Naive Quark Research Articles

NeutralBmixing in staggered chiral perturbation theory

I calculate, at one loop in staggered chiral perturbation theory, the matrix elements of the complete set of five local operators that may contribute to $B$ mixing both in the standard model and in beyond-the-standard-model theories. Lattice computations of these matrix elements by the Fermilab Lattice/MILC collaborations (and earlier by the HPQCD collaboration) convert a light staggered quark into a naive quark, and construct the relevant four-quark operators as local products of two local bilinears, each involving the naive light quark and the heavy quark. This particular representation of the operators turns out to be important in the chiral calculation, and it results in the presence of ``wrong-spin'' operators, whose contributions however vanish in the continuum limit. If the matrix elements of all five operators are computed on the lattice, then no additional low-energy constants are required to describe wrong-spin chiral effects.

THE STATUS OF MESONIC EXOTICA

Over twenty years ago QCD-inspired models of hadronic states began a rigorous search for meson outside of Naive Quark Model. Here we briefly review the current status of Exotica.

Medium modifications of nucleon electromagnetic form factors

We use the Nambu–Jona-Lasinio model as an effective quark theory to investigate the medium modifications of the nucleon electromagnetic form factors. By using the equation of state of nuclear matter derived in this model, we discuss the results based on the naive quark–scalar diquark picture, the effects of finite diquark size, and the meson cloud around the constituent quarks. We apply this description to the longitudinal response function for quasielastic electron scattering. RPA correlations, based on the nucleon–nucleon interaction derived in the same model, are also taken into account in the calculation of the response function.

Further Improvements to staggered quarks

The improved staggered quark action has enabled breakthrough calculations of QCD with three light dynamical quarks. The largest remaining $a^2$ scaling violations come from "taste-changing" interactions, which are significantly reduced over naive quarks because of carefully chosen smearing of the gauge links. Here we examine further improvements to the quark action and show that additional smearing of the links can reduce the taste-changing errors by another factor of two, whilst retaining a fully $a^2$-improved action. The taste-changing errors can be understood within a perturbative calculation of the four-quark operators that appear at one-loop. We also discuss unquenching techniques with such a highly improved action.

Nucleon-nucleon, γp and γγ scattering using factorization: the Aspen model and analytic amplitude analysis

Using the Aspen, model, a QCD-inspired eikonal model, we fit high energy nucleon-nucleon total cross sections, nuclear slopes and ϱ-values, including cosmic ray p-air data. This model which is crossing symmetric, insures unitarity and analyticity, as well as satisfying the Froissrt bound. Assuming that the ratio of σelastic/σtotal is independent of process, the Aspen model predicts factorization. Utilizing the model parameters found for nucleon-nucleon scattering, we predict the total cross sections and nuclear slope parameters for γp reactions, as well as total cross sections for γγ scattering, by invoking the naive quark model and vector dominance. In particular, by making a χ2 fit to the nuclear slope parameters for the ‘elastic’ reactions γ + p → ϱ+p, γ+p → φ+p, γ+p → ω+p, we find the proportionality constant κ = 0.661±0.008, in excellent agreement with the value of κ = 23 predicted by the naive vector quark model (2 quarks in the vector meson-like photon and 3 quarks in the nucleon). We next test the (lack of) sensitivity of the factorization theorems to the assumed distributions of the impact parameter b, for nucleon-nucleon, γp and γγ scattering, that are used in the eikonal. Finally, making an independent analysis utilizing real analytical amplitudes and assuming factorization, we make a simultaneous fit to the total cross sections for pp, pp, γp and γγ cross sections, as well as the ϱ-values for pp and pp. In this global fit we separately fit to the results of L3 and OPAL obtained using either the PHOJET or PYTHIA Monte Carlo program—the published values use the average of the two. It should be noted that these two Monte Carlos give different normalizations as well as different energy behaviors. The overall fit is good using PHOJET and poor using PYTHIA, lending strong support for the factorization hypothesis, as well as the naive quark model and vector dominance.

Why the Naive Quark Model Can Yield a Good Account of the Baryon Magnetic Moments

The chiral quark model suggests that the baryon quark sea is negatively polarized. This modifies the spin structure as given by the naive quark model and agrees with experimental data. However, for the magnetic moments, there is significant cancellation between the contributions from this sea spin polarization and the orbital angular momentum so that effectively the moments are given by the valence constituent quarks alone, as in the naive quark model.

Implications of the strange spin of the nucleon for the neutron electric dipole moment in supersymmetric theories

Supersymmetric model contributions to the neutron electric dipole moment arise via quark electric dipole moment operators, whose matrix elements are usually calculated using the Naive Quark Model (NQM). However, experiments indicate that the NQM does not describe well the quark contributions Δq to the nucleon spin, and so may provide misleading estimates of electric dipole operator matrix elements. Taking the Δq from experiment, we indeed find consistently smaller estimates of the neutron electric dipole moment for given values of the supersymmetric model parameters. This weakens previous constraints on CP violation in supersymmetric models, which we exemplify analytically in the case where the lightest supersymmetric particle (LSP) is a U(1) gaugino B , and display numerically for other LSP candidates.

Current algebra and nonleptonic weak decays of charmed baryons

We have calculated the nonleptonic weak-decay rates of Λc+(c[ud]), A+(c[su]), A0(c[sd]) and T0(css). We used current algebra and an evaluation of the matrix elements 〈Bf|Hwp.c.|Bi〉 in the context of nonrelativisticSU6 wave functions. These matrix elements are evaluated with and without including short-distance QCD effects. The results are compared with the available experimental data. It is found that the naive quark Hamiltonian, without short-distance QCD effects included, gives results which are in reasonable agreement with experiment, whereas the QCD-corrected Hamiltonian gives widths which are much too large. The results are also compared with earlier calculations using the MIT bag model and the quark model.

Incorporating pion effects into the naive quark model

A hybrid of the naive nonrelativistic quark model and the Chew-Low model is proposed. The pion is treated as an elementary particle which interacts with the "bare baryon" or "baryon core" via the Chew-Low interaction. The baryon core, which is the source of the pion interaction, is described by the naive nonrelativistic quark model. It turns out that the baryon-core radius has to be as large as 0.8 fm, and consequently the cutoff momentum $\ensuremath{\Lambda}$ for the pion interaction is $\ensuremath{\lesssim}3{m}_{\ensuremath{\pi}}$, ${m}_{\ensuremath{\pi}}$ being the pion mass. Because of this small $\ensuremath{\Lambda}$ (as compared with $\ensuremath{\Lambda}\ensuremath{\sim}\mathrm{nucleon}$ mass in the old Chew-Low model) the effects of the pion cloud are strongly suppressed. The baryon masses, baryon magnetic moments (except for ${\ensuremath{\Xi}}^{\ensuremath{-}}$), and the nucleon charge radii can be reproduced quite well. However, we found it singularly difficult to fit the axial-vector weak decay constant ${g}_{A}$.

Analysis of Decay Angular Distributions of Electroproduced by the Naive Quark Model

Analysis of Decay Angular Distributions of Electroproduced by the Naive Quark Model