537 research outputs found
Unambiguous one-loop quantum energies of 1+1 dimensional bosonic field configurations
We calculate one-loop quantum energies in a renormalizable self-interacting
theory in one spatial dimension by summing the zero-point energies of small
oscillations around a classical field configuration, which need not be a
solution of the classical field equations. We unambiguously implement standard
perturbative renormalization using phase shifts and the Born approximation. We
illustrate our method by calculating the quantum energy of a
soliton/antisoliton pair as a function of their separation. This energy
includes an imaginary part that gives a quantum decay rate and is associated
with a level crossing in the solutions to the classical field equation in the
presence of the source that maintains the soliton/antisoliton pair.Comment: Email correspondence to [email protected] ; 10 pages, 2 figures,
REVTeX, BoxedEPS; v2: Fixed description of level crossing as a function of
; v3: Fixed numerical error in figure dat
Quark and Gluon Orbital Angular Momentum and Spin in Hard Processes
We suggest a method of constructing gauge invariant quark and gluon
distributions that describe an abstract QCD observable and apply this method to
analyze angular momentum of a hadron. In addition to the known quark and gluon
polarized structure functions, we obtain gauge invariant distributions for
quark and gluon orbital angular momenta, and consider some basic properties of
these distributions and their moments.Comment: 16 pages, no figures, RevTe
The Casimir Energy in a Separable Potential
The Casimir energy is the first-order-in-\hbar correction to the energy of a
time-independent field configuration in a quantum field theory. We study the
Casimir energy in a toy model, where the classical field is replaced by a
separable potential. In this model the exact answer is trivial to compute,
making it a good place to examine subtleties of the problem. We construct two
traditional representations of the Casimir energy, one from the Greens
function, the other from the phase shifts, and apply them to this case. We show
that the two representations are correct and equivalent in this model. We study
the convergence of the Born approximation to the Casimir energy and relate our
findings to computational issues that arise in more realistic models.Comment: 16 pages, 4 EPS figures, REVTeX using BoxedEPS macros; email to
[email protected]
Heavy Quark Fragmentation into Heavy Mesons
We present a QCD based interpretation of heavy quark fragmentation which
utilizes the heavy quark mass expansion. By distinguishing between perturbative
and non-perturbative QCD effects, we show how to reliably extract mass
independent parameters characterizing the fragmentation function. Because these
parameters are quark mass independent, this procedure should permit tests of
heavy quark symmetry. Furthermore, we show that heavy quark mass corrections
vanish at order in QCD. There also exist higher twist corrections of
order and
which we relate to the leading twist fragmentation function.Comment: 36 pages (Plain TeX with a PostScript figure appended at end), MIT
CTP #218
Spin structure and longitudinal polarization of hyperon in e+e- annihilation at high energies
Longitudinal polarizations of different kinds of hyperons produced in e+e-
annihilation at LEP I and LEP II energies in different event samples are
calculated using two different pictures for the spin structure of hyperon: that
drawn from polarized deep inelastic lepton-nucleon scattering data or that
using SU(6) symmetric wave functions. The result shows that measurements of
such polarizations should provide useful information to the question of which
picture is more suitable in describing the spin effects in the fragmentation
processes.Comment: 26 pages with 10 figures. Submitted to Phys. Rev.
Charmed Strange Pentaquarks in the Large Limit
The properties of pentaquarks containing a heavy anti-quark and strange
quarks are studied in the bound state picture. In the flavor SU(3) limit, there
are many pentaquark states with the same binding energy. When the SU(3)
symmetry breaking effects are included, however, three states become
particularly stable due to a ``Gell-Mann--Okubo mechanism''. They are the and states discussed by Lipkin, and a a previously
unstudied state. These states will have and
their masses are estimated. These states, if exist, may be seen in experiments
in the near future.Comment: 12 pages in REVTeX, no figure
A Heavy Fermion Can Create a Soliton: A 1+1 Dimensional Example
We show that quantum effects can stabilize a soliton in a model with no
soliton at the classical level. The model has a scalar field chirally coupled
to a fermion in 1+1 dimensions. We use a formalism that allows us to calculate
the exact one loop fermion contribution to the effective energy for a spatially
varying scalar background. This energy includes the contribution from
counterterms fixed in the perturbative sector of the theory. The resulting
energy is therefore finite and unambiguous. A variational search then yields a
fermion number one configuration whose energy is below that of a single free
fermion.Comment: 10 pages, RevTeX, 2 figures composed from 4 .eps files; v2: fixed
minor errors, added reference; v3: corrected reference added in v
Space-time properties of the higher twist amplitudes
A consistent and intuitive description of the twist-4 corrections to the
hadron structure functions is presented in a QCD-improved parton model using
time-ordered perturbative theory, where the collinear singularities are
naturally eliminated. We identify the special propagators with the backward
propagators of partons in time order.Comment: 18 Pages, Latex, 8 Ps figures, To appear in Phys. Rev.
Gluon Spin in the Nucleon
We study the operator description of the gluon spin contribution ()
to the nucleon's spin as it is measured in deep inelastic processes.
can be related to the forward matrix element of a local gluon operator in
gauge. In quark models the nucleon contains ambient color electric and
magnetic fields. The latter are thought to be responsible for spin splittings
among the light baryons. We show that these fields give rise to a significant
{\it negative\/} contribution to at the quark model renormalization
scale, . The non-Abelian character of QCD is responsible for the sign
of . In a generic non-relativistic quark model , in the bag model
. These correspond to
and at .Comment: 12 pages in REVTeX. The paper has been entirely revise
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