arXiv subjects

C. D. Roberts

Publications and source records attributed to C. D. Roberts.

At least 127 records · Page 7Linked to original sources

arXiv1999

Diquarks: condensation without bound states

We employ a bispinor gap equation to study superfluidity at nonzero chemical potential: mu .neq. 0, in two- and three-colour QCD. The two-colour theory, QC2D, is an excellent exemplar: the order of truncation of the quark-quark scattering kernel: K, has no qualitative impact, which allows a straightforward elucidation of the effects of mu when the coupling is strong. In rainbow-ladder truncation, diquark bound states appear in the spectrum of the three-colour theory, a defect that is eliminated by an improvement of K. The corrected gap equation describes a superfluid phase that is semi-quantitatively similar to that obtained using the rainbow truncation. A model study suggests that the width of the superfluid gap and the transition point in QC2D provide reliable quantitative estimates of those quantities in QCD.

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arXiv1999

Pair creation: back-reactions and damping

We solve the quantum Vlasov equation for fermions and bosons, incorporating spontaneous pair creation in the presence of back-reactions and collisions. Pair creation is initiated by an external impulse field and the source term is non-Markovian. A simultaneous solution of Maxwell's equation in the presence of feedback yields an internal current and electric field that exhibit plasma oscillations with a period tau_pl. Allowing for collisions, these oscillations are damped on a time-scale, tau_r, determined by the collision frequency. Plasma oscillations cannot affect the early stages of the formation of a quark-gluon plasma unless tau_r >> tau_pl and tau_pl approx. 1/Lambda_QCD approx 1 fm/c.

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arXiv1999

Describing a1 and b1 decays

Two-body pion-radiating and weak decays of light axial-vector mesons and the rho are studied as a phenomenological application of the QCD Dyson-Schwinger equations. Models based on the rainbow-ladder truncation are capable of providing a good description and, in particular, yield the correct sign and magnitude of the a1 -> rho pi and b1 -> omega pi D/S ratios, with no additional mechanism necessary.

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arXiv1999

Survey of heavy-meson observables

We employ a Dyson-Schwinger equation model to effect a unified and uniformly accurate description of light- and heavy-meson observables, which we characterise by heavy-meson leptonic decays, semileptonic heavy-to-heavy and heavy-to-light transitions - B -> D*, D, rho, pi; D -> K*, K, pi, radiative and strong decays - B(s)* -> B(s) gamma; D(s)* -> D(s) gamma, D pi, and the rare B-> K* gamma flavour-changing neutral-current process. We elucidate the heavy-quark limit of these processes and, using a model-independent mass formula valid for all nonsinglet pseudoscalar mesons, demonstrate that their mass rises linearly with the mass of their heaviest constituent. In our numerical calculations we eschew a heavy-quark expansion and rely instead on the observation that the dressed c,b-quark mass functions are well approximated by a constant, interpreted as their constituent-mass: we find M_c=1.32 GeV and M_b=4.65 GeV. The calculated heavy-meson leptonic decay constants and transition form factors are a necessary element in the experimental determination of CKM matrix elements. The results also show that this framework, as employed hitherto, is well able to describe vector meson polarisation observables.

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arXiv1999

Heavy Meson Observables

An accurate and unified description of the non-hadronic electroweak interactions of light- and heavy-mesons is possible when the dressing of quark propagators and the finite size of hadronic bound states is explicitly accounted for.

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arXiv1999

A dynamical, confining model and hot quark stars

We explore the consequences of an equation of state (EOS) obtained in a confining Dyson-Schwinger equation model of QCD for the structure and stability of nonstrange quark stars at finite-T, and compare the results with those obtained using a bag-model EOS. Both models support a temperature profile that varies over the star's volume and the consequences of this are model independent. However, in our model the analogue of the bag pressure is (T,mu)-dependent, which is not the case in the bag model. This is a significant qualitative difference and comparing the results effects a primary goal of elucidating the sensitivity of quark star properties to the form of the EOS.

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arXiv1998

Heavy Meson Observables and Dyson-Schwinger Equations

Dyson-Schwinger equation (DSE) studies show that the b-quark mass-function is approximately constant, and that this is true to a lesser extent for the c-quark. These observations provide the basis for a study of the leptonic and semileptonic decays of heavy pseudoscalar mesons using a ``heavy-quark'' limit of the DSEs. When exact, this limit reduces the number of independent form factors. Semileptonic decays with light mesons in the final state are also accessible because the DSEs provide a description of light-quark propagation characteristics and light-meson structure. A description of B-meson decays is straightforward, however, the study of decays involving the D-meson indicates that c-quark mass-corrections are quantitatively important.

