839 research outputs found
Chiral Symmetry restoration in the massive Thirring model at finite T and : Dimensional reduction and the Coulomb gas
We show that in certain limits the (1+1)-dimensional massive Thirring model
at finite temperature is equivalent to a one-dimensional Coulomb gas of
charged particles at the same . This equivalence is then used to explore the
phase structure of the massive Thirring model. For strong coupling and
(the fermion mass) the system is shown to behave as a free gas of "molecules"
(charge pairs in the Coulomb gas terminology) made of pairs of chiral
condensates. This binding of chiral condensates is responsible for the
restoration of chiral symmetry as . In addition, when a fermion
chemical potential is included, the analogy with a Coulomb gas
still holds with playing the role of a purely imaginary external electric
field. For small and we find a typical massive Fermi gas behaviour
for the fermion density, whereas for large it shows chiral restoration by
means of a vanishing effective fermion mass. Some similarities with the chiral
properties of low-energy QCD at finite and baryon chemical potential are
discussed.Comment: 28 pages, 6 figures, better resolution figures are available upon
reques
Are Occupation Numbers Observable?
The question of whether occupation numbers and momentum distributions of
nucleons in nuclei are observables is considered from an effective field theory
perspective. Field redefinitions lead to variations that imply the answer is
negative, as illustrated in the interacting Fermi gas at low density.
Implications for the interpretation of (e,e'p) experiments with nuclei are
discussed.Comment: 8 pages, revtex4, 4 ps figure
Finite Temperature Effective Potential for the Abelian Higgs Model to the Order
A complete calculation of the finite temperature effective potential for the
abelian Higgs model to the order is presented and the result is
expressed in terms of physical parameters defined at zero temperature. The
absence of a linear term is verified explicitly to the given order and proven
to survive to all orders. The first order phase transition has weakened in
comparison with lower order calculation, which shows up in a considerable
decrease of the surface tension. The only difference from the original version
is the splitting of some overlong lines causing problems with certain mailers.Comment: 13 pages LaTex ( figures not included , hardcopy available on request
: [email protected] or t00heb@dhhdesy3 ) , DESY 93-08
The Paradox of Virtual Dipoles in the Einstein Action
The functional integral of pure Einstein 4D quantum gravity admits abnormally
large and long-lasting "dipolar fluctuations", generated by virtual sources
with the property Int d^4x Sqrt{g(x)} Tr T(x) = 0. These fluctuations would
exist also at macroscopic scales, with paradoxical consequences. We set out
their general features and give numerical estimates of possible suppression
processes.Comment: LaTeX, 5 pages; reference adde
TeV Astrophysics Constraints on Planck Scale Lorentz Violation
We analyze observational constraints from TeV astrophysics on Lorentz
violating nonlinear dispersion for photons and electrons without assuming any a
priori equality between the photon and electron parameters. The constraints
arise from thresholds for vacuum Cerenkov radiation, photon decay and
photo-production of electron-positron pairs. We show that the parameter plane
for cubic momentum terms in the dispersion relations is constrained to an order
unity region in Planck units. We find that the threshold configuration can
occur with an asymmetric distribution of momentum for pair creation, and with a
hard photon for vacuum Cerenkov radiation.Comment: 4 pages, RevTeX4, 1 figure. Some references and a footnote added,
improved discussion on the photon annihilation and GZK cutoff. Minor changes
of wording. Main results unchanged. Version to appear as a Rapid
Communication in PR
Celebrity culture and public connection: bridge or chasm?
Media and cultural research has an important contribution to make to recent debates about declines in democratic engagement: is for example celebrity culture a route into democratic engagement for those otherwise disengaged? This article contributes to this debate by reviewing qualitative and quantitative findings from a UK project on 'public connection'. Using self-produced diaries (with in-depth multiple interviews) as well as a nationwide survey, the authors argue that while celebrity culture is an important point of social connection sustained by media use, it is not linked in citizens' own accounts to issues of public concern. Survey data suggest that those who particularly follow celebrity culture are the least engaged in politics and least likely to use their social networks to involve themselves in action or discussion about public-type issues. This does not mean 'celebrity culture' is 'bad', but it challenges suggestions of how popular culture might contribute to effective democracy
Maxwell - Chern - Simons topologically massive gauge fields in the first-order formalism
We find the canonical and Belinfante energy-momentum tensors and their
nonzero traces. We note that the dilatation symmetry is broken and the
divergence of the dilatation current is proportional to the topological mass of
the gauge field. It was demonstrated that the gauge field possesses the `scale
dimensionality' d=1/2. Maxwell - Chern - Simons topologically massive gauge
field theory in 2+1 dimensions is formulated in the first-order formalism. It
is shown that 6x6-matrices of the relativistic wave equation obey the Duffin -
Kemmer - Petiau algebra. The Hermitianizing matrix of the relativistic wave
equation is given. The projection operators extracting solutions of field
equations for states with definite energy-momentum and spin are obtained. The
5x5-matrix Schrodinger form of the equation is derived after the exclusion of
non-dynamical components, and the quantum-mechanical Hamiltonian is obtained.
Projection operators extracting physical states in the Schrodinger picture are
found.Comment: 18 pages, correction in Ref. [5
Gap ratio in anharmonic charge-density-wave systems
Many experimental systems exist that possess charge-density-wave order in
their ground state. While this order should be able to be described with models
similar to those used for superconductivity, nearly all systems have a ratio of
the charge-density-wave order parameter to the transition temperature that is
too high for conventional theories. Recent work explained how this can happen
in harmonic systems, but when the lattice distortion gets large, anharmonic
effects must play an increasingly important role. Here we study the gap ratio
for anharmonic charge-density wave systems to see whether the low-temperature
properties possess universality as was seen previously in the transition
temperature and to see whether the explanation for the large gap ratios
survives for anharmonic systems as well.Comment: (5 pages, 3 figures, ReVTeX
The Path-Integral Approach to the N=2 Linear Sigma Model
In QFT the effective potential is an important tool to study symmetry
breaking phenomena. It is known that, in some theories, the canonical approach
and the path-integral approach yield different effective potentials. In this
paper we investigate this for the Euclidean N=2 linear sigma model. Both the
Green's functions and the effective potential will be computed in three
different ways. The relative merits of the various approaches are discussed.Comment: 2 figure
Magnetotransport near a quantum critical point in a simple metal
We use geometric considerations to study transport properties, such as the
conductivity and Hall coefficient, near the onset of a nesting-driven spin
density wave in a simple metal. In particular, motivated by recent experiments
on vanadium-doped chromium, we study the variation of transport coefficients
with the onset of magnetism within a mean-field treatment of a model that
contains nearly nested electron and hole Fermi surfaces. We show that most
transport coefficients display a leading dependence that is linear in the
energy gap. The coefficient of the linear term, though, can be small. In
particular, we find that the Hall conductivity is essentially
unchanged, due to electron-hole compensation, as the system goes through the
quantum critical point. This conclusion extends a similar observation we made
earlier for the case of completely flat Fermi surfaces to the immediate
vicinity of the quantum critical point where nesting is present but not
perfect.Comment: 11 pages revtex, 4 figure
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