arXiv subjects

P. C. Tandy

Publications and source records attributed to P. C. Tandy.

34 records · Page 2Linked to original sources

arXiv2001

Finite T meson correlations and quark deconfinement

Finite temperature spatial $\bar q q$ correlation modes in the $π$ and $ρ$ channels are studied with the rainbow-ladder truncated quark Dyson-Schwinger equation and Bethe-Salpeter equation in the Matsubara formalism. To retain the finite range of the effective interaction while facilitating summation over fermion Matsubara modes necessary to ensure continuity at {T=0}, a separable kernel is used. The model is fixed by {T=0} properties and it implements dynamical chiral symmetry breaking and quark confinement. Above and below the deconfinement and chiral restoration transitions, we study $M_π(T)$, $f_π(T)$ and the 3-space transverse and longitudinal masses $M_ρ^T(T)$ and $M_ρ^L(T)$. For the $ρ$ mode we also study the strong and electromagnetic partial widths to determine the $T$-dependence that is generated by the quark content. The $ρ\to ππ$ width decreases sharply just above the chiral restoration transition leaving the $ρ$ with only its narrow {$e^+ e^-$} width. We discuss improvements needed by this model, especially in regard to the high $T$ behavior of the masses where we compare to lattice QCD simulations.

nucl-th↗

arXiv2000

The $π$, $K^+$, and $K^0$ electromagnetic form factors

The rainbow truncation of the quark Dyson-Schwinger equation is combined with the ladder Bethe-Salpeter equation for the meson amplitudes and the dressed quark-photon vertex in a self-consistent Poincaré-invariant study of the pion and kaon electromagnetic form factors in impulse approximation. We demonstrate explicitly that the current is conserved in this approach and that the obtained results are independent of the momentum partitioning in the Bethe-Salpeter amplitudes. With model gluon parameters previously fixed by the condensate, the pion mass and decay constant, and the kaon mass, the charge radii and spacelike form factors are found to be in good agreement with the experimental data.

nucl-th↗

arXiv2000

Dyson-Schwinger Equation Approach to the QCD Deconfinement Transition and J/Psi Dissociation

We consider an extension of the finite-temperature Dyson-Schwinger equation (DSE) approach to heavy mesons and quarkonia and apply it to calculate the cross section for the J/Psi breakup reaction J/Psi + pi --> D + D. We study the effects of chiral symmetry restoration in the light quark sector on this process and obtain a critical enhancement of the reaction rate at the chiral/ deconfinement transition. Implications for the kinetics of charmonium production in ultrarelativistic heavy-ion collisions are discussed with particular emphasis on the recently observed anomalous J/Psi suppression as a possible signal for quark matter formation.

hep-ph↗

arXiv2000

T-dependence of pseudoscalar and scalar correlations

Inhomogeneous pseudoscalar and scalar Bethe-Salpeter equations solved using a renormalisation-group-improved rainbow-ladder truncation exhibit bound state poles below and above T_c, the critical temperature for chiral symmetry restoration. Above T_c the bound state amplitudes are identical, as are the positions and residues of the pseudoscalar and scalar poles in the vertices. In the chiral limit the pi^0 -> gamma gamma coupling vanishes at T_c, as do f_pi, m_sigma, g_{sigma pi pi}. For light current-quark masses the 2-pi decay channel of the isoscalar-scalar meson remains open until very near T_c, and the widths of the dominant pion decay modes remain significant in the vicinity of the crossover.

nucl-th↗

arXiv1999

The Quark-Photon Vertex and the Pion Charge Radius

The rainbow truncation of the quark Dyson-Schwinger equation is combined with the ladder Bethe-Salpeter equation for the dressed quark-photon vertex to study the low-momentum behavior of the pion electromagnetic form factor. With model gluon parameters previously fixed by the pion mass and decay constant, the pion charge radius $r_π$ is found to be in excellent agreement with the data. When the often-used Ball-Chiu Ansatz is used to construct the quark-photon vertex directly from the quark propagator, less than half of $r_π^2$ is generated. The remainder of $r^2_π$ is seen to be attributable to the presence of the $ρ$-pole in the solution of the ladder Bethe-Salpeter equation.

