arXiv subjects

A. W. Thomas

Publications and source records attributed to A. W. Thomas.

At least 343 records · Page 19Linked to original sources

arXiv1999

Structure and Production of Lambda Baryons

We discuss the quark parton structure of the $Λ$ baryon and the fragmentation of quarks into $Λ$ baryons. We show that the hyperfine interaction, responsible for the $Δ$-$N$ and $Σ^0$-$Λ$ mass splittings, leads not only to sizeable SU(3) and SU(6) symmetry breaking in the quark distributions of the $Λ$, but also to significant polarized non-strange quark distributions. The same arguments suggest flavor asymmetric quark fragmentation functions and non-zero polarized non-strange quark fragmentation functions. The calculated fragmentation functions give a good description of all measured observables. We predict significant positive $Λ$ polarization in semi-inclusive DIS experiments while models based on SU(3) flavor symmetry predict zero or negative $Λ$ polarization. Our approach also provides a natural explanation for the dependence of the maximum of the $ξ=\ln(1/z)$ spectrum on the mass of the particles produced in $e^+e^-$ annihilation.

hep-ph↗

arXiv1999

A New Analysis of Charge Symmetry Violation in Parton Distributions

To date, the strongest indication of charge symmetry violation in parton distributions has been obtained by comparing the $F_2$ structure functions from CCFR neutrino data and NMC muon data. We show that in order to make precise tests of charge symmetry with the neutrino data, two conditions must be satisfied. First, the nuclear shadowing calculations must be made explicitly for neutrinos, not simply taken from muon data on nuclei. Second, the contribution of strange and charm quarks should be calculated explicitly using next-to-leading order [NLO] QCD, and the ``slow rescaling'' charm threshold correction should not be applied to the neutrino data. When these criteria are satisfied, the comparison is consistent with charge symmetry within the experimental errors and the present uncertainty in the strange quark distribution of the nucleon.

hep-ph↗

arXiv1999

Precision determination of the pi-N scattering lengths and the charged pi-NN coupling constant

We critically evaluate the isovector GMO sumrule for the charged $πN N$ coupling constant using recent precision data from $π^-$p and $π^-$d atoms and with careful attention to systematic errors. From the $π^-$d scattering length we deduce the pion-proton scattering lengths ${1/2}(a_{π^-p}+a_{π^-n})=(-20\pm 6$(statistic)$ \pm 10$ (systematic))~$\cdot 10^{-4}m_{π_c}^{-1}$ and ${1/2}(a_{π^-p}-a_{π^-n})=(903 \pm 14)\cdot 10^{-4}m_{π_c}^{-1}$. From this a direct evaluation gives $g^2_c(GMO)/4π=14.20\pm 0.07$(statistic)$\pm 0.13$(systematic) or $f^2_c/4π= 0.0786\pm 0.0008$.

hep-ph↗

arXiv1999

Direct CP Violation in Charmed Hadron Decays via $ρ-ω$ Mixing

We study the possibility of obtaining large direct CP violation in the charmed hadron decays $D^+ \to ρ^+ ρ^0 (ω) \to ρ^+ π^+π^-$, $D^+ \to π^+ ρ^0 (ω) \to π^+ π^+π^-$, $D^0 \to ϕρ^0 (ω) \toϕπ^+π^-$, $D^0 \toηρ^0 (ω) \toηπ^+π^-$, $D^0 \to η' ρ^0 (ω) \toη' π^+π^-$, $D^0 \to π^0 ρ^0 (ω) \to π^0 π^+π^-$, and $Λ_c \ra p ρ^0 (ω) \to p π^+π^-$ via $ρ-ω$ mixing. The analysis is carried out in the factorization approach. The CP violation parameter depends on the effective parameter, $N_{c}$, which is relevant to hadronization dynamics of each decay channel and should be determined by experiments. It is found that for fixed $N_{c}$ the CP violation parameter reaches its maximum value when the invariant mass of the $π^+π^-$ pair is in the vicinity of the $ω$ resonance. For $D^0 \to π^0 ρ^0 (ω) \to π^0 π^+π^-$ and $Λ_c \to p ρ^0 (ω) \to p π^+π^-$ the maximum CP violating asymmetry is bigger than 1% when $1.98\leq N_c \leq 1.99$ and $1.95\leq N_c \leq 2.02$, respectively. We also estimate the decay branching ratios for $D^0 \to π^0 ρ^0$ and $Λ_c \to p ρ^0$ for these values of $N_c$, which should be tested by future experimental data.

