182 research outputs found
Coupling running through the Looking-Glass of dimensional Reduction
The dimensional reduction, in a form of transition from four to two
dimensions, was used in the 90s in a context of HE Regge scattering. Recently,
it got a new impetus in quantum gravity where it opens the way to
renormalizability and finite short-distance behavior. We consider a QFT model
with running coupling defined in both the two domains of
different dimensionality; the \gbar(Q^2)\, evolutions being duly conjugated
at the reduction scale Beyond this scale, in the deep UV 2-dim
region, the running coupling does not increase any more. Instead, it {\it
slightly decreases} and tends to a finite value \gbar_2(\infty) \,< \,
\gbar_2(M^2)\, from above. As a result, the global evolution picture looks
quite peculiar and can propose a base for the modified scenario of gauge
couplings behavior with UV fixed points provided by dimensional reduction
instead of leptoquarks.Comment: 8 pages, 4 figures,Version to match the one which (besides the
Appendix) will appear in "Particles and Nuclei (PEPAN), Letters", v.7, No
6(162) 2010 pp 625-631. Slightly edited, one more reference and related
numerical estimate adde
Asymptotically free scalar curvature-ghost coupling in Quantum Einstein Gravity
We consider the asymptotic-safety scenario for quantum gravity which
constructs a non-perturbatively renormalisable quantum gravity theory with the
help of the functional renormalisation group. We verify the existence of a
non-Gaussian fixed point and include a running curvature-ghost coupling as a
first step towards the flow of the ghost sector of the theory. We find that the
scalar curvature-ghost coupling is asymptotically free and RG relevant in the
ultraviolet. Most importantly, the property of asymptotic safety discovered so
far within the Einstein-Hilbert truncation and beyond remains stable under the
inclusion of the ghost flow.Comment: 8 pages, 3 figures, RevTe
Critical behavior of the (2+1)-dimensional Thirring model
We investigate chiral symmetry breaking in the (2+1)-dimensional Thirring
model as a function of the coupling as well as the Dirac flavor number Nf with
the aid of the functional renormalization group. For small enough flavor number
Nf < Nfc, the model exhibits a chiral quantum phase transition for sufficiently
large coupling. We compute the critical exponents of this second order
transition as well as the fermionic and bosonic mass spectrum inside the broken
phase within a next-to-leading order derivative expansion. We also determine
the quantum critical behavior of the many-flavor transition which arises due to
a competition between vector and chiral-scalar channel and which is of second
order as well. Due to the problem of competing channels, our results rely
crucially on the RG technique of dynamical bosonization. For the critical
flavor number, we find Nfc ~ 5.1 with an estimated systematic error of
approximately one flavor.Comment: 28 pages, 14 figure
Questionable and unquestionable in the perturbation theory of non-Abelian models
We show, by explicit computation, that bare lattice perturbation theory in
the two-dimensional O(n) nonlinear models with superinstanton boundary
conditions is divergent in the limit of an infinite number of points
. This is the analogue of David's statement that renormalized
perturbation theory of these models is infrared divergent in the limit where
the physical size of the box tends to infinity. We also give arguments which
support the validity of the bare perturbative expansion of short-distance
quantities obtained by taking the limit term by term in
the theory with more conventional boundary conditions such as Dirichlet,
periodic, and free.Comment: One reference added to the published version, 28 pages, 3 figure
Quantum Einstein Gravity
We give a pedagogical introduction to the basic ideas and concepts of the
Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum
approach based upon the effective average action, we summarize the state of the
art of the field with a particular focus on the evidence supporting the
existence of the non-trivial renormalization group fixed point at the heart of
the construction. As an application, the multifractal structure of the emerging
space-times is discussed in detail. In particular, we compare the continuum
prediction for their spectral dimension with Monte Carlo data from the Causal
Dynamical Triangulation approach.Comment: 87 pages, 13 figures, review article prepared for the New Journal of
Physics focus issue on Quantum Einstein Gravit
Emergence of a 4D World from Causal Quantum Gravity
Causal Dynamical Triangulations in four dimensions provide a
background-independent definition of the sum over geometries in nonperturbative
quantum gravity, with a positive cosmological constant. We present evidence
that a macroscopic four-dimensional world emerges from this theory dynamically.Comment: 11 pages, 3 figures; some short clarifying comments added; final
version to appear in Phys. Rev. Let
Relativistic Quantum Gravity at a Lifshitz Point
We show that the Horava theory for the completion of General Relativity at UV
scales can be interpreted as a gauge fixed theory, and it can be extended to an
invariant theory under the full group of four-dimensional diffeomorphisms. In
this respect, although being fully relativistic, it results to be locally
anisotropic in the time-like and space-like directions defined by a family of
irrotational observers. We show that this theory propagates generically three
degrees of freedom: two of them are related to the four-dimensional
diffeomorphism invariant graviton (the metric) and one is related to a
propagating scalar mode. Finally, we note that in the present formulation,
matter can be consistently coupled to gravity.Comment: v4: Erratum added: explanation on the true dynamical fields of the
relativistic theory added. The theory is interpreted as a Tensor-Scalar
relativistic theory. Reference added. Version accepted in JHE
Coulomb excitation of Ni at safe energies
The value in Ni has been measured using Coulomb
excitation at safe energies. The Ni radioactive beam was
post-accelerated at the ISOLDE facility (CERN) to 2.9 MeV/u. The emitted
rays were detected by the MINIBALL detector array. A kinematic
particle reconstruction was performed in order to increase the measured c.m.
angular range of the excitation cross section. The obtained value of
2.8 10 efm is in good agreement with the value
measured at intermediate energy Coulomb excitation, confirming the low
transition probability.Comment: 4 pages, 5 figure
From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework
Using the Einstein-Hilbert approximation of asymptotically safe quantum
gravity we present a consistent renormalization group based framework for the
inclusion of quantum gravitational effects into the cosmological field
equations. Relating the renormalization group scale to cosmological time via a
dynamical cutoff identification this framework applies to all stages of the
cosmological evolution. The very early universe is found to contain a period of
``oscillatory inflation'' with an infinite sequence of time intervals during
which the expansion alternates between acceleration and deceleration. For
asymptotically late times we identify a mechanism which prevents the universe
from leaving the domain of validity of the Einstein-Hilbert approximation and
obtain a classical de Sitter era.Comment: 47 pages, 17 figure
Infrared fixed point in quantum Einstein gravity
We performed the renormalization group analysis of the quantum Einstein
gravity in the deep infrared regime for different types of extensions of the
model. It is shown that an attractive infrared point exists in the broken
symmetric phase of the model. It is also shown that due to the Gaussian fixed
point the IR critical exponent of the correlation length is 1/2. However,
there exists a certain extension of the model which gives finite correlation
length in the broken symmetric phase. It typically appears in case of models
possessing a first order phase transitions as is demonstrated on the example of
the scalar field theory with a Coleman-Weinberg potential.Comment: 9 pages, 7 figures, final version, to appear in JHE
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