nucl-th↗

arXiv1999

The quark-photon vertex and meson electromagnetic form factors

The ladder Bethe-Salpeter solution for the dressed photon-quark vertex is used to study the low-momentum behavior of the pion electromagnetic and the $γ^\star π^0 γ$ transition form factors. With model parameters previously fixed by light meson masses and decay constants, the low-momentum slope of both form factors is in excellent agreement with the data. In comparison, the often-used Ball-Chiu Ansatz for the vertex is found to be deficient; less than half of the obtained $r_π^2$ is generated by that Ansatz while the remainder of the charge radius could be attributed to the tail of the $ρ$ resonance.

nucl-th↗

arXiv1999

Mesonic correlations and quark deconfinement

A simple confining separable interaction Ansatz for the rainbow-ladder truncated QCD Dyson-Schwinger equations is used to implement chiral restoration and quark deconfinement in a study of $\bar q q$ meson states at finite temperature. The model is fixed at T=0 by reproducing selected pi and rho properties. Deconfinement and chiral restoration are found to both occur at T_c=146 MeV. In the pion sector, we investigate M_pi(T) and f_pi(T) along with the exact QCD mass relation and the GMOR relation. For the vector mode, we investigate the 3-space transverse and longitudinal masses M_rho^T(T) and M_ρ^L(T), along with the widthfor the decay rho^0 --> e^+e^-. The equation of state (EOS) for the model is investigated in the T-mu plane, a tri-critical point is identified and the relationship to a bag model is discussed. The deconfinement transition in rapidly rotating neutron stars is considered and a new signal from the pulsar timing in binary systems with mass accretion is suggested. The model is used to discuss the possibility of a superconducting quark matter phase.

nucl-th↗

arXiv1998

Quark deconfinement and meson properties at finite temperature

A simple confining separable interaction Ansatz for the rainbow-ladder truncated QCD Dyson-Schwinger equations is used to study quark deconfinement and meson states at finite temperature. The model is fixed at T=0 to implement quark confinement while preserving the Goldstone mechanism for the $π$. Within the Matsubara formalism, a very slow temperature dependence is found for the $π$ and $ρ$ meson masses $M_H(T)$ until near the deconfinement temperature $T_c=143$ MeV. Related to rapid decrease of the dynamically-generated quark mass function for $T>T_c$, this model produces $π$ and $ρ$ masses that rise significantly and are better interpreted as spatial screening masses. The $T$-dependent screening mass defect $ΔM_H = 2πT-M_H(T)$ is compared to results of lattice gauge theory simulations and also to those of an infrared dominant analytic model.

hep-ph↗

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.

nucl-th↗

arXiv1997

Hadron Physics from the Global Color Model of QCD

We review recent progress in modeling the quark-gluon content of mesons and their low-energy interactions through the Global Color Model field theory. An emphasis is placed on techniques that are shared with the approach based on truncations of the Dyson-Schwinger equations of QCD. In contrast to most other field theory models for QCD degrees of freedom in hadron physics, this approach directly deals with the derived intrinsic space-time extent of the meson modes in their role as field variables and can accommodate confinement as well as dynamical breaking of chiral symmetry. Various theoretical techniques and approximations found useful in this approach are described. Selected applications reviewed here include the properties and interactions of the Goldstone bosons, interaction vertex functions, low energy chiral observables, electromagnetic interactions and form factors, and transition form factors. Some initial considerations of Vector Meson Dominance and pion loop processes are discussed.

nucl-th↗

arXiv1996

Pion Loop Contribution to Rho-Omega Mixing and Mass Splitting

We study the self-energy amplitudes for the rho-omega meson system produced by an effective field theory model at the quark level characterized by a finite range quark-quark interaction and an isospin breaking bare quark mass difference $m_u - m_d$. Dynamical effects associated with confinement and dressing of quarks and the finite size of the produced $\bar{q}q$ meson modes are treated. Parameters of this approach, previously constrained by soft pion physics, are here supplemented in a minimal way to reproduce $m_ω$ and $g_{ρππ}$. Calculations are carried out up to the one pion loop level and the predictions for rho-omega mixing, mass splitting, the rho width and the symmetry breaking omega-pi-pi coupling constant and form factor are presented. The contribution of the direct $ω\rightarrow ππ$ process relative to the $ω\rightarrow ρ\rightarrow ππ$ process is discussed within this model.