hep-ph↗

arXiv1999

Spectral change of sigma and omega mesons in a dense nuclear medium

The spectra of the scalar ($σ$) and vector ($ω$) mesons in nuclear matter are studied in detail using the quark-meson coupling (QMC) model. It is shown that above normal nuclear matter density the effects of $σ$-$ω$ mixing and the decay of the $σ$ into $2π$ change the spectra of $σ$ and $ω$ mesons considerably. As in Quantum Hadrodynamics we find a remarkable spectral change in the $σ$ meson and the longitudinal mode of the $ω$ meson, namely a two-peak structure.

nucl-th↗

arXiv1999

Non-perturbative chiral corrections for lattice QCD

We explore the chiral aspects of extrapolation of observables calculated within lattice QCD, using the nucleon magnetic moments as an example. Our analysis shows that the biggest effects of chiral dynamics occur for quark masses corresponding to a pion mass below 600 MeV. In this limited range chiral perturbation theory is not rapidly convergent, but we can develop some understanding of the behaviour through chiral quark models. This model dependent analysis leads us to a simple Padé approximant which builds in both the limits $m_π\to 0$ and $m_π\to \infty$ correctly and permits a consistent, model independent extrapolation to the physical pion mass which should be extremely reliable.

hep-ph↗

arXiv1999

Correlator mixing and mass reduction as signals of chiral symmetry restoration

Chiral symmetry restoration in a dense medium is to some extent a consequence of the nuclear pion cloud. These pions induce a mixing of the axial and vector current contributions in the axial and vector correlators. We discuss their influence on hadron masses and investigate the signal produced by the remaining contribution associated with chiral symmetry restoration. Using the quark-meson coupling model we find that the latter is associated with the reduction of hadron masses.

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arXiv1999

Effect of nucleon structure variation on the longitudinal response function

Using the quark-meson coupling (QMC) model, we study the longitudinal response function for quasielastic electron scattering from nuclear matter. In QMC the coupling constant between the scalar ($σ$) meson and the nucleon is expected to decrease with increasing nuclear density, because of the self-consistent modification of the structure of the nucleon. The reduction of the coupling constant then leads to a smaller contribution from relativistic RPA than in the Walecka model. However, since the electromagnetic form factors of the in-medium nucleon are modified at the same time, the longitudinal response function and the Coulomb sum are reduced by a total of about 20% in comparison with the Hartree contribution. We find that the relativistic RPA and the nucleon structure variation both contribute about fifty-fifty to the reduction of the longitudinal response.

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arXiv1999

Chiral corrections in hadron spectroscopy

We show that the implementation of chiral symmetry in recent studies of the hadron spectrum in the context of the constituent quark model is inconsistent with chiral perturbation theory. In particular, we show that the leading nonanalytic (LNA) contributions to the hadron masses are incorrect in such approaches. The failure to implement the correct chiral behaviour of QCD results in incorrect systematics for the corrections to the masses.

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arXiv1999

Resonance model study of kaon production in baryon baryon reactions for heavy ion collisions

The energy dependence of the total kaon production cross sections in baryon baryon ($N$ and $Δ$) collisions are studied in the resonance model, which is a relativistic, tree-level treatment. This study is the first attempt to complete a systematic, consistent investigation of the elementary kaon production reactions for both the pion baryon and baryon baryon reactions. Our model suggests that the magnitudes of the isospin-averaged total cross sections for the $N N \to N Y K$ and $ΔN \to N Y K$ ($Y = Λ$ or $Σ$) reactions are almost equal at energies up to about 200 MeV above threshold. However, the magnitudes for the $ΔN$ reactions become about 6 times larger than those for the $N N$ reactions at energies about 1 GeV above threshold. Furthermore, the magnitudes of the isospin-averaged total cross sections for the $N N \to ΔY K$ reactions turn out to be comparable to those for the $N N \to N Y K$ reactions at $N N$ invariant collision energies about 3.1 GeV, and about 5 to 10 times larger at $N N$ invariant collision energies about 3.5 GeV. The microscopic cross sections are parametrized in all isospin channels necessary for the transport model studies of kaon production in heavy ion collisions. These cross sections are then applied in the relativistic transport model to study the sensitivity to the underlying elementary kaon production cross sections.

