Search results for "Chromodynamics"
showing 10 items of 1030 documents
Geometric approacch to condensates in halographic QCD
2005
An SU(Nf)xSU(Nf) Yang-Mills theory on an extra-dimensional interval is considered, with appropriate symmetry-breaking boundary conditions on the IR brane. UV-brane to UV-brane correlators at high energies are compared with the OPE of two-point functions of QCD quark currents. Condensates correspond to departure from AdS of the (different) metrics felt by vector and axial combinations, away from the UV brane. Their effect on hadronic observables is studied: the extracted condensates agree with the signs and orders of magnitude expected from QCD.
Heavy quark symmetry at large recoil
1992
Abstract We analyze the large recoil behaviour of heavy meson transition form factors using the Brodsky-Lepage hard scattering formalism. At the leading order of the heavy mass scale the large recoil form factors exhibit a new type of heavy quark symmetry. We discuss next-to-leading mass effects and present explicit 1/MQ expressions for the form factors in the peaking approximation.
One-loop amplitudes for four-point functions with two external massive quarks and two external massless partons up toO(ε2)
2006
We present complete analytical O({epsilon}{sup 2}) results on the one-loop amplitudes relevant for the next-to-next-to-leading order (NNLO) quark-parton model description of the hadroproduction of heavy quarks as given by the so-called loop-by-loop contributions. All results of the perturbative calculation are given in the dimensional regularization scheme. These one-loop amplitudes can also be used as input in the determination of the corresponding NNLO cross sections for heavy flavor photoproduction, and in photon-photon reactions.
Nucleon axial charge in lattice QCD with controlled errors
2012
We report on our calculation of the nucleon axial charge ${g}_{\mathrm{A}}$ in QCD with two flavors of dynamical quarks. A detailed investigation of systematic errors is performed, with a particular focus on contributions from excited states to three-point correlation functions. The use of summed operator insertions allows for a much better control over such contamination. After performing a chiral extrapolation to the physical pion mass, we find ${g}_{\mathrm{A}}=1.223\ifmmode\pm\else\textpm\fi{}0.063(\mathrm{stat}{)}_{\ensuremath{-}0.060}^{+0.035}(\mathrm{syst})$, in good agreement with the experimental value.
Quark transverse charge densities in the from lattice QCD
2009
Abstract We extend the formalism relating electromagnetic form factors to transverse quark charge densities in the light-front frame to the case of a spin-3/2 baryon and calculate these transverse densities for the Δ ( 1232 ) isobar using lattice QCD. The transverse charge densities for a transversely polarized spin-3/2 particle are characterized by monopole, dipole, quadrupole, and octupole patterns representing the structure beyond that of a pure point-like spin-3/2 particle. We present lattice QCD results for the Δ-isobar electromagnetic form factors for pion masses down to approximatively 350 MeV for three cases: quenched QCD, two-degenerate flavors of dynamical Wilson quarks, and three…
QCD sum rules for heavy baryons
2001
We construct the heavy baryonic currents by using the Bethe-Salpeter wave functions in the heavy quark limit. We discuss the one-loop renormalization of these heavy baryonic currents as well as their two-point correlators up to the order $1/M_h$. For a special case, we do the QCD sum rule for masses of the doublet (3/2,5/2).
Measurement of spin correlation in tt¯ production using dilepton final states
2011
We measure the correlation between the spin of the top quark and the spin of the anti-top quark in (ttbar -> W+ W- b bbar -> l+ nu b l- nubar bbar) final states produced in ppbar collisions at a center of mass energy sqrt(s)=1.96 TeV, where l is an electron or muon. The data correspond to an integrated luminosity of 5.4 fb-1 and were collected with the D0 detector at the Fermilab Tevatron collider. The correlation is extracted from the angles of the two leptons in the t and tbar rest frames, yielding a correlation strength C= 0.10^{+0.45}_{-0.45}, in agreement with the NLO QCD prediction within two standard deviations, but also in agreement with the no correlation hypothesis.
Strange quark condensate from QCD sum rules to five loops
2007
It is argued that it is valid to use QCD sum rules to determine the scalar and pseudoscalar two-point functions at zero momentum, which in turn determine the ratio of the strange to non-strange quark condensates $R_{su} = \frac{}{}$ with ($q=u,d$). This is done in the framework of a new set of QCD Finite Energy Sum Rules (FESR) that involve as integration kernel a second degree polynomial, tuned to reduce considerably the systematic uncertainties in the hadronic spectral functions. As a result, the parameters limiting the precision of this determination are $\Lambda_{QCD}$, and to a major extent the strange quark mass. From the positivity of $R_{su}$ there follows an upper bound on the latt…
Nucleon average quark momentum fraction with Nf = 2+1 Wilson fermions
2018
We report on an analysis of the average quark momentum fraction of the nucleon and related quantities using Nf = 2 + 1 Wilson fermions. Computations are performed on four CLS ensembles covering three values of the lattice spacing at pion masses down to Mπ ≈ 200 MeV. Several source-sink separations (~ 1:0 fm to ~ 1:4 fm) are used to assess the excited-state contamination. To gain further insight, the generalized pencil-of-functions approach has been implemented to reduce the excited-state contamination in the relevant two-and three-point functions. Preliminary results are shown for the isovector nucleon charges from vector, axial vector and tensor derivative (twist-2) operators.
Light quark condensates from QCD sum rules
1985
The light quark condensates have been determined by two different methods: By Laplace transformed QCD sum rules together with an improved hadronic continuum from extended PCAC and by analytic continuation by duality (ACD) of the asymptotic QCD amplitude. Both methods yield compatible results. The PCAC corrections are considerably large: for theu, d quarks near 8% and for theu, s quarks of order 60%.