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A. W. Thomas

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

At least 325 records · Page 18Linked to original sources
arXiv2000

Lattice QCD Calculations of Hadron Structure: Constituent Quarks and Chiral Symmetry

New data from parity-violating experiments on the deuteron now allow isolation of the strange-quark contribution to the nucleon magnetic moment, G_M^s(0), without the uncertainty surrounding the anapole moment of the nucleon. Still, best estimates place G_M^s(0) > 0. It is illustrated how this experimental result challenges the very cornerstone of the constituent quark model. The chiral physics giving rise to G_M^s(0) \sim 0 is illustrated.

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arXiv2000

HiX2000: Theory Summary

We summarize the theoretical consensus of the HiX workshop concerning those measurements which seem most appropriate for inclusion in the ``white paper'' being prepared to justify the proposed 12 GeV upgrade at Jefferson Lab. The criteria for inclusion are that such measurements should be decisive, not possible elsewhere and should answer crucial physics questions with broad implications.

hep-ex↗
arXiv2000

Hadron structure after 25 years of QCD

We briefly review the status of our understanding of hadron structure based on QCD. This includes the role of symmetries, especially chiral symmetry, and the insights provided by lattice QCD. The main focus is on baryon structure and especially the nucleon, but this cannot be treated realistically without reference to spectroscopy. Our aim is to highlight recent insights and promising directions for future work.

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arXiv2000

Novel features of $J/Ψ$ dissociation in matter

We make a detailed study of the effect that the recently predicted modification of the in-medium masses of charmed mesons would have on $J/Ψ$ dissociation on pion and $ρ$-meson comovers in relativistic heavy ion collisions. We find a substantial dependence of the $J/Ψ$ absorption rates on the density and temperature of the nuclear matter. This suggests that a quantitative analysis of $J/Ψ$ dissociation in nucleus nucleus collisions should include the effects of the modification of meson masses in dense matter.

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arXiv2000

On the $Λ$ to $Σ^0$ ratio from proton-proton collisions

We compare the recent COSY data on the total $pp{\to}pΛ{K^+}$ and $pp{\to}pΣ^0{K^+}$ cross sections with the predictions from two boson exchange models that either are based solely on $π$ and $K$ exchange or include $π$, $η$ and $ρ$ exchange as well as the virtual excitation of intermediate baryon resonances. Both models are found to reproduce the data after the inclusion of final state hyperon-nucleon interactions. Thus, within the experimental uncertainties, both models also roughly reproduce the strong dependence of the $Λ$ to $Σ^0$ ratio as a function of the excess energy near threshold, as well as at higher energies.

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arXiv2000

Incorporating Chiral Symmetry in Extrapolations of Octet Baryon Magnetic Moments

We explore methods of extrapolating lattice calculations of hadronic observables to the physical regime, while respecting the constraints of chiral symmetry and heavy quark effective theory. In particular, we extrapolate lattice results for magnetic moments of the spin-1/2 baryon octet to the physical pion mass and compare with experimental measurements. The success previously reported for extrapolations of the nucleon magnetic moments carries over to the Sigma baryons. A study of the residual discrepancies in the Xi baryon moments suggests that it is important to have new simulation data with a more realistic strange quark mass.

hep-lat↗
arXiv2000

Chiral aspects of hadron structure

Chiral loop corrections for hadronic properties are considered in a constituent quark model. It is emphasized that the correct implementation of such corrections requires a sum over intermediate hadronic states. The leading non-analytic corrections are very important for baryon magnetic moments and explain the failure of the sum rule $(μ_{Σ^{+}}+2μ_{Σ^{-}})/μ_Λ=-1$ predicted by the constituent quark model.

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arXiv2000

Comment on ``Parton distributions, d/u, and higher twist effects at high x''

In a recent paper Yang and Bodek extracted the free neutron structure function at large x by extrapolating the density dependence of the nuclear EMC ratios to the deuteron. We clarify why this approach is ill-defined for light nuclei, and introduces a large theoretical bias into the extraction of F_2n at large x.

hep-ex↗
arXiv2000

Lambda Polarization in Polarized Proton-Proton Collisions at RHIC

We discuss Lambda polarization in semi-inclusive proton-proton collisions, with one of the protons longitudinally polarized. The hyperfine interaction responsible for the $Δ$-$N$ and $Σ$-$Λ$ mass splittings gives rise to flavor asymmetric fragmentation functions and to sizable polarized non-strange fragmentation functions. We predict large positive Lambda polarization in polarized proton-proton collisions at large rapidities of the produced Lambda, while other models, based on SU(3) flavor symmetric fragmentation functions, predict zero or negative Lambda polarization. The effect of $Σ^0$ and $Σ^*$ decays is also discussed. Forthcoming experiments at RHIC will be able to differentiate between these predictions.

hep-ph↗
arXiv2000

How precisely can we determine the pion-nucleon coupling constant from the isovector GMO sum rule?

