34 research outputs found
Mass Corrections to the Tau Decay Rate
In this note radiative corrections to the total hadronic decay rate of the
-lepton are studied employing perturbative QCD and the operator product
expansion. We calculate quadratic quark mass corrections to the decay rate
ration to the order and find that they
contribute appreciably to the Cabbibo supressed decay modes of the
-lepton. We also discuss corrections of mass dimension D=4, where we
emphasize the need of a suitable choice of the renormalization scale of the
quark and gluon condensates.Comment: 13 pages, LaTeX, no figures. This version fixes a typo in eq. (25) of
the original paper (Z. Phys. C59 (1993) 525) and an errror in a numerical
integration procedure which has resulted to a significant increase of the
O(\alpha_s^2) coefficient in eq. (27). As a consequence also some tables in
Section 4 have been modifie
Strong coupling constant from decay within renormalization scheme invariant treatment
We extract a numerical value for the strong coupling constant \alpha_s from
the \tau-lepton decay rate into nonstrange particles. A new feature of our
procedure is the explicit use of renormalization scheme invariance in
analytical form in order to perform the actual analysis in a particular
renormalization scheme. For the reference coupling constant in the
\MSsch-scheme we obtain \alpha_s(M_\tau)= 0.3184 \pm 0.0060_{exp} which
corresponds to \al_s(M_Z)= 0.1184 \pm 0.0007_{exp} \pm 0.0006_{hq mass}. This
new numerical value is smaller than the standard value from -data quoted
in the literature and is closer to \al_s(M_Z)-values obtained from high energy
experiments.Comment: 8 page
Contour-improved versus fixed-order perturbation theory in hadronic tau decays
The hadronic decay rate of the tau lepton serves as one of the most precise
determinations of the QCD coupling alpha_s. The dominant theoretical source of
uncertainty at present resides in the seeming disparity of two approaches to
improving the perturbative expansion with the help of the renormalisation
group, namely fixed-order and contour-improved perturbation theory. In this
work it is demonstrated that in fact both approaches yield compatible results.
However, the fixed-order series is found to oscillate around the
contour-improved result with an oscillation frequency of approximately six
perturbative orders, approaching it until about the 30th order, after which the
expansion reveals its asymptotic nature. Additionally, the renormalisation
scale and scheme dependencies of the perturbative series for the tau hadronic
width are investigated in detail.Comment: 20 pages, 5 eps-figures; discussion on scale and scheme dependence
added as compared to published journal version JHEP 09 (2005) 05
Liquid krypton electromagnetic calorimeter
Abstract A calorimeter using 30 tons of liquid krypton for the KEDR detector is being constructed. The main effects which determine the energy and space resolution have been studied. An energy resolution of 1.7% at 1.2 GeV was obtained with the prototype. A space resolution of 0.4 mm for relativistic particles has been reached with the prototype
Scale setting for alpha_s beyond leading order
We present a general procedure for incorporating higher-order information
into the scale-setting prescription of Brodsky, Lepage and Mackenzie. In
particular, we show how to apply this prescription when the leading coefficient
or coefficients in a series in the strong coupling alpha_s are anomalously
small and the original prescription can give an unphysical scale. We give a
general method for computing an optimum scale numerically, within dimensional
regularization, and in cases when the coefficients of a series are known. We
apply it to the heavy quark mass and energy renormalization in lattice NRQCD,
and to a variety of known series. Among the latter, we find significant
corrections to the scales for the ratio of e+e- to hadrons over muons, the
ratio of the quark pole to MSbar mass, the semi-leptonic B-meson decay width,
and the top decay width. Scales for the latter two decay widths, expressed in
terms of MSbar masses, increase by factors of five and thirteen, respectively,
substantially reducing the size of radiative corrections.Comment: 39 pages, 15 figures, 5 tables, LaTeX2
Decay constants, light quark masses and quark mass bounds from light quark pseudoscalar sum rules
The flavor and pseudoscalar correlators are investigated using
families of finite energy sum rules (FESR's) known to be very accurately
satisfied in the isovector vector channel. It is shown that the combination of
constraints provided by the full set of these sum rules is sufficiently strong
to allow determination of both the light quark mass combinations ,
and the decay constants of the first excited pseudoscalar mesons in
these channels. The resulting masses and decay constants are also shown to
produce well-satisfied Borel transformed sum rules, thus providing non-trivial
constraints on the treatment of direct instanton effects in the FESR analysis.
