62 research outputs found
Possible duality violations in tau decay and their impact on the determination of alpha_s
We discuss the issue of duality violations in hadronic tau decay. After
introducing a physically motivated ansatz for duality violations, we estimate
their possible size by fitting this ansatz to the tau experimental data
provided by the ALEPH collaboration. Our conclusion is that these data do not
exclude significant duality violations in tau decay. This may imply an
additional systematic error in the value of alpha_s(m_tau), extracted from tau
decay, as large as \delta alpha_s(m_tau) \sim 0.003-0.010 .Comment: 20 pages, 4 figures. Minor fixes in the Appendi
The hadronic light by light contribution to the with holographic models of QCD
We study the anomalous electromagnetic pion form factor
with a set of holographic models. By comparing with
the measured value of the linear slope, some of these models can be ruled out.
From the remaining models we obtain predictions for the low-energy quadratic
slope parameters of , currently out of experimental
reach but testable in the near future. We find it particularly useful to encode
this low-energy information in a form factor able to satisfy also QCD
short-distance constraints. We choose the form factor introduced by D'Ambrosio,
Isidori and Portoles in kaon decays, which has the right short distance for a
particular value of the quadratic slope, which is later shown to be compatible
with our holographic predictions. We then turn to a determination of the
(dominant) pion exchange diagram in the hadronic light by light scattering
contribution to the muon anomalous magnetic moment. We quantify the theoretical
uncertainty in coming from the different input we use: QCD short
distances, experimental input and low-energy holographic predictions. We also
test the pion-pole approximation. Our final result is
, where the error is driven by the
linear slope of , soon to be measured with precision
at KLOE-2. Our numerical analysis also indicates that large values of the
magnetic susceptibility are disfavored, therefore pointing at a mild
effect from the pion off-shellness. However, in the absence of stronger bounds
on , an additional systematic uncertainty on the previous
value for cannot be excluded.Comment: 20 pages, 5 figures. Substantial improvements throughout the text to
match the published version. Enhanced discussion of the analysis in Section
IV with the addition of two appendices. Conclusions unchange
Issues in determining alpha_s from hadronic tau decay and electroproduction data
We discuss some key issues associated with duality-violating and
non-perturbative OPE contributions to the theoretical representations of light
quark current-current two-point functions and relevant to precision
determinations of alpha_s from hadronic tau decay and electroproduction
cross-section data. We demonstrate that analyses with an explicit
representation of duality-violating effects are required to bring theoretical
errors in such extractions under control, motivating the accompanying paper in
these proceedings, which presents the results of such an analysis.Comment: 5 pages, 4 figures. Prepared for the Proceedings of the International
Workshop on e+e- collisions from Phi to Psi (PHIPSI11), Sep. 19-22, 2011,
BINP, Novosibirsk, Russi
Violation of quark-hadron duality and spectral chiral moments in QCD
We analyze the spectral moments of the V-A two-point correlation function. Using all known short-distance constraints and the most recent experimental data from tau decays, we determine the lowest spectral moments, trying to assess the uncertainties associated with the so-called violations of quark-hadron duality. We have generated a large number of 'acceptable' spectral functions, satisfying all conditions, and have used them to extract the wanted hadronic parameters through a careful statistical analysis. We obtain accurate values for the ChPT couplings L10 and C87, and a realistic determination of the dimension six and eight contributions in the operator product expansion, O6=(-5.4^{+3.6}_{-1.6})*10^-3 GeV^6 and O8=(-8.9^{+12.6}_{-7.4})*10^-3 GeV^8, showing that the duality-violation effects have been usually underestimated in previous literature
What two models may teach us about duality violations in QCD
Though the operator product expansion is applicable in the calculation of
current correlation functions in the Euclidean region, when approaching the
Minkowskian domain, violations of quark-hadron duality are expected to occur,
due to the presence of bound-state or resonance poles. In QCD finite-energy sum
rules, contour integrals in the complex energy plane down to the Minkowskian
axis have to be performed, and thus the question arises what the impact of
duality violations may be. The structure and possible relevance of duality
violations is investigated on the basis of two models: the Coulomb system and a
model for light-quark correlators which has already been studied previously. As
might yet be naively expected, duality violations are in some sense "maximal"
for zero-width bound states and they become weaker for broader resonances whose
