4,582 research outputs found
A (1+1)-Dimensional Reduced Model of Mesons
We propose an extension of 't Hooft's large- light-front QCD in two
dimensions to include helicity and physical gluon degrees of freedom, modelled
on a classical dimensional reduction of four dimensional QCD. A
non-perturbative renormalisation of the infinite set of coupled integral
equations describing boundstates is performed. These equations are then solved,
both analytically in a phase space wavefunction approximation and numerically
by discretising momenta, for (hybrid) meson masses and (polarized) parton
structure functions.Comment: LaTex 13 pages; 2 figures, uuencoded file (PostScript
On the Transition from Confinement to Screening in QCD_{1+1} Coupled to Adjoint Fermions at Finite N
We consider SU(N) QCD_{1+1} coupled to massless adjoint Majorana fermions,
where N is finite but arbitrary. We examine the spectrum for various values of
N, paying particular attention to the formation of multi-particle states, which
were recently identified by Gross, Hashimoto and Klebanov in the N -> infinity
limit of the theory. It is believed that in the limit of vanishing fermion
mass, there is a transition from confinement to screening in which string-like
states made out of adjoint fermion bits dissociate into stable constituent
``single particles''. In this work, we provide numerical evidence that such a
transition into stable constituent particles occurs not only at large N, but
for any finite value of N. In addition, we discuss certain issues concerning
the ``topological'' properties exhibited by the DLCQ spectrum.Comment: 14 pages, Late
Quantum Chromodynamics and Other Field Theories on the Light Cone
We discuss the light-cone quantization of gauge theories as a calculational
tool for representing hadrons as QCD bound-states of relativistic quarks and
gluons, and also as a novel method for simulating quantum field theory on a
computer. The light-cone Fock state expansion of wavefunctions provides a
precise definition of the parton model and a general calculus for hadronic
matrix elements. We present several new applications of light-cone Fock
methods, including calculations of exclusive weak decays of heavy hadrons, and
intrinsic heavy-quark contributions to structure functions. Discretized
light-cone quantization, is outlined and applied to several gauge theories. We
also discuss the construction of the light-cone Fock basis, the structure of
the light-cone vacuum, and outline the renormalization techniques required for
solving gauge theories within the Hamiltonian formalism on the light cone.Comment: 206 pages Latex, figures included, Submitted to Physics Report
Renormalization of an effective Light-Cone QCD-inspired theory for the Pion and other Mesons
The renormalization of the effective QCD-Hamiltonian theory for the
quark-antiquark channel is performed in terms of a renormalized or fixed-point
Hamiltonian that leads to subtracted dynamical equations. The fixed
point-Hamiltonian brings the renormalization conditions as well as the
counterterms that render the theory finite. The approach is renormalization
group invariant. The parameters of the renormalized effective QCD-Hamiltonian
comes from the pion mass and radius, for a given constituent quark mass. The 1s
and excited 2s states of are calculated as a function of the mass of
the quark being s, c or b, and compared to the experimental values.Comment: 39 pages, 10 figure
Light-Front QCD(1+1) Coupled to Adjoint Scalar Matter
We consider adjoint scalar matter coupled to QCD(1+1) in light-cone
quantization on a finite `interval' with periodic boundary conditions. We work
with the gauge group SU(2) which is modified to by the
non-trivial topology. The model is interesting for various nonperturbative
approaches because it is the sector of zero transverse momentum gluons of pure
glue QCD(2+1), where the scalar field is the remnant of the transverse gluon
component. We use the Hamiltonian formalism in the gauge .
What survives is the dynamical zero mode of , which in other theories
gives topological structure and degenerate vacua. With a point-splitting
regularization designed to preserve symmetry under large gauge transformations,
an extra dependent term appears in the current . This is reminiscent
of an (unwanted) anomaly. In particular, the gauge invariant charge and the
similarly regulated no longer commute with the Hamiltonian. We show that
nonetheless one can construct physical states of definite momentum which are
not {\it invariant} under large gauge transformations but do {\it transform} in
a well-defined way. As well, in the physical subspace we recover vanishing {\it
expectation values} of the commutators between the gauge invariant charge,
momentum and Hamiltonian operators. It is argued that in this theory the vacuum
is nonetheless trivial and the spectrum is consistent with the results of
others who have treated the large N, SU(N), version of this theory in the
continuum limit.Comment: LaTex, 13 pages. Submitted to Physics Letters
Transient effects on electron spin observation
In an earlier publication we addressed the problem of splitting an electron beam in the Stern-Gerlach experiment. In contrast to arguments put forward in the early days of quantum theory, we concluded that there are no issues of principle preventing the observation of electron spin during free flight. In that paper, however, we considered only a sudden switch off of the separating magnetic field. In this work we consider the possible effects of finite switching times at the beginning and the end of the interaction period. We consider a model where the coupling between the electron and the field is time dependent. As a result of the time dependence, the field also acquires an electric component, but this seems to cause no significant change of our conclusions. On the other hand, the smooth change of the interaction enforces the same longitudinal velocity on the electron both at the beginning and end of the interaction period because of conservation laws; this effect was missing in our earlier calculations. As the electrons are supposed to travel as a beam, this feature helps by restoring the beam quality after the interaction
Towards Solving QCD in Light-Cone Quantization -- On the Spectrum of the Transverse Zero Modes for SU(2)
The formalism for a non-abelian pure gauge theory in (2+1) dimensions has
recently been derived within Discretized Light-Cone Quantization, restricting
to the lowest {\it transverse} momentum gluons. It is argued why this model can
be a paradigm for full QCD. The physical vacuum becomes non-trivial even in
light-cone quantization. The approach is brought here to tractable form by
suppressing by hand both the dynamical gauge and the constraint zero mode, and
by performing a Tamm-Dancoff type Fock-space truncation. Within that model the
Hamiltonian is diagonalized numerically, yielding mass spectra and
wavefunctions of the glue-ball states. We find that only color singlets have a
stable and discrete bound state spectrum. The connection with confinement is
discussed. The structure function of the gluons has a shape like . The existence of the continuum limit is verified by deriving a
coupled set of integral equations.Comment: 1 Latex file & 9 Postscript files; tarred, compressed and uuencode
Transverse Lattice QCD in 2+1 Dimensions
Following a suggestion due to Bardeen and Pearson, we formulate an effective
light-front Hamiltonian for large-N gauge theory in (2+1)-dimensions. Two
space-time dimensions are continuous and the remaining space dimension is
discretised on a lattice. Eguchi-Kawai reduction to a (1+1)-dimensional theory
takes place. We investigate the string tension and glueball spectrum, comparing
with Euclidean Lattice Monte Carlo data.Comment: 4 pages LaTeX with 2 Postscript figures, uses boxedeps.tex and e
spcrc2.sty. Poster session contribution to LATTICE96(poster). Minor changes
in new versio
Tube Model for Light-Front QCD
We propose the tube model as a first step in solving the bound state problem
in light-front QCD. In this approach we neglect transverse variations of the
fields, producing a model with 1+1 dimensional dynamics. We then solve the two,
three, and four particle sectors of the model for the case of pure glue SU(3).
We study convergence to the continuum limit and various properties of the
spectrum.Comment: 29 page
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