Search results for "quantum chromodynamics"
showing 10 items of 1027 documents
Duality in semileptonicτdecay
1984
The branching ratios for all channels of tau decay are reconsidered within the framework of perturbative and nonperturbative QCD. The predictions agree with experiment.
Two-flavour lattice QCD correlation functions in the deconfinement transition region
2013
We report on a lattice QCD calculation with two dynamical flavors of the isovector vector correlator in the high-temperature phase. We analyze the correlator in terms of the associated spectral function by performing a fit for the difference of the thermal and vacuum spectral functions, using also an exact sum rule that constrains this difference. Additonally we carry out a direct fit for the thermal spectral function, and obtain good agreement between the two analyses for frequencies below the two-pion threshold. Under the assumption that the spectral function is smooth in that region, we give an estimate of the electrical conductivity.
The generation of the ϱ-resonance by QCD
1992
By showing that the imaginary part of a suitable QCD amplitude, after extrapolation up to the cut, exhibits indeed a prominent bump structure where the ϱ-resonance is expected to be, a rather direct indication for the generation of the ϱ-resonance by QCD is given. This is achieved by using a mathematically rigorous method of stable analytic extrapolation, based on the theory of maximally converging sequences of polynomials and the application of conformal mappings.
Method of analytic continuation by duality in QCD: Beyond QCD sum rules
1986
We present the method of analytic continuation by duality which allows the approximate continuation of QCD amplitudes to small values of the momentum variables where direct perturbative calculations are not possible. This allows a substantial extension of the domain of applications of hadronic QCD phenomenology. The method is illustrated by a simple example which shows its essential features.
Positivity bound on the imaginary part of the right-chiral tensor coupling $g_R$ in polarized top quark decay
2017
We derive a positivity bound on the right-chiral tensor coupling Im $g_R$ in polarized top quark decay by analyzing the angular decay distribution of the three-body polarized top quark decay $t(\uparrow)\to b+\ell^+ +\nu_\ell$ in NLO QCD. We obtain the bound $-0.0420 \le$ Im $g_R \le 0.0420$.
The top quark right coupling in the tbW-vertex
2015
The most general parametrization of the $tbW$ vertex includes a right coupling $V_R$ that is zero at tree level in the standard model. This quantity may be measured at the Large Hadron Collider where the physics of the top decay is currently investigated. This coupling is present in new physics models at tree level and/or through radiative corrections, so its measurement can be sensitive to non standard physics. In this paper we compute the leading electroweak and QCD contributions to the top $V_R$ coupling in the standard model. This value is the starting point in order to separate the standard model effects and, then, search for new physics. We also propose observables that can be address…
Determination of alpha_s at NLO*+NNLL from a global fit of the low-z parton-to-hadron fragmentation functions in e+e- and DIS collisions
2014
The QCD coupling alpha_s is determined from a combined analysis of experimental e+e- and e-p jet data confronted to theoretical predictions of the energy evolution of the parton-to-hadron fragmentation functions (FFs) moments --multiplicity, peak, width, skewness-- at low fractional hadron momentum z. The impact of approximate next-to-leading order (NLO*) corrections plus next-to-next-to-leading log (NNLL) resummations, compared to previous LO+NLL calculations, is discussed. A global fit of the full set of existing data, amounting to 360 FF moments at collision energies sqrt(s)~1--200 GeV, results in alpha_s(m_Z^2)=0.1189^{+0.0025}_{-0.0014} at the Z mass.
Ratio of strange to non-strange quark condensates in QCD
2001
Laplace transform QCD sum rules for two-point functions related to the strangeness-changing scalar and pseudoscalar Green's functions $\psi(Q^2)$ and $\psi_5(Q^2)$, are used to determine the subtraction constants $\psi(0)$ and $\psi_5(0)$, which fix the ratio $R_{su}\equiv \frac{}{}$. Our results are $\psi(0)= - (1.06 \pm 0.21) \times 10^{-3} {GeV}^4$, $\psi_5(0)= (3.35 \pm 0.25) \times 10^{-3} {GeV}^4$, and $R_{su}\equiv \frac{}{} = 0.5 \pm 0.1$. This implies corrections to kaon-PCAC at the level of 50%, which although large, are not inconsistent with the size of the corrections to Goldberger-Treiman relations in $SU(3)\otimes SU(3)$.
$\eta$-$\eta\prime$ mixing in Large-Nc ChPT: discussion, phenomenology, and prospects
2016
A systematic study of the η-η ′ mixing in Large-Nc chiral perturbation theory is presented [1] with special emphasis on the role of the next-to-next-to-leading-order contributions in the combined momentum, quark-mass, and 1/Nc expansions. At this order, loop corrections as well as OZIrule-violating pieces need to be included. Mixing angles as well as pseudoscalar decay constants are discussed within this framework. The results are compared with recent phenomenological approaches.
The spectrum of bosonized QCD2 in the chiral limit
1991
Abstract By studying an equivalent non-abelian bosonic theory we resolve the spectrum of Quantum Chromodynamics in one space-one time dimensions for massless quarks. The emphasis is placed in the non chiral sector described by colored meson fields. Two and four point functions of these fields are explicitly calculated in the large N limit. Some of the relevant issues: chiral symmetry realization, phases, baryon spectrum, topology etc …, are revisited.