arXiv subjects

B. D. Keister

Publications and source records attributed to B. D. Keister.

14 recordsLinked to original sources

arXiv2015

Numerical and Physical Stability of Supernova Neutrino Flavor Evolution

This paper examines neutrino flavor evolution outside a supernova neutrinosphere using a one-dimensional model that retains the non-linear nature of neutrino-neutrino interactions as well as some aspects of the full geometry. In some limiting cases analytic results can be obtained that display different behavior from their counterparts in (linear) solar neutrino flavor evolution. For more general cases, numerical solutions require extended numerical algorithms to achieve stable solutions, and these solutions exhibit standard chaotic behavior.

astro-ph.HE↗

arXiv2012

Cluster properties in Poincare invariant quantum mechanics

Using a simple model we provide a quantitative study of the size of the corrections needed to restore cluster properties to the construction of Poincare invariant dynamical models with kinematic spins, first provided by B. Bakamjian and L. H. Thomas. Our model calculations suggest that these corrections are too small to have a quantitative impact on nuclear physics observables calculated using models with meson and nucleon degrees of freedom.

nucl-th↗

arXiv2012

Model tests of cluster separability in relativistic quantum mechanics

A relativistically invariant quantum theory first advanced by Bakamjian and Thomas has proven very useful in modeling few-body systems. For three particles or more, this approach is known formally to fail the constraint of cluster separability, whereby symmetries and conservation laws that hold for a system of particles also hold for isolated subsystems. Cluster separability can be restored by means of a recursive construction using unitary transformations, but implementation is difficult in practice, and the quantitative extent to which the Bakamjian-Thomas approach violates cluster separability has never been tested. This paper provides such a test by means of a model of a scalar probe in a three-particle system for which (1) it is simple enough that there is a straightforward solution that satisfies Poincaré invariance and cluster separability, and (2) one can also apply the Bakamjian-Thomas approach. The difference between these calculations provides a measure of the size of the corrections from the Sokolov construction that are needed to restore cluster properties. Our estimates suggest that, in models based on nucleon degrees of freedom, the corrections that restore cluster properties are too small to effect calculations of observables.

nucl-th↗

arXiv2010

Relativistic quantum theories and neutrino oscillations

Neutrino oscillations are examined under the broad requirements of Poincaré-invariant scattering theory in an S-matrix formulation. This approach can be consistently applied to theories with either field or particle degrees of freedom. The goal of this paper is to use this general framework to identify all of the unique physical properties of this problem that lead to a simple oscillation formula. We discuss what is in principle observable, and how many factors that are important in principle end up being negligible in practice.

hep-ph↗

arXiv2005

Quantitative Relativistic Effects in the Three-Nucleon Problem

The quantitative impact of the requirement of relativistic invariance in the three-nucleon problem is examined within the framework of Poincaré invariant quantum mechanics. In the case of the bound state, and for a wide variety of model implementations and reasonable interactions, most of the quantitative effects come from kinematic factors that can easily be incorporated within a non-relativistic momentum-space three-body code.

nucl-th↗

arXiv1997

Relativity and the Minimum Slope of the Isgur-Wise Function

Sum rules based upon heavy quark effective theory indicate that the Isgur-Wise function $ξ(w)$ has a minimum slope as w approaches 1, which is zero for light degrees of freedom with zero spin and 1/4 for light spin 1/2. Quark-model studies reveal sources for a minimum slope from a variety of relativistic effects. In this paper the origins of the minimum slope in the sum-rule and quark-model approaches are compared by considering hadrons with arbitrary light spin. In both approaches the minimum slope increases with the light spin, but there appears to be no detailed correspondence between the quark-model and sum-rule approaches.

