormalization group running is significant. The
next-to-leading-log coefficient for the heavy quark production current near
threshold is determined. The velocity renormalization group result includes the
alpha_s^3 ln^2(alpha_s) ``non-renormalization group logarithms'' of Kniehl and
Penin.Comment: 30 pages, journal versio
Top Quark Pair Production close to Threshold: Top Mass, Width and Momentum Distribution
The complete NNLO QCD corrections to the total cross section in the kinematic region close to the top-antitop
threshold are calculated by solving the corresponding Schroedinger equations
exactly in momentum space in a consistent momentum cutoff regularization
scheme. The corrections coming from the same NNLO QCD effects to the top quark
three-momentum distribution are determined. We discuss
the origin of the large NNLO corrections to the peak position and the
normalization of the total cross section observed in previous works and propose
a new top mass definition, the 1S mass M_1S, which stabilizes the peak in the
total cross section. If the influence of beamstrahlung and initial state
radiation on the mass determination is small, a theoretical uncertainty on the
1S top mass measurement of 200 MeV from the total cross section at the linear
collider seems possible. We discuss how well the 1S mass can be related to the
mass. We propose a consistent way to implement the top quark width
at NNLO by including electroweak effects into the NRQCD matching coefficients,
which then can become complex.Comment: 53 pages, latex; minor changes, a number of typos correcte
Stability of twin circular tunnels in cohesive-frictional soil using the node-based smoothed finite element method (NS-FEM)
This paper presents an upper bound limit analysis procedure using the node-based smoothed finite element method (NS-FEM) and second order cone programming (SOCP) to evaluate the stability of twin circular tunnels in cohesive-frictional soils subjected to surcharge loading. At first stage, kinematically admissible displacement fields of the tunnel problems are approximated by NS-FEM using triangular elements (NS-FEM-T3). Next, commercial software Mosek is employed to deal with the optimization problems, which are formulated as second order cone. Collapse loads as well as failure mechanisms of plane strain tunnels are obtained directly by solving the optimization problems. For twin circular tunnels, the distance between centers of two parallel tunnels is the major parameter used to determine the stability. In this study, the effects of mechanical soil properties and the ratio of tunnel diameter and the depth to the tunnel stability are investigated. Numerical results are verified with those available to demonstrate the accuracy of the proposed method
Top quark mass definition and top quark pair production near threshold at the NLC
We suggest an infrared-insensitive quark mass, defined by subtracting the
soft part of the quark self energy from the pole mass. We demonstrate the deep
relation of this definition with the static quark-antiquark potential. At
leading order in 1/m this mass coincides with the PS mass which is defined in a
completely different manner. Going beyond static limit, the small normalization
point introduces recoil corrections which are calculated here as well. Using
this mass concept and other concepts for the quark mass we calculate the cross
section of e+ e- -> t t-bar near threshold at NNLO accuracy adopting three
alternative approaches, namely (1) fixing the pole mass, (2) fixing the PS
mass, and (3) fixing the new mass which we call the PS-bar mass. We demonstrate
that perturbative predictions for the cross section become much more stable if
we use the PS or the PS-bar mass for the calculations. A careful analysis
suggests that the top quark mass can be extracted from a threshold scan at NLC
with an accuracy of about 100-200 MeV.Comment: published version, 21 pages in LaTeX including 11 PostScript figure
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