arXiv subjects

M. L. Nekrasov

Publications and source records attributed to M. L. Nekrasov.

At least 19 recordsLinked to original sources

arXiv2026

PDF at small $x$ in the non-perturbative region

Parton distribution function (PDF) at small $x$ in a fast-moving proton is investigated within an upgraded parton model that includes parton splitting with branching cascades and parton fusion. In the region of moderately small $x$, we obtain a power-law behavior of the parton density $x f(x)$ with an exponent proportional to the logarithm of the probability of parton splitting. Taking into account parton fusion leads to a nonlinear equation for the PDF. In the region of very small $x$, the phenomenon of saturation of the parton density is detected and a model estimate of its value in this regime is obtained. The results are compared with those obtained previously based on the analysis of equations in logarithmic approximations of perturbative QCD.

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arXiv2025

High-energy Coulomb scattering of spatially extended particles

We analyze pure Coulomb high-energy elastic scattering of charged particles (hadrons or nuclei), discarding their strong interactions. We distinguish three scattering modes, determined by the magnitude of the momentum transfer, in which particles behave as point-like, structureless extended, and structured composite objects. The results are compared in the potential and QFT approaches of the eikonal model. In the case of proton Coulomb scattering at the LHC the difference between these two approaches is significant. This indicates the unsuitability of the potential approach. However, in the case of Coulomb scattering of heavy nuclei, the leading one is the optical approximation, which formally reproduces the prescription of the potential approach.

hep-ph↗

arXiv2024

Coulomb-nuclear interference in elastic proton scattering in the eikonal approach

We find exact solution in the Cahn eikonal model, which describes Coulomb-nuclear interference in elastic scattering of charged hadrons. The cases of both point-like and extended particles equipped with electromagnetic form factors are considered. According to the solution obtained the Coulomb-nuclear contributions are not exponentiated and cannot be added to the Coulomb phase. At the same time, the $O(α)$-approximation of the amplitude is ambiguous, which makes it unsuitable for data processing.

hep-ph↗

arXiv2023

pp elastic scattering at ISR and LHC energies

We describe data on elastic $pp$ scattering at ISR and LHC energies using the analytical representation of the amplitude as the sum of three exponentials equipped with complex phases. In addition, we take into account the Coulomb and Coulomb-nuclear contributions in the entire transfer region and strong "perturbative" contributions in the large transfer region. Refined values of basic scattering characteristics are obtained. Significant changes in the behavior of the amplitude are detected at the transition from ISR to LHC energies. The results obtained indicate a two-layer structure of the colliding protons. In the transition to 13 TeV, a sharp increase in the size of the inner layer and its activity in the scattering is observed. A possible interpretation of this effect is discussed.

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arXiv2022

Resonant mode of elastic scattering of protons at ultra-high energies

An interpretation is given for elastic proton scattering at ultra-high energies with impact-parameter amplitude exceeding the black disk limit. It is shown that this scattering mode can arise due to the contribution of an exceptional intermediate state that unites correlated partons of colliding protons at ultra-high energies into a single coherent system. The behavior of the real part of the amplitude in this mode is discussed.

hep-ph↗

arXiv2021

On the shape of nucleons at high energies

A scenario of evolution of the shape of nucleons with increasing energy is described in the framework of extended parton model, which consistently takes into account the transverse motion of the partons. At the energy $E$ up to LHC, the nucleons have the form of a spheroid which expands as $\sqrt{\ln \!E}$ in the transverse directions and grows linearly in $E$ in the longitudinal direction. With a further increase in the energy a mode of correlated behavior of the partons is established, which stops the longitudinal growth. Simultaneously, the expansion in the transverse directions changes to $\ln \!E$, and a hollow mostly free of partons is formed inside the nucleons along the central axis in the direction of their motion. Numerical estimates of the corresponding parameters are obtained.

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arXiv2020

On the transverse size of hadrons at asymptotically high energies

We show that Gribov diffusion of the partons in the impact parameter plane, which leads to the square-root-of-logarithmic growth of the transverse size of the hadrons, can occur only simultaneously with a similar diffusion in the transverse-momentum space. At the same time, a restriction of the partons in the transverse momenta entails an increase in their propagation in the impact parameter plane. Ultimately this leads to a logarithmic growth of the transverse size of hadrons at asymptotically high energies.

hep-ph↗

arXiv2019

Contributions of unstable and virtual particles to nuclear form factors

The coherent meson scattering off heavy nuclei with the production of two particles in the final state is investigated. We obtain the form factors for the direct production of the final state and through intermediate particles, unstable ones that can decay inside the nucleus and virtual ones. The cases of scattering both in the Coulomb and in the strong field of the nucleus are considered. This work is stimulated by an experimental study of the chiral anomaly in a beam of charged kaons in the OKA facility.

