arXiv subjects

Bohdan Grzadkowski

Publications and source records attributed to Bohdan Grzadkowski.

At least 55 records · Page 3Linked to original sources

arXiv2014

Thick-Brane Cosmology

We search for time-dependent solutions for the 5-dimensional system of a scalar field canonically coupled to gravity. Time-independent and time-dependent scalar field configurations with the most general homogeneous and isotropic 4D metric are considered. For the case of time-independent scalar field, the time evolution of the scale factor is obtained for different values of the spatial curvature $k=0,\pm 1$. In the case of time-dependent scalar field, two classes of solutions are discussed and an extension of the superpotential formalism is proposed.

hep-th↗

arXiv2014

Generalized Randall-Sundrum model with a single thick brane

A generalized version of the Randall-Sundrum model-2 with different cosmological constants on each side of a brane has been discussed. A possibility of replacing the singular brane by a configuration of a scalar field has been also considered, the Einstein equations for this setup were solved and stability of the solution discussed. It has been shown that under mild assumptions the relation between cosmological constants and the brane tension obtained in the brane limit does not depend on the particular choice of the regularizing profile of the scalar field.

hep-th↗

arXiv2013

Cosmology of Multi-Singlet-Scalar Extensions of the Standard Model

An extension of the Standard Model by extra scalar singlets was considered. Theoretical (unitarity, vacuum stability, triviality) and cosmological (dark matter relic abundance, direct detection experiments, constraints on dark matter self-interaction) constraints were discussed.

hep-ph↗

arXiv2013

Two-Higgs-Doublet Models and Enhanced Rates for a 125 GeV Higgs

We examine the level of enhancement that can be achieved in the ZZ and γγchannels for a two-Higgs-doublet model Higgs boson (either the light h or the heavy H) with mass near 125 GeV after imposing all constraints from LEP data, B physics, precision electroweak data, vacuum stability, unitarity and perturbativity. The latter constraints restrict substantially the possibilities for enhancing the gg -> h -> γγor gg -> H -> γγsignal relative to that for the SM Higgs, hSM. Further, we find that a significant enhancement of the gg -> h -> γγor gg -> H -> γγsignal in Type II models is possible only if the gg -> h -> ZZ or gg -> H -> ZZ mode is even more enhanced, a situation disfavored by current data. In contrast, in the Type I model one can achieve enhanced rates in the γγfinal state for the h while having the ZZ mode at or below the SM rate - the largest [gg -> h -> γγ]/[gg -> hSM -> γγ] ratio found is of order ~1.3 when the two Higgs doublet vacuum expectation ratio is tanβ= 4 or 20 and the charged Higgs boson has its minimal LEP-allowed value of m_{H^\pm} = 90 GeV.

hep-ph↗

arXiv2013

Brane modeling in warped extra-dimension

Five-dimensional scenarios with infinitesimally thin branes replaced by appropriate configurations of a scalar field were considered. A possibility of periodic extra dimension was discussed in the presence on non-minimal scalar-gravity coupling and a generalized Gibbons-Kallosh-Linde sum rule was found. In order to avoid constraints imposed by periodicity, a non-compact spacial extra dimension was introduced. A five dimensional model with warped geometry and two thin branes mimicked by a scalar profile was constructed and discussed. In the thin brane limit the model corresponds to a set-up with two positive-tension branes. The presence of two branes allows to address the issue of the hierarchy problem which could be solved by the standard warping of the four dimensional metric provided the Higgs field is properly localized. Stability of the background solution was discussed and verified in the presence of the most general perturbations of the metric and the scalar field.

hep-th↗

arXiv2012

Multi-Scalar-Singlet Extension of the Standard Model - the Case for Dark Matter and an Invisible Higgs Boson

We consider a simple extension of the Standard Model by the addition of N real scalar gauge singlets $\vp$ that are candidates for Dark Matter. By collecting theoretical and experimental constraints we determine the space of allowed parameters of the model. The possibility of ameliorating the little hierarchy problem within the multi-singlet model is discussed. The Spergel-Steinhardt solution of the Dark Matter density cusp problem is revisited. It is shown that fitting the recent CRESST-II data for Dark Matter nucleus scattering implies that the standard Higgs boson decays predominantly into pairs of Dark Matter scalars. It that case discovery of the Higgs boson at LHC and Tevatron is impossible. The most likely mass of the dark scalars is in the range 15 GeV $\lsim \mvp \lsim$ 50 GeV with $BR(h \to \vp\vp)$ up to 96%.

hep-ph↗

arXiv2012

Higgs-Radion interpretation of the LHC data?

