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A. Ghinculov

Publications and source records attributed to A. Ghinculov.

At least 19 recordsLinked to original sources
arXiv2004

The rare decay B --> X_s l^+ l^- to NNLL precision for arbitrary dilepton invariant mass

We present a new phenomenological analysis of the inclusive rare decay $B \to X_s \ell^+\ell^-$. In particular, we present the first calculation of the NNLL contributions due to the leading two-loop matrix elements, evaluated for arbitrary dilepton invariant mass. This allows to obtain the first NNLL estimates of the dilepton mass spectrum and the lepton forward-backward asymmetry in the high $ M^2_{\ell^+ \ell^-}$ region, and to provide an independent check of previously published results in the low $ M^2_{\ell^+ \ell^-}$ region. The numerical impact of these NNLL corrections in the high-mass region ($ M^2_{\ell^+ \ell^-} > 14.4 GeV^2$) amounts to -13% in the integrated rate, and leads to a reduction of the scale uncertainty to $\pm 3%$. The impact of non-perturbative contributions in this region is also discussed in detail.

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arXiv2002

NNLL QCD Corrections to the Decay $B \to X_s \ell^+ \ell^-$

We briefly discuss the status of the NNLL QCD calculations in the inclusive rare B decay $B \to X_s \ell^+ \ell^-$. Two important ingredients, the two-loop matrix elements of the four quark operator ${\cal O}_2$ and the bremsstrahlung contributions, were quite recently finalised. The new contributions significantly improve the sensitivity of the inclusive decay $B \to X_s l^+ l^-$ decay in testing extensions of the standard model in the sector of flavour dynamics; for instance the two-loop calculation cuts the low- scale uncertainty in half and the bremsstrahlung calculation leads to a 10% shift of the position of the zero of the forward- backward asymmetry.

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arXiv2002

Forward--Backward Asymmetry in B -> X_s l^+ l^- at the NNLL Level

We report the results of a new calculation of soft-gluon corrections in B -> X_s l^+ l^- decays. In particular, we present the first calculation of bremsstrahlung and corresponding virtual terms to the lepton forward-backward asymmetry, which allows us to systematically include all contributions to this observable beyond the lowest non-trivial order. The new terms are important, for instance the position of the zero of the asymmetry receives corrections of O(10%). Using a different method, we also provide an independent check of recently published results on bremsstrahlung and infrared virtual corrections to the dilepton-invariant mass distribution.

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arXiv2001

QCD and QED Corrections to Light-by-Light Scattering

We present the QCD and QED corrections to the fermion-loop contributions to light-by-light scattering, gamma gamma to gamma gamma, in the ultrarelativistic limit where the kinematic invariants are much larger than the masses of the charged fermions.

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arXiv2001

Electroweak radiative corrections:Towards a full two-loop analysis

In calculating electroweak radiative corrections at two-loop level, one encounters Feynman graphs with several different masses on the internal propagators and on the external legs, which lead to complicated scalar functions. We describe a general analytic-numerical reduction scheme for evaluating any two-loop diagrams with general kinematics and general renormalizable interactions, whereby ten basic functions form a complete set after tensor reduction. We illustrate this scheme by applying it to two- and three-point functions. We discuss the treatment of infrared singularities within this numerical approach.

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arXiv2000

Two-Loop Correction to Bhabha Scattering

We present the two-loop virtual QED corrections to e^+ e^- to mu^+ mu^- and Bhabha scattering in dimensional regularization. The results are expressed in terms of polylogarithms. The form of the infrared divergences agrees with previous expectations. These results are a crucial ingredient in the complete next-to-next-to-leading order QED corrections to these processes. A future application will be to reduce theoretical uncertainties associated with luminosity measurements at e^+ e^- colliders. The calculation also tests methods that may be applied to analogous QCD processes.

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arXiv2000

Reduction and evaluation of two-loop graphs with arbitrary masses

We describe a general analytic-numerical reduction scheme for evaluating any 2-loop diagrams with general kinematics and general renormalizable interactions, whereby ten special functions form a complete set after tensor reduction. We discuss the symmetrical analytic structure of these special functions in their integral representation, which allows for optimized numerical integration. The process Z -> bb is used for illustration, for which we evaluate all the 3-point, non-factorizable g^2*alpha_s mixed electroweak-QCD graphs, which depend on the top quark mass. The isolation of infrared singularities is detailed, and numerical results are given for all two-loop three-point graphs involved in this process.

