1,143 research outputs found
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
Determination of |V_us| from hadronic tau decays
The recent update of the strange spectral function and the moments of the
invariant mass distribution by the OPAL collaboration from hadronic tau decay
data are employed to determine |V_us| as well as m_s. Our result,
|V_us|=0.2208\pm0.0034, is competitive to the standard extraction of |V_us|
from K_e3 decays and to the new proposals to determine it. Furthermore, the
error associated to our determination of |V_us| can be reduced in the future
since it is dominated by the experimental uncertainty that will be eventually
much improved by the B-factories hadronic tau data. Another improvement that
can be performed is the simultaneous fit of both |V_us| and m_s to a set of
moments of the hadronic tau decays invariant mass distribution, which will
provide even a more accurate determination of both parameters.Comment: 6 pages. Invited talk given by E.G. at the XXXXth Rencontres de
Moriond on Electroweak Interactions and Unified Theories, La Thuile, Italy,
5-12 Mar 200
Quantum Loops in the Resonance Chiral Theory: The Vector Form Factor
We present a calculation of the Vector Form Factor at the next-to-leading
order in the 1/N_C expansion, within the framework of Resonance Chiral Theory.
The calculation is performed in the chiral limit, and with two dynamical quark
flavours. The ultraviolet behaviour of quantum loops involving virtual
resonance propagators is analyzed, together with the kind of counterterms
needed in the renormalization procedure. Using the lowest-order equations of
motion, we show that only a few combinations of local couplings appear in the
final result. The low-energy limit of our calculation reproduces the standard
Chiral Perturbation Theory formula, allowing us to determine the resonance
contribution to the chiral low-energy couplings, at the next-to-leading order
in 1/N_C, keeping a full control of their renormalization scale dependence.Comment: 27+1 pages, 9 figure
V(us) Determination from Hyperon Semileptonic Decays
We analyze the numerical determination of the quark mixing factor V(us) from
hyperon semileptonic decays. The discrepancies between the results obtained in
two previous studies are clarified. Our fits indicate sizeable SU(3) breaking
corrections, which unfortunately can only be fully determined from the data at
the first order. The lack of a reliable theoretical calculation of second-order
symmetry breaking effects translates into a large systematic uncertainty, which
has not been taken into account previously. Our final result, V(us) = 0.226 +/-
0.005, is not competitive with the existing determinations from K(l3), K(l2)
and \tau decays.Comment: 16 pages, no figures. References added and other minor change
Strange Quark Mass from the Invariant Mass Distribution of Cabibbo-Suppressed Tau Decays
Quark mass corrections to the tau hadronic width play a significant role only
for the strange quark, hence providing a method for determining its mass. The
experimental input is the vector plus axial-vector strange spectral function
derived from a complete study of tau decays into strange hadronic final states
performed by ALEPH. New results on strange decay modes from other experiments
are also incorporated. The present analysis determines the strange quark mass
at the Mtau mass scale using moments of the spectral function. Justified
theoretical constraints are applied to the nonperturbative components and
careful attention is paid to the treatment of the perturbative expansions of
the moments which exhibit convergence problems. The result obtained,
m_s(Mtau^2) = (120 +- 11_exp +- 8_Vus +- 19_th) MeV = (120^+21_-26) MeV, is
stable over the scale from Mtau down to about 1.4 GeV. Evolving this result to
customary scales yields m_s(1 GeV^2) = (160^+28_-35) MeV and m_s(4 GeV^2) =
(116^+20_-25) MeV.Comment: LaTex, 8 pages, 4 figures (EPS
Determination of the Chiral Couplings L_10 and C_87 from Semileptonic Tau Decays
Using recent precise hadronic tau-decay data on the V-A spectral function,
and general properties of QCD such as analyticity, the operator product
expansion and chiral perturbation theory, we get accurate values for the QCD
chiral order parameters L_10^r(M_rho) and C_87^r(M_rho). These two low-energy
constants appear at order p^4 and p^6, respectively, in the chiral perturbation
theory expansion of the V-A correlator. At order p^4 we obtain L_10^r(M_rho) =
-(5.22\pm 0.06)10^{-3}. Including in the analysis the two-loop (order p^6)
contributions, we get L_10^r(M_rho) = -(4.06\pm 0.39)10^{-3} and C_87^r(M_rho)
= (4.89\pm 0.19)10^{-3}GeV^{-2}. In the SU(2) chiral effective theory, the
corresponding low-energy coupling takes the value \overline l_5 = 13.30 \pm
0.11 at order p^4, and \overline l_5 = 12.24 \pm 0.21 at order p^6.Comment: 17 pages, 3 figures, v2: Added reference, published versio
The Standard Model prediction for eps'/eps
We review fundamental aspects of a new Standard Model analysis of eps'/eps
which takes into account the strong enhancement induced by final state
interactions.Comment: 3 pages, LaTex, uses ws-p10x7.cls. Talk given by E.P. at ICHEP2000,
Osaka, July 27-August 2, 200
epsilon'/eps in the Standard Model
We overview the detailed analysis of epsilon'/epsilon within the Standard
Model, presented in ref. 1. When all sources of large logarithms are
considered, both at short and long distances, it is possible to perform a
reliable Standard Model estimate of epsilon'/epsilon. The strong S-wave
rescattering of the final pions has an important impact on this observable. The
Standard Model prediction is found to be Re(epsilon'/epsilon) =(1.7 +-
0.9)*10^{-3}, in good agreement with the most recent experimental measurements.
A better estimate of the strange quark mass would reduce the uncertainty to
about 30%.Comment: Talk presented at the International Europhysics Conference on High
Energy Physics, July 12-18, 2001,Budapest, Hungar
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