arXiv subjects

Archil Kobakhidze

Publications and source records attributed to Archil Kobakhidze.

At least 73 records · Page 4Linked to original sources

arXiv2014

Criticality in the scale invariant standard model (squared)

We consider first the standard model Lagrangian with $μ_h^2$ Higgs potential term set to zero. We point out that this clasically scale invariant theory potentially exhibits radiative electroweak/scale symmetry breaking with very high vacuum expectation value (VEV) for the Higgs field, $< ϕ> \approx 10^{17-18}$ GeV. Furthermore, if such a vacuum were realized then cancellation of vacuum energy automatically implies that this nontrivial vacuum is degenerate with the trivial unbroken vacuum. Such a theory would therefore be critical with the Higgs self-coupling and its beta function nearly vanishing at the symmetry breaking minimum, $λ(μ=< ϕ>)\approx β_λ (μ=< ϕ>)\approx 0$. A phenomenologically viable model that predicts this criticality property arises if we consider two copies of the standard model Lagrangian, with exact $Z_2$ symmetry swapping each ordinary particle with a partner. The spontaneously broken vacuum can then arise where one sector gains the high scale VEV, while the other gains the electroweak scale VEV. The low scale VEV is perturbed away from zero due to a Higgs portal coupling, or via the usual small Higgs mass terms $μ_h^2$, which softly break the scale invariance. In either case, the cancellation of vacuum energy requires $M_t = (171.53 \pm 0.42)$ GeV, which is close to its measured value of $(173.34 \pm 0.76)$ GeV.

hep-ph↗

arXiv2014

Inflationary Baryogenesis in a Model with Gauged Baryon Number

We argue that inflationary dynamics may support a scenario where significant matter-antimatter asymmetry is generated from initially small-scale quantum fluctuations that are subsequently stretched out over large scales. This scenario can be realised in extensions of the Standard Model with an extra gauge symmetry having mixed anomalies with the electroweak gauge symmetry. Inflationary baryogenesis in a model with gauged baryon number is considered in detail.

hep-ph↗

arXiv2014

Comments on the Hierarchy Problem in Effective Theories

We discuss aspects of the hierarchy problem in effective theories with light scalars and a large, physical ultraviolet (UV) cutoff. We make two main points: (1) The (naive) fine-tuning observed in an effective theory does not automatically imply that the UV completion is fine tuned. Instead, it gives a type of upper bound on the severity of the actual tuning in the UV completion; the actual tuning can be less severe than the naive tuning or even non-existent. (2) Within an effective theory, there appear to be two types of parameter relations that can alleviate the sensitivity of the scalar mass to the cutoff --- relationships among dimensionless couplings or relationships among dimensionful parameters. Supersymmetric models provide symmetry-motivated examples of the former, while scale-invariant models give symmetry-motivated examples of the latter.

hep-ph↗

arXiv2014

The Higgs vacuum is unstable

So far, the experiments at the Large Hadron Collider (LHC) have shown no sign of new physics beyond the Standard Model. Assuming the Standard Model is correct at presently available energies, we can accurately extrapolate the theory to higher energies in order to verify its validity. Here we report the results of new high precision calculations which show that absolute stability of the Higgs vacuum state is now excluded. Combining these new results with the recent observation of primordial gravitational waves by the BICEP Collaboration, we find that the Higgs vacuum state would have quickly decayed during cosmic inflation, leading to a catastrophic collapse of the universe into a black hole. Thus, we are driven to the conclusion that there must be some new physics beyond the Standard Model at energies below the instability scale $Λ_I \sim 10^{9}$ GeV, which is responsible for the stabilisation of the Higgs vacuum.

hep-ph↗

arXiv2014

Probing Light Higgsinos in Natural SUSY from Monojet Signals at the LHC

We investigate a strategy to search for light, nearly degenerate higgsinos within the natural MSSM at the LHC. We demonstrate that the higgsino mass range $μ$ in $100-150$ GeV, which is preferred by the naturalness, can be probed at $2σ$ significance through the monojet search at 14 TeV HL-LHC with 3000 fb$^{-1}$ luminosity. The proposed method can also probe certain region in the parameter space for the lightest neutralino with a high higgsino purity, that cannot be reached by planned direct detection experiments at XENON-1T(2017).

hep-ph↗

arXiv2013

Excluding a Generic Spin-2 Higgs Impostor

We argue that the current experimental data in conjunction with perturbative unitarity considerations exclude the possibility that the LHC 125-126 GeV resonance is a generic massive spin-2 particle of either parity. We analyse tree-level Z-spin-2 elastic scattering and demonstrate that perturbative unitarity breaks down at energies $Λ\sim 600$ GeV. Furthermore, we find that W,Z-spin-2 interactions contribute to the electroweak oblique parameters in a way that is in gross disagreement with observations.

hep-ph↗

arXiv2013

Poincare 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.

hep-ph↗

arXiv2013

Time-symmetric quantization in spacetimes with event horizons

The standard quantization formalism in spacetimes with event horizons implies a non-unitary evolution of quantum states, as initial pure states may evolve into thermal states. This phenomenon is behind the famous black hole information loss paradox which provoked long-standing debates on the compatibility of quantum mechanics and gravity. In this paper we demonstrate that within an alternative time-symmetric quantization formalism thermal radiation is absent and states evolve unitarily in spacetimes with event horizons. We also discuss the theoretical consistency of the proposed formalism. We explicitly demonstrate that the theory preserves the microcausality condition and suggest a "reinterpretation postulate" to resolve other apparent pathologies associated with negative energy states. Accordingly as there is a consistent alternative, we argue that choosing to use time-asymmetric quantization is a necessary condition for the black hole information loss paradox.

