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S. R. Gevorkyan

Publications and source records attributed to S. R. Gevorkyan.

At least 19 recordsLinked to original sources

Impact of vector mesons polarization on its interaction with matter

The production of unstable particles on different nuclei provides the possibility to determine the total cross section of the interaction of vector mesons $V=ρ,ω,φ, K^{*0}(892),J/ψ$ etc. with nucleons. This interaction is defined by a set of amplitudes that correspond to the transverse (helicity $λ=\pm 1$) or longitudinal ($λ=0$) polarization of the vector meson. The total cross section for the interaction of transversely polarized vector mesons with nucleon $σ_T=σ(V_T N)$ has been extracted from the coherent photoproduction of vector mesons off nuclei, while the vector meson production in charge exchange reactions as $π^\pm(K^\pm)+A\to V^0(K^{*0})+A'$ provides the unique opportunity to obtain the not yet measured total cross section for longitudinally polarized vector meson interacting with nucleon $σ_L=σ(V_L N)$. We shortly discuss the importance of the knowledge of $σ_L$ and possibility to extract its value from experiments on nuclear target.

hep-ph

Enhancement of low-mass dileptons in ultraperipheral collisions

It is shown that production of low-mass $e^+e^-$-pairs in ultraperipheral nuclear collisions is enhanced due to the Sommerfeld-Gamow-Sakharov (SGS) factor. This effect is especially strong near the threshold of creation of unbound $e^+e^-$-pairs with low masses in the two-photon fusion. Coulomb attraction of the non-relativistic components of such pairs may lead to the increased intensity of 511 keV photons. It can be recorded at the NICA collider and has some astrophysical implications. The analogous effect can be observed at LHC in dilepton production.

hep-ph

Dimesoatom breakup in the Coulomb field

Momentum and angular distributions of charged meson pairs $h^+h^-$ ($h=π, K$) from elementary atoms (EA) breakup (ionization) in the Coulomb field of a target atom is considered in the Born and Glauber approximations. Exploiting the fact that the atomic screening of the target Coulomb potential is important at small transfer momenta, while multi-photon exchanges are essential at large transfer momenta we express the cross sections of EA breakup as a sum of two terms. In the region of modest transfer momenta the cross section is determined by the single-photon exchange (first Born approximation) accounting for the target atoms screening, whereas at large transfer momenta using the unscreened potential allows to take into account all multi-photon exchanges and obtain the cross section of EA breakup in the close analytical form.

hep-ph

Different approaches to calculate the $K^{\pm}\to π^{\pm}π^0e^+e^-$ decay width

The rare $K^\pm\toπ^\pmπ^0 e^+e^-$ decay, currently under analysis by the NA48/2 Collaboration, is considered. We have performed two theoretical approaches to calculate the differential decay width -- in the kaon rest frame, where we use Cabibbo-Maksimovicz variables, and in the center-of-mass system of the lepton pair. The latter essentially simplifies the computations. A comparison between the two approaches has been performed. We have also found the dependencies of the differential decay rate as a function of the virtual photon and dipion system masses

hep-ph

Vector exchanges in production of light meson pairs and elementary atoms

The production of pseudoscalar and scalar mesons pairs and bound states (positronium or pionium atoms) in high energy $γγ$ collisions at high energies provided by photon or vector meson exchanges are considered. The vector exchanges lead to nondecreasing with energy cross section of binary process $γ+γ\to h_a+h_b$ with $h_a, h_b$ states in the fragmentation regions of initial particles. The production of light mesons pairs $ππ, ηη, η'η', σσ$ as well as a pairs of positronium $Ps$ and pionium $A_π$ atoms in peripheral kinematics are discussed. Unlike the photon exchange the vector meson exchange needs a reggeization, leading to fall with energy. Nevertheless due to peripheral kinematics out of very forward production angles the vector meson exchanges dominated. The proposed approach allows to express the matrix elements of the considered processes through impact factors, which can be calculated in perturbation models like Chiral Perturbation Theory (ChPT) or Nambu-Jona-Lasinio (NJL) model or determined from $γγ$ sub-processes or vector mesons radiative decay widths. We obtain the cross sections for pionium atom production in collisions of high energy pions and electrons with protons. The possibility to measure these processes in experiment are discussed.

