arXiv subjects

Bohdan Grzadkowski

Publications and source records attributed to Bohdan Grzadkowski.

At least 73 records · Page 4Linked to original sources

arXiv2006

5-Dimensional Difficulties of Gauge-Higgs Unifications

We consider 5 dimensional gauge theories where the 5th direction is compactified on the orbifold S^1/Z_2, and where the 5th components of the gauge bosons play the role of the Standard Model Higgs boson (gauge-Higgs unification). The gauge symmetry breaking is realized through the appropriate orbifold boundary conditions and through the Hosotani mechanism. We show that for {\em any such theory} the assumption that the low-energy vector-boson spectrum consists of the W^\pm, Z and γonly, is inconsistent with the experimental requirements \sin^2θ_W\simeq 1/4 and ρ\equiv m_W^2/(m_Z^2\cos^2θ_w)=1.

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arXiv2006

New-Physics Search through gamma gamma -> t tbar -> lX/bX

We probe optimal beam polarizations for new-physics search in top-quark and Higgs-boson sectors at Photon Linear Colliders (PLC). Expressing possible non-standard effects generated by SU(2)xU(1) gauge-invariant dimension-6 effective operators as anomalous top and Higgs couplings, we estimate expected statistical sensitivities of these couplings in gamma gamma -> t tbar -> lX/bX, using the optimal-observable method.

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arXiv2005

Light excitations in 5-dimensional gauge theories

We consider general five-dimensional gauge theories compactified on on an orbifold S1/Z2 with all fields propagating in the bulk. We propose a generalized set of boundary conditions and derive the general features of the low energy-spectrum. The results are illustrated with two simple examples.

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arXiv2005

Optimal Beam Polarizations for New-Physics Search through gamma gamma -> t tbar -> lX/bX

We perform an optimal-observable analysis of the final charged-lepton/b-quark momentum distributions in gamma+gamma -> t+tbar -> lX/bX for various beam polarizations in order to study possible anomalous t-tbar-gamma, tbW and gamma-gamma-H couplings, which could be generated by SU(2)xU(1) gauge-invariant dimension-6 effective operators. We find optimal beam polarizations that will minimize the uncertainty in determination of those non-standard couplings. We also compare e+ebar and gamma+gamma colliders from the viewpoint of the anomalous-top-quark-coupling determination.

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arXiv2005

Studying Possible Anomalous Top-Quark Couplings at Photon Colliders

Search for new-physics through possible anomalous t tbar gamma, tbW and gamma gamma H couplings which are generated by SU(2) X U(1) gauge-invariant dimension-6 effective operators is discussed, using energy and angular distributions of final charged-lepton/ b-quark in gamma gamma -> t tbar -> l X / bX for various beam polarizations. Optimal beam polarizations that minimize uncertainty in determination of those non-standard couplings are found performing an optimal-observable analysis.

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arXiv2005

Majorana Fermions and CP Violation from 5-dimensional Theories; a Systematic Approach

Within five-dimensional compactified theories we discuss generalized periodicity and orbifold boundary conditions that allow for mixing between particles and anti-particles after a shift by the size of extra dimensions or after the orbifold reflection. A systematic strategy for constructing 4-dimensional models is presented, in particular we find a general form of the periodicity and orbifold conditions that are allowed by consistency requirements. We formulate general conditions for a presence of massless Kaluza-Klein modes and discuss remaining gauge symmetry of the zero-mode sector. It is shown that if the orbifold twist operation transforms particles into anti-particles then the zero-mode fermions are 4-dimensional Majorana fermions. The possibility of explicit and spontaneous CP violation is discussed. General considerations are illustrated by many Abelian and non-Abelian examples.

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arXiv2004

Optimal-Observable Analysis of Possible New Physics Using the b-quark in gamma gamma -> tt -> bX

We study possible anomalous top-quark couplings generated by SU(2) x U(1) gauge invariant dimension-6 effective operators, using the final b-quark momentum distribution in gamma gamma -> tt -> bX. Taking into account non-standard tt gamma, tbW and gamma gamma H couplings, we perform an optimal-observable analysis in order to estimate the precision for the determination of all relevant non-standard couplings.

