1,650 research outputs found
Integrating out the Dirac sea in the Walecka model
We derive a purely fermionic no-sea effective theory, featuring
positive-energy states only for the Walecka model. In contrast to the so-called
mean-field theory approach with the no-sea approximation, where the Dirac sea
is simply omitted from the outset, we turn to the relativistic Hartree
approximation and explicitly construct a no-sea effective theory from the
underlying quantum field theory. Several results obtained within these two
approaches are confronted with each other. This sheds new light on the
reliability of the mean-field theory with the no-sea approximation as well as
the role of the Dirac sea. Restricting to 1+1 dimensions, we obtain new
analytical insights into nonuniform nuclear matter.Comment: 15 pages, 8 figures, several points clarified, Fig.7 replaced,
references adde
Expected Precision of Higgs Boson Partial Widths within the Standard Model
We discuss the sources of uncertainty in calculations of the partial widths
of the Higgs boson within the Standard Model. The uncertainties come from two
sources: the truncation of perturbation theory and the uncertainties in input
parameters. We review the current status of perturbative calculations and note
that these are already reaching the parts-per-mil level of accuracy for the
major decay modes. The main sources of uncertainty will then come from the
parametric dependences on alpha_s, m_b, and m_c. Knowledge of these parameters
is systematically improvable through lattice gauge theory calculations. We
estimate the precision that lattice QCD will achieve in the next decade and the
corresponding precision of the Standard Model predictions for Higgs boson
partial widths.Comment: 20 pages, 1 figure; v2: minor typo correction
Metacolor
A new mechanism is presented for the generation of quark and lepton masses,
based on a heavy fourth family and a new sector of massless fermions. The
massless fermions have only discrete chiral symmetries and they are confined by
the metacolor force. The resulting electroweak corrections may be smaller than
in technicolor theories.Comment: 11 pages, LaTeX, UTPT-93-09. (The title has changed from "Massless
Technicolor" and more discussion of dynamics has been added.
Parton showers as sources of energy-momentum deposition in the QGP and their implication for shockwave formation at RHIC and at the LHC
We derive the distribution of energy and momentum transmitted from a primary
fast parton and its medium-induced bremsstrahlung gluons to a thermalized
quark-gluon plasma. Our calculation takes into account the important and thus
far neglected effects of quantum interference between the resulting color
currents. We use our result to obtain the rate at which energy is absorbed by
the medium as a function of time and find that the rate is modified by the
quantum interference between the primary parton and secondary gluons. This
Landau-Pomeranchuk-Migdal type interference persists for time scales relevant
to heavy ion phenomenology. We further couple the newly derived source of
energy and momentum deposition to linearized hydrodynamics to obtain the bulk
medium response to realistic parton propagation and splitting in the
quark-gluon plasma. We find that because of the characteristic large angle
in-medium gluon emission and the multiple sources of energy deposition in a
parton shower, formation of well defined Mach cones by energetic jets in heavy
ion reactions is not likely.Comment: 8 pages, 4 figure
Higgs Triplets, Decoupling, and Precision Measurements
Electroweak precision data has been extensively used to constrain models
containing physics beyond that of the Standard Model. When the model contains
Higgs scalars in representations other than SU(2) singlets or doublets, and
hence rho not equal to one at tree level, a correct renormalization scheme
requires more inputs than the three needed for the Standard Model. We discuss
the connection between the renormalization of models with Higgs triplets and
the decoupling properties of the models as the mass scale for the scalar
triplet field becomes much larger than the electroweak scale. The requirements
of perturbativity of the couplings and agreement with electroweak data place
strong restrictions on models with Higgs triplets. Our results have important
implications for Little Higgs type models and other models with rho not equal
to one at tree level.Comment: 23 page
Dynamical Hartree-Fock-Bogoliubov Theory of Vortices in Bose-Einstein Condensates at Finite Temperature
We present a method utilizing the continuity equation for the condensate
density to make predictions of the precessional frequency of single off-axis
vortices and of vortex arrays in Bose-Einstein condensates at finite
temperature. We also present an orthogonalized Hartree-Fock-Bogoliubov (HFB)
formalism. We solve the continuity equation for the condensate density
self-consistently with the orthogonalized HFB equations, and find stationary
solutions in the frame rotating at this frequency. As an example of the utility
of this formalism we obtain time-independent solutions for
quasi-two-dimensional rotating systems in the co-rotating frame. We compare
these results with time-dependent predictions where we simulate stirring of the
condensate.Comment: 13 pages, 11 figures, 1 tabl
V,W and X in Technicolour Models
Light techni-fermions and pseudo Goldstone bosons that contribute to the
electroweak radiative correction parameters V,W and X may relax the constraints
on technicolour models from the experimental values of the parameters S and T.
Order of magnitude estimates of the contributions to V,W and X from light
techni-leptons are made when the the techni-neutrino has a small Dirac mass or
a large Majorana mass. The contributions to V,W and X from pseudo Goldstone
bosons are calculated in a gauged chiral Lagrangian. Estimates of V,W and X in
one family technicolour models suggest that the upper bounds on S and T should
be relaxed by between 0.1 and 1 depending upon the precise particle spectrum.Comment: 19 pages + 2 pages of ps figs, SWAT/1
Breakdown of the Fermi-liquid regime in the 2D Hubbard model from a two-loop field-theoretical renormalization group approach
We analyze the particle-hole symmetric two-dimensional Hubbard model on a
square lattice starting from weak-to-moderate couplings by means of the
field-theoretical renormalization group (RG) approach up to two-loop order.
This method is essential in order to evaluate the effect of the
momentum-resolved anomalous dimension which arises in the
normal phase of this model on the corresponding low-energy single-particle
excitations. As a result, we find important indications pointing to the
existence of a non-Fermi liquid (NFL) regime at temperature displaying
a truncated Fermi surface (FS) for a doping range exactly in between the
well-known antiferromagnetic insulating and the -wave singlet
superconducting phases. This NFL evolves as a function of doping into a
correlated metal with a large FS before the -wave pairing
susceptibility finally produces the dominant instability in the low-energy
limit.Comment: 9 pages, 9 figures; published in Phys. Rev.
Gauge invariance in two-particle scattering
It is shown how gauge invariance is obtained for the coupling of a photon to
a two-body state described by the solution of the Bethe-Salpeter equation. This
is illustrated both for a complex scalar field theory and for interaction
kernels derived from chiral effective Lagrangians.Comment: 16 pages, 2 figures, references added and commented o
Vacuum entanglement enhancement by a weak gravitational field
Separate regions in space are generally entangled, even in the vacuum state.
It is known that this entanglement can be swapped to separated Unruh-DeWitt
detectors, i.e., that the vacuum can serve as a source of entanglement. Here,
we demonstrate that, in the presence of curvature, the amount of entanglement
that Unruh-DeWitt detectors can extract from the vacuum can be increased.Comment: 6 pages, 1 figur
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