arXiv subjects

A. W. Thomas

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

At least 253 records · Page 14Linked to original sources

arXiv2003

Neutrino Physics Without Oscillations

There is a tremendous potential for neutrinos to yield valuable new information about strongly interacting systems. Here we provide a taste of this potential, beginning with the existence of a rigorous sum rule for the proton and neutron spin structure functions based on the measurement of the flavor singlet axial charge of the nucleon. We also comment on the NuTeV report of a 3$σ$ deviation of the value of $\sin^2 θ_W$ measured in neutrino (and anti-neutrino) scattering from that expected within the Standard Model.

hep-ex↗

arXiv2003

Simulation Methods of the Processes $B \to π^+ π^- V$ including $ρ^0-ω$ Mixing Effects

Simulation methods for the decays $ B \to π^+ π^- V$, where $V$ is a $1^{--}$ vector-meson, are presented in detail. Emphasis is put on the use of the helicity formalism and the use of effective Lagrangians. We show the importance of ${ρ^{0}}-ω$ mixing in enhancing the direct $CP$ violation (DCPV) when the pion-pion invariant mass is near the mass of the $ω$.

hep-ph↗

arXiv2003

Direct CP Violation in $B \to π^{+} π^{-} π$: Determination of $α$ without discrete ambiguity

Direct CP violation in the hadronic decays $\Bar{B}^{0} \to π^{+}π^{-} π^{0}$ is investigated near the peak of the $ρ^{0}$ taking into account the effect of $ρ- ω$ mixing. Branching ratios for processes $B^{\pm,0} \to ρ^{\pm,0}π^{\pm,0}$ and $B^{-} \to ωπ^{-}$ are calculated as well. We find that the CP violating asymmetry is strongly dependent on the CKM matrix elements. For a fixed $N_{c}^{eff}$, the CP violating asymmetry, $a$, has a maximum of order -40% to -70% for $\Bar{B}^{0} \to ρ^{0}(ω) π^{0}$ when the invariant mass of the $π^{+}π^{-}$ pair is in the vicinity of the $ω$ resonance. The sensitivity of the asymmetry to $N_{c}^{eff}$ is small in that case. Moreover, we find that in the range of $N_{c}^{eff}$ which is allowed by the most recent experimental branching ratios from the BABAR, BELLE and CLEO Collaborations, the sign of $\sin δ$ is always positive. Thus, a measurement of direct CP violation in decays $\Bar{B}^{0} \to π^{+}π^{-} π^{0}$ would remove the mod$(π)$ ambiguity in the determination of the CP violating phase angle $α$.

hep-ph↗

arXiv2003

Indication of anomalous exchange in exclusive charmonium photoproduction

We find that available data on $J/Ψ$ photoproduction at $|t|{\ge}$1 GeV are energy independent over the range $22{\le}\sqrt{s}{\le}170$ GeV and show a very soft $t$ dependence. Through a systematic analysis of the data we detect a new trajectory, which manifests itself at threshold and is energy independent. The trajectory couples to the axial form factor of the nucleon and can reproduce the $J/Ψ$ exclusive photoproduction data at large -$t$.

nucl-th↗

arXiv2003

Extraction of parton distributions from lattice QCD

We review the calculation of moments of both the polarized and unpolarized parton distribution functions of the nucleon in lattice QCD, and in particular their extrapolation to the physical region. We also discuss the reconstruction of the x dependence of the valence quark distributions in the nucleon from a finite number of lattice moments.

hep-lat↗

arXiv2003

Chiral Structure in Baryon Magnetic Moments

We highlight recent advances in finite-range regularised chiral effective field theory. Application to baryon magnetic moments show an unambiguous signal of quenched chiral physics in FLIC fermion simulations.

hep-lat↗

arXiv2003

Matching functions for heavy particles

We introduce matching functions as a means of summing heavy-quark logarithms to any order. Our analysis is based on Witten's approach, where heavy quarks are decoupled one at a time in a mass-independent renormalization scheme. The outcome is a generalization of the matching conditions of Bernreuther and Wetzel: we show how to derive closed formulas for summed logarithms to any order, and present explicit expressions for leading order and next-to-leading order contributions. The decoupling of heavy quarks in theories lacking asymptotic freedom is also considered.

hep-ph↗

arXiv2003

Physical Baryon Resonance Spectroscopy from Lattice QCD

We complement recent advances in the calculation of the masses of excited baryons in quenched lattice QCD with finite-range regulated chiral effective field theory enabling contact with the physical quark mass region. We examine the P-wave contributions to the low-lying nucleon and delta resonances.

nucl-th↗

arXiv2003

Observing Chiral Nonanalytic Behavior with FLIC Fermions

First results from lattice QCD revealing the chiral nonanalytic behavior of nucleon and Delta baryon magnetic moments are presented. Numerical simulations in the light quark mass regime employing the nonperturbatively O(a)-improved conserved vector current are enabled via FLIC fermions. Quenched chiral perturbation theory for the nucleon and Delta magnetic moments is derived to next to next to leading nonanalytic order. Numerical simulation results for the proton and Delta baryon magnetic moments in quenched QCD reveal dramatic signatures of the quenched meson cloud, which are in accord with the predictions of quenched chiral perturbation theory.