poles lie further away from the physical axis. Furthermore, to a certain
extent, they can be suppressed by choosing appropriate weight functions in the
finite-energy sum rules. A simplified Ansatz for including effects of duality
violations in phenomenological QCD sum rule analyses is discussed as well.Comment: 17 pages, 6 figures; version to appear in JHE
A holographic approach to low-energy weak interactions of hadrons
We apply the double-trace formalism to incorporate nonleptonic weak
interactions of hadrons into holographic models of the strong interactions. We
focus our attention upon nonleptonic kaon decays. By working with
a Yang-Mills--Chern-Simons 5-dimensional action, we explicitly show how, at low
energies, one recovers the weak chiral Lagrangian for both the
anomalous and nonanomalous sectors. We provide definite predictions for the low
energy coefficients in terms of the AdS metric and argue that the double-trace
formalism is a 5-dimensional avatar of the Weak Deformation Model introduced
long ago by Ecker et al. As a significant phenomenological application, we
reassess the decays in the light of the holographic model. Previous
models found a fine-tuned cancellation of resonance exchange in these decays,
which was both conceptually puzzling and quantitatively in disagreement with
experimental results. The holographic model we build is an illustrative
counterexample showing that the cancellation encountered in the literature is
not generic but a model-dependent statement and that agreement with experiment
can be obtained.Comment: 20 page
alpha_s from tau decays revisited
Being a determination at low energies, the analysis of hadronic tau decay
data provides a rather precise determination of the strong coupling alpha_s
after evolving the result to M_Z. At such a level of precision, even small
non-perturbative effects become relevant for the central value and error. While
those effects had been taken into account in the framework of the operator
product expansion, contributions going beyond it, so-called duality violations,
have previously been neglected. The following investigation fills this gap
through a finite-energy sum rule analysis of tau decay spectra from the OPAL
experiment, including duality violations and performing a consistent fit of all
appearing QCD parameters. The resulting values for alpha_s(M_tau) are 0.307(19)
in fixed-order perturbation theory and 0.322(26) in contour-improved
perturbation theory, which translates to the n_f=5 values 0.1169(25) and
0.1187(32) at M_Z, respectively.Comment: 4 pages, 3 figures. Prepared for the Proceedings of the International
Workshop on e+e- collisions from Phi to Psi (PHIPSI11), Sep. 19-22, 2011,
BINP, Novosibirsk, Russi
Kaon mixing and the charm mass
We study contributions to the Delta S=2 weak Chiral Lagrangian producing
K0-K0bar mixing which are not enhanced by the charm mass. For the real part,
these contributions turn out to be related to the box diagram with up quarks
but, unlike in perturbation theory, they do not vanish in the limit m_u->0.
They increase the leading contribution to the K_L-K_S mass difference by ~10%.
This means that short distances amount to (90+-15)% of this mass difference.
For the imaginary part, we find a correction to the lambda_c^2 m_c^2 term of
-5% from the integration of charm, which is a small contribution to epsilon_K.
The calculation is done in the large-Nc limit and we show explicitly how to
match short and long distances.Comment: 20 pages, 5 figures. Typos fixe
Long-distance dimension-eight operators in B_K
Besides their appearance at short distances \gtrsim 1/M_W, local
dimension-eight operators also contribute to kaon matrix elements at long
distances of order \gtrsim 1/mu_ope, where mu_ope is the scale controlling the
Operator Product Expansion in pure QCD, without weak interactions. This comes
about in the matching condition between the effective quark Lagrangian and the
Chiral Lagrangian of mesons. Working in dimensional regularization and in a
framework where these effects can be systematically studied, we calculate the
correction from these long-distance dimension-eight operators to the
renormalization group invariant B_K factor of K^0-K^0bar mixing, to
next-to-leading order in the 1/Nc expansion and in the chiral limit. The
correction is controlled by the matrix element <0|\bar s_L \tilde{G}_{mu
nu}gamma^mu d_L|K^0>, is small, and lowers B_K.Comment: 14 pages, LaTeX. Explanatory comments added to match version in
journa
An example of resonance saturation at one loop
We argue that the large-Nc expansion of QCD can be used to treat a Lagrangian
of resonances in a perturbative way. As an illustration of this we compute the
L_10 coupling of the Chiral Lagrangian by integrating out resonance fields at
one loop. Given a Lagrangian and a renormalization scheme, this is how in
principle one can answer in a concrete and unambiguous manner questions such as
at what scale resonance saturation takes place.Comment: 9 pages, 5 figures. Enlarged discussion, results unchanged. To be
published in Phys. Rev.
- …