nucl-th↗

arXiv1996

Meson Transition Form Factors From A QCD Model Field Theory

We discuss form factors and coupling constants for the $γ^* π^0 γ$, $ρππ$ and $γπρ$ interactions generated by a model field theory that produces finite size $\bar{q}q$ meson modes. The approach implements dressing of the vertices and propagators consistent with dynamical chiral symmetry breaking, gauge invariance, quark confinement and perturbative QCD.

nucl-th↗

arXiv1996

Ground-state Spectrum of Light-quark Mesons

A confining, Goldstone theorem preserving, separable Ansatz for the ladder kernel of the two-body Bethe-Salpeter equation is constructed from phenomenologically efficacious $u$, $d$ and $s$ dressed-quark propagators. The simplicity of the approach is its merit. It provides a good description of the ground-state isovector-pseudoscalar, vector and axial-vector meson spectrum; facilitates an exploration of the relative importance of various components of the two-body Bethe-Salpeter amplitudes, showing that sub-leading Dirac components are quantitatively important in the isovector-pseudoscalar meson channels; and allows a scrutiny of the domain of applicability of ladder truncation studies. A colour-antitriplet diquark spectrum is obtained. Shortcomings of separable Ansätze and the ladder kernel are highlighted.

nucl-th↗

arXiv1994

Off-Shell Axial Anomaly via the γ^* π^0 -> γTransition

The $γ^* π^0 \rightarrow γ$ form factor, including the extension off the pion mass-shell, is obtained from a generalized impulse approximation within a QCD-based model field theory known to provide an excellent description of the pion charge form factor. This approach implements dressing of the vertex functions and propagators consistent with dynamical chiral symmetry breaking, gauge invariance, quark confinement and perturbative QCD. Soft nonperturbative behavior, dictated by the axial anomaly, is found to evolve to the perturbative QCD limit only for \mbox{$Q^2 \geq 20~{\rm GeV}^2$}.

hep-ph↗

arXiv1994

Charge symmetry breaking via rho-omega mixing from model quark-gluon dynamics

The quark-loop contribution to the $ρ^0-ω$ mixing self-energy function is calculated using a phenomenologically successful QCD-based model field theory in which the $ρ^0$ and $ω$ mesons are composite $\bar{q}q$ bound states. In this calculation the dressed quark propagator, obtained from a model Dyson-Schwinger equation, is confining. In contrast to previous studies, the meson-$\bar{q}q$ vertex functions are characterised by a strength and range determined by the dynamics of the model; and the calculated off-mass-shell behaviour of the mixing amplitude includes the contribution from the calculated diagonal meson self-energies. The mixing amplitude is shown to be very sensitive to the small isovector component of dynamical chiral symmetry breaking. The spacelike quark-loop mixing-amplitude generates an insignificant charge symmetry breaking nuclear force.

hep-ph↗

arXiv1993

Gauge Invariance and the Electromagnetic Current of Composite Pions

The Global Color-symmetry Model of QCD is extended to deal with a background electromagnetic field and the associated conserved current is identified for composite $\bar{q}q$ pion modes of the model. Although the analysis is limited to tree level in the bilocal fields that bosonize the model, the identified photon-pion vertex produces the charge form factor associated with ladder Bethe-Salpeter pion amplitudes. A Ward-Takahashi identity for this vertex is derived in terms of the effective inverse propagator for the equivalent local pion field and the intrinsic ladder Bethe-Salpeter amplitudes. This identity is then used to illustrate gauge invariance by showing that identical vertex information is produced from the gauge change of the free action once proper account is taken of the gauge transformation properties of the bilocal pion fields. Comments are made on the location of the vector dominance mechanism in this treatment.

nucl-th↗