hep-ph

Formation of $πμ$ atoms in $K_{μ4} decay

We derive the decay rate of $πμ$ atom formation in $K_{μ4}$ decay. Using the obtained expressions we calculate the decay rate of atom formation and point out that considered decay can give a noticeable contribution as a background to the fundamental decay $K^+\to π^+ν\barν$.

hep-ph

Final state interaction in kaons decays

The kaons decays to the pairs of charged and neutral pions are considered in the framework of the non-relativistic quantum mechanics. The general expressions for the decay amplitudes to the two different channels accounting for the strong interaction between pions are obtained. The developed approach allows one to estimate the contribution of terms of any order in strong interaction and correctly takes into account the electromagnetic interaction between the pions in the final state.

hep-ph

Radiative corrections in $K_{e4}$ decay

The final state interaction of pions in the decay $K^\pm\to π^+π^-e^\pmν$ allows to obtain the value of the isospin and angular momentum zero pion-pion scattering length $a_0^0$. To extract this quantity from experimental data the radiative corrections (RC) have to be taken into account. Basing on the lowest order results and the factorization hypothesis, we get the expressions for RC in the leading and next-to leading logarithmical approximation. It is shown that the decay width dependence on the lepton mass $m_e$ through the parameter $σ=\fracα{2π}\br{\ln\frac{M^2}{m_e^2}-1}$ has a standard form of the Drell-Yan process and is proportional to the Sommerfeld-Sakharov factor. The numerical estimations are presented.

hep-ph

The electromagnetic effects in $K_{e4}$ decay

The final state interaction of pions in $K_{e4}$ decay allows one to obtain the value of the isospin and angular momentum zero $ππ$ scattering length $a_0^0$.We take into account the electromagnetic interaction of pions and isospin symmetry breaking effects caused by different masses of neutral and charged pions and estimate the impact of these effects on the procedure of scattering length extraction from $K_{e4}$ decay.

hep-ph

The isospin symmetry breaking effects in $K_{e4}$ decays

The Fermi-Watson theorem is generalized to the case of two coupled channels with different masses and applied to final state interaction in $K_{e4}$ decays. The impact of considered effect on the phase of the $ππ$ scattering is estimated and shown that it can be crucial for scattering lengths extraction from experimental data on $K_{e4}$ decays.

hep-ph

Electromagnetic effects and scattering lengths extraction from experimental data on $K \to 3π$ decays

The final state interactions in $K^\pm\toπ^\pmπ^0π^0$ decays are considered using the methods of non-relativistic quantum mechanics. We show how to take into account the largest electromagnetic effects in the analysis of experimental data, using the amplitudes calculated earlier. We propose the relevant expressions for amplitude corrections valid both above and below the two charged pion production threshold $M_{π^0π^0}=2m_{π^\pm}$, including the average effect for the threshold bin. These formulae can be used in the procedure of pion scattering lengths measurement from $M_{π^0π^0}$ spectrum.

hep-ph

Electromagnetic corrections to final state interactions in $K\to 3π$ decays

The final state interactions of pions in decays $K^\pm\toπ^\pmπ^0π^0$ are considered using the methods of quantum mechanics. We show how to incorporate the electromagnetic effects in the amplitudes of these decays and to work out the relevant expressions valid above and below the two charged pions production threshold $M_c=2m$. The electromagnetic corrections are given as evaluated in a potential model.

hep-ph

Challenge of lepton pair production in peripheral collisions of relativistic ions