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arXiv1998

Analysis of chiral and thermal susceptibilities

We calculate the chiral and thermal susceptibilities for two confining Dyson-Schwinger equation models of QCD with two light flavours, a quantitative analysis of which yields the critical exponents, beta and delta, that characterise the second-order chiral symmetry restoration transition. The method itself is of interest, minimising the influence of numerical noise in the calculation of the order parameter for chiral symmetry breaking near the critical temperature. For the more realistic of the two models we find: T_c approx. 153 MeV, and the non-mean-field values: beta = 0.46 +/- 0.04, delta = 4.3 +/- 0.3 and 1/(beta delta)= 0.54 +/- 0.05, which we discuss in comparison with the results of other models.

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arXiv1998

Chemical potential dependence of pi and rho properties

Using a confining, Dyson-Schwinger equation model of QCD at finite temperature (T) and chemical potential (mu) we study the behaviour of , m_pi, f_pi and the masses of the rho- and phi-mesons. For each of these quantities there is a necessary anticorrelation between its response to T and its response to mu. (- ) and f_pi decrease with T and increase with mu; m_pi is almost insensitive to T and mu until very near the border of the confinement domain; and the mass of the longitudinal component of the vector mesons increases with T and decreases with mu. At T=0, the rho-meson mass is reduced by approximately 15% at nuclear matter density. These results are a consequence of the necessary momentum-dependence of the dressed-quark self-energy.

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arXiv1997

Heavy- to light-meson transition form factors

Semileptonic heavy -> heavy and heavy -> light meson transitions are studied as a phenomenological application of a heavy-quark limit of Dyson-Schwinger equations. Employing two parameters: E, the difference between the mass of the heavy meson and the effective-mass of the heavy quark; and Lambda, the width of the heavy-meson Bethe-Salpeter amplitude, we calculate f_+(t) for all decays on their entire kinematically accessible t-domain. Our study favours f_B in the range 0.135-0.17 GeV and with E=0.44 GeV and 1/Lambda = 0.14 fm we obtain f_+^{B pi}(0) = 0.46. As a result of neglecting 1/m_c-corrections, we estimate that our calculated values of ρ^2 = 0.87 and f_+^{DK}(0)=0.62 are too low by approximately 15%. However, the bulk of these corrections should cancel in our calculated values of Br(D -> πl nu)/Br(D -> K l nu)=0.13 and f_+^{D pi}(0)/f_+^{DK}(0) = 1.16.

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arXiv1997

Deconfinement at finite chemical potential

In a confining, renormalisable, Dyson-Schwinger equation model of two-flavour QCD we explore the chemical-potential dependence of the dressed-quark propagator, which provides a means of determining the behaviour of the chiral and deconfinement order parameters, and low-energy pion observables. We find coincident, first order deconfinement and chiral symmetry restoration transitions at μ_c = 375 MeV. f_πis insensitive to μuntil μ\approx μ_0 = 0.7 mu_c when it begins to increase rapidly. m_πis weakly dependent on μ, decreasing slowly with μand reaching a minimum 6% less than its μ=0 value at μ=μ_0. In a two-flavour free-quark gas at μ=μ_c the baryon number density would be approximately 3 ρ_0, where ρ_0=0.16 fm^{-3}; while in such a gas at μ_0 the density is ρ_0.

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arXiv1997

Differences between heavy and light quarks

The quark Dyson-Schwinger equation shows that there are distinct differences between light and heavy quarks. The dynamical mass function of the light quarks is characterised by a sharp increase below 1 GeV, whereas the mass function of the heavy quarks is approximately constant in this infrared region. As a consequence, the heavy-meson masses increase linearly with the current-quark masses, whereas the light pseudoscalar meson masses are proportional to the square root of the current-quark masses.

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arXiv1997

Leptonic and semileptonic decays of heavy mesons

A heavy-quark limit of the Dyson-Schwinger equations is introduced and used in an exploratory study of leptonic and semileptonic decays of heavy mesons. The application of this framework to heavy-to-light semileptonic decays, which require a good knowledge of light-quark propagation characteristics and light-meson bound state amplitudes, is illustrated.

