Archil Kobakhidze

Solution to the hierarchy problem from an almost decoupled hidden sector within a classically scale invariant theory

If scale invariance is a classical symmetry then both the Planck scale and the weak scale should emerge as quantum effects. We show that this can be realized in simple scale invariant theories with a hidden sector. The weak/Planck scale hierarchy emerges in the (technically natural) limit in which the hidden sector decouples from the ordinary sector. In this limit, finite corrections to the weak scale are consequently small, while quadratic divergences are absent by virtue of classical scale invariance, so there is no hierarchy problem.

Electroweak Higgs as a pseudo-Goldstone boson of broken scale invariance

We point out that it is possible to associate the electroweak Higgs boson with the pseudo-Goldstone boson of broken scale invariance, thus resolving the hierarchy problem in a technically natural way. We illustrate this idea with two specific gauge models. Besides being consistent with all currently available experimental data, both models maintain the predictive power of the standard model, since the first model has only one additional parameter beyond the Standard Model, and the second has the same number of free parameters as the Standard Model.

Noncommutative general relativity

We define a theory of noncommutative general relativity for canonical noncommutative spaces. We find a subclass of general coordinate transformations acting on canonical noncommutative spacetimes to be volume-preserving transformations. Local Lorentz invariance is treated as a gauge theory with the spin connection field taken in the so(3,1) enveloping algebra. The resulting theory appears to be a noncommutative extension of the unimodular theory of gravitation. We compute the leading order noncommutative correction to the action and derive the noncommutative correction to the equations of motion of the weak gravitation field.

Proton stability in TeV-scale GUTs

Abstract We discuss the proton decay problem in theories with low gravity and/or GUT scales. We pointed out that the gravity induced proton decay can be indeed suppressed up to a desired level, while the GUT origin of the proton instability is rather problematic. To solve this problem we suggest the GUT model where the proton is stable in all orders of perturbation theory. This can be simply achieved by the replication of quark–lepton families with ordinary quarks and leptons residing in different GUT representations and by an appropriate dimensional reduction. The model predicts extra mirror states which along with the GUT particles and the excitations of extra dimensions could be observable at high-energy colliders providing the unification scale is in the TeV range.

750 GeV diphoton resonance in a top and bottom seesaw model

Abstract The top and bottom seesaw model, which extends the top seesaw in order to accommodate a 125 GeV Higgs boson, predicts vector-like top/bottom partners and these partners can be bounded to form several neutral and charged singlet composite scalars by some new strong dynamics. In this letter, we use such a singlet scalar to interpret the 750 GeV diphoton resonance. This singlet scalar is dominantly produced through the gluon fusion process induced by the partners and its diphoton decay is induced by both the partners and the charged singlet scalars. We show that this scenario can readily account for the observed 750 GeV diphoton signal under the current LHC constraints. Further, this scenario predicts some other phenomenology, such as a strong correlation between the decays to γγ , Zγ and ZZ , a three-photon signal from the associate production of a singlet scalar and a photon, as well as some signals from the partner cascade decays. These signals may jointly allow for a test of this framework in future 100 TeV hadron collider and ILC experiments.

Neutrino mass in radiatively broken scale-invariant models

Scale invariance may be a classical symmetry which is broken radiatively. This provides a simple way to stabilize the scale of electroweak symmetry breaking against radiative corrections. The simplest phenomenologically successful model of this type involves the addition of one real scalar field to the standard model. In this minimal model the electroweak Higgs can be interpreted as the pseudo-Goldstone boson of broken scale invariance. We study the possible origin of neutrino mass in such models, both at tree-level and radiatively.

Electroweak Vacuum (In)Stability in an Inflationary Universe

Abstract Recent analysis shows that if the 125–126 GeV LHC resonance turns out to be the Standard Model Higgs boson, the electroweak vacuum would be a metastable state at 98% C.L. In this Letter we argue that, during inflation, the electroweak vacuum can actually be very short-lived, contrary to the conclusion that follows from the flat spacetime analysis. Namely, in the case of a pure Higgs potential the electroweak vacuum decays via the Hawking–Moss transition, which has no flat spacetime analogue. As a result, the Higgs vacuum is unstable, unless the rate of inflation is low enough: H inf ≲ 10 9 – 10 12 GeV . Models of inflation with such a low rate typically predict negligible tensor perturbations in the cosmic microwave background radiation (CMBR). This is also true for models in which the perturbations are produced by a curvaton field. We also find that if the effective curvature of the Higgs potential at a local maximum (which may be induced by inflaton-Higgs interactions) is large enough, then the decay of the electroweak vacuum is dominated by the Coleman–de Luccia transition. The electroweak vacuum is also short-lived in this case, due to a negative effective self-interaction coupling. Based on our analysis of Higgs vacuum stability during inflation, we conclude that the observation of tensor perturbations by the Planck satellite would provide strong indirect evidence for new physics beyond the Standard Model responsible for stabilisation of the electroweak vacuum.

On the infrared limit of Horava's gravity with the global Hamiltonian constraint

We show that Horavas theory of gravitation with the global Hamiltonian constraint does not reproduce General Rela- tivity in the infrared domain. There is one extra propagatin g degree of freedom, besides those two associated with the massless graviton, which does not decouple.

Poincaré protection for a natural electroweak scale

We discuss a class of technically-natural UV extensions of the Standard Model in which the electroweak scale is shielded from large radiative corrections from heavy UV physics due to an enhanced Poincare symmetry. Such heavy sectors can be invoked to provide solutions to known shortcomings of the Standard Model, such the strong-CP problem, the absence of dark matter, and the lack of neutrino masses. We discuss the relationship to scale-invariant models.

Gravity is not an entropic force

We argue that experiments with ultra-cold neutrons in the gravitational field of Earth disprove recent speculations on the entropic origin of gravitation. Introduction. Recently, E. Verlinde presented a thought-provoking paper (1) claiming that inertia and gravitation have intrinsically thermodynamic origin. This claim is in accord with some earlier observations on the possible relation between thermodynamics and gravity (2), (3). If correct, E. Verlindes work suggests a radically different approach to gravitational interactions. Namely, the force of gravity is associated with an entropic force caused by changes in the entropy associated with positions of material bodies. The entropy, in accord with the holographic picture, is stored on holographic screens and the space is emergent between two such screens. The gradient of Newtonian potential (gravitational force) in the emergent space measures the entropy difference between the screens. In this picture, because of macroscopic (thermodynamic) nature of gravity, there is no place for gravitons and hence no need to worry about quantization of gravity and the related problems. It seems that for macroscopic material bodies E. Verlindes the- ory do indeed reproduce results of ordinary Newtonian (and perhaps also Einsteinian) gravity 1 . However, as we will argue in this short note, for microscopic (quantum me- chanical) systems the situation is different. In particular, the results of experiments with ultra-cold neutrons in the gravitational field of Earth (4) are in disagreement with E. Verlindes idea.