arXiv subjects

G. Cvetic

Publications and source records attributed to G. Cvetic.

At least 37 records · Page 2Linked to original sources

arXiv2000

No CPT Violation from Tilted Brane in Neutral Meson--Antimeson Systems

Tilted brane in theories with large compact extra dimensions leads to spontaneous symmetry breaking of the Lorentz and rotational invariance in four dimensions, as shown by Dvali and Shifman. In this brief report, we point out that the mentioned Lorentz symmetry breaking, although leading to the CPT--violating interaction terms, cannot lead to the CPT violation in the experimentally interesting $K$--${\bar K}$ and analogous systems.

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arXiv2000

Heavy Majorana neutrino production at electron-muon colliders

Possibilities for detecting heavy Majorana neutrinos ($N$'s) at future $eμ$ colliders are investigated. In contrast to the $e^-e^+$ colliders (LEP200 and NLC), the center-of-mass (CMS) energies achieved at $eμ$ colliders can be much higher and the $Z$-mediated $s$ channel is excluded automatically. This opens the attractive possibility of having high production cross sections for $N$'s and at the same time probing only the strength of charged current couplings of $N$'s ($NWe$ and $NWμ$). The production cross sections and the expected numbers of events for the reaction $e^{\mp} μ^{\pm} \to NN \to W^{\pm} {\ell}^{\mp} W^{\pm} {\ell}^{\prime \mp}$ are calculated for various masses $M$ of the Majorana neutrinos and for the CMS energies $\sqrt{s} = 0.5-6.0$ TeV. The values of the charged current coupling parameters are set equal to their present upper bounds. We obtain reasonably high production cross sections. Further, the effects of the off-shell intermediate $N$'s turn out to be significant only at $\sqrt{s} \stackrel{>}{\approx} 2$ TeV.

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arXiv1999

Off-diagonal generalized vector dominance in DIS and QCD

We review the generalized vector dominance (GVD) approach to DIS at small values of the scaling variable, x. In particular, we concentrate on a recent formulation of GVD that explicitly incorporates the configuration of the gamma^* -> q qbar transition and a QCD-inspired ansatz for the (qqbar)p scattering amplitude. The destructive interference, originally introduced in off-diagonal GVD is traced back to the generic strcuture of two-gluon exchange. Asymptotically, the transverse photoabsorption cross section behaves as (ln Q^2)/Q^2, implying a logarithmic violation of scaling for F_2, while the longitudinal-to-transverse ratio decreases as 1/\ln Q^2. We also briefly comment on vector-meson production.

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arXiv1999

Top quark condensation

Top quark condensation, in particular the minimal framework where the neutral Higgs scalar is (predominantly) an effective ${\bar t} t$ condensate of the Standard Model, is reviewed. Computational approaches are compared and similarities, differences and deficiencies pointed out. Extensions of the minimal framework, including scenarios with two composite Higgs doublets, additional neutrino condensates, and ${\bar t}t$ condensation arising from four-fermion interactions with enlarged symmetries, are described. Possible renormalizable models of underlying physics potentially responsible for the condensation, including topcolor assisted technicolor frameworks, are discussed. Phenomenological implications of top condensate models are outlined. Outstanding theoretical issues and problems for future investigation are pointed out. Progress in the field after this article was accepted has been briefly covered in a Note added at the end.

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arXiv1999

Deep inelastic Scattering, QCD, and generalised vector dominance

We provide a formulation of generalised vector dominance (GVD) for low-x deep-inelastic scattering that explicitly incorporates the $γ^{\ast} \to q{\bar q}$ transition and a QCD-inspired ansatz for the $(q{\bar q})p$ forward-scattering amplitude. The destructive interference originally introduced in off-diagonal GVD is recovered in the present formulation and traced back to the generic structure of two-gluon-exchange as incorporated into the notion of colour transparency. Asymptotically, the transverse photoabsorption cross section behaves as (ln Q^2)/Q^2, implying a logarithmic violation of scaling for F_2, while the longitudinal-to-transverse ratio decreases as 1/ln Q^2.

