393 research outputs found
Fermionic Corrections to the Three-Loop Matching Coefficient of the Vector Current
In this paper we consider the matching coefficient of the vector current
between Quantum Chromodynamics (QCD) and Non-Relativistic QCD (NRQCD) to
three-loop order in perturbation theory. We evaluate the fermionic corrections
containing a closed massless fermion loop. The results are building blocks both
for the bottom and top quark system at threshold. We explain in detail the
methods used for the evaluation of the Feynman diagrams, classify the occurring
master integrals and provide results for the latter. The numerical effects are
significant. They have the tendency to improve the behaviour of the
perturbative series -- both for the bottom and top quark system.Comment: 21 page
The static potential: lattice versus perturbation theory in a renormalon-based approach
We compare, for the static potential and at short distances, perturbation
theory with the results of lattice simulations. We show that a
renormalon-dominance picture explains why in the literature sometimes
agreement, and another disagreement, is found between lattice simulations and
perturbation theory depending on the different implementations of the latter.
We also show that, within a renormalon-based scheme, perturbation theory agrees
with lattice simulations.Comment: 18 pages, 11 figures, lattice data of Necco and Sommer introduced,
references added, some lengthier explanations given, physical results
unchange
Bottonium mass - evaluation using renormalon cancellation
We present a method of calculating the bottonium mass M[Upsilon(1S)] = [2 mb
+ E(b barb)]. The binding energy is separated into the soft and ultrasoft
components E(b barb)=[E(s)+E(us)] by requiring the reproduction of the correct
residue parameter value of the renormalon singularity for the renormalon
cancellation in the sum [2 mb + E(s)]. The Borel resummation is then performed
separately for (2 mb) and E(s), using the infrared safe MSbar mass [bar mb] as
input. E(us) is estimated. Comparing the result with the measured value of
M[Upsilon(1S)], the extracted value of the quark mass is [bar mb](mu=[bar mb])
= 4.241 +- 0.068 GeV (for the central value alphas(MZ)=0.1180). This value of
[bar mb] is close to the earlier values obtained from the QCD spectral sum
rules, but lower than from pQCD evaluations without the renormalon structure
for heavy quarkonia.Comment: 4 pages, uses espcrc2.sty, presented at QCD0
NRQCD Analysis of Bottomonium Production at the Tevatron
Recent data from the CDF collaboration on the production of spin-triplet
bottomonium states at the Tevatron p \bar p collider are analyzed within the
NRQCD factorization formalism. The color-singlet matrix elements are determined
from electromagnetic decays and from potential models. The color-octet matrix
elements are determined by fitting the CDF data on the cross sections for
Upsilon(1S), Upsilon(2S), and Upsilon(3S) at large p_T and the fractions of
Upsilon(1S) coming from chi_b(1P) and chi_b(2P). We use the resulting matrix
elements to predict the cross sections at the Tevatron for the spin-singlet
states eta_b(nS) and h_b(nP). We argue that eta_b(1S) should be observable in
Run II through the decay eta_b -> J/psi + J/psi.Comment: 20 pages, 3 figure
Electroweak non-resonant NLO corrections to e+ e- -> W+ W- b bbar in the t tbar resonance region
We analyse subleading electroweak effects in the top anti-top resonance
production region in e+ e- collisions which arise due to the decay of the top
and anti-top quarks into the W+ W- b bbar final state. These are NLO
corrections adopting the non-relativistic power counting v ~ alpha_s ~
sqrt(alpha_EW). In contrast to the QCD corrections which have been calculated
(almost) up to NNNLO, the parametrically larger NLO electroweak contributions
have not been completely known so far, but are mandatory for the required
accuracy at a future linear collider. The missing parts of these NLO
contributions arise from matching coefficients of non-resonant production-decay
operators in unstable-particle effective theory which correspond to off-shell
top production and decay and other non-resonant irreducible background
processes to t tbar production. We consider the total cross section of the e+
e- -> W+ W- b bbar process and additionally implement cuts on the invariant
masses of the W+ b and W- bbar pairs.Comment: LaTeX, 33 pages, 6 figure
NNLO tau+tau- production cross section at threshold
The threshold behaviour of the cross section sigma(e+e- -> tau+tau-) is
analysed, taking into account the known higher-order corrections. At present,
this observable can be determined to next-to-next-to-leading order (NNLO) in a
combined expansion in powers of alpha_s and fermion velocities.Comment: 4 pages, 3 figures. To appear in the proceedings of QCD 02:
High-Energy Physics International Conference in Quantum Chromodynamics,
Montpellier, France, 2-9 Jul 200
Heavy-Light Meson Decay Constant from QCD Sum Rules in Three-Loop Approximation
In this paper we compute the decay constant of the pseudo-scalar heavy-light
mesons in the heavy quark effective theory framework of QCD sum rules. In our
analysis we include the recently evaluated three-loop result of order
for the heavy-light current correlator. The value of the bottom
quark mass, which essentially limits the accuracy of the sum rules for
meson, is extracted from the nonrelativistic sum rules for
resonances in the next-to-next-to-leading approximation. We find stability of
our result with respect to all types of corrections and the specific form of
the sum rule which reduces the uncertainty. Our results MeV and
MeV for the and meson decay constants are in impressive
agreement with recent lattice calculations.Comment: minor editorial changes, references added, to appear in PR
Threshold production of unstable top
We develop a systematic approach to describe the finite lifetime effects in
the threshold production of top quark-antiquark pairs. It is based on the
nonrelativistic effective field theory with an additional scale rho^(1/2) m_t
characterizing the dynamics of the top-quark decay, which involves a new
expansion parameter rho=1-m_W/m_t. Our method naturally resolves the problem of
spurious divergences in the analysis of the unstable top production. Within
this framework we compute the next-to-leading nonresonant contribution to the
total cross section of the top quark-antiquark threshold production in
electron-positron annihilation through high-order expansion in rho and confirm
the recently obtained result. We extend the analysis to the
next-to-next-to-leading O(alpha_s) nonresonant contribution which is derived in
the leading order in rho. The dominant nonresonant contribution to the
top-antitop threshold production in hadronic collisions is also obtained.Comment: 20 pages, 7 figures; v2: added a section on invariant mass cuts and
one reference, minor changes in Introduction, results unchanged, matches
published versio
Statistical Reconstruction of Qutrits
We discuss a procedure of measurement followed by the reproduction of the
quantum state of a three-level optical system - a frequency- and spatially
degenerate two-photon field. The method of statistical estimation of the
quantum state based on solving the likelihood equation and analyzing the
statistical properties of the obtained estimates is developed. Using the root
approach of estimating quantum states, the initial two-photon state vector is
reproduced from the measured fourth moments in the field . The developed
approach applied to quantum states reconstruction is based on the amplitudes of
mutually complementary processes. Classical algorithm of statistical estimation
based on the Fisher information matrix is generalized to the case of quantum
systems obeying Bohr's complementarity principle. It has been experimentally
proved that biphoton-qutrit states can be reconstructed with the fidelity of
0.995-0.999 and higher.Comment: Submitted to Physical Review
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
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