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arXiv2017

Non-coherent contributions in charge-exchange reactions and $η$--$η'$ mixing

We analyse $K^- p \to (η, η',π^0) Λ$ on the basis of the fit of data in a wide region of energies, and $π^- p \to (η, η') n$ at the energies of GAMS-4$π$. We show that disagreements between the data and the predictions of Regge theory may be explained by the mode change of summation of intermediate contributions at increasing energy, from coherent to non-coherent. A method of experimental measurement of the non-coherent contributions is proposed. On the basis of available data on the charge-exchange reactions the $η$--$η'$ mixing is estimated.

hep-ph↗

arXiv2015

Charge-exchange reactions from the standpoint of the parton model

Using simple arguments, we show that charge-exchange reactions at high energies go through the hard scattering of fast quarks. On this basis we describe $π^- p \to M^0 n$ and $K^- p \to M^0 Λ$, $M^0 =π^0, η, η'$, in a combined approach which defines hard contributions in the parton model and soft ones in Regge phenomenology. The disappearance of a dip according to recent GAMS-4$π$ data in the differential cross-section $K^- p \to ηΛ$ at $|t| \approx$ 0.4--0.5 (GeV$\!$/c)$^2$ at transition to relatively high momenta, is explained as a manifestation of a mode change of summation of hard contributions from coherent to incoherent. Other manifestations of the mentioned mode change are discussed. Constraints on the $η$--$η'$ mixing and gluonium admixture in $η'$ are obtained.

hep-ph↗

arXiv2014

$K^{-}p$ and $π^{-}p$ charge-exchange processes and $η- η'$ mixing

The processes of charge-exchanges of $π^{-}$ and $K^{-}$ on protons at high energies and zero transfers are described on the basis of the parton model. It is shown that at the mentioned kinematic conditions the scattering occurs on the valence quarks only, while scattering on sea quarks leads to disruption of final state. Thereby a substantiation is given of the valence approximation for the specified reaction. On the basis of GAMS-4$π$ data it is shown that the description of $η- η'$ mixing does not require an introduction of additional states that do not fit into the scheme of basis states $|η_N >$ and $|η_S >$. The resulting value of the mixing angle is $ϕ_p=(36.1\pm 0.9)^0$.

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arXiv2012

Modified perturbation theory for angular distribution in W-pair production

We examine the capabilities of the modified perturbation theory (MPT) for description of $W$-pair production and decay in $e^{+} e^{-}$ annihilation. In a model with Dyson-resummed propagators of unstable particles, we calculate even and odd contributions to the distribution in the cosine of the $W^{-}$ production angle relative to the $e^{-}$ beam. On comparing the results of calculations in the NNLO approximation of MPT with the exact results in the model, a coincidence of outcomes at the ILC energies is detected at the per-mille level.

hep-ph↗

arXiv2012

Results of numerical simulations for unstable-particles pair production in modified perturbation theory in NNLO

We consider pair production and decay of fundamental unstable particles in the framework of a modified perturbation theory (MPT), which treats resonant contributions of unstable particles in the sense of distributions. The cross-sections for the top-quark pair production and for the $W$-boson pair production in $e^{+}e^{-}$ annihilation are calculated within the NNLO in models that admit exact solutions. In both cases an excellent convergence of the MPT is detected at the energies close to and above the maximum of the cross section. In the case of $W$-boson pair production a precision of the description at the ILC energies is ensured at the level of one per-mille or higher.

hep-ph↗

arXiv2011

W-pair production in modified perturbation theory

We examine capabilities of the modified perturbation theory (MPT) for description of the processes with productions and decays of fundamental unstable particles. We calculate total cross-section for $e^{+} e^{-} \to γ,Z \to W^{+} W^{-} \to 4f$ in a model with the Dyson-resummed and with the MPT-expanded up to the NNLO Breit-Wigner factors, and compare the outcomes. At the ILC energies a coincidence of the outcomes is detected with precision better than one per-mille.

hep-ph↗

arXiv2011

Pair production of fundamental unstable particles in modified perturbation theory in NNLO

We consider pair production and decay of fundamental unstable particles in the framework of a modified perturbation theory (MPT) treating resonant contributions in the sense of distributions. The cross-section of the process is calculated within the NNLO of the MPT in a model that admits exact solution. Universal massless-particles contributions are taken into consideration. A comparison of the outcomes with the exact solution demonstrates excellent convergence of the MPT series at the energies near and above the maximum of the cross-section.

hep-ph↗

arXiv2010

Unstable-particles pair production in modified perturbation theory in NNLO

We consider pair production and decay of fundamental unstable particles in the framework of a modified perturbation theory (MPT), which treats resonant contributions of unstable particles in the sense of distributions. The cross-section of the process is calculated within the NNLO of the MPT in a model that admits exact solution. Universal massless-particles contributions are taken into consideration. The calculations are carried out by means of FORTRAN code with double precision, which ensures per mille accuracy of computations. A comparison of the outcomes with the exact solution reveals an excellent convergence of the MPT series at the energies close to and above the maximum of the cross-section. Near the maximum of the cross-section the discrepancy of the NNLO approximation makes up a few per mille.

hep-ph↗

arXiv2010

Unstable-particles pair production in MPTapproach in NNLO

We consider pair production and decay of fundamental unstable particles in the framework of a modified perturbation theory treating resonant contributions in the sense of distributions. Outcomes of a numerical simulation up to the NNLO with taking into account universal massless-particles contributions are presented.

hep-ph↗

arXiv2009

Modified perturbation theory for pair production and decay of fundamental unstable particles

We construct an asymptotic expansion in powers of the coupling constant directly of the cross-section for pair production and decay of fundamental unstable particles. The resonant and kinematic singularities arising in the expansion we treat in the sense of distributions. This mode allows us to transform formally divergent integrals into absolutely convergent ones with keeping the asymptotic property of the expansion. The appropriate procedure is elaborated up to an arbitrary order of the expansion. The peculiarity of application of the procedure in the threshold region is analysed. The scheme of the calculations within the NNLO approximation is explicitly discussed.

hep-ph↗