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arXiv1999

Heavy Majorana neutrinos in e+e- colliders

We investigate possibilities for detecting heavy Majorana neutrinos ($N$'s) in $e^+e^-$ at LEP200 and future Linear Colliders. We concentrate on the processes where the pairs of intermediate heavy $N$'s produce a clear signal of total lepton number violation ($e^+e^- \to NN \to W^+l^-W^+l^{\prime -}$). Such a signal is not possible if the heavy neutrinos are of Dirac nature. Our approach is general in the sense that the intermediate $N$'s can be either on shell or off shell. Discussion of the relative numerical importance of the $s$ and the $t+u$ channels of the $NN$ production is also included.

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arXiv1998

Higgs-mediated flavor-changing neutral currents in the general framework with two Higgs doublets -- an RGE analysis

We consider the standard model with two Higgs doublets with the most general Yukawa coupling terms (``type III''). In the model, the neutral-Higgs-mediated flavor-changing neutral currents (FCNC's) are allowed, but must be reasonably suppressed at low energies of probes. It has been known that the existing hierarchies of quark masses render this suppression at low energies rather natural. On the other hand, the model has been regarded by many as unnatural because of the absence of any symmetry that would ensure persistence of this suppression as the energy of probes increases. The opinion has been based on the expectation that the mentioned FCNC's would increase by large factors at increasing energies. We performed a numerical analysis of the flow of these FCN coupling parameters as governed by the one-loop renormalization group equations (RGE's), in a simplified case when Yukawa couplings of the first quark generation are neglected. The analysis shows a remarkable persistence of the mentioned FCNC suppression and thus indicates that the model is not unnatural in the RGE sense. Further, we point out two mistakes in the Yukawa RGE's of Machacek and Vaughn at one-loop level.

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arXiv1998

Global approximants with renormalization scale invariance in pQCD

Truncated perturbative series (TPS's) of any observable have the unphysical dependence on the choice of the renormalization scale (RScl). The diagonal Padé approximants (dPA's) to any TPS of an observable possess the favorable property of being invariant in the large-${β_0}$ limit. This means that they are invariant under the change of the RScl $μ^2$ when the ``running'' coupling parameter $α_s(μ^2)$ evolves according to the one-loop renormalization group equation. We present a method which generalizes this result -- the resulting new approximants are fully RScl-invariant in the perturbative QCD (pQCD). Further, we present some numerical examples. Both the dPA's and the new approximants are global, i.e., their structure goes beyond the usual (polynomial) TPS form and thus they could reveal some non-perturbative effects.

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arXiv1998

Towards a physical expansion in perturbative gauge theories by using improved Baker-Gammel approximants

Applicability of the previously introduced method of modified diagonal Baker-Gammel approximants is extended to truncated perturbative series (TPS) of any order in gauge theories. The approximants reproduce the TPS when expanded in power series of the gauge coupling parameter to the order of that TPS. The approximants have the favorable property of being exactly invariant under the change of the renormalization scale, and that property is arrived at by a generalization of the method of the diagonal Padé approximants. The renormalization scheme dependence is subsequently eliminated by a variant of the method of the principle of minimal sensitivity (PMS). This is done by choosing the values of the renormalization-scheme-dependent coefficients, which appear in the beta function of the gauge coupling parameter, in such a way that the diagonal Baker-Gammel approximants have zero values of partial derivatives with respect to these coefficients. The resulting approximants are then independent of the renormalization scale and of the renormalization scheme.

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arXiv1998

Renormalization-scale-invariant continuation of truncated QCD (QED) series -- an analysis beyond large-beta_0 approximation

An approximation algorithm is proposed to transform truncated QCD (or QED) series for observables. The approximation is a modification of the Baker-Gammel approximants, and is independent of the renormalization scale (RScl) $μ$ -- the coupling parameter $α(μ)$ in the series and in the resulting approximants can evolve according to the perturbative renormalization group equation (RGE) to any chosen loop order. The proposed algorithm is a natural generalization of the recently proposed method of diagonal Padé approximants, the latter making the result RScl-invariant in large-$β_0$ approximation for $α(μ)$. The algorithm described below can extract large amount of information from a calculated available truncated perturbative series for an observable, by implicitly resumming large classes of diagrams.

