arXiv subjects

D. I. Kazakov

Publications and source records attributed to D. I. Kazakov.

At least 73 records · Page 4Linked to original sources

arXiv2007

Renormalizable 1/N_f Expansion for Field Theories in Extra Dimensions

We demonstrate how one can construct renormalizable perturbative expansion in formally nonrenormalizable higher dimensional field theories. It is based on $1/N_f$-expansion and results in a logarithmically divergent perturbation theory in arbitrary high space-time dimension. First, we consider a simple example of $N$-component scalar filed theory and then extend this approach to Abelian and non-Abelian gauge theories with $N_f$ fermions. In the latter case, due to self-interaction of non-Abelian fields the proposed recipe requires some modification which, however, does not change the main results. The resulting effective coupling is dimensionless and is running in accordance with the usual RG equations. The corresponding beta function is calculated in the leading order and is nonpolynomial in effective coupling. It exhibits either UV asymptotically free or IR free behaviour depending on the dimension of space-time. The original dimensionful coupling plays a role of a mass and is also logarithmically renormalized. We analyze also the analytical properties of a resulting theory and demonstrate that in general it acquires several ghost states with negative and/or complex masses. In the former case, the ghost state can be removed by a proper choice of the coupling. As for the states with complex conjugated masses, their contribution to physical amplitudes cancels so that the theory appears to be unitary.

hep-th↗

arXiv2007

Conformal Invariance = Finiteness and Beta Deformed N=4 SYM Theory

We claim that if by a choice of the couplings the theory can be made conformally invariant (vanishing of the beta functions) it is automatically finite and vice versa. This is demonstrated by explicit example in supersymmetric gauge theory. The formalism is then applied to the beta deformed ${\cal N}=4$ SYM theory and it is shown that the requirement of conformal invariance = finiteness can be achieved for any complex parameter of deformations.

hep-th↗

arXiv2007

Light stops in the MSSM parameter space

We consider the regions of the MSSM parameter space where the top squarks become light and even may be the LSP. This happens when the triple scalar coupling A becomes very big compared to m_0. We show that in this case the requirement that the LSP is neutral imposes noticeable constraint on the parameter space excluding low m_0 and m_{1/2} similar to constraint from the Higgs mass limit. In some cases these constraints overlap. This picture takes place in a wide region of \tanβ. In a narrow band close to the border line the stops are long-lived particles and decay into quarks and neutralino (chargino). The cross-section of their production at LHC via gluon fusion mechanism in this region may reach a few pb.

hep-ph↗

arXiv2007

Renormalizable Expansion for Nonrenormalizable Theories: II. Gauge Higher Dimensional Theories

The previously developed renormalizable perturbative 1/N-expansion in higher dimensional scalar field theories is extended to gauge theories with fermions. It is based on the $1/N_f$-expansion and results in a logarithmically divergent perturbation theory in arbitrary high odd space-time dimension. Due to the self-interaction of non-Abelian fields the proposed recipe requires some modification which, however, does not change the main results. The new effective coupling is dimensionless and is running in accordance with the usual RG equations. The corresponding beta function is calculated in the leading order and is nonpolynomial in effective coupling. The original dimensionful gauge coupling plays a role of mass and is also logarithmically renormalized. Comments on the unitarity of the resulting theory are given.

hep-th↗

arXiv2006

Renormalizable Expansion for Nonrenormalizable Theories: I. Scalar Higher Dimensional Theories

We demonstrate how one can construct renormalizable perturbative expansion in formally nonrenormalizable higher dimensional scalar theories. It is based on 1/N-expansion and results in a logarithmically divergent perturbation theory in arbitrary high odd space-time dimension. The resulting effective coupling is dimensionless and is running in accordance with the usual RG equations. The corresponding beta function is calculated in the leading order and is nonpolynomial in effective coupling. It exhibits either UV asymptotically free or IR free behaviour depending on the dimension of space-time.

hep-th↗

arXiv2006

Light Superpartners at Hadron Colliders

Uncertainties of the MSSM predictions are due to an unknown SUSY breaking mechanism. To reduce these uncertainties, one usually imposes constraints on the MSSM parameter space. Recently, two new constraints became available, both from astrophysics: WMAP precise measurement of the amount of the Dark Matter in the Universe and EGRET data on an excess in diffuse gamma ray flux. Being interpreted as a manifestation of supersymmetry these data lead to severe constraints on parameter space and single out a very restricted area. The key feature of this area is the splitting of light gauginos from heavy squarks and sleptons. We study the phenomenological properties of this scenario, in particular, the cross-sections of superparticle production, their decay patterns and signatures for observation at hadron colliders, Tevatron and LHC. We found that weakly interacting particles in this area are very light so that the cross-sections may reach fractions of a pb with jets and/or leptons as final states accompanied by missing energy taken away by light neutralino with a mass around 100 GeV.