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arXiv2000

Loops and legs beyond perturbation theory

Within the non-perturbative 1/N expansion, we discuss numerical methods for calculating multi-loop Feynman graph needed to derive physical scattering amplitudes. We apply higher order 1/N methods to the scalar sector of the standard model, and show the existence of a mass saturation effect. The mass saturation has direct implications for future searches at the LHC and at possible muon colliders.

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arXiv1999

Momentum expansion of massive two-loop Feynman graphs around a finite value

We give an algorithm for obtaining expansions of massive two-loop Feynman graphs in powers of the external momentum around a finite, nonzero value of the momentum. This is based on our general two-loop formalism to reduce massive two-loop graphs with renormalizable interactions into a standard set of special functions. After the algebraic reduction, the final results are obtained by numerical integration. We apply the expansion algorithm to treat the top-dependent corrections of O(g^2 alpha_s) to the b quark self-energy and extract its momentum expansion on-shell.

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arXiv1999

Considerations on anomalous vector boson couplings

We discuss the meaning of anomalous vector boson self couplings. Implications of present experimental constraints for future colliders are discussed. Results for triple vector boson production at the LHC are given.

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arXiv1998

Nonperturbative three-point functions of the O(N) sigma model in the 1/N expansion at NLO

We present a calculation of the three-point functions of the O(N)-symmetric sigma model. The calculation is done nonperturbatively by means of a higher-order 1/N expansion combined with a tachyonic regularization which we proposed in previous publications. We use the results for calculating the standard model process ff -> H -> WW nonperturbatively in the quartic coupling of the scalar sector.

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arXiv1998

Testing nonperturbative techniques in the scalar sector of the standard model

We discuss the current picture of the standard model's scalar sector at strong coupling. We compare the pattern observed in the scalar sector in perturbation theory up to two-loop with the nonperturbative solution obtained by a next-to-leading order 1/N expansion. In particular, we analyze two resonant Higgs scattering processes, ff -> H -> f'f' and ff -> H -> ZZ, WW. We describe the ingredients of the nonperturbative calculation, such as the tachyonic regularization, the higher order 1/N intermediate renormalization, and the numerical methods for evaluating the graphs. We discuss briefly the perspectives and usefulness of extending these nonperturbative methods to other theories.

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arXiv1998

New developments in the 1/N expansion and nonperturbative Higgs physics

We show in this paper that the 1/N expansion is a reliable tool to calculate the properties of a heavy Higgs boson. The 1/N expansion sums up all orders in perturbation theory, and therefore avoids the renormalization scheme dependence of the conventional perturbative approach. It is explained how effects due to the Landau pole of the Higgs sector are isolated and subtracted, and how to perform actual calculations, by computing the Higgs line shape for the processes f\bar f\to H \to ZZ,f'\bar f' at next-to-leading order in the 1/N expansion. The results are compared to the perturbative results to show the agreement between the perturbative and the nonperturbative approach for Higgs masses up to 1 TeV. We conclude that the theoretical predictions for Higgs observables are well under control for the entire kinematical region of the LHC.

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arXiv1998

Higgs mass saturation effect and the LHC discovery potential

In view of recent perturbative and nonperturbative evidence that the peak of the Higgs resonance saturates as the coupling increases, we examine the potential of the future Large Hadron Collider to discover a heavy Higgs resonance by gluon fusion.

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arXiv1998

Massive two--loop integrals in renormalizable theories

We propose a framework for calculating two-loop Feynman diagrams which appear within a renormalizable theory in the general mass case and at finite external momenta. Our approach is a combination of analytical results and of high accuracy numerical integration, similar to a method proposed previously for treating diagrams without numerators. We reduce all possible tensor structures to a small set of scalar integrals, for which we provide integral representations in terms of four basic functions. The algebraic part is suitable for implementing in a computer program for the automatic generation and evaluation of Feynman graphs. The numerical part is essentially the same as in the case of Feynman diagrams without numerators.

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arXiv1997

NLO correction to Higgs boson parameters in the 1/N expansion

We present the result of a calculation of the next-to-leading correction to the Higgs propagator in the 1/N expansion, where the Higgs sector is treated as an O(N) symmetric sigma-model. The results are compared with two-loop perturbation theory. The existing discrepancy between the lowest order of the 1/N expansion and perturbation theory is dramatically reduced by including the NLO in 1/N. We find a maximum effective Higgs mass of 930-980 GeV. We give an approximate relation between Higgs width and mass, which can be used for phenomenological purposes.

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