gr-qc↗

arXiv2013

Neutrino Masses and Higgs Vacuum Stability

The Standard Model electroweak vacuum has been found to be metastable, with the true stable vacuum given by a large, phenomenologically unacceptable vacuum expectation value $\approx M_{P}$. Moreover, it may be unstable in an inflationary universe. Motivated by the necessity of physics beyond the Standard Model and to accommodate non-zero neutrino masses, we investigate vacuum stability within type-II seesaw and left-right symmetric models. Our analysis is performed by solving the renormalisation group equations, carefully taking into account the relevant threshold corrections. We demonstrate that a phenomenologically viable left-right symmetric model can be constructed by matching it with the SM at one-loop. In both models we demonstrate the existence of a large area of parameter space where the Higgs vacuum is absolutely stable.

hep-ph↗

arXiv2013

Cosmological Zitterbewegung

We describe a new phenomenon of zitterbewegung of a free Dirac particle in cosmological spacetimes. Unlike the similar effect theorized by Schrodinger in 1930, the cosmological zitterbewegung is a real, physically attainable effect, which originates from the mixing of positive and negative frequency modes of a field operator in cosmological spacetimes. We briefly discuss the potential for observing this effect in laboratory experiments with trapped ions.

gr-qc↗

arXiv2013

Electroweak Vacuum (In)Stability in an Inflationary Universe

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 paper 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: $H_{\rm inf}\lesssim 10^9-10^{12}$ 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.

hep-ph↗

arXiv2013

Scale invariance and the electroweak symmetry breaking

We argue that classical scale invariance provides a technically natural solution to the problem of the radiative stability of the electroweak scale. Some realistic electroweak scale-invariant models are considered and their possible manifestations at the LHC are briefly discussed.

hep-ph↗

arXiv2013

Electroweak scale invariant models with small cosmological constant

We consider scale invariant models where the classical scale invariance is broken perturbatively by radiative corrections at the electroweak scale. These models offer an elegant and simple solution to the hierarchy problem. If we further require the cosmological constant to be small then such models are also highly predictive. Indeed, the minimal such model, comprising a Higgs doublet and a real singlet, has the same number of parameters as the standard model. Although this minimal model is disfavoured by recent LHC data, we show that two specific extensions incorporating neutrino masses and dark matter are fully realistic. That is, consistent with all experiments and observations. These models predict a light pseudo-Goldstone boson, $h$, with mass around 10 GeV or less. A fermionic-bosonic mass relation is also predicted. The specific models considered, as well as more generic scale invariant models, can be probed at the LHC.

hep-ph↗

arXiv2012

Standard Model with a distorted Higgs sector and the enhanced Higgs diphoton decay rate

We propose a theoretical justification for the anomalous Higgs couplings without extending the particle content of the Standard Model, but rather assuming different realization of the electroweak symmetry and the representation of the Higgs field. Namely, the electroweak symmetry in our model is non-linearly realised with the Higgs field residing in the singlet representation of the electroweak gauge group. Within this framework we identify the simplest scenario with CP-violating Higgs-top coupling which explains the enhanced rate of $h\toγγ$, while the Higgs production cross section is unaffected. Remarkably, this can be achieved with a reduced strength of the Higgs-top Yukawa coupling, $| y_t|<| y^{\rm SM}_t|$, and, thus, the electroweak vacuum stability problem may also be resolved in our scenario without invoking new physics below the Planck scale. In addition, models with the distorted Higgs sector provide a new framework for potentially successful electroweak baryogenesis.

hep-ph↗

arXiv2012

Stochastic superspace phenomenology at the Large Hadron Collider

We analyse restrictions on the stochastic superspace parameter space arising from 1 fb$^{-1}$ of LHC data, and bounds on sparticle masses, cold dark matter relic density and the branching ratio of the process $B_s \rightarrow μ^+ μ^-$. A region of parameter space consistent with these limits is found where the stochasticity parameter, ξ, takes values in the range -2200 GeV < ξ< -900 GeV, provided the cutoff scale is $\mathcal{O}(10^{18})$ GeV.

hep-ph↗

arXiv2011

ATLAS and CMS hints for a mirror Higgs boson

ATLAS and CMS have provided hints for the existence of a Higgs-like particle with mass of about 144 GeV with production cross section into standard decay channels which is about 50% that of the standard model Higgs boson. We show that this 50% suppression is exactly what the mirror matter model predicts when the two scalar mass eigenstates, each required to be maximal admixtures of a standard and mirror-Higgs boson, are separated in mass by more than their decay widths but less than the experimental resolution. We discuss prospects for the future confirmation of this interesting hint for non-standard Higgs physics.

hep-ph↗

arXiv2011

Cosmological constant in scale-invariant theories

The incorporation of a small cosmological constant within radiatively-broken scale-invariant models is discussed. We show that phenomenologically consistent scale-invariant models can be constructed which allow a small positive cosmological constant, providing certain relation between the particle masses is satisfied. As a result, the mass of the dilaton is generated at two-loop level. Another interesting consequence is that the electroweak symmetry-breaking vacuum in such models is necessarily a metastable `false' vacuum which, fortunately, is not expected to decay on cosmological time scales.

hep-ph↗

arXiv2011

Once more: gravity is not an entropic force

We argue that neutron interference experiments and experiments on gravitational bound states of neutron unambiguously disprove entropic origin of gravitation. The criticism expressed in a recent paper [arXiv:1104.4650] concerning our arguments against entropic gravity is shown to be invalid.

hep-th↗