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arXiv1998

Improvement of the method of diagonal Pade approximants for perturbative series in gauge theories

Recently it has been pointed out that diagonal Padé approximants to truncated perturbative series in gauge theories have the remarkable property of being independent of the choice of the renormalization scale as long as the gauge coupling parameter $α(p^2)$ is taken to evolve according to the one-loop renormalization group equation -- i.e., in the large-$β_0$ approximation. In this letter we propose and describe an improvement to the method of diagonal Padé approximants. The improved method results in approximants which are independent of the chosen renormalization scale when $α(p^2)$ evolves at any chosen (in principle arbitrary) loop-level.

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arXiv1997

Has the general two-Higgs-doublet model unnatural FCNC suppression? An RGE analysis

There is a widespread belief that the general two-Higgs-doublet model (G2HDM) behaves unnaturally with respect to evolution of the flavor-changing neutral Yukawa coupling parameters (FCNYCP's) -- i.e., that the latter, although being suppressed at low energies of probes, in general increase by a large factor as the energy of probes increases. We investigate this, by evolving Yukawa parameters by one-loop renormalization group equations (RGE's) and neglecting contributions of the first quark generation. For patterns of FCNYCP suppression at low energies suggested by the existing quark mass hierarchies, FCNYCP's remain remarkably stable (suppressed) up to energies very close to the Landau pole. This indicates that G2HDM preserves FCNYCP suppression, for reasonably chosen patterns of that suppression at low energies.

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arXiv1997

Flavor democracy and quark mass matrices

Based on experimental mass hierarchy, a set of flavor-democratic (FD) quark mass matrices at low energies is discussed. The model predicts CP violation parameters $J_{CP}=(0.3 \pm 0.2) 10^{-4}$ and ${ε'/ε} = (0.6 \pm 0.5) 10^{-3}$. However, this simple FD model also predicts a physical top quark mass not much higher than 100 GeV. As a next step, we assume that the Standard Model (SM) breaks down around some high energy $Λ$, and is replaced by a new FD flavor gauge theory (FGT). This possibility can be investigated by studying renormalization group equations for the Yukawa couplings of the SM with two Higgs doublets for various $m_t$ and VEV ratios $v_U/v_D$. With appropriate flavor-democratic boundary conditions at $Λ_{FGT}$, bounds on masses of top quark and tau-neutrino are derived, which are compatible with experimental bounds.

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arXiv1997

One-loop renormalization group equations of the general framework with two Higgs doublets

We derive one-loop renormalization group equations (RGE's) for Yukawa coupling parameters of quarks and for the vacuum expectation values of the Higgs doublets in a general framework of the Standard Model with two Higgs doublets (2HDM ``type III''). In the model, the neutral-Higgs-mediated flavor-changing neutral currents are allowed but are assumed to be reasonably suppressed at low energies. The popular ``type II'' and ``type I'' models are just special cases of this framework. We also present a numerical example for the RGE flow of Yukawa coupling parameters and masses of quarks. Detailed investigation, through RGE's, of connection between the low energy and the high energy structure of these 2HDM's would give us some additional insights into the viability of such frameworks, i.e., it would tell us for which choices of phenomenologically acceptable values of low energy parameters such frameworks can be regarded as reasonably natural (without excessive changes in the parameter structure as energy of probes increases over a wide range). Furthermore, such analyses could provide us with possible signals of new physics at high energies.

