4,302 research outputs found
New Results on the Hadronic Contributions to alpha(M_Z) and to (g-2)_mu
We reevaluate the dispersion integrals of the leading order hadronic
contributions to the running of the QED fine structure constant alpha(s) at
s=M_Z^2, and to the anomalous magnetic moments of the muon and the electron.
Finite-energy QCD sum rule techniques complete the data from e+e- annihilation
and tau decays at low energy and at the cc-bar threshold. Global quark-hadron
duality is assumed in order to resolve the integrals using the Operator Product
Expansion wherever it is applicable.
We obtain delta_alpha_had(M_Z) = (276.3 +/- 1.6)x10^{-4} yielding
alpha^{-1}(M_Z) = 128.933 +/- 0.021, and a_mu^had = (692.4 +/- 6.2)x10^{-10}
with which we find for the complete Standard Model prediction a_mu^SM =
(11659159.6 +/- 6.7)x10^{-10}. For the electron, the hadronic contribution
reads a_e^had = (187.5 +/- 1.8)x10^{-14}.Comment: 16 pages, 3 figure
Two-photon exchange model for production of neutral meson pairs in e+e- annihilation
A vector-dominance two-photon exchange model is proposed to explain the
recently observed production of and pairs in
annihilation at 10.58 GeV with the BaBar detector. All the observed
features of the data --angular and decay distributions, rates-- are in
agreement with the model. Predictions are made for yet-unobserved final states.Comment: 7 pages, 2 figures, 1 tabl
Updated Estimate of the Muon Magnetic Moment Using Revised Results from e+e- Annihilation
A new evaluation of the hadronic vacuum polarization contribution to the muon
magnetic moment is presented. We take into account the reanalysis of the
low-energy e+e- annihilation cross section into hadrons by the CMD-2
Collaboration. The agreement between e+e- and tau spectral functions in the pi
pi channel is found to be much improved. Nevertheless, significant
discrepancies remain in the center-of-mass energy range between 0.85 and 1.0
GeV, so that we refrain from averaging the two data sets. The values found for
the lowest-order hadronic vacuum polarization contributions are a_mu[had,LO] =
(696.3 +- 6.2[exp] +- 3.6[rad])e-10 (e+e- -based) and a_mu[had,LO] = (711.0 +-
5.0[exp] +- 0.8[rad] +- 2.8[SU2])e-10 (tau-based), where the errors have been
separated according to their sources: experimental, missing radiative
corrections in e+e- data, and isospin breaking. The corresponding Standard
Model predictions for the muon magnetic anomaly read a_mu = (11,659,180.9 +-
7.2[had] +- 3.5[LBL] +- 0.4[QED+EW])e-10 (e+e- -based) and a_mu = (11,659,195.6
+- 5.8[had] +- 3.5[LBL] +- 0.4[QED+EW])e-10 (tau-based), where the errors
account for the hadronic, light-by-light (LBL) scattering and electroweak
contributions. The deviations from the measurement at BNL are found to be (22.1
+- 7.2 +- 3.5 +- 8.0)e-10 (1.9 sigma) and (7.4 +- 5.8 +- 3.5 +- 8.0)e-10 (0.7
sigma) for the e+e- and tau-based estimates, respectively, where the second
error is from the LBL contribution and the third one from the BNL measurement.Comment: 14 pages, 7 figures (to be submitted to Phys Lett B
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
Improved Determination of alpha_QED(M_Z^2) and the Anomalous Magnetic Moment of the Muon
We reevaluate the hadronic contribution to the running of the QED fine
structure constant alpha(s) at s = M_Z^2. We use data from e+e- annihilation
and tau decays at low energy and at the qq-bar thresholds, where resonances
occur. Using so-called spectral moments and the Operator Product Expansion
(OPE), it is shown that a reliable theoretical prediction of the hadronic
production rate R(s) is available at relatively low energies. Its application
improves significantly the precision on the hadronic vacuum polarization
contribution. We obtain delta_alpha^had = (277.8 +/- 2.6) x 10^-4 yielding
alpha^-1(M_Z^2) = 128.923 +/- 0.036$. Inserting this value in a global
electroweak fit using current experimental input, we constrain the mass of the
Standard Model Higgs boson to be M_Higgs = (129 +103 -62) GeV. Analogously, we
improve the precision of the hadronic contribution to the anomalous magnetic
moment of the muon for which we obtain a_mu^had = (695.1 +/- 7.5) x 10^-10.Comment: tex, 18 pages and 3 figure
Status of the Fermilab Muon (g-2) Experiment
The New Muon Collaboration at Fermilab has proposed to measure the
anomalous magnetic moment of the muon, , a factor of four better than
was done in E821 at the Brookhaven AGS, which obtained ppm. The last digit of is changed
from the published value owing to a new value of the ratio of the
muon-to-proton magnetic moment that has become available. At present there
appears to be a difference between the Standard-Model value and the measured
value, at the standard deviation level when electron-positron
annihilation data are used to determine the lowest-order hadronic piece of the
Standard Model contribution. The improved experiment, along with further
advances in the determination of the hadronic contribution, should clarify this
difference. Because of its ability to constrain the interpretation of
discoveries made at the LHC, the improved measurement will be of significant
value, whatever discoveries may come from the LHC.Comment: Proceedings of the PhiPsi09, Oct. 13-16, 2009, Beijing, China, 4
pages 2 figures. Version 2 includes Fermilab report number, minor corrections
and one additional referenc
Improved Determination of the Hadronic Contribution to the Muon (g-2) and to alpha(M_Z**2) Using new Data from Hadronic Tau Decays
We have reevaluated the hadronic contribution to the anomalous magnetic
moment of the muon (g-2) and to the running of the QED fine structure constant
alpha(s) at s=M_Z**2. We incorporated new data from hadronic tau decays,
recently published by the ALEPH Collaboration. In addition, compared to
previous analyses, we use more extensive e+e- annihilation data sets. The
integration over the total hadronic cross section is performed using
experimental data up to 40 GeV and results from perturbative QCD above 40 GeV.