We explore the parameter choices in the five-dimensional Randall-Sundrum model with the inclusion of Higgs-radion mixing that can describe current LHC hints for one or more Higgs boson signals.

hep-ph↗

arXiv2011

Anomalous Wtb coupling effects in the weak radiative B-meson decay

We study the effect of anomalous Wtb couplings on the B -> Xs gamma branching ratio. The considered couplings are introduced as parts of gauge-invariant dimension-six operators that are built out of the Standard Model fields only. One-loop contributions from the charged-current vertices are assumed to be of the same order as the tree-level flavour-changing neutral current ones. Bounds on the corresponding Wilson coefficients are derived.

hep-ph↗

arXiv2010

Tuned Two-Higgs-Doublet Model

We consider a Two-Higgs-Doublet Model (2HDM) constrained by the condition that assures cancellation of quadratic divergences up to the leading two-loop order. Regions in the parameter space consistent with existing experimental constraints and with the cancellation condition are determined. The possibility for CP violation in the scalar potential is discussed and regions of \tan_β-M_{H^\pm} with substantial amount of CP violation are found. The model allows to ameliorate the little hierarchy problem by lifting the minimal scalar Higgs boson mass and by suppressing the quadratic corrections to scalar masses. The cutoff originating from the naturality arguments is therefore lifted from ~0.6 TeV in the Standard Model to ~2.5 TeV in the 2HDM, depending on the mass of the lightest scalar.

hep-ph↗

arXiv2010

Natural Two-Higgs-Doublet Model

We show that the Two-Higgs-Doublet Model (2HDM) constrained by the two-loop-order requirement of cancellation of quadratic divergences is consistent with the existing experimental constraints. The model allows to ameliorate the little hierarchy problem by suppressing the quadratic corrections to scalar masses and lifting the mass of the lightest Higgs boson. A strong source of CP violation emerges from the scalar potential. The cutoff originating from the naturality arguments is shifted from 0.6 TeV in the Standard Model to >6 TeV in the 2HDM, depending on the mass of the lightest scalar.

hep-ph↗

arXiv2009

Uncosmology

We discuss some cosmological features of a hypothetical type of new physics characterized by begin asymptotically free in the UV regime and conformally invariant in the IR. We show that nucleosynthesis data generates non-trivial constrains this type of models.

hep-ph↗

arXiv2009

Cosmology with Unparticles

We discuss cosmological consequences of the existence of physics beyond the standard model that exhibits Banks-Zaks and unparticle behavior in the UV and IR respectively. We first derive the equation of state for unparticles and use it to obtain the temperature dependence of the corresponding energy and entropy densities. We then formulate the Boltzmann and Kubo equations for both the unparticles and the Banks-Zaks particles, and use these results to determine the equilibrium conditions between the standard model and the new physics. We conclude by obtaining the constraints on the effective number of degrees of freedom of unparticles imposed by Big-Bang nucleosynthesis.

hep-ph↗

arXiv2009

Naive solution of the little hierarchy problem and its physical consequences

We argue that adding gauge-singlet real scalars to the Standard Model can both ameliorate the little hierarchy problem and provide a realistic source of Dark Matter. Masses of the scalars should be in the 1-3 TeV range, while the lowest cutoff of the (unspecified) UV completion of the model must be ~> 5 TeV, depending on the Higgs boson mass and the number of singlets present. The scalars couple to the Majorana mass term for right-handed neutrinos implying one massless neutrino. The resulting mixing angles are consistent with the tri-bimaximal mixing scenario.

