No Arabic abstract
The quark condensate is calculated within the world-line effective-action formalism, by using for the Wilson loop an ansatz provided by the stochastic vacuum model. Starting with the relation between the quark and the gluon condensates in the heavy-quark limit, we diminish the current quark mass down to the value of the inverse vacuum correlation length, finding in this way a 64%-decrease in the absolute value of the quark condensate. In particular, we find that the conventional formula for the heavy-quark condensate cannot be applied to the c-quark, and that the corrections to this formula can reach 23% even in the case of the b-quark. We also demonstrate that, for an exponential parametrization of the two-point correlation function of gluonic field strengths, the quark condensate does not depend on the non-confining non-perturbative interactions of the stochastic background Yang-Mills fields.
We investigate a connection between a renormalon ambiguity of heavy quark mass and the gluon condensate contribution into the quark dispersion law related with a virtuality defining a displacement of the heavy quark from the perturbative mass-shell, which happens inside a hadron.
We study the static potential between external quark-antiquark pairs in a strongly coupled gauge theory with a large number of colors and massive dynamical flavors, using a dual string description. When the constituent mass of the dynamical quarks is set below a certain critical value, we find a first order phase transition between a linear and a Coulomb-like regime. Above the critical mass the two phases are smoothly connected. We also study the dependence on the theory parameters of the quark-antiquark separation at which the static configuration decays into specific static-dynamical mesons.
The gluon condensate is very sensitive to the QCD deconfinement transition since its value changes drastically with the deconfinement transition. We calculate the gluon condensate dependence of the heavy quark potential in AdS/CFT to study how the property of the heavy quarkonium is affected by a relic of the deconfinement transition. We observe that the heavy quark potential becomes deeper as the value of the gluon condensate decreases. We interpret this as a dropping of the heavy quarkonium mass just above the deconfinement transition, which is similar to the results obtained from QCD sum rule and from a bottom-up AdS/QCD model.
We show that one can re-arrange the Heavy Quark Expansion for inclusive weak decays of charmed hadrons in such a way that the resulting expansion is an expansion in $Lambda_{rm QCD} / m_c$ and $alpha_s (m_c)$ with order-one coefficients. Unlike in the case of the bottom quark, the leading term includes not only the contribution of the free-quark decay, but also a tower of terms related to matrix elements of four quark operators.
We present results on an analysis of the decay constants f_B and f_Bs with two flavours of sea quark. The calculation has been carried out on 3 different bare gauge couplings and 4 sea quark masses at each gauge coupling, with m_pi/m_rho ranging from 0.8 to 0.6. We employ the Fermilab formalism to perform calculations with heavy quarks whose mass is in the range of the b-quark. A detailed comparison with a quenched calculation using the same action is made to elucidate the effects due to the sea quarks.