Radial excitations of heavy-light mesons from QCD sum rules

QCD sum rules are commonly used to predict the characteristics of ground-state hadrons. We demonstrate that two-point sum rules for the decay constants of charmed ($D^{(*)},D_s^{(*)}$) and bottom ($B^{(*)},B_s^{(*)}$) mesons can also be modified to estimate the decay constants of the first radial excitations, $D^{(*)'},D_s^{(*)'}$ and $B^{(*)'},B_s^{(*)'}$, respectively, provided the masses of these resonances are used as an input. For the radially excited charmed mesons we use available experimental data, whereas the masses of analogous bottom mesons are estimated from the heavy-quark limit. The decay constants predicted for the radial excitations of heavy-light pseudoscalar and vector mesons are systematically smaller than those of the ground states and we comment on the possible origin of this difference. Our results can be used in the sum rule calculations of heavy-to-light form factors and in the factorization approximations for nonleptonic $B$-meson decays where the decay constants of charmed mesons enter as input parameters.Comment: 16 pages, a few comments added, version to appear in EPJ

The Challenge of Light-Front Quantisation: Recent Results

We explain what is the challenge of light-front quantisation, and how we can now answer it because of recent progress in solving the problem of zero modes in the case of non-Abelian gauge theories. We also give a description of the light-front Hamiltonian for SU(2) finite volume gluodynamics resulting from this recent solution to the problem of light-front zero modes.Comment: 17 pages, lecture delivered by GBP at the XXXIV PNPI Winter School, Repino, St.Petersburg, Russia, February 14-20, 2000, version to appear in the Proceeding

Next-to-Leading Order perturbative QCD corrections to baryon correlators in matter

We compute the next-to-leading order perturbative QCD corrections to the correlators of nucleon interpolating currents in relativistic nuclear matter. The main new result is the calculation of the O(alpha_s) perturbative corrections to the coefficient functions of the vector quark condensate in matter. This condensate appears in matter due to the violation of Lorentz invariance. The NLO perturbative QCD corrections turn out to be large which implies that the NLO corrections must be included in a sum rule analysis of the properties of both bound nucleons and relativistic nuclear matter.Comment: 19 pages in LaTeX, including 5 Postscript figure

Plasmon channels in the electronic relaxation of diamond under high-order harmonics femtosecond irradiation

We used high order harmonics of a femtosecond titanium-doped sapphire system (pulse duration 25 fs) to realise Ultraviolet Photoelectron Spectroscopy (UPS) measurements on diamond. The UPS spectra were measured for harmonics in the range 13 to 27. We also made ab initio calculations of the electronic lifetime of conduction electrons in the energy range produced in the UPS experiment. Such calculations show that the lifetime suddenly diminishes when the conduction electron energy reaches the plasmon energy, whereas the UPS spectra show evidence in this range of a strong relaxation mechanism with an increased production of low energy secondary electrons. We propose that in this case the electronic relaxation proceeds in two steps : excitation of a plasmon by the high energy electron, the latter decaying into individual electron-hole pairs, as in the case of metals. This process is observed for the first time in an insulator and, on account of its high efficiency, should be introduced in the models of laser breakdown under high intensity

Resummation analysis of the τ\tau decay width using the four-loop β\beta-function

We extract the strong coupling constant $\alpha_s(m_\tau^2)$ from the semileptonic $\tau$ decay width taking into account resummation effects from the running of the strong coupling constant. In the $\overline{\rm MS}$ scheme the result reads $\alpha_s=0.375\pm 0.007$ to third order and $\alpha_s=0.378\pm 0.007$ to fourth order in the $\beta$-function, respectively, where we use the recently computed four-loop coefficient $\beta_3$. These values for the coupling constant have to be compared with the value $\alpha_s=0.354\pm 0.005$ derived from a third order analysis of $\tau$ decays. We determine the exact value of the convergence radius of the perturbation series by analyzing the singularity structure of the complex coupling constant plane.Comment: 12 pages, LaTeX, more detailed explanation of the analysis done for the determination of the convergence radius, one PostScript figure adde

Up and down quark masses from Finite Energy QCD sum rules to five loops

The up and down quark masses are determined from an optimized QCD Finite Energy Sum Rule (FESR) involving the correlator of axial-vector divergences, to five loop order in Perturbative QCD (PQCD), and including leading non-perturbative QCD and higher order quark mass corrections. This FESR is designed to reduce considerably the systematic uncertainties arising from the (unmeasured) hadronic resonance sector, which in this framework contributes less than 3-4% to the quark mass. This is achieved by introducing an integration kernel in the form of a second degree polynomial, restricted to vanish at the peak of the two lowest lying resonances. The driving hadronic contribution is then the pion pole, with parameters well known from experiment. The determination is done in the framework of Contour Improved Perturbation Theory (CIPT), which exhibits a very good convergence, leading to a remarkably stable result in the unusually wide window $s_0 = 1.0 - 4.0 {GeV}^2$, where $s_0$ is the radius of the integration contour in the complex energy (squared) plane. The results are: $m_u(Q= 2 {GeV}) = 2.9 \pm 0.2$ MeV, $m_d(Q= 2 {GeV}) = 5.3 \pm 0.4$ MeV, and $(m_u + m_d)/2 = 4.1 \pm 0.2$ Mev (at a scale Q=2 GeV).Comment: Additional references to lattice QCD results have been adde

O(αs)O(\alpha_s) corrections to the correlator of finite mass baryon currents

We present analytical next-to-leading order results for the correlator of baryonic currents at the three loop level with one finite mass quark. We obtain the massless and the HQET limit of the correlator from the general formula as particular cases. We also give explicit expressions for the moments of the spectral density.Comment: 12 pages, 2 Postscript figure

Analytic Perturbation Theory: A New Approach to the Analytic Continuation of the Strong Coupling Constant αS\alpha_S into the Timelike Region

The renormalization group applied to perturbation theory is ordinarily used to define the running coupling constant in the spacelike region. However, to describe processes with timelike momenta transfers, it is important to have a self-consistent determination of the running coupling constant in the timelike region. The technique called analytic perturbation theory (APT) allows a consistent determination of this running coupling constant. The results are found to disagree significantly with those obtained in the standard perturbative approach. Comparison between the standard approach and APT is carried out to two loops, and threshold matching in APT is applied in the timelike region.Comment: 16 pages, REVTeX, 7 postscript figure

Bulk Nanocrystalline Thermoelectrics Based on Bi-Sb-Te Solid Solution

A nanopowder from p-Bi-Sb-Te with particles ~ 10 nm were fabricated by the ball milling using different technological modes. Cold and hot pressing at different conditions and also SPS process were used for consolidation of the powder into a bulk nanostructure and nanocomposites. The main factors allowing slowing-down of the growth of nanograins as a result of recrystallization are the reduction of the temperature and of the duration of the pressing, the increase of the pressure, as well as addition of small value additives (like MoS2, thermally expanded graphite or fullerenes). It was reached the thermoelectric figure of merit ZT=1.22 (at 360 K) in the bulk nanostructure Bi0,4Sb1,6Te3 fabricated by SPS method. Some mechanisms of the improvement of the thermoelectric efficiency in bulk nanocrystalline semiconductors based on BixSb2-xTe3 are studied theoretically. The reduction of nanograin size can lead to improvement of the thermoelectric figure of merit. The theoretical dependence of the electric and heat conductivities and the thermoelectric power as the function of nanograins size in BixSb2-xTe3 bulk nanostructure are quite accurately correlates with the experimental data.Comment: 35 pages, 24 figures, 4 tables, 52 reference