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arXiv2004

Probing Anomalous Top-Quark Couplings Induced by Dim.6 Operators at Photon Colliders

Possible anomalous top-quark couplings induced by SU(2) x U(1) gauge-invariant dimension-6 effective operators were studied in the process of t t-bar productions and decays at polarized gamma gamma colliders. Two CP-violating asymmetries, a linear-polarization asymmetry and a circular-polarization asymmetry, were computed including both non-standard [t t-bar gamma] and [gamma gamma H] couplings. An optimal-observable analysis for the process gamma gamma --> t t-bar --> l^{+-} ... was performed in order to estimate the precision for determination of all relevant non-standard couplings, including the anomalous tbW coupling.

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arXiv2004

Electroweak symmetry breaking and radion stabilization in universal extra dimensions

We discuss the stabilization of the scalar sector, including the radion, in the gauge model with one universal extra dimension, within Higgs and Higgsless scenarios. The stabilization occurs at the one-loop level, through the fermionic contribution to the effective potential; in the Higgs case, for stabilization to take place the bosonic contribution must be balanced by the fermionic one, hence the scales of these two cannot differ too much. However, there is no need for (softly broken) supersymmetry to achieve the stabilization - it can be arranged for a reasonably wide range of couplings and mass scales. The primary instability in the model is the run-away of the radion vacuum expectation value. It turns out that the requirement of the radion stability, in the Higgs case, favours a Higgs boson mass below 0.26 TeV, which is consistent with the Standard Model upper bound that follows from the electroweak precision measurements. The typical radion mass is of the order of 10^{-6} eV. The radion mass can be made larger by rising the scale of fermion masses, as clearly seen in the Higgsless case. The cosmological constant may be cancelled by suitable counterterms, in such a way that the stabilization is not affected.

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arXiv2004

CP Violation from 5-dimensional QED

It has been shown that QED in (1+4)-dimensional space-time, with the fifth dimension compactified on a circle, leads to CP violation (CPV). Depending on fermionic boundary conditions, CPV may be either explicit (through the Scherk--Schwarz mechanism), or spontaneous (via the Hosotani mechanism). The fifth component of the gauge field acquires (at the one-loop level) a non-zero vacuum expectation value. In the presence of two fermionic fields, this leads to spontaneous CPV in the case of CP-symmetric boundary conditions. Phenomenological consequences are illustrated by a calculation of the electric dipole moment for the fermionic zero-modes.

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arXiv2004

Low-energy effective theory from a non-trivial scalar background in extra dimensions

Consequences of a non-trivial scalar field background for an effective 4D theory were studied in the context of one compact extra dimension. The periodic background that appears within the (1+4)-dimensional $ϕ^4$ theory was found and the excitations above the background (and their spectrum) were determined analytically. It was shown that the presence of the non-trivial solution leads to the existence of a minimal size of the extra dimension that is determined by the mass parameter of the scalar potential. It was proved that imposing orbifold antisymmetry boundary conditions allows us to eliminate a negative mass squared Kaluza-Klein ground-state mode that otherwise would cause an instability of the system. The localization of fermionic modes in the presence of the non-trivial background was discussed in great detail varying the size of the extra dimension and the strength of the Yukawa coupling. A simple exact solution for the zero-mode fermionic states was found and the solution for non-zero modes in terms of trigonometric series was constructed. The fermionic mass spectrum, which reveals a very interesting structure, was found numerically. It was shown that the natural size of the extra dimension is twice as large as the period of the scalar background solution.

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arXiv2003

Higgs-Boson Mass Limits and Precise Measurements beyond the Standard Model

The triviality and vacuum stability bounds on the Higgs-boson mass ($\mh$) were revisited in presence of weakly-coupled new interactions parameterized in a model-independent way by effective operators of dimension 6. The constraints from precision tests of the Standard Model were taken into account. It was shown that for the scale of new physics in the region $Λ\simeq 2 ÷50 \tev$ the Standard Model triviality upper bound remains unmodified whereas it is natural to expect that the lower bound derived from the requirement of vacuum stability is substantially modified depending on the scale $\La$ and strength of coefficients of effective operators. A natural generalization of the standard triviality condition leads also to a substantial reduction of the allowed region in the ($Λ,\mh$) space.