The values of and obtained are in good agreement with the
values implied by recent hadronic decay analyses and the ratios obtained
from ChPT. New light quark mass bounds based on FESR's involving weight
functions which strongly suppress spectral contributions from the excited
resonance region are also presented.Comment: 28 pages, 10 figure
The Determination of alpha_s from Tau Decays Revisited
We revisit the determination of alpha_s(m_tau) using a fit to inclusive tau
hadronic spectral moments in light of (1) the recent calculation of the
fourth-order perturbative coefficient K_4 in the expansion of the Adler
function, (2) new precision measurements from BABAR of e+e- annihilation cross
sections, which decrease the uncertainty in the separation of vector and
axial-vector spectral functions, and (3) improved results from BABAR and Belle
on tau branching fractions involving kaons. We estimate that the fourth-order
perturbative prediction reduces the theoretical uncertainty, introduced by the
truncation of the series, by 20% with respect to earlier determinations. We
discuss to some detail the perturbative prediction and show that the effect of
the incomplete knowledge of the series is reduced by using the so-called
contour-improved calculation, as opposed to fixed-order perturbation theory
which manifests convergence problems. The corresponding theoretical
uncertainties are studied at the tau and Z mass scales. Nonperturbative
contributions extracted from the most inclusive fit are small, in agreement
with earlier determinations. Systematic effects from quark-hadron duality
violation are estimated with simple models and found to be within the quoted
systematic errors. The fit gives alpha_s(m_tau) = 0.344 +- 0.005 +- 0.007,
where the first error is experimental and the second theoretical. After
evolution to M_Z we obtain alpha_s(M_Z) = 0.1212 +- 0.0005 +- 0.0008 +- 0.0005,
where the errors are respectively experimental, theoretical and due to the
evolution. The result is in agreement with the corresponding NNNLO value
derived from essentially the Z width in the global electroweak fit. The
alpha_s(M_Z) determination from tau decays is the most precise one to date.Comment: 22 pages, 7 figure
The check of QCD based on the tau-decay data analysis in the complex q^2-plane
The thorough analysis of the ALEPH data on hadronic tau-decay is performed in
the framework of QCD. The perturbative calculations are performed in 3 and
4-loop approximations. The terms of the operator product expansion (OPE) are
accounted up to dimension D=8. The value of the QCD coupling constant
alpha_s(m_tau^2)=0.355 pm 0.025 was found from hadronic branching ratio R_tau.
The V+A and V spectral function are analyzed using analytical properties of
polarization operators in the whole complex q^2-plane. Borel sum rules in the
complex q^2 plane along the rays, starting from the origin, are used. It was
demonstrated that QCD with OPE terms is in agreement with the data for the
coupling constant close to the lower error edge alpha_s(m_tau^2)=0.330. The
restriction on the value of the gluonic condensate was found
=0.006 pm 0.012 GeV^2. The analytical perturbative QCD was
compared with the data. It is demonstrated to be in strong contradiction with
experiment. The restrictions on the renormalon contribution were found. The
instanton contributions to the polarization operator are analyzed in various
sum rules. In Borel transformation they appear to be small, but not in spectral
moments sum rules.Comment: 24 pages; 1 latex + 13 figure files. V2: misprints are corrected,
uncertainty in alpha_s is explained in more transparent way, acknowledgement
is adde
Resummation of the hadronic tau decay width with the modified Borel transform method
A modified Borel transform of the Adler function is used to resum the
hadronic tau decay width ratio. In contrast to the ordinary Borel transform,
the integrand of the Borel integral is renormalization--scale invariant. We use
an ansatz which explicitly accounts for the structure of the leading infrared
renormalon. Further, we use judiciously chosen conformal transformations for
the Borel variable, in order to map sufficiently away from the origin the other
ultraviolet and infrared renormalon singularities. In addition, we apply Pade
approximants for the corresponding truncated perturbation series of the
modified Borel transform, in order to further accelerate the convergence.
Comparing the results with the presently available experimental data on the tau
hadronic decay width ratio, we obtain . These predictions
virtually agree with those of our previous resummations where we used ordinary
Borel transforms instead.Comment: 32 pages, 2 eps-figures, revtex; minor changes in the formulations; a
typo in Eq.(47) corrected; version as appearing in Phys. Rev.
Measurement of and between 3.12 and 3.72 GeV at the KEDR detector
Using the KEDR detector at the VEPP-4M collider, we have measured
the values of and at seven points of the center-of-mass
energy between 3.12 and 3.72 GeV. The total achieved accuracy is about or
better than at most of energy points with a systematic uncertainty of
about . At the moment it is the most accurate measurement of in
this energy range