hep-ph↗

arXiv1997

Baryon Current Matrix Elements in a Relativistic Quark Model

Current matrix elements and observables for electro- and photo-excitation of baryons from the nucleon are studied in a light-front framework. Relativistic effects are examined by comparison to a nonrelativistic model and can typically be of order 20-25%, but can be larger for certain matrix elements, such as radial transitions conventionally used to describe the Roper resonance. A systematic study shows that the violation of rotational covariance of the baryon transition matrix elements stemming from the use of one-body currents is generally small.

nucl-th↗

arXiv1996

Baryon Magnetic Moments in a Relativistic Quark Model

Magnetic moments of baryons in the ground-state octet and decuplet are calculated in a light-front framework. We investigate the effects of quark mass variation both in the current operator and in the wavefunctions. A simple fit uses single oscillator wavefunctions for the baryons and allows the three flavors of quark to have nonzero anomalous magnetic moments. We find a good fit to the data without allowing for strange quark contributions to the nucleon moments. A slightly better fit is obtained by allowing for explicit SU(3)_f breaking in the wavefunctions through a simple mechanism. The predictions for magnetic moments in our relativistic model are also much less sensitive to the values chosen for the constituent quark masses than those of nonrelativistic models. Relativistic effects can be of order 20% in general, and can alter familiar relationships between the moments based on SU(3)_f and a nonrelativistic treatment of spin.

nucl-th↗

arXiv1996

Useful Bases for Problems in Nuclear and Particle Physics

A set of exactly computable orthonormal basis functions that are useful in computations involving constituent quarks is presented. These basis functions are distinguished by the property that they fall off algebraically in momentum space and can be exactly Fourier-Bessel transformed. The configuration space functions are associated Laguerre polynomials multiplied by an exponential weight, and their Fourier-Bessel transforms can be expressed in terms of Jacobi polynomials in $Λ^2/(k^2 + Λ^2)$. A simple model of a meson containing a confined quark-antiquark pair shows that this basis is much better at describing the high-momentum properties of the wave function than the harmonic-oscillator basis.

nucl-th↗

arXiv1996

Relativistic Dynamics and the Deuteron Axial Current

The deuteron axial current is sensitive both to the form of the implementation of relativistic dynamics as well as to the details of the deuteron D state at moderate momentum transfer, making it a natural partner to the magnetic form factor for exploring details of nucleon-nucleon dynamics and associated electroweak properties.

nucl-th↗

arXiv1996

Causality in Dense Matter

The possibility of non-causal signal propagation is examined for various theories of dense matter. This investigation requires a discussion of definitions of causality, together with interpretations of spacetime position. Specific examples are used to illustrate the satisfaction or violation of causality in realistic calculations.

hep-th↗

arXiv1994

Baryon Current Matrix Elements in a Light-Front Framework

Current matrix elements and observables for electro- and photo-excitation of baryons from the nucleon are studied in a light-front framework. Relativistic effects are estimated by comparison to a nonrelativistic model, where we use simple basis states to represent the baryon wavefunctions. Sizeable relativistic effects are found for certain transitions, for example, to radial excitations such as that conventionally used to describe to the Roper resonance. A systematic study shows that the violation of rotational covariance of the baryon transition matrix elements stemming from the use of one-body currents is generally small.

nucl-th↗

arXiv1994

Forms of Relativistic Dynamics: What Are the Possibilities?

Various methods of constructing solvable few-body models are reviewed, with an emphasis on direct interactions with few degrees of freedom, as an alternative to the use of local quantum field theories. Several applications are discussed.

nucl-th↗

arXiv1993

Rotational covariance and light-front current matrix elements

Light-front current matrix elements for elastic scattering from hadrons with spin~1 or greater must satisfy a nontrivial constraint associated with the requirement of rotational covariance for the current operator. Using a model $ρ$ meson as a prototype for hadronic quark models, this constraint and its implications are studied at both low and high momentum transfers. In the kinematic region appropriate for asymptotic QCD, helicity rules, together with the rotational covariance condition, yield an additional relation between the light-front current matrix elements.

hep-ph↗