6,190 research outputs found
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
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
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
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
Running Gauge Coupling in Asymptotically Safe Quantum Gravity
We investigate the non-perturbative renormalization group behavior of the
gauge coupling constant using a truncated form of the functional flow equation
for the effective average action of the Yang-Mills-gravity system. We find a
non-zero quantum gravity correction to the standard Yang-Mills beta function
which has the same sign as the gauge boson contribution. Our results fit into
the picture according to which Quantum Einstein Gravity (QEG) is asymptotically
safe, with a vanishing gauge coupling constant at the non-trivial fixed point.Comment: 27 page
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
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Infrared remote sensing of cometary parent volatiles from the ground, air, and space
A balanced view of the present generation of infrared instruments for cometary compositional studies is presented. Ground-based instruments are compared with airborne and spaceborne capabilities. An attempt to give examples of the unique science achievable with each is made, and particular emphasis is on the unique aspects of a dedicated Cometary Composition Telescope in earth orbit for investigating the chemical and structural heterogeneity of the cometary nucleus
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