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arXiv2003

Triviality and stability limits on the Higgs boson mass in effective theories

The impact of new interactions on the triviality and stability Higgs-boson mass bounds has been studied. The interactions have been parame- trized in a model-independent way by a set of effective operators of dimension 6. Constraints from electroweak observables at 1-loop level have been included. In the analyzed region of scale of new physics $\La \simeq 2 ÷50 \tev$ the classic triviality bound remains unchanged. An extension of the triviality condition that has been introduced leads to strong constraints on the possible models. The stability bound on the Higgs boson mass is substantially modified depending on the scale $\La$ and strength of coefficients of relevant effective operators.

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arXiv2003

The Effective Potential and Vacuum Stability within Universal Extra Dimensions

The one-loop effective potential calculated for a generic model that originates from 5-dimensional theory reduced down to 4 dimensions is considered. The cut-off and dimensional regularization schemes are discussed and compared. It is demonstrated that the prescriptions are consistent with each other and lead to the same physical consequences. Stability of the ground state is discussed for a U(1) model that is supposed to mimic the Standard Model extended to 5 dimensions. It has been shown that fermionic Kaluza-Klein modes can dramatically influence the shape of the effective potential shifting the instability scale even by several orders of magnitude.

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arXiv2003

Decoupling of Anomalous Top-Quark-Decay Vertices in Angular Distribution of Secondary Particles

Angular distribution of a secondary particle from top-quark decays is studied in a simple and general manner, paying careful attention to how relevant the top-quark production mechanism is. The conditions that lead to the distribution free from any possible anomalous top-quark decay interactions are specified. It is discussed how important the approximations adopted in earlier papers are.

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arXiv2002

Higgs-Boson Interactions within the Randall-Sundrum Model

The scalar sector of the Randall-Sundrum model is discussed. The effective potential for the Standard Model Higgs-boson (h) interacting with Kaluza-Klein excitations of the graviton and the radion (phi) has been derived and it has been shown that only the Standard Model-like vacuum is allowed. The theoretical and experimental consequences of the curvature-scalar mixing xi RHH introduced on the visible brane are considered and simple sum rules that relate the couplings of the mass eigenstates (h) and (phi) to pairs of vector bosons and fermions are derived. The sum rule for the ZZh and ZZ phi couplings in combination with LEP/LEP2 data implies that not both the h and phi can be light. We present explicit results for the still allowed region in the (mh,mphi) plane that remains after imposing the LEP upper limits for non-standard scalar couplings to a ZZ pair. The phenomenological consequences of the mixing are investigated and, in particular, it is shown that the Higgs-boson decay h -> phi phi would provide an experimental signature for non-zero xi and can have a very substantial impact on the Higgs-boson searches, having BR(h->phi phi) as large as 30 - 40 %.

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arXiv2002

Angular Distribution of Leptons in General ttbar Production and Decay

Angular distribution of the secondary lepton in top-quark production followed by subsequent semi-leptonic decay is studied assuming general top-quark couplings. It is shown that the distribution does not depend on any possible anomalous tbW couplings and is determined only by the standard V-A decay vertex for any production mechanism if certain well-justified conditions are satisfied.

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arXiv2001

Triviality and Vacuum Stability Bounds on the Higgs Boson Mass beyond the Standard Model

The triviality and vacuum stability bounds on the Higgs-boson mass were revisited in presence of weakly-coupled new interactions parameterized in a model-independent way by effective operators of dimension 6. It was shown that for the scale of new physics in the region Lambda=.5-50 TeV the Standard Model triviality upper bound remains unmodified whereas it is natural to expect that the lower bound derived from the requirement of vacuum stability is increased by 40-60 GeV depending on the scale Lambda and strength of coefficients of effective operators. It turns out that if the Higgs-boson mass is close to its lower LEP limit then the upper bound on the scale of new physics that follows from the vacuum stability requirement would be decreased dramatically even for modest values of coefficients of effective operators implying new physics already at the scale of a few TeV.

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