7,628 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
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
The exact renormalization group equation for pure quantum gravity is used to
derive the non-perturbative \Fbeta-functions for the dimensionless Newton
constant and cosmological constant on the theory space spanned by the
Einstein-Hilbert truncation. The resulting coupled differential equations are
evaluated for a sharp cutoff function. The features of these flow equations are
compared to those found when using a smooth cutoff. The system of equations
with sharp cutoff is then solved numerically, deriving the complete
renormalization group flow of the Einstein-Hilbert truncation in . The
resulting renormalization group trajectories are classified and their physical
relevance is discussed. The non-trivial fixed point which, if present in the
exact theory, might render Quantum Einstein Gravity nonperturbatively
renormalizable is investigated for various spacetime dimensionalities.Comment: 58 pages, latex, 24 figure
Renormalization group improved gravitational actions: a Brans-Dicke approach
A new framework for exploiting information about the renormalization group
(RG) behavior of gravity in a dynamical context is discussed. The
Einstein-Hilbert action is RG-improved by replacing Newton's constant and the
cosmological constant by scalar functions in the corresponding Lagrangian
density. The position dependence of and is governed by a RG
equation together with an appropriate identification of RG scales with points
in spacetime. The dynamics of the fields and does not admit a
Lagrangian description in general. Within the Lagrangian formalism for the
gravitational field they have the status of externally prescribed
``background'' fields. The metric satisfies an effective Einstein equation
similar to that of Brans-Dicke theory. Its consistency imposes severe
constraints on allowed backgrounds. In the new RG-framework, and
carry energy and momentum. It is tested in the setting of homogeneous-isotropic
cosmology and is compared to alternative approaches where the fields and
do not carry gravitating 4-momentum. The fixed point regime of the
underlying RG flow is studied in detail.Comment: LaTeX, 72 pages, no figure
Renormalization of the Topological Charge in Yang-Mills Theory
The conditions leading to a nontrivial renormalization of the topological
charge in four--dimensional Yang--Mills theory are discussed. It is shown that
if the topological term is regarded as the limit of a certain nontopological
interaction, quantum effects due to the gauge bosons lead to a finite
multiplicative renormalization of the theta--parameter while fermions give rise
to an additional shift of theta. A truncated form of an exact renormalization
group equation is used to study the scale dependence of the theta--parameter.
Possible implications for the strong CP--problem of QCD are discussed.Comment: 31 pages, late
The contribution of methanol to the 3.4 micron feature in comets
With the advent of improved detectors and improved moderate resolution spectrometers several interesting features have been seen in the infrared spectra of comets. In particular, an emission excess at 3.52 microns was observed in several comets, and has recently been tentatively assigned to the nu 3 band of methanol (CH3OH). Using a developed model it is possible to calculate the relative strengths of the CH3OH features. The 3.52 microns emission strengths were used in a number of comets to retrieve methanol amounts, and the model was used to predict the fraction of the 3.4 micron flux which is contributed by the species. Implications for cometary formation are discussed
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
We discuss various basic conceptual issues related to coarse graining flows
in quantum gravity. In particular the requirement of background independence is
shown to lead to renormalization group (RG) flows which are significantly
different from their analogs on a rigid background spacetime. The importance of
these findings for the asymptotic safety approach to Quantum Einstein Gravity
(QEG) is demonstrated in a simplified setting where only the conformal factor
is quantized. We identify background independence as a (the ?) key prerequisite
for the existence of a non-Gaussian RG fixed point and the renormalizability of
QEG.Comment: 2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum
Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to
appear in General Relativity and Gravitatio
Automated NLO QCD Corrections with WHIZARD
We briefly discuss the current status of NLO QCD automation in the Monte
Carlo event generator WHIZARD. The functionality is presented for the explicit
study of off-shell top quark production with associated backgrounds at a lepton
collider.Comment: 9 pages, 5 figures, to appear in the proceedings of the European
Physical Society Conference on High Energy Physics 2015 (EPS-HEP 2015),
Vienna, Austria, 22nd to 29th of July 201
Matching NLO QCD Corrections in WHIZARD with the POWHEG scheme
Building on the new automatic subtraction of NLO amplitudes in WHIZARD, we
present our implementation of the POWHEG scheme to match radiative corrections
consistently with the parton shower. We apply this general framework to two
linear collider processes, and
.Comment: 7 pages, 4 figures, to appear in the proceedings of the European
Physical Society Conference on High Energy Physics 2015 (EPS-HEP 2015),
Vienna, Austria, 22nd to 29th of July 201
Density-functional theory investigation of oxygen adsorption at Pd(11N)(N=3,5,7) vicinal surfaces
We present a density-functional theory study addressing the on-surface
adsorption of oxygen at the Pd(11N) (N =3,5,7) vicinal surfaces, which exhibit
(111) steps and (100) terraces of increasing width. We find the binding to be
predominantly governed by the local coordination at the adsorption site. This
leads to very similar bonding properties at the threefold step sites of all
three vicinal surfaces, while the binding at the central fourfold hollow site
in the four atomic row terrace of Pd(117) is already very little disturbed by
the presence of the neighboring steps.Comment: 9 pages including 4 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
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