1,721,078 research outputs found
One loop beta functions and fixed points in higher derivative sigma models
Full text linkWe calculate the one loop beta functions for nonlinear sigma models in four dimensions containing general two and four derivative terms. In the O(N) model there are four such terms and nontrivial fixed points exist for all N geq 4. In the chiral SU(N) models there are in general six couplings, but only five for N=3 and four for N=2; we find fixed points only for N=2,3. In the approximation considered, the four derivative couplings are asymptotically free but the coupling in the two derivative term has a nonzero limit. These results support the hypothesis that certain sigma models may be asymptotically safe
The renormalization group, systems of units and the hierarchy problem
No full textIn the context of the Renormalization Group (RG) for gravity I discuss the role of field rescalings and their relation to choices of units. I concentrate on a simple Higgs model coupled to gravity, where natural choices of units can be based on Newton's constant or on the Higgs mass. These quantities are not invariant under the RG, and the ratio between the units is scale-dependent. In the toy model, strong RG running occurs in the intermediate regime between the Higgs and the Planck scale, reproducing the results of the Randall-Sundrum I model. Possible connections with the problem of the mass hierarchy are pointed out
Unimodular quantum gravity and the cosmological constant
No full textIt is shown that the one-loop effective action of unimodular gravity is the same as that of ordinary gravity, restricted to unimodular metrics. The only difference is in the treatment of the global scale degree of freedom and of the cosmological term. A constant vacuum energy does not gravitate, addressing one aspect of the cosmological constant problem
Towards metric-affine quantum gravity
No full textI review here some motivations to consider a theory of gravity based on independent metric and connection, and its status as a quantum theory
Chirality in unified theories of gravity
No full textWe show how to obtain a single chiral family of an SOð10Þ grand unified theory (GUT), starting from a Majorana-Weyl representation of a unifying (GraviGUT) group SOð3; 11Þ, which contains the gravita- tional Lorentz group SOð3; 1Þ. An action is proposed, which reduces to the correct fermionic grand unified theory action in the broken phase.We show how to obtain a single chiral family of an SO(10) grand unified theory ( GUT), starting from a Majorana-Weyl representation of a unifying (GraviGUT) group SO(3, 11), which contains the gravitational Lorentz group SO(3, 1). An action is proposed, which reduces to the correct fermionic grand unified theory action in the broken phase
New class of ghost- and tachyon-free metric affine gravities
Full text linkWe construct the spin-projection operators for a theory containing a symmetric two-index tensor and a general three-index tensor. We then use them to analyze, at linearized level, the most general action for a metric affine theory of gravity with terms up to second order in curvature, which depends on 28 parameters. In the metric case, we recover known results. In the torsion-free case, we are able to determine the most general six-parameter class of theories that are projective invariant, contain only one massless spin 2 and no spin 3, and are free of ghosts and tachyons
Asymptotic safety of gravity coupled to matter
No full textNonperturbative treatments of the UV limit of pure gravity suggest that it admits a stable fixed point with positive Newton's constant and cosmological constant. We prove that this result is stable under the addition of a scalar field with a generic potential and nonminimal couplings to the scalar curvature. There is a fixed point where the mass and all nonminimal scalar interactions vanish while the gravitational couplings have values which are almost identical to the pure gravity case. We discuss the linearized flow around this fixed point and find that the critical surface is four-dimensional. In the presence of other, arbitrary, massless minimally coupled matter fields, the existence of the fixed point, the sign of the cosmological constant and the dimension of the critical surface depend on the type and number of fields. In particular, for some matter content, there exist polynomial asymptotically free scalar potentials, thus providing a solution to the well-known problem of triviality
On Target-Space Duality in p-Branes
No full textWe study the target space duality transformations in p-branes as transformations which mix the world volume field equations with Bianchi identities. We consider an (m+p+1)-dimensional space-time with p + 1 dimensions compactified, and a particular form of the background fields. We find that while a GL(2) = SL(2) x R group is realized when m = 0, only a two-parameter group is realized when m > 0
Renormalization group effects in the conformal sector of 4d quantom gravity with matter
No full textWe discuss the ''gravitationally dressed'' beta functions in the Gross-Neveu model interacting with 2d Liouville theory and in SU(N) gauge theory interacting with the conformal sector of 4d quantum gravity. Among the effects we suggest one may feel that the gravitational dressing are the minimum of the effective potential and the running of the gauge coupling
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