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Register a new userParton Distribution Functions for diquarks based on an AdS/QCD quark-diquark nucleon model
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We present a phenomenological unpolarized Parton Distribution Functions for diquarks based on a soft-wall light front AdS/QCD quark-diquark nucleon model. From a probed model consistent with the Drewll-Yan-West relation and quark counting rule, we have performed a fit of some free parameters using known phenomenological quark PDF data. The model considers the entire set of possible diquarks within the nucleon valence, in the present work we focus on the spin-0 $ud_0$, spin-1 $ud_1$ and spin-1 $uu_1$ diquarks into the valence of protons. The diquark PDFs obtained are able to used in proton-proton collision simulations.
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We present the CTEQ6HQ parton distribution set which is determined in the general variable flavor number scheme which incorporates heavy flavor mass effects; hence, this set provides advantages for precision observables which are sensitive to charm and bottom quark masses. We describe the analysis procedure, examine the predominant features of the new distributions, and compare with previous distributions. We also examine the uncertainties of the strange quark distribution and how the the recent NuTeV dimuon data constrains this quantity.
Polarized parton distribution functions are determined by using world data from the longitudinally polarized deep inelastic scattering experiments. A new parametrization of the parton distribution functions is adopted by taking into account the positivity and the counting rule. From the fit to the asymmetry data A_1, the polarized distribution functions of u and d valence quarks, sea quarks, and gluon are obtained. The results indicate that the quark spin content is DeltaSigma=0.20 and 0.05 in the leading order (LO) and the next-to-leading-order (NLO) MS-bar scheme, respectively. However, if x dependence of the sea-quark distribution is fixed at small x by perturbative QCD and Regge theory, it becomes Delta Sigma=0.24 ~ 0.28 in the NLO. The small-x behavior cannot be uniquely determined by the existing data, which indicates the importance of future experiments. From our analysis, we propose one set of LO distributions and two sets of NLO ones as the longitudinally-polarized parton distribution functions.
The theoretical predictions are given for the forward limit of the unpolarized spin-flip isovector generalized parton distribution function $(E^u - E^d)(x, xi, t)$ within the framework of the chiral quark soliton model, with full inclusion of the polarization of Dirac sea quarks. We observe that $[(H^u - H^d) + (E^u - E^d)](x,0,0)$ has a sharp peak around $x=0$, which we interpret as a signal of the importance of the pionic $q bar{q}$ excitation with large spatial extension in the transverse direction. Another interesting indication given by the predicted distribution in combination with Jis angular momentum sum rule is that the $bar{d}$-quark carries more angular momentum than the $bar{u}$-quark in the proton, which may have some relation with the physics of the violation of the Gottfried sum rule.
Holographic soft-wall model is successful in the phenomenology of hadrons. Here with the use of generalized parton distributions (GPDs) obtained from AdS/QCD, perturbative effects are entered into the formalism. Perturbations are incorporated in the formalism through the evolution of GPDs according to the DGLAP like equations. Evolved proton GPDs are compared with a phenomenological model to show that we can get good improvements of the holographic model. It seems that combining the holographic soft-wall model with perturbative effects to some extent, gives the correct physics of GPDs.
We investigate the uncertainties of the heavy-quark parton distribution functions in the variable flavor number scheme. Because the charm- and bottom-quark parton distribution functions (PDFs) are constructed predominantly from the gluon PDF, it is a common practice to assume that the heavy-quark and gluon uncertainties are the same. We show that this approximation is a reasonable first guess, but it is better for bottom quarks than charm quarks. We calculate the PDF uncertainty for t-channel single-top-quark production using the Hessian matrix method, and predict a cross section of 2.12+0.32-0.29 pb at run II of the Tevatron.
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