527 research outputs found
Hadronic contribution to the muon g-2: a theoretical determination
The leading order hadronic contribution to the muon g-2, , is
determined entirely from theory using an approach based on Cauchy's theorem in
the complex squared energy s-plane. This is possible after fitting the
integration kernel in with a simpler function of . The
integral determining in the light-quark region is then split
into a low energy and a high energy part, the latter given by perturbative QCD
(PQCD). The low energy integral involving the fit function to the integration
kernel is determined by derivatives of the vector correlator at the origin,
plus a contour integral around a circle calculable in PQCD. These derivatives
are calculated using hadronic models in the light-quark sector. A similar
procedure is used in the heavy-quark sector, except that now everything is
calculable in PQCD, thus becoming the first entirely theoretical calculation of
this contribution. Using the dual resonance model realization of Large
QCD to compute the derivatives of the correlator leads to agreement with the
experimental value of . Accuracy, though, is currently limited by the
model dependent calculation of derivatives of the vector correlator at the
origin. Future improvements should come from more accurate chiral perturbation
theory and/or lattice QCD information on these derivatives, allowing for this
method to be used to determine accurately entirely from theory,
independently of any hadronic model.Comment: Several additional clarifying paragraphs have been added. 1/N_c
corrections have been estimated. No change in result
Charm-quark mass from weighted finite energy QCD sum rules
The running charm-quark mass in the scheme is determined from
weighted finite energy QCD sum rules (FESR) involving the vector current
correlator. Only the short distance expansion of this correlator is used,
together with integration kernels (weights) involving positive powers of ,
the squared energy. The optimal kernels are found to be a simple {\it pinched}
kernel, and polynomials of the Legendre type. The former kernel reduces
potential duality violations near the real axis in the complex s-plane, and the
latter allows to extend the analysis to energy regions beyond the end point of
the data. These kernels, together with the high energy expansion of the
correlator, weigh the experimental and theoretical information differently from
e.g. inverse moments FESR. Current, state of the art results for the vector
correlator up to four-loop order in perturbative QCD are used in the FESR,
together with the latest experimental data. The integration in the complex
s-plane is performed using three different methods, fixed order perturbation
theory (FOPT), contour improved perturbation theory (CIPT), and a fixed
renormalization scale (FMUPT). The final result is , in a wide region of stability against changes in the
integration radius in the complex s-plane.Comment: A short discussion on convergence issues has been added at the end of
the pape
Bottom-quark mass from finite energy QCD sum rules
Finite energy QCD sum rules involving both inverse and positive moment
integration kernels are employed to determine the bottom quark mass. The result
obtained in the scheme at a reference scale of
is . This value translates into
a scale invariant mass . This result
has the lowest total uncertainty of any method, and is less sensitive to a
number of systematic uncertainties that affect other QCD sum rule
determinations.Comment: An appendix has been added with explicit expressions for the
polynomials used in Table
QCD sum rule determination of the charm-quark mass
QCD sum rules involving mixed inverse moment integration kernels are used in order to determine the running charm-quark mass in the MS¯ scheme. Both the high and the low energy expansion of the vector current correlator are involved in this determination. The optimal integration kernel turns out to be of the form p(s)=1−(s0/s)2, where s0 is the onset of perturbative QCD. This kernel enhances the contribution of the well known narrow resonances, and reduces the impact of the data in the range s≃20−25GeV2. This feature leads to a substantial reduction in the sensitivity of the results to changes in s0, as well as to a much reduced impact of the experimental uncertainties in the higher resonance region. The value obtained for the charm-quark mass in the MS¯ scheme at a scale of 3 GeV is m¯c(3GeV)=987±9MeV, where the error includes all sources of uncertainties added in quadrature
B meson decay constants f(Bc), f(Bs) and f(B) from QCD sum rules
Finite energy QCD sum rules with Legendre polynomial integration kernels are used to determine the heavy meson decay constant f(Bc), and revisit f(B) and f(Bs). Results exhibit excellent stability in a wide range of values of the integration radius in the complex squared energy plane, and of the order of the Legendre polynomial. Results are f(Bc) = 528 +/- 19 MeV, f(B) = 186 +/- 14 MeV, and f(Bs) = 222 +/- 12 MeV
Low- and High-Energy Expansion of Heavy-Quark Correlators at Next-To-Next-To-Leading Order
We calculate three-loop corrections to correlation functions of heavy-quark
currents in the low- and high-energy regions. We present 30 coefficients both
in the low-energy and the high-energy expansion of the scalar and the vector
correlator with non-diagonal flavour structure. In addition we compute 30
coefficients in the high-energy expansion of the diagonal vector, axial-vector,
scalar and pseudo-scalar correlators. Possible applications of our new results
are improvements of lattice-based quark-mass determinations and the approximate
reconstruction of the full momentum dependence of the correlators.Comment: 15 pages, 4 figures; corrected diagram in example and extended
discussio
Precise Charm- and Bottom-Quark Masses: Theoretical and Experimental Uncertainties
Recent theoretical and experimental improvements in the determination of
charm and bottom quark masses are discussed. A new and improved evaluation of
the contribution from the gluon condensate to the
charm mass determination and a detailed study of potential uncertainties in the
continuum cross section for production is presented, together with a
study of the parametric uncertainty from the -dependence of our
results. The final results, MeV and
MeV, represent, together with a closely related lattice
determination MeV, the presently most precise
determinations of these two fundamental Standard Model parameters. A critical
analysis of the theoretical and experimental uncertainties is presented.Comment: 12 pages, presented at Quarks~2010, 16th International Seminar of
High Energy Physics, Kolomna, Russia, June 6-12, 2010; v2: references adde
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