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arXiv1996

Regularization at the next-to-leading order in the top-mode standard model without gauge bosons

We study Higgs condensation in the top-mode standard model at the next-to-leading (NTL) order in 1/Nc, by calculating the effective potential as a function of a hard mass term of the top quark. We include the effects of the top and bottom quarks, all the scalars, and the leading QCD effects, but not the effects of the transverse components of the electroweak gauge bosons. The model contains finite UV energy cutoff parameters Lf, Lb for the fermionic and the bosonic momenta. Condensation takes place at energies L \sim Lf \sim Lb. We describe how to regularize the integrals over the fermionic momenta in a way free of momentum branching ambiguities and treat the terms of 1/Nc expansion mutually consistently. We use three different cutoff procedures. We also discuss how to ensure the validity of Goldstone theorem. Demanding that the NTL contributions not ``wash out'' the leading ones, we obtain for the cutoffs: L = O(1 TeV), a result largely independent of the regularization procedure.

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arXiv1996

Goldstone modes and the Higgs condensation beyond one loop

We study the Higgs condensation $H={\bar t}t$ mechanism in the Top-mode Standard Model at the next-to-leading order in 1/Nc. The calculation includes the effects of the Goldstone fields, but not the effects of the transverse components of the electroweak gauge bosons. The resulting effective theory is parametrized by means of a finite energy cut-off $Ł$ at which the condensation is supposed to take place. Demanding that the next-to-leading order contributions not dominate over the leading order ones, we get a rather low bound for the cut-off: $Ł= O(1 TeV)$. QCD effects can change the results somewhat, but the basic conclusions remain unchanged. The inclusion of the Goldstone degrees of freedom tends to decrease the bound on $Ł$.

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arXiv1996

Top quark effects on the scalar sector of the minimal Standard Model

We study effects of the heavy top quark ($m_t \approx 180 GeV$) on the scalar sector of the minimal Standard Model. We construct the effective potential for the scalar doublet, by first taking into account the leading contributions of the top quark loops. Minimizing this potential gives us a condition analogous to the leading-Nc gap equation of the standard ${\bar t}t$-condensation model (Top-mode standard model). This essentially nonperturbative condition leads to a low ultraviolet cutoff $Ł= O(1 TeV)$ in the case when the bare mass $μ$ of the scalar doublet in the tree level potential satisfies $μ^2 = -M^2_0 \leq 0$ and the scalar doublet there is selfinteracting ($λ> 0$). We demand that the scalar selfinteraction behave perturbatively -- in the sense that its 1-loop contributions influence the effective poten- tial distinctly less than those of the Yukawa coupling of the heavy top quark. When we subsequently include the 1-loop contributions of the scalar and the gauge bosonic sectors in a perturbative manner, the results change numerically, but the cutoff $Ł$ remains $O(1 TeV)$. The resulting Higgs mass $M_H$ is then in the range 150-250 GeV. Furthermore, the results of the paper survive even in the case when the square of the bare mass $μ^2$ is positive, as long as $μ^2 \leq O(λv^2)$, where $λ/4!$ is the usual bare coupling parameter of the quartic selfinteraction of the scalars and $v$ is the vacuum expectation value ($v=246.2 \GeV$, $M^2_H \approx λv^2/3$).

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arXiv1995

Condensation effects beyond one loop in the Top-mode Standard Model without gauge bosons

We study dynamical symmetry breaking in the Standard Model including the next-to-leading order terms. We introduce at a high, but finite, energy scale Lambda a top quark condensate H={t {bar t}} and derive, using path integral methods, the effective potential including quadratic fluctuations in the scalar field H. We neglect the contributions of all components of the massive electroweak gauge bosons. The existence of a non-trivial minimum in the effective potential leads to the condition that the cut-off Lambda is limited from above: Lambda < Lambda_{crit} approx 4.7 m_t^{phys.} (for N_c = 3), and to a new lower bound for the 4-fermion coupling a = (G N_c Lambda^2)/(8 pi^2) > 1.60. Similar results are obtained if we demand, instead, that the next-to-leading order contributions not shift the location z=(m_t^{bare}/Lambda)^2 of the minimum drastically, e.g.~by not more than a factor of 2. The results are reproduced diagrammatically, where the leading plus all the next-to-leading order diagrams in the (1/N_c)-expansion are included. Dominant QCD effects are also included, but their impact on the numerical results is shown to be small.

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