The isovector GMO sum rule for zero energy forward pion-nucleon scattering is critically studied to obtain the charged pion-nucleon coupling constant using the precise negatively charged pion-proton and pion-deuteron scattering lengths deduced recently from pionic atom experiments. This direct determination leads to a pseudoscalar charged pion-nucleon coupling constant of 14.23 +- 0.09 (statistic) +- 0.17 (systematic). We obtain also accurate values for the pion-nucleon scattering lengths.

hep-ph↗
arXiv2000

Lattice QCD Calculations of the Sigma Commutator

As a direct source of information on chiral symmetry breaking within QCD, the sigma commutator is of considerable importance. With recent advances in the calculation of hadron masses within full QCD it is of interest to see whether the sigma commutator can be calculated directly from the dependence of the nucleon mass on the input quark mass. We show that provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses one can indeed obtain a fairly reliable determination of the sigma commutator using present lattice data. Within two-flavour, dynamical-fermion QCD the value obtained lies in the range 45 to 55 MeV.

hep-lat↗
arXiv1999

1/m_Q Corrections to the Bethe-Salpeter Equation for Λ_{Q} in the Diquark Picture

Corrections of order 1/m_Q (Q=b or c) to the Bethe-Salpeter (B-S) equation for Λ_Q are analyzed on the assumption that the heavy baryon Λ_Q is composed of a heavy quark and a scalar, light diquark. It is found that in addition to the one B-S scalar function in the limit m_Q -> \infty, two more scalar functions are needed at the order 1/m_Q. These can be related to the B-S scalar function in the leading order. The six form factors for the weak transition Λ_b -> Λ_c are expressed in terms of these wave functions and the results are consistent with heavy quark effective theory to order 1/m_Q. Assuming the kernel for the B-S equation in the limit m_Q -> \infty to consist of a scalar confinement term and a one-gluon-exchange term we obtain numerical solutions for the B-S wave functions, and hence for the $Λ_b \to Λ_c$ form factors to order 1/m_Q. Predictions are given for the differential and total decay widths for Λ_b -> Λ_c l \barν, and also for the nonleptonic decay widths for Λ_b -> Λ_c plus a pseudoscalar or vector meson, with QCD corrections being also included.

hep-ph↗
arXiv1999

Baryon Masses from Lattice QCD: Beyond the Perturbative Chiral Regime

Consideration of the analytic properties of pion-induced baryon self-energies leads to new functional forms for the extrapolation of light baryon masses. These functional forms reproduce the leading non-analytic behavior of chiral perturbation theory, the correct non-analytic behavior at the $N π$ threshold and the appropriate heavy-quark limit. They involve only three unknown parameters, which may be obtained by fitting to lattice data. Recent dynamical fermion results from CP-PACS and UKQCD are extrapolated using these new functional forms. We also use these functions to probe the limit of applicability of chiral perturbation theory to the extrapolation of lattice QCD results.

hep-lat↗
arXiv1999

Scaling law for the electromagnetic form factors of the proton

The violation of the scaling law for the electric and magnetic form factors of the proton are examined within the cloudy bag model. The suppression of the ratio of the electric and magnetic form factors is natural in the bag model. The pion cloud plays a moderate role in understanding the recent data from TJNAF.

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arXiv1999

On Studying Charm in Nuclei through Antiproton Annihilation

We examine the production of open charm in antiproton annihilation on finite nuclei. The enhancement of the subthreshold production cross section, even in a nucleus as light as carbon, should provide a clean signature of the reduction in the masses of these mesons in-medium. We also show that a careful analysis of the $D^+$ and $D^-$ spectra can yield important information on the cross section for $D^{\pm}N$ scattering.

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arXiv1999

On the $ξ$-Distribution of Inclusively Produced Particles in $e^+e^-$ Annihilation

We discuss the momentum distributions of inclusively produced particles in $e^+e^-$ annihilation. We show that the dependence of the position of the maxima of the $ξ=\ln(1/z)$ spectra on the mass of the produced particles follows naturally from the general definition of fragmentation functions when energy-momentum conservation is correctly incorporated.

hep-ph↗
arXiv1999

Charge symmetry breaking in mirror nuclei from quarks

The binding energy differences of the valence proton and neutron of the mirror nuclei, $^{15}$O -- $^{15}$N, $^{17}$F -- $^{17}$O, $^{39}$Ca -- $^{39}$K and $^{41}$Sc -- $^{41}$Ca, are calculated using the quark-meson coupling (QMC) model. The calculation involves nuclear structure and shell effects explicitly. It is shown that binding energy differences of a few hundred keV arise from the strong interaction, even after subtracting all electromagnetic corrections. The origin of these differences may be ascribed to the charge symmetry breaking effects set in the strong interaction through the u and d current quark mass difference.

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