hep-ph↗

arXiv2006

Search for gluinos with ATLAS at LHC

Prospects for ATLAS observation of a SUSY-like signal from two gluinos are investigated within a certain region of the mSUGRA parameter space, where the cross section of the two gluinos production via gluon-gluon fusion is estimated at a rather high level of 13 pb. The event selection trigger uses a very clear signature of the process (4 jets + 4 muons + up to 4 secondary vertices topology) when final decay products of each gluino are b-anti-b and muon-anti-muon pairs and the lightest SUSY particle, the neutralino. Rather high transverse missing energy carried away by two neutralinos is an essential signature of the event and also allows the relevant Standard Model background to be reduced significantly. The generation and reconstruction processes are performed by means of the ATLAS common software framework ATHENA.

hep-ex↗

arXiv2006

Supersymmetry and LHC

The motivation for introduction of supersymmetry in high energy physics as well as a possibility for supersymmetry discovery at LHC (Large Hadronic Collider) are discussed. The main notions of the Minimal Supersymmetric Standard Model (MSSM) are introduced. Different regions of parameter space are analyzed and their phenomenological properties are compared. Discovery potential of LHC for the planned luminosity is shown for different channels. The properties of SUSY Higgs bosons are studied and perspectives of their observation at LHC are briefly outlined.

hep-ph↗

arXiv2006

The supersymmetric interpretation of the EGRET excess of diffuse Galactic gamma rays

Recently it was shown that the excess of diffuse Galactic gamma rays above 1 GeV traces the Dark Matter halo, as proven by reconstructing the peculiar shape of the rotation curve of our Galaxy from the gamma ray excess. This can be interpreted as a Dark Matter annihilation signal. In this paper we investigate if this interpretation is consistent with Supersymmetry. It is found that the EGRET excess combined with all electroweak constraints is fully consistent with the minimal mSUGRA model for scalars in the TeV range and gauginos below 500 GeV.

hep-ph↗

arXiv2006

EGRET Excess of diffuse Galactic Gamma Rays interpreted as a Signal of Dark Matter Annihilation

Elsaesser and Mannheim fit a contribution of Dark Matter Annihilation (DMA) to the extragalactic contribution of the galactic diffuse gamma ray flux, as deduced from the EGRET data by Strong, Moskalenko and Reimer.They find a WIMP mass of 515{+110}{-75} GeV and quote a systematic error of 30\%. However, they do not include large systematic uncertainties from the fact that the determination of the extragalactic flux (EGF) requires a model for the subtraction of the Galactic flux from the data.The data used were obtained with a model without Galactic DM, so one expects additional uncertainty in the region where DMA contributes. Including a Galactic DMA contribution reduces the significance and the WIMP mass. The latter then becomes compatible with the Galactic excess of diffuse gamma rays, which posseses all the properties of DMA with a much higher significance than the extragalactic excess.

astro-ph↗

arXiv2005

Renormalization Group Treatment of Nonrenormalizable Interactions

The structure of the UV divergencies in higher dimensional nonrenormalizable theories is analysed. Based on renormalization operation and renormalization group theory it is shown that even in this case the leading divergencies (asymptotics) are governed by the one-loop diagrams the number of which, however, is infinite. Explicit expression for the one-loop counter term in an arbitrary D-dimensional quantum field theory without derivatives is suggested. This allows one to sum up the leading asymptotics which are independent of the arbitrariness in subtraction of higher order operators. Diagrammatic calculations in a number of scalar models in higher loops are performed to be in agreement with the above statements. These results do not support the idea of the naïve power-law running of couplings in nonrenormalizable theories and fail (with one exception) to reveal any simple closed formula for the leading terms.

hep-th↗

arXiv2005

Production of Long-Lived Sleptons at LHC

We analyse the MSSM parameter space and discuss the narrow band near the so-called co-annihilation region where sleptons may be long-lived particles. This region is consistent with the WMAP restrictions on the Dark matter and depends on the value of $\tanβ$. In this region staus are long-lived and may go through the detector. Due to a relatively small mass (150 $÷$ 850 GeV) their production cross-section at LHC may reach a few % pb.