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arXiv1999

Charm in the Nucleon

A next-to-leading order analysis of inelastic electroproduction of charm is performed using an interpolating scheme which maps smoothly onto massless QCD evolution at large Q^2 and photon-gluon fusion at small Q^2. In contrast with earlier analyses, this scheme allows the inclusion of quark and target mass effects and heavy quark thresholds, as well as possible non-perturbative, or intrinsic, charm contributions. We find no conclusive evidence in favor of an intrinsic charm component in the nucleon, although several data points which disagree with perturbative QCD expectations will need to be checked by future experiments.

hep-ph↗

arXiv1999

Fock terms in the quark-meson coupling model

The mean field description of nuclear matter in the quark-meson coupling model is improved by the inclusion of exchange contributions (Fock terms). The inclusion of Fock terms allows us to explore the momentum dependence of meson-nucleon vertices and the role of pionic degrees of freedom in matter. It is found that the Fock terms maintain the previous predictions of the model for the in-medium properties of the nucleon and for the nuclear incompressibility. The Fock terms significantly increase the absolute values of the single-particle, four-component scalar and vector potentials, a feature that is relevant for the spin-orbit splitting in finite nuclei.

nucl-th↗

arXiv1999

Pion Exchange and the H1 Forward Spectrometer Data

We point out that the $Δπ$ component of the nucleon wave function is vital to the interpretation of the recent H1 data for leading baryon production. While the $n/p$ ratio is equal to two with the $Nπ$ component alone, the inclusion of the $Δπ$ component brings this ratio very near to unity, as observed in the experiment. This result demonstrates that pion exchange can not only account for leading neutron but also for a large fraction of the leading proton production.

hep-ph↗

arXiv1999

Bethe-Salpeter Equation for Heavy Baryons ω_{Q}^{(*)} in the Diquark Picture

In the heavy quark limit, the heavy baryons ω_{Q}^{(*)} (ωcould be Σ, Ξor Ωand Q=b or c) are regarded as composed of a heavy quark and an axial vector, light diquark with good spin and isospin quantum numbers. Based on this diquark picture we establish the Bethe-Salpeter (B-S) equation for ω_{Q}^{(*)} in the limit where the heavy quark has infinite mass, m_Q -> \infty. It is found that in this limit there are three components in the B-S wave function for ω_{Q}^{(*)}. Assuming the kernel to consist of a scalar confinement term and a one-gluon-exchange term we derive three coupled integral equations for the three B-S scalar functions in the covariant instantaneous approximation. Numerical solutions for the three B-S scalar functions are presented, including a discussion about their dependence on the various input parameters. These solutions are applied to calculate the Isgur-Wise functions ξ(ω) and ζ(ω) for the weak transitions Ω_{b}^{(*)} \to Ω_{c}^{(*)}. Using these we give predictions for the Cabibbo-allowed nonleptonic decay widths and up-down asymmetries for Ω_{b} \to Ω_{c}^{(*)} plus a pseudoscalar or vector meson.

hep-ph↗

arXiv1999

Parton Distributions for the Octet and Decuplet Baryons

We calculate the parton distributions for both polarized and unpolarized octet and decuplet baryons, using the MIT bag, dressed by mesons. We show that the hyperfine interaction responsible for the $Δ- N$ and $Σ^0 - Λ$ splittings leads to large deviations from SU(3) and SU(6) predictions. For the $Λ$ we find significant polarized, non-strange parton distributions which lead to a sizable $Λ$ polarization in polarized, semi-inclusive $ep$ scattering. We also discuss the flavour symmetry violation arising from the meson-cloud associated with the chiral structure of baryons.

hep-ph↗

arXiv1999

Charmed Mesic Nuclei: Bound D and $\bar{D}$ states with ^{208}Pb

We show that the D^- meson will form narrow bound states with ^{208}Pb. Mean field potentials for the D^0, D^0-bar and D^- in ^{208}Pb are calculated self-consistently using the quark-meson coupling (QMC) model in local density approximation. The meson-^{208}Pb bound state energies are then calculated by solving the Klein-Gordon equation with these potentials. The experimental confirmation and comparison with the D^0-bar and D^0 will provide distinctive information on the nature of the interaction between the charmed meson and matter.

nucl-th↗

arXiv1999

A quark-meson coupling model with short-range quark-quark correlations

Short-range quark-quark correlations are introduced into the quark-meson coupling (QMC) model in a simple way. The effect of these correlations on the structure of the nucleon in dense nuclear matter is studied. We find that the short-range correlations may serve to reduce a serious problem associated with the modified quark-meson coupling model (within which the bag constant is allowed to decrease with increasing density), namely the tendency for the size of the bound nucleon to increase rapidly as the density rises. We also find that, with the addition of correlations, both QMC and modified QMC are consistent with the phenomenological equation of state at high density.

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arXiv1998

The role of the $P_{11}(1710)$ in the $NN{\to}NΣK$ reaction

Using the resonance model, which was successfully applied for the study of the $pp{\to}pΛK^+$ reaction, we investigate $NN{\to}NΣK$ reactions that should provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the $ΣK$ system, as well as the Dalitz plot for the $NN{\to}NΣK$ reaction, provide direct information about the $ΣK$ production mechanism, which can be tested in the near future by experiments at COSY.

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