nucl-th↗

arXiv2003

Nucleon electromagnetic form factors from lattice QCD

It is imperative that lattice QCD serve to develop our understanding of hadron structure and, where possible, to guide the interpretation of experimental data. There is now a great deal of effort directed at the calculation of the electroweak form factors of the nucleon, where for example, measurements at Jefferson Laboratory have recently revealed surprising behaviour in the ratio $G_E/G_M$. While, for the present, calculations within the framework of lattice QCD are limited to relatively large quark mass, there has been considerable progress in our understanding of how to extrapolate to the chiral limit. Here we report the results of the application of these techniques to the most recent form factor data from the QCDSF Collaboration. The level of agreement with all of the form factors, for $Q^2$ below 1 GeV$^2$, is already impressive.

hep-lat↗

arXiv2003

QCD Factorization in $B$ Decays into $ρπ$

Based on the QCD factorization approach we analyse the branching ratios for the channel $B \to ρπ$. From the comparisons with experimental data provided by CLEO, BELLE and BABAR we constrain the form factor $F^{B \to π}(m_ρ^{2})$ and propose boundaries for this form factor depending on the CKM matrix element parameters $ρ$ and $η$.

hep-ph↗

arXiv2003

Direct CP Violation In B --> pi^+ pi^- V With rho^0--omega Mixing Effects: Phenomenological Approach

We present a detailed study of direct CP violation and branching ratios in the channels $B^{0,\pm} \to π^{+}π^{-} V^{0,\pm}$, where $V$ is a vector meson ($K^{* 0,\pm}$ or $ρ^{\pm}$). Emphasis is placed upon the important role played by ${ρ^{0}}-ω$ mixing effects in the estimation of the CP-violating asymmetry parameter, $a_{cp}$, associated with the difference of $B$ and $\bar B$ decay amplitudes. A thorough study of the helicity amplitudes is presented as a function of the pion-pion invariant mass. All of the calculations and simulations considered correspond to channels which will be analyzed at the LHCb facility.

hep-ph↗

arXiv2003

Parton distribution functions in the pion from lattice QCD

We analyze the moments of parton distribution functions in the pion calculated in lattice QCD, paying particular attention to their chiral extrapolation. Using the lowest three non-trivial moments calculated on the lattice, we assess the accuracy with which the x dependence of both the valence and sea quark distributions in the pion can be extracted. The resulting valence quark distributions at the physical pion mass are in fair agreement with existing Drell-Yan data, but the statistical errors are such that one cannot yet confirm (or rule out) the large-x behavior expected from hadron helicity conservation in perturbative QCD. However, one can expect that the next generation of calculations in lattice QCD will allow one to extract parton distributions with a level of accuracy comparable with current experiments.

hep-lat↗

arXiv2003

Deep inelastic scattering from A=3 nuclei and the neutron structure function

We present a comprehensive analysis of deep inelastic scattering from He-3 and H-3, focusing in particular on the extraction of the free neutron structure function, F_2^n. Nuclear corrections are shown to cancel to within 1-2% for the isospin-weighted ratio of He-3 to H-3 structure functions, which leads to more than an order of magnitude improvement in the current uncertainty on the neutron to proton ratio F_2^n/F_2^p at large x. Theoretical uncertainties originating from the nuclear wave function, including possible non-nucleonic components, are evaluated. Measurement of the He-3 and H-3 structure functions will, in addition, determine the magnitude of the EMC effect in all A < 4 nuclei.

nucl-th↗

arXiv2003

Charge Symmetry Violation Corrections to Determination of the Weinberg Angle in Neutrino Reactions

We show that the correction to the Paschos-Wolfenstein relation associated with charge symmetry violation in the valence quark distributions is essentially model independent. It is proportional to a ratio of quark momenta that is independent of Q^2. This result provides a natural explanation of the surprisingly good agreement found between our earlier estimates within several different models. When applied to the recent NuTeV measurement, this effect significantly reduces the discrepancy with other determinations of the Weinberg angle.

hep-ph↗

arXiv2003

Charge Symmetry Violating Contributions to Neutrino Reactions

The NuTeV group has measured charged and neutral current reactions for neutrinos on iron targets. Ratios of these cross sections provide an independent measurement of the Weinberg angle. The NuTeV value for sin^2 theta_W is three standard deviations larger than the value measured in other electroweak processes. By reviewing theoretical estimates of parton charge symmetry violation (CSV), we study CSV contributions to the NuTeV measurement. We conclude that charge symmetry violating effects should remove roughly 30% of the discrepancy between the NuTeV result and other determinations of sin^2 theta_W.

hep-ph↗

arXiv2003

Meson-Nucleon Vertex Form Factors at Finite Temperature Using a Soft Pion Form Factor

The temperature and density dependence of the meson-nucleon vertex form factors is studied in the framework of thermofield dynamics. Results are obtained for two rather different nucleon-nucleon potentials: the usual Bonn potential and the variation with a softer $πNN$ form factor, due to Holinde and Thomas. In general, the results show only a modest degree of sensitivity to the choice of interaction.

nucl-th↗

arXiv2003

Delta Baryon Magnetic Moments From Lattice QCD

Theoretical predictions for the magnetic moments of the physical Delta baryons are extracted from lattice QCD calculations. We utilize finite-range regulated effective field theory that is constructed to have the correct Dirac moment mass dependence in the region where the up and down quark masses are heavy. Of particular interest is the chiral nonanalytic behaviour encountered as the nucleon-pion decay channel opens. We find a Delta^++ magnetic moment (at the Delta pole) of 4.99 \pm 0.56 μ_N. This result is within the Particle Data Group range of 3.7-7.5 μ_N and compares well with the experimental result of Bosshard et al. of 4.52 \pm 0.51 \pm 0.45 μ_N. The interplay between the different pion-loop contributions to the Delta^+ magnetic moment leads to the surprising result that the proton moment may exceed that of the Delta^+, contrary to conventional expectations.

hep-lat↗