[en] Taking into account all available data on the mass sector, we obtain unitary rotation matrices that diagonalize the quark matrices by using a specific parametrization of the Cabibbo-Kobayashi-Maskawa mixing matrix. In this way, we find mass matrices for the up- and down-quark sectors of a specific, symmetric form, with traces of a democratic texture.

[en] The first moment of longitudinal and transverse spin densities of quarks in the nucleon are calculated in a light-front constituent quark model for the different cases of quark and nucleon polarization. Significant distortions are found for the transverse spin densities. In particular the Sivers function is predicted with opposite sign for up and down quarks and the Boer-Mulders function is predicted large and negative for both up and down quarks, in agreement with lattice calculations. Quite a different spin distribution is obtained for up and down quarks in the cases of quarks and proton transversely or longitudinally polarized in the same direction

[en] A new formulation of chiral fermions on the lattice is presented. It is a version of overlap fermions, but built from the computationally efficient staggered fermions rather than the previously used Wilson fermions. The construction reduces the four quark flavors described by the staggered fermion to two quark flavors; this pair can be taken as the up and down quarks in Lattice QCD. A domain wall formulation giving a truncation of this overlap construction is also outlined.

[en] I review quark momentum distributions in the nucleon at large momentum fractions x. Particular attention is paid to the impact of nuclear effects in deuterium on the d/u quark distribution ratio as x → 1. A new global study of parton distributions, using less restrictive kinematic cuts in Q² and W², finds strong suppression of the d quark distribution once nuclear corrections are accounted for.

[en] We measure the mass dependence of the forward-backward charge asymmetry in 157,553 p(bar p) → Z/γ* → e⁺e^- interactions, corresponding to 5.0 fb^-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider at √s = 1.96 TeV. The effective weak mixing angle (θ_eff^#ell#) from this process involving predominantly the first generation of quarks is extracted as sin² θ_eff^#ell# = 0.2309 ± 0.0008 (stat.) ± 0.0006 (syst.). We also present the most precise direct measurement of the vector and axial-vector couplings of u and d quarks to the Z boson.

[en] Hierarchical quark masses and small mixing angles are shown to lead to a simple triangular form for U- and D-type quark mass matrices. In the basis where one of the matrices is diagonal, each matrix element of the other is, to a good approximation, the product of a quark mass and a CKM matrix element. The physical content of a general mass matrix can be easily deciphered in its triangular form. This parametrization could serve as a useful starting point for model building. Examples of mass textures are analyzed using this method. copyright 1999 The American Physical Society

[en] We report results of a new global next-to-leading order fit of parton distribution functions in which cuts on W and Q are relaxed, thereby including more data at high values of x. Effects of target mass corrections, higher twist contributions, and nuclear corrections for deuterium data are significant in the large-x region. The leading twist parton distributions are found to be stable to target mass correction model variations as long as higher twist contributions are also included. The behavior of the d quark as x→1 is particularly sensitive to the deuterium corrections, and using realistic nuclear smearing models the d-quark distribution at large x is found to be softer than in previous fits performed with more restrictive cuts.

[en] We present a study of impact parameter dependent parton distributions (ipdpdfs) using a recent parametrization of u and d quark GPDs H(x,t) and E(x,t) at zero skewness ζ = 0 and discuss the region of validity of the positivity condition which puts limits on the range of x and impact parameter b_perpendicular used in this parametrization. We also present the GPDs in transverse and longitudinal position space using the same parametrization for non-zero ζ. For GPDs in longitudinal position space in the kinematical region x > ζ, a diffraction pattern is observed, as seen before in several other models.

[en] All lattice-QCD calculations of the hadronic-vacuum-polarization contribution to the muon’s anomalous magnetic moment to date have been performed with degenerate up- and down-quark masses. Here we calculate directly the strong-isospin-breaking correction to $a_{\mathrm{\mu <!--\; ??\; -->}}^{\mathrm{H}\mathrm{V}\mathrm{P}}$ for the first time with physical values of $m_u$ and $m_d$ and dynamical $u$, $d$, $s$, and $c$ quarks, thereby removing this important source of systematic uncertainty. We obtain a relative shift to be applied to lattice-QCD results obtained with degenerate light-quark masses of $\mathrm{\delta <!--\; ??\; -->}{a}_{\mathrm{\mu <!--\; ??\; -->}}^{\mathrm{H}\mathrm{V}\mathrm{P},m_u\ne <!--\; ???\; -->m_d}=+1.5(7)$%, in agreement with estimates from phenomenology.

[en] We study the status of S₃, i.e. a slightly broken symmetry of quarks and propose a new model in which the S₃ symmetry among the three generation up-quarks is slightly broken into the C₂ symmetry while the S₃ symmetry of the down-quarks is completely broken in a different way. (author)