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arXiv1997

pi- and K-meson Bethe-Salpeter Amplitudes

Independent of assumptions about the form of the quark-quark scattering kernel, K, we derive the explicit relation between the flavour-nonsinglet pseudoscalar meson Bethe-Salpeter amplitude, Gamma_H, and the dressed-quark propagator in the chiral limit. In addition to a term proportional to gamma_5, Gamma_H necessarily contains qualitatively and quantitatively important terms proportional to gamma_5 gamma.P and gamma_5 gamma.k k.P, where P is the total momentum of the bound state. The axial-vector vertex contains a bound state pole described by Gamma_H, whose residue is the leptonic decay constant for the bound state. The pseudoscalar vertex also contains such a bound state pole and, in the chiral limit, the residue of this pole is related to the vacuum quark condensate. The axial-vector Ward-Takahashi identity relates these pole residues; with the Gell-Mann--Oakes-Renner relation a corollary of this identity. The dominant ultraviolet asymptotic behaviour of the scalar functions in the meson Bethe-Salpeter amplitude is fully determined by the behaviour of the chiral limit quark mass function, and is characteristic of the QCD renormalisation group. The rainbow-ladder Ansatz for K, with a simple model for the dressed-quark-quark interaction, is used to illustrate and elucidate these general results. The model preserves the one-loop renormalisation group structure of QCD. The numerical studies also provide a means of exploring procedures for solving the Bethe-Salpeter equation without a three-dimensional reduction.

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arXiv1997

Dyson-Schwinger Equations and the Application to Hadronic Physics

We review the current status of nonperturbative studies of gauge field theory using the Dyson-Schwinger equation formalism and its application to hadronic physics. We begin with an introduction to the formalism and a discussion of renormalisation in this approach. We then review the current status of studies of Abelian gauge theories [e.g., strong coupling quantum electrodynamics] before turning our attention to the non-Abelian gauge theory of the strong interaction, quantum chromodynamics. We discuss confinement, dynamical chiral symmetry breaking and the application and contribution of these techniques to our understanding of the strong interactions.

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arXiv1997

Pion mass and decay constant

Independent of assumptions about the form of the quark-antiquark scattering kernel we derive the explicit relation between the pion Bethe-Salpeter amplitude, Gamma_pi, and the quark propagator in the chiral limit; Gamma_pi necessarily involves a non-negligible gamma_5 gamma.P term (P is the pion four-momentum). We also obtain exact expressions for the pion decay constant, f_pi, and mass, both of which depend on Gamma_pi; and demonstrate the equivalence between f_pi and the pion Bethe-Salpeter normalisation constant in the chiral limit. We stress the importance of preserving the axial-vector Ward-Takahashi identity in any study of the pion itself, and in any study whose goal is a unified understanding of the properties of the pion and other hadronic bound states.

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arXiv1997

Thermodynamic properties of a simple, confining model

We study the equilibrium thermodynamics of a simple, confining, DSE-model of 2-flavour QCD at finite temperature and chemical potential. The model has two phases: one characterised by confinement and dynamical chiral symmetry breaking; and the other by their absence. The phase boundary is defined by the zero of the vacuum-pressure difference between the confined and deconfined phases. Chiral symmetry restoration and deconfinement are coincident with the transition being of first order, except for $μ=0$, where it is second order. Nonperturbative modifications of the dressed-quark propagator persist into the deconfined domain and lead to a dispersion law modified by a dynamically-generated, momentum-dependent mass-scale. This entails that the Stefan-Boltzmann limit for the bulk thermodynamic quantities is attained only for large values of temperature and chemical potential.

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arXiv1997

Semileptonic decays of heavy mesons

The semileptonic and leptonic decays of heavy mesons are studied as a phenomenological application and exploration of a heavy-quark limit of Dyson-Schwinger equations. The single form factor, $ξ(w)$, which characterises the semileptonic decay in this limit, is calculated and compares well with recent experimental extractions. We obtain a lower bound of 1/3 on the slope-parameter $ρ^2 \equiv -ξ^\prime(1)$, which, in calculations with realistic input, is exceeded by a great deal: agreement with experimental data requiring $ρ^2 \sim 1.2 - 1.6$. The flavour and momentum dependence of the light-quark propagators has observable consequences.

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