hep-ph↗

arXiv2005

EGRET Excess of Diffuse Galactic Gamma Rays as Tracer of Dark Matter

The public data from the EGRET space telescope on diffuse Galactic gamma rays in the energy range from 0.1 to 10 GeV are reanalyzed with the purpose of searching for signals of Dark Matter annihilation (DMA). The analysis confirms the previously observed excess for energies above 1 GeV in comparison with the expectations from conventional Galactic models. In addition, the excess was found to show all the key features of a signal from Dark Matter Annihilation (DMA): a) the excess is observable in all sky directions and has the same shape everywhere, thus pointing to a common source; b) the shape corresponds to the expected spectrum of the annihilation of non-relativistic massive particles into - among others - neutral $π^0$ mesons, which decay into photons. From the energy spectrum of the excess we deduce a WIMP mass between 50 and 100 GeV, while from the intensity of the excess in all sky directions the shape of the halo could be reconstructed. The DM halo is consistent with an almost spherical isothermal profile with substructure in the Galactic plane in the form of toroidal rings at 4 and 14 kpc from the center. These rings lead to a peculiar shape of the rotation curve, in agreement with the data, which proves that the EGRET excess traces the Dark Matter.

astro-ph↗

arXiv2005

On the two-loop O(α_s^2) corrections to the pole mass of the t-quark in the MSSM

The paper is devoted to the calculation of additional two-loop O(α_s^2) MSSM corrections to the relation between the pole mass of the t-quark and its running mass in the DR scheme. The value of the second order contribution from large mass expansion is studied. As a by-product we calcutated two-loop anomalous dimension of the running quark DR-mass in the supersymmetric QCD.

hep-ph↗

arXiv2004

Beyond The Standard Model

The present lectures contain an introduction to possible new physics beyond the Standard Model. Having in mind first of all accelerator experiments of the nearest future we concentrate on supersymmetry, a new symmetry that relates bosons and fermions, as the first target of experimental search. Since supersymmetry is widely covered in the literature, we mostly consider novel developments and applications to hadron colliders. We describe then the so-called extra dimensional models in less detail and discuss their possible manifestations.

hep-ph↗

arXiv2004

On High Energy Scattering in Extra Dimensions

We analyze the behaviour of the high-energy scattering amplitude within the brane world scenario in extra dimensions. We argue that contrary to the popular opinion based on the Kaluza-Klein approach, the cross-section does not increase with energy, but changes the slope close to the compactification scale and then decreases like in the 4-dimensional theory. A particular example of the quark-antiquark scattering due to the gluon exchange in the bulk is considered.

hep-ph↗

arXiv2004

Excess of EGRET Galactic Gamma Ray Data interpreted as Dark Matter Annihilation

The diffuse galactic EGRET gamma ray data show a clear excess for energies above 1 GeV in comparison with the expectations from conventional galactic models. The excess is seen with the same spectrum in all sky directions, as expected for Dark Matter (DM) annihilation. This hypothesis is investigated in detail. The energy spectrum of the excess is used to limit the WIMP mass to the 50-100 GeV range, while the sky maps are used to determine the halo structure, which is consistent with a triaxial isothermal halo with additional enhancement of Dark Matter in the disc. The latter is strongly correlated with the ring of stars around our galaxy at a distance of 14 kpc, thought to originate from the tidal disruption of a dwarf galaxy. It is shown that this ring of DM with a mass of $\approx 2\cdot 10^{11} M_\odot$ causes the mysterious change of slope in the rotation curve at $R=1.1R_0$ and the large local surface density of the disc. The total mass of the halo is determined to be $3\cdot 10^{12} M_\odot$. A cuspy profile is definitely excluded to describe the gamma ray data. These signals of Dark Matter Annihilation are compatible with Supersymmetry for boost factors of 20 upwards and have a statistical significance of more than $10σ$ in comparison with the conventional galactic model. The latter combined with all features mentioned above provides an intriguing hint that the EGRET excess is indeed a signal from Dark Matter Annihilation.

astro-ph↗

arXiv2003

Field Theory in Extra Dimensions

We analyze the possibility to construct a self-consistent gauge field theory in $D>4$. We first look for the cancellation of the UV divergences in SUSY theories. Then, following the Wilson RG approach, we study the RG equation for the gauge coupling in perturbative and nonperturbative regimes. In the first case the power low running is discussed. In the second case it is shown that there exist the ultraviolet fixed point where the gauge coupling is dimensionless in any space-time dimension. This fixed point is nonperturbative and corresponds to scale invariant theory. The same phenomenon also happens in supersymmetric theory in D=6.

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