2,411 research outputs found
The Ward Identity from the Background Field Dependence of the Effective Action
The dependence of the effective action for gauge theories on the background
field obeys an exact identity. We argue that for Abelian theories the Ward
identity follows from the more general background field identity. This
observation is particularly relevant for the anomalous Ward identity valid for
gauge theories with an effective infrared cutoff as used for flow equations.Comment: 8 page
Wilson Renormalization Group for Supersymmetric Gauge Theories and Gauge Anomalies
We extend the Wilson renormalization group (RG) to supersymmetric theories.
As this regularization scheme preserves supersymmetry, we exploit the
superspace technique. To set up the formalism we first derive the RG flow for
the massless Wess-Zumino model and deduce its perturbative expansion. We then
consider N=1 supersymmetric Yang-Mills and show that the local gauge symmetry
-broken by the regularization- can be recovered by a suitable choice of the RG
flow boundary conditions. We restrict our analysis to the first loop, the
generalization to higher loops presenting no difficulty due to the iterative
nature of the procedure. Furthermore, adding matter fields, we reproduce the
one-loop supersymmetric chiral anomaly to the second order in the vector field.Comment: 22 pages, 1 Postscript figure, uses amssym
Gauge invariance and background field formalism in the exact renormalisation group
We discuss gauge symmetry and Ward-Takahashi identities for Wilsonian flows
in pure Yang-Mills theories. The background field formalism is used for the
construction of a gauge invariant effective action. The symmetries of the
effective action under gauge transformations for both the gauge field and the
auxiliary background field are separately evaluated. We examine how the
symmetry properties of the full theory are restored in the limit where the
cut-off is removed.Comment: version to be published in PL
Topological invariants in interacting Quantum Spin Hall: a Cluster Perturbation Theory approach
Using Cluster Perturbation Theory we calculate Green's functions,
quasi-particle energies and topological invariants for interacting electrons on
a 2-D honeycomb lattice, with intrinsic spin-orbit coupling and on-site e-e
interaction. This allows to define the parameter range (Hubbard U vs spin-orbit
coupling) where the 2D system behaves as a trivial insulator or Quantum Spin
Hall insulator. This behavior is confirmed by the existence of gapless
quasi-particle states in honeycomb ribbons. We have discussed the importance of
the cluster symmetry and the effects of the lack of full translation symmetry
typical of CPT and of most Quantum Cluster approaches. Comments on the limits
of applicability of the method are also provided.Comment: 7 pages, 7 figures: discussion improved, one figure added, references
updated. Matches version published in New J. Phy
Bulk Aluminum at High Pressure: A First-Principles Study
The behavior of metals at high pressure is of great importance to the fields
of shock physics, geophysics, astrophysics, and nuclear materials. In order to
further understand the properties of metals at high pressures we studied the
equation of state of aluminum using first-principles techniques up to 2500 GPa,
pressures within reach of the planned L.L.N.L. National Ignition Facility. Our
simulations use density-functional theory and density-functional perturbation
theory in the generalized gradient approximation at 0K. We found core overlaps
to become relevant beyond pressures of 1200 GPa. The equations of state for
three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp,
fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa
respectively. From the phonon dispersions at increasing pressure, we predict a
softening of the lowest transverse acoustic vibrational mode along the [110]
direction, which corresponds to a Born instability of the fcc phase at 725 GPa.Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This
version has update many figures. Moreover we provided updated and more
accurate numbers based on further in-depth analyses of potential
computational error
Sex hormones in allergic conjunctivitis: altered levels of circulating androgens and estrogens in children and adolescents with vernal keratoconjunctivitis
PURPOSE:
Vernal keratoconjunctivitis (VKC) is a chronic allergic disease mainly affecting boys in prepubertal age and usually recovering after puberty. To evaluate a possible role of sex hormones in VKC, serum levels of sex hormones in children and adolescents with VKC were assessed.
METHODS:
12 prepubertal and 7 early pubertal boys with active VKC and 6 male patients with VKC in remission phase at late pubertal age and 48 healthy age and sex-matched subjects were included. Serum concentration of estrone, 17 beta-estradiol, dehydroepiandrosterone-sulfate, total testosterone and free testosterone, dihydrotestosterone (DHT), cortisol, delta-4-androstenedione, follicle-stimulating hormone, luteinizing hormone, and sex-hormones binding globuline (SHBG) were evaluated.
RESULTS:
Serum levels of Estrone were significantly increased in all groups of patients with VKC when compared to healthy controls (P < 0.001). Prepubertal and early pubertal VKC showed a significant decrease in DHT (P = 0.007 and P = 0.028, resp.) and SHBG (P = 0.01 and P = 0.002, resp.) when compared to controls and serum levels of SHBG were increased in late pubertal VKC in remission phase (P = 0.007).
CONCLUSIONS AND RELEVANCE:
VKC patients have different circulating sex hormone levels in different phases of the disease and when compared to nonallergic subjects. These findings suggest a role played by sex hormones in the pathogenesis and/or activity of VKC
Chiral gauge theories and anomalies in the Wilson renormalization group approach
We extend the Wilson renormalization group (RG) formulation to chiral gauge
theories and show that local gauge symmetry can be implemented by a suitable
choice of the RG flow boundary conditions. Since the space-time dimension is
four, there is no ambiguity in handling the matrix \g_5 and left and right
fermions are not coupled. As a result the ultraviolet action contains all
possible globally chiral invariant interactions. Nevertheless, the correct
chiral anomaly is reproduced.Comment: 16 pages, 4 figures, LaTex, uses epsfig, amssym
Scheme Independence at First Order Phase Transitions and the Renormalisation Group
We analyse approximate solutions to an exact renormalisation group equation
with particular emphasis on their dependence on the regularisation scheme,
which is kept arbitrary. Physical quantities related to the coarse-grained
potential of scalar QED display universal behaviour for strongly first-order
phase transitions. Only subleading corrections depend on the regularisation
scheme and are suppressed by a sufficiently large UV scale. We calculate the
relevant coarse-graining scale and give a condition for the applicability of
Langer's theory of bubble nucleation.Comment: 12 pages, LaTeX, 4 figures included (needs epsfig.sty), two equations
added, typo correcte
Exact Flow Equations and the U(1)-Problem
The effective action of a SU(N)-gauge theory coupled to fermions is evaluated
at a large infrared cut-off scale k within the path integral approach. The
gauge field measure includes topologically non-trivial configurations
(instantons). Due to the explicit infrared regularisation there are no gauge
field zero modes. The Dirac operator of instanton configurations shows a zero
mode even after the infrared regularisation, which leads to U_A(1)-violating
terms in the effective action. These terms are calculated in the limit of large
scales k.Comment: 22 pages, latex, no figures, with stylistic changes and some
arguments streamlined, typos corrected, References added, to appear in Phys.
Rev.
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