The improvement from tau data concerns mainly the pion form factor, where the
uncertainty in the corresponding integral could be reduced by more than a
factor of two. We obtain for the lowest order hadronic vacuum polarization
graph a_mu(had) = (695.0 +/- 15.0) x 10^{-10} and delta(alpha(M_Z**2))(had) =
(280.9 +/- 6.3) x 10^{-4} using e+e- data only. The corresponding results for
combined e+e- and tau data are a_mu(had) = (701.1 +/- 9.4) x 10^{-10} and
delta(alpha(M_Z**2))(had) = (281.7 +/- 6.2) x 10^{-4}, where the latter is
calculated using the contribution from the five lightest quarks.Comment: 23 pages, LaTex, 6 figures, Paper submitted to Zeitschrift fuer
Physik
New results on the hadronic vacuum polarization to the muon g-2
Results on the lowest-order hadronic vacuum polarization contribution to the
muon magnetic anomaly are presented. They are based on the latest published
experimental data used as input to the dispersion integral. Thus recent results
on tau to nutau pi pi0 decays from Belle and on e+ e- annihilation to pi+ pi-
from BABAR and KLOE are included. The new data, together with improved
isospin-breaking corrections for tau decays, result into a much better
consistency among the different results. A discrepancy between the Standard
Model prediction and the direct g-2 measurement is found at the level of 3
sigma.Comment: proceedings of the PhiPsi09 conference, Oct. 13-16, 2009, Beijing,
Chin
Hadronic light-by-light scattering contribution to the muon g-2
We review recent developments concerning the hadronic light-by-light
scattering contribution to the anomalous magnetic moment of the muon. We first
discuss why fully off-shell hadronic form factors should be used for the
evaluation of this contribution to the g-2. We then reevaluate the numerically
dominant pion-exchange contribution in the framework of large-N_C QCD, using an
off-shell pion-photon-photon form factor which fulfills all QCD short-distance
constraints, in particular, a new short-distance constraint on the off-shell
form factor at the external vertex in g-2, which relates the form factor to the
quark condensate magnetic susceptibility in QCD. Combined with available
evaluations of the other contributions to hadronic light-by-light scattering
this leads to the new result a_{\mu}(LbyL; had) = (116 \pm 40) x 10^{-11}, with
a conservative error estimate in view of the many still unsolved problems. Some
potential ways for further improvements are briefly discussed as well. For the
electron we obtain the new estimate a_{e}(LbyL; had) = (3.9 \pm 1.3) x
10^{-14}.Comment: 6 pages, 1 figure, to be published in the proceedings of the PhiPsi09
workshop, Oct. 13-16, 2009, Beijing, Chin
The Physics of Hadronic Tau Decays
Hadronic tau decays represent a clean laboratory for the precise study of
quantum chromodynamics (QCD). Observables (sum rules) based on the spectral
functions of hadronic tau decays can be related to QCD quark-level calculations
to determine fundamental quantities like the strong coupling constant,
parameters of the chiral Lagrangian, |V_us|, the mass of the strange quark, and
to simultaneously test the concept of quark-hadron duality. Using the best
available measurements and a revisited analysis of the theoretical framework,
the value alpha_s(m_tau) = 0.345 +- 0.004[exp] +- 0.009[theo] is obtained.
Taken together with the determination of alpha_s(m_Z) from the global
electroweak fit, this result leads to the most accurate test of asymptotic
freedom: the value of the logarithmic slope of 1/alpha_s(s) is found to agree
with QCD at a precision of 4%. In another approach, the tau spectral functions
can be used to determine hadronic quantities that, due to the nonperturbative
nature of long-distance QCD, cannot be computed from first principles. An
example for this is the contribution from hadronic vacuum polarization to
loop-dominated processes like the anomalous magnetic moment of the muon. This
article reviews the measurements of nonstrange and strange tau spectral
functions and their phenomenological applications.Comment: 89 pages, 32 figures; final version accepted for publication by
Reviews of Modern Physic
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