93 research outputs found
Charm and Bottom Masses from Sum Rules with a Convergence Test
In this talk we discuss results of a new extraction of the MS-bar charm quark
mass using relativistic QCD sum rules at O(as**3) based on moments of the
vector and the pseudoscalar current correlators and using the available
experimental measurements from e+e- collisions and lattice results,
respectively. The analysis of the perturbative uncertainties is based on
different implementations of the perturbative series and on independent
variations of the renormalization scales for the mass and the strong coupling
following a work we carried out earlier. Accounting for the perturbative series
that result from this double scale variation is crucial since some of the
series can exhibit extraordinarily small scale dependence, if the two scales
are set equal. The new aspect of the work reported here adresses the problem
that double scale variation might also lead to an overestimate of the
perturbative uncertainties. We supplement the analysis by a convergence test
that allows to quantify the overall convergence of QCD perturbation theory for
each moment and to discard series that are artificially spoiled by specific
choices of the renormalization scales. We also apply the new method to an
extraction of the MS-bar bottom quark mass using experimental moments that
account for a modeling uncertainty associated to the continuum region where no
experimental data is available. We obtain m_c(m_c) = 1.287 +- 0.020 GeV and
m_b(m_b) = 4.167 +- 0.023 GeV.Comment: 6 pages, 2 figures. Presented at the International Workshop on the
CKM Unitarity Triangle Vienna, Austria, September 8-12, 201
Hadron Mass Effects in Power Corrections to Event Shapes
We study the effect of hadron masses on the leading power correction of dijet
event-shape distributions. We define the transverse velocity operator, that
describes the effects of hadron masses. It depends on the "transverse velocity"
r, which is different from one only for non-vanishing hadron masses. We find
that hadron-mass effects in general break universality. However we provide a
simple method to identify universality classes of event shapes with a common
power correction. We also compute the anomalous dimension of the power
correction and the structure of the corresponding Wilson coefficient, finding a
nontrivial result.Comment: 8 pages, 3 figures, 2 tables, to appear in the proceedings of Xth
Quark Confinement and the Hadron Spectru
Bottom and Charm Mass Determinations with a Convergence Test
We present new determinations of the MS-bar charm quark mass using
relativistic QCD sum rules at O(alpha_s^3) from the moments of the vector and
the pseudoscalar current correlators. We use available experimental
measurements from e+e- collisions and lattice simulation results, respectively.
Our analysis of the theoretical uncertainties is based on different
implementations of the perturbative series and on independent variations of the
renormalization scales for the mass and the strong coupling. Taking into
account the resulting set of series to estimate perturbative uncertainties is
crucial, since some ways to treat the perturbative expansion can exhibit
extraordinarily small scale dependence when the two scales are set equal. As an
additional refinement, we address the issue that double scale variation could
overestimate the perturbative uncertainties. We supplement the analysis with a
test that quantifies the convergence rate of each perturbative series by a
single number. We find that this convergence test allows to determine an
overall and average convergence rate that is characteristic for the series
expansions of each moment, and to discard those series for which the
convergence rate is significantly worse. We obtain mc(mc) = 1.288 +- 0.020 GeV
from the vector correlator. The method is also applied to the extraction of the
MS-bar bottom quark mass from the vector correlator. We compute the
experimental moments including a modeling uncertainty associated to the
continuum region where no data is available. We obtain mb(mb) = 4.176 +- 0.023
GeV.Comment: 53 pages, 16 figures, 19 tables; v2 typos fixed, references added,
modification of section 6.3, results for bottom moments and bottom mass
updated, matches published versio
Calibrating the Na\"ive Cornell Model with NRQCD
Along the years, the Cornell Model has been extraordinarily successful in
describing hadronic phenomenology, in particular in physical situations for
which an effective theory of the strong interactions such as NRQCD cannot be
applied. As a consequence of its achievements, a relevant question is whether
its model parameters can somehow be related to fundamental constants of QCD. We
shall give a first answer in this article by comparing the predictions of both
approaches. Building on results from a previous study on heavy meson
spectroscopy, we calibrate the Cornell model employing NRQCD predictions for
the lowest-lying bottomonium states up to NLO, in which the bottom mass is
varied within a wide range. We find that the Cornell model mass parameter can
be identified, within perturbative uncertainties, with the MSR mass at the
scale GeV. This identification holds for any value of or
the bottom mass, and for all perturbative orders investigated. Furthermore, we
show that: a) the "string tension" parameter is independent of the bottom mass,
and b) the Coulomb strength of the Cornell model can be related to the
QCD strong coupling constant at a characteristic non-relativistic
scale. We also show how to remove the renormalon of the static QCD
potential and sum-up large logs related to the renormalon subtraction by
switching to the low-scale, short-distance MSR mass, and using R-evolution. Our
R-improved expression for the static potential remains independent of the heavy
quark mass value and agrees with lattice QCD results for values of the radius
as large as fm, and with the Cornell model potential at long distances.
Finally we show that for moderate values of , the R-improved NRQCD and
Cornell static potentials are in head-on agreement.Comment: 22 pages, 13 figures, 3 table
Chiral Perturbation Theory with tensor sources
We construct the most general chirally-invariant Lagrangian for mesons in the
presence of external sources coupled to the tensor current
\bar{\psi}\sigma_{\mu\nu}\psi. In order to have only even terms in the chiral
expansion, we consider the new source of O(p^2). With this choice, we build the
even-parity effective Lagrangian up to the p^6-order (NLO). While there are
only 4 new terms at the p^4-order, at p^6-order we find 78 terms for n_f=2 and
113 terms for n_f=3. We provide a detailed discussion on the different
mechanisms that ensure that our final set of operators is complete and
non-redundant. We also examine the odd-parity sector, to conclude that the
first operators appear at the p^8-order (NNLO).Comment: 23 pages, one figure; typos corrected, one paragraph added, new
section added, references added, published versio
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