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V. A. Nikonov

Publications and source records attributed to V. A. Nikonov.

116 records · Page 7Linked to original sources

Partial widths for the decays eta(1295) -> gamma gamma and eta(1440) -> gamma gamma

We discuss gamma gamma partial widths of pseudoscalar/isoscalar mesons eta(M) in the mass region M = 1000-1500 MeV. The transition amplitudes eta(1295) -> gamma gamma and eta(1440) -> gamma gamma are studied within an assumption that the decaying mesons are the members of the first radial excitation nonet 2^1S_0 q\bar q. The calculations show that partial widths being of the order of 0.1 keV are dominantly due to the n\bar n meson component while the contribution of the s\bar s component is small.

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Quark Model and Neutral Strange Secondary Production by Neutrino and Antineutrino Beams

The experimental data on $K^0$ and $Λ$ production by $ν$ and $\barν$ beams are compared with the predictions of quark model assuming that the direct production of secondaries dominates. Disagreement of these predictions with the data allows one to suppose that there exists considerable resonance decay contribution to the multiplicities of produced secondaries.

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Partial widths a_0(980)\toγγ, f_0(980)\toγγand q\bar q-classification of the lightest scalar mesons

We calculate partial widths for the decays a_0(980)\toγγand f_0(980)\toγγunder the assumption that a_0(980) and f_0(980) are members of the basic 1^3P_0 q\bar q nonet. The results are in a reasonable agreement with data thus giving an argument for a q\bar q origin of these mesons. We also calculate the γγpartial widths for the other scalar mesons, members of the 2^3P_0 q\bar q nonet.

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Pomeron in diffractive processes $γ^*(Q^2)p\toρ^0 p$ and $γ^*(Q^2)p\toγ^*(Q^2) p$ at large Q^2: the onset of pQCD

We study the reactions $γ^*(Q^2)p\toρ^0 p$ and $γ^*(Q^2)p\toγ^*(Q^2) p$ at large Q^2 and $W^2/Q^2$ and small momentum transfer, $κ^2_\perp$, to the nucleon where the pomeron exchange dominates. At large Q^2 the virtual photon selects a hard $q\bar q$ pair, thus selecting the hard pomeron component (the BFKL pomeron). The amplitudes for both transverse and longitudinal polarizations of the initial photon and outgoing $ρ$-meson (photon) are calculated in the framework of the BFKL pomeron exchange. Our calculations show that one cannot expect the early onset of the pure perturbative regime in the discussed diffractive processes: the small interquark distances, $ρ_{q\bar q} <0.2$ fm, start to dominate not earlier than at $Q^2 \simeq 100 GeV^2, W^2/Q^2 \simeq 10^7$ in $γ^*(Q^2)p\toρ^0 p$ and $Q^2 \simeq 50 GeV^2, W^2/Q^2 \simeq 10^6$ in $γ^*(Q^2)p\toγ^*(Q^2) p$.

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Hadron Diffractive Processes: the Structure of Soft Pomeron and Colour Screening

On the basis of the experimental data on diffractive processes in $πp$, $pp$ and $p \bar p$ collisions at intermediate, moderately high and high energies, we restore the scattering amplitude related to the $t$-channel exchange by vacuum quantum numbers by taking account of the diffractive $s$-channel rescatterings. At intermediate and moderately high energies, the $t$-channel exchange amplitude turns, with a good accuracy, into an effective pomeron which renders the results of the additive quark model. At superhigh energies the scattering amplitude provides a Froissart-type behaviour, with an asymptotic universality of cross sections such as $σ^{tot}_{πp}/σ^{tot}_{pp} \to 1$ at $s \to \infty$. The quark structure of hadrons being taken into account at the level of constituent quarks, the cross sections of pion and proton (antiproton) in the impact parameter space of quarks, $σ_π(\vec {r}_{1\perp},\vec {r}_{2\perp};s)$ and $σ_p(\vec {r}_{1\perp},\vec {r}_{2\perp},\vec {r}_{3\perp};s)$, are found as functions of $s$. These cross sections implicate the phenomenon of colour screening: they tend to zero at $|\vec r_{i\perp}-\vec r_{k\perp}| \to 0$. The effective colour screening radius for pion (proton) is found for different $s$. The predictions for the diffractive cross sections at superhigh energies are presented.

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η-η' -Glueball mixing from photon-meson transition form factors and decay ratio D_s --> ηl ν/ η' l ν

The background due to the direct diffractive dissociation of the photon into the $π^+π^-$-pair to the "elastic" diffractive $ρ^0$-meson production in electron-proton collisions is calculated. The amplitude for the background process $γp \to π^+ π^- p$ is proportional to the $π$-meson - proton cross section. Therefore, describing the HERA data, we can estimate $σ(πp)$ at energy $s_{πp}\sim (2 - 3) 10^{3} GeV^2$ that is considerably higher the existing data. At large $Q^2$ the interference between resonant and non-resonant $π^+ π^-$ production leads also to the more slow increase of the $σ^L/σ^T$ ratio with the mass of the $2π$ (i.e. $ρ^0$-meson) state.

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Photon--meson transition form factors $γπ^0$, $γη$ and $γη'$ at low and moderately high $Q^2$

We study photon--meson transition form factors $γ^*(Q^2)γ\to π^0,η,η'$ at low and moderately high virtualities $Q^2$ of one of the photons, the second photon being real. For the description of the form factor at low $Q^2$, a nontrivial quark--antiquark $(q\bar q)$ structure of the photon in the soft region is assumed, i.e. the photon is treated much like an ordinary vector meson. At large $Q^2$, along with a perturbative tail of the soft wave function, as it is for a hadron, the photon wave function contains also a standard QED point-like $q\bar q$ component. The latter provides the $1/Q^2$ behavior of the transition form factor at large $Q^2$ in accordance with perturbative QCD. Using the experimental results on $γπ^0$ form factor, we reconstruct the soft photon wave function which is found to have the structure similar to pion's, which has been formerly determined from a study of the elastic pion form factor. Assuming the universality of the ground--state pseudoscalar meson wave functions we calculate the transition form factors $γη$, $γη'$ and partial widths $η\toγγ$, $η'\toγγ$, in a perfect agreement with data.

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QCD-Motivated Pomeron and High Energy Hadronic Diffractive Cross Sections

Soft diffractive cross sections in $pp$ (or $\bar pp$), $πp$ and $γp$ collisions are calculated using QCD-motivated pomeron. The $s$-channel unitarization of the amplitude is performed by the eikonal approach, and in the framework of the quark structure of hadrons the colour screening is taken into account. The performed description of the diffractive processes has led to the parameters of the bare pomeron $P$ which occurred to be close to the corresponding parameters of the Lipatov's pomeron. Parameters of the bare pomeron and those of the three-reggeon block $PGG$ ($G$ is a reggeized gluon) has been fixed by the data at moderately high energies; for superhigh energies the predictions are made. The intercept of the bare pomeron is found to be in remarkable agreement with the low-$x$ data for deep inelastic scattering. An evaluation of the effective colour transparency radius is done which can be used in hadron-nuclei reactions.

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