The new approach to the lepton pair production in the Coulomb field of two highly relativistic nuclei was developed.Solving the operator equation for lepton scattering in arbitrary Coulomb field, we obtain the amplitudes for lepton scattering in the Coulomb potential in terms of light cone variables. Using the Watson expansion for the amplitude of lepton scattering on two centers we propose prescription which allows one to construct the amplitude for lepton pair production in the Coulomb field of two highly relativistic ions. We show that for the certain sums of finite terms of the Watson series numerous cancellations lead to infrared stability of amplitude.

hep-ph

The inelastic photon-electron collisions with polarized beams

We discussed the photoproduction of pair of charged particles $a\bar{a}\quad (a=e,μ,π)$ as well as the double photon emission processes off an electron accounting for the polarization of colliding particles. In the kinematics when all the particles can be considered as a massless, we obtain the compact analytical expressions for the differential cross sections of these processes. As the application of obtained results the special cases of production by circular and linear polarized photons are considered.

hep-ph

Multiple exchanges in lepton pair production in high--energy heavy ion collisions

The recent analysis of nuclear distortions in DIS off nuclei revealed a breaking of the conventional hard factorization for multijet observable. The related pQCD analysis of distortion effects for jet production in nucleus-nucleus collisions is as yet lacking. As a testing ground for such an analysis we consider the Abelian problem of higher order Coulomb distortions of the spectrum of lepton pairs produced in peripheral nuclear collisions. We report an explicit calculation of the contribution to the lepton pair production in the collision of two photons from one nucleus with two photons from the other nucleus, $2γ+ 2γ\to \m{l}^+\m{l}^-$. The dependence of this amplitude on the transverse momenta has a highly nontrivial form the origin of which can be traced to the mismatch of the conservation of the Sudakov components for the momentum of leptons in the Coulomb field of the oppositely moving nuclei. The result suggests that the familiar eikonalization of Coulomb distortions breaks down for the oppositely moving Coulomb centers, which is bad news from the point of view of extensions to the pQCD treatment of jet production in nuclear collisions. On the other hand, we notice that the amplitude for the $2γ+ 2γ\to \m{l}^+\m{l}^-$ process has a logarithmic enhancement for the lepton pairs with large transverse momentum, which is absent for $nγ+ mγ\to \m{l}^+\m{l}^-$ processes with $m,n > 2$. We discuss the general structure of multiple exchanges and show how to deal with higher order terms which cannot be eikonalized.

hep-ph

The lowest order inelastic QED processes at polarized photon-electron high energy collisions

The compact expressions for cross sections of photoproduction of a pair of charged particles $\mathrm{e}^+,\mathrm{e}^-$; $μ^+,μ^-$; $π^+,π^-$ as well as the double Compton scattering process are given. The explicit analytic expressions for the case of polarized photon and the initial electron in the kinematics when all the particles can be considered as a massless ones are presented. The photon polarization is described in the terms of Stokes parameters.

hep-ph

Ultraperipheral photoproduction of vector mesons in the nuclear Coulomb field and the size of neutral vector mesons

We point out a significance of ultraperipheral photoproduction of vector mesons in the Coulomb field of nuclei as a means of measuring the radius of the neutral vector meson. This new contribution to the production amplitude is very small compared to the conventional diffractive amplitude, but because of large impact parameters inherent to the ultraperipheral Coulomb mechanism its impact on the diffraction slope is substantial. We predict appreciable and strongly energy dependent increase of the diffraction slope towards very small momentum transfer.The magnitude of the effect is proportional to the mean radius squared of the vector meson and is within the reach of high precision photoproduction experiments, which gives a unique experimental handle on the size of vector mesons.

hep-ph

Lepton pair production in relativistic ion collisions to all orders in $Zα$ with logarithmic accuracy

The problem of summation of the perturbation series for the process of lepton pair production in relativistic ion collisions is investigated. We show that the amplitude of this process can be obtained in the compact analytical form, if one confines to the terms growing as a powers of logarithm energy in the cross section, the approximation of which is completely justified for such colliders as RHIC and LHC. Using this result we calculate the Coulomb corrections to the cross section of the process under consideration.

hep-ph