hep-ph↗

arXiv2009

Pragmatic approach to the little hierarchy problem: the case for Dark Matter and neutrino physics

We show that the addition of real scalars (gauge singlets) to the Standard Model can both ameliorate the little hierarchy problem and provide realistic Dark Matter candidates. To this end, the coupling of the new scalars to the standard Higgs boson must be relatively strong and their mass should be in the 1-3 TeV range, while the lowest cutoff of the (unspecified) UV completion must be >~ 5 TeV, depending on the Higgs boson mass and the number of singlets present. The existence of the singlets also leads to realistic and surprisingly reach neutrino physics. The resulting light neutrino mass spectrum and mixing angles are consistent with the constraints from the neutrino oscillations.

hep-ph↗

arXiv2009

Note on the strong CP problem from a 5-dimensional perspective

We consider 5 dimensional gauge theories where the 5th direction is compactified on an interval. The Chern-Simons (CS) terms (favored by naive dimensional analysis) are discussed. A simple scenario with an extra U(1)_X gauge field that couples to SU(3)_{color} through a CS term in the bulk is constructed. The extra component of the Abelian gauge field plays a role of the axion, which in the standard manner solves the strong CP problem easily avoiding most of experimental constraints. This version has an expanded discussion of the phenomenology of the model and various clarifications in the estimation of the operator coefficients in the appendix.

hep-ph↗

arXiv2007

Higgs-Boson Mass Limit within the Randall-Sundrum Model

Perturbative unitarity for $\wlwl$ scattering is discussed within the Randall-Sundrum model. It is shown that the exchange of massive 4D Kaluza-Klein gravitons leads to amplitudes growing linearly with the CM energy squared. Summing over KK gravitons up to a scale $\lbar$ and testing unitarity at $\rts=\lbar$, one finds that unitarity is violated for $\lbar$ below the 'naive dimensional analysis' scale, $\lnda$. It is also shown that the exchange of gravitons can substantially relax the upper limit from unitarity on the mass of the Standard Model Higgs boson - consistency with unitarity for all $\rts$ below $\lbar$ is possible for $\mh$ as large as $ 1.4\tev$, depending on the curvature of the background metric. Observation of the mass and width (or cross section) of one or more KK gravitons at the LHC will directly determine the curvature and the scale $\lw$ specifying the couplings of matter to the KK gravitons. With this information and a measurement of the Higgs boson mass it will be possible to determine the precise $\rts$ value below which unitarity will remain valid.

hep-ph↗

arXiv2007

KK Gravitons and Unitarity Violation in the Randall-Sundrum Model

We show that perturbative unitarity for $W_LW_L$ scattering places significant constraints on the Randall-Sundrum theory with two 3-branes, with matter confined to the TeV brane. The exchange of massive 4D Kaluza-Klein gravitons leads to amplitudes growing linearly with the CM energy squared. Summing over KK gravitons up to a scale $\lbar$ and testing unitarity at $\sqrt s=\lbar$, one finds that unitarity is violated for $\lbar$ below the 'naive dimensional analysis' scale, $Λ_{NDA}$. We evaluate $\lbar$ as a function of the curvature ratio $m_0/M_{Planck}$ for the pure gravity theory. We then demonstrate that unitarity need not be violated at $\lbar$ in the presence of a heavy Higgs boson. In fact, much larger Higgs masses are consistent with unitarity than if no KK gravitons are present. Observation of the mass and width (or cross section) of one or more KK gravitons at the LHC will directly determine $m_0/M_{Planck}$ and the scale $\hat Λ$ specifying the couplings of matter to the KK gravitons. With this information in hand and a measurement of the Higgs boson mass, one can determine the precise scale $\lbar$ below which unitarity will remain valid.

hep-ph↗

arXiv2006

Strategies and obstacles in constructing realistic higher-dimensional models

We discuss several aspects of higher dimensional models that contain bulk gauge and fermion fields only. In particular we argue that non-standard boundary conditions involving charge-conjugate fermion fields offer attractive model building possibilities. We also discuss a no-go theorem for 5-dimensional models which severely limits their phenomenological relevance.

hep-ph↗