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G. N. Shestakov

Publications and source records attributed to G. N. Shestakov.

54 records · Page 3Linked to original sources
arXiv2008

Lightest Scalar Resonances and the Dynamics of the γγ\toππReactions

The high-statistics Belle data on the γγ\toπ^+π^- and γγ\toπ^0π^0 reactions have been jointly analyzed. The main dynamical mechanisms of these reactions for energies below 1.5 GeV have been revealed. It has been shown that the direct coupling constants of the σ(600) and f_0(980) resonances with a γγpair are small and that the σ(600)\toγγand f_0(980)\toγγdecays are four-quark transitions due primarily to π^+π^- and K^+K^- loop mechanisms, respectively. The role of the chiral shielding of the σ(600) resonance is emphasized. The widths of the f_0(980)\toγγand σ(600)\toγγdecays averaged over the resonance mass distributions, as well as the width of the f_2(1270)\toγγdecay, are estimated as (Γ_{f_0\toγγ})_{ππ}\approx 0.19 keV, (Γ_{σ\toγγ})_{ππ}\approx 0.45 keV, and Γ_{f_2\toγγ}(m^2_{f_2})\approx 3.8$ keV.

hep-ph↗
arXiv2008

Theory of Scalars

Outline: 1. Introduction, 2. Confinement, chiral dynamics and light scalar mesons, 3. Chiral shielding of the σ(600), chiral constraints (the CGL band), the σ(600) and the f0(980) in ππ->ππ, ππ->KKbar, ϕ->γ\pi0\pi0, 4. The ϕmeson radiative decays on light scalar resonances, 5. Light scalars in γγcollisions. Evidence for four-quark components of light scalars is given. The priority of Quantum Field Theory in revealing the light scalar mystery is emphasized.

hep-ph↗
arXiv2008

Lightest scalar and tensor resonances in $γγ\toππ$ after the Belle experiment

New high statistics Belle data on the $γγ\toπ^+π^-$ reaction cross section measured in the range of pion-pair invariant masses $\sqrt{s}$ between 0.8 GeV and 1.5 GeV are analyzed to clarify the current situation around the $σ(600)$, $f_0(980)$, and $f_2(1270)$ resonances in $γγ$ collisions. The present analysis shows that the direct coupling constants of the $σ(600)$ and $f_0(980)$ resonances to $γγ$ are small, and the $σ(600)\toγγ$ and $f_0(980)\toγγ$ decays are four-quark transitions caused by the $π^+π^-$ and $K^+K^-$ loop mechanisms, respectively. The chiral shielding of the $σ(600)$ resonance takes place in the reactions $γγ\toππ$ as well as in $ππ$ scattering. Some results of a simultaneous description of the $γγ\toπ^+π^-$ and $γγ\toπ^0π^0$ Belle data are also presented. In particular, the following tentative estimate of the $f_2(1270)\toγγ$ decay width is obtained: $Γ_{f_2\toγγ}(m^2_{f_2})\approx 3.68$ keV.

hep-ph↗
arXiv2007

Lightest scalar in the SU_L(2)*SU_R(2) linear σmodel

We consider the lightest scalar meson in the frame of the $SU_L(2)\times SU_R(2)$ linear $σ$ model, keeping in mind that this model could be the low energy realization of the two-flavour QCD. We show that the $σ$ field is described by its four-quark component at least in the $σ$ resonance energy (virtuality) region and the $σ\toγγ$ decay is the four quark transition. We emphasize that residues of the $σ$ pole in the $ππ\toππ$ and $γγ\toππ$ amplitudes do not give an idea about the $σ$ meson nature, and the progress in studying the $σ$ meson production mechanisms in different processes could essentially further us in understanding its nature.

hep-ph↗
arXiv2006

Light Scalar Mesons from Phi to Psi

The following topics are considered. 1) Confinement, chiral dynamics, and light scalar mesons. 2) Chiral shielding of the $σ(600)$. 3) The $ϕ$ meson radiative decays about nature of light scalar resonances. 4) The $J/ψ$ decays about nature of light scalar resonances. 5) The $a_0(980)\toγγ$ and $f_0(980)\toγγ$ decays about nature of light scalar resonances. 6) New round in $γγ\toπ^+π^-$, the Belle data. Arguments in favor of the four-quark model of the $a_0(980)$ and $f_0(980)$ mesons are given.

hep-ph↗
arXiv2005

Shape of the $f_0(980)$ in $γγ\toπ^+π^-$

The Belle Collaboration results on the observation of the $f_0(980)$ resonance in the reaction $γγ\toπ^+π^-$ are analyzed. It is argued that they point to the presence of mechanisms which give rise to a strong distortion of the $f_0(980)$ resonance shape in comparison with the shape of a solitary Breit-Wigner resonance. It is shown that the main factors responsible for the formation of the specific, steplike, shape of the $f_0(980)$ resonance in the $γγ\toπ^+π^-$ reaction cross section are the $K^+K^-$ loop mechanism of the $f_0(980)$ coupling to the $γγ$ system and the destructive interference between the background and $f_0(980)$ resonance contributions in the $π^+π^-$ invariant mass region below the $K^+K^-$ threshold.

hep-ph↗
arXiv2004

Manifestation of the $a^0_0(980)-f_0(980)$ mixing in the reaction $π^-p\toηπ^0n$ on a polarized target

It is argued that the single-spin asymmetry in the reaction $π^-p\to a_0^0(980)n\to(ηπ^0)_S n$ is extremely sensitive to the mixing of the $a^0_0(980)$ and $f_0(980)$ resonances. It is shown that at low momentum transfers (namely, in any one of the intervals $0\leq-t\leq0.025,...,0.1$ GeV$^2$) the normalized asymmetry, which can take the values from -1 to 1, must undergo a jump in magnitude close to 1 in the $ηπ^0$ invariant mass region between 0.965 GeV and 1.01 GeV. The strong asymmetry jump is the straightforward manifestation of the $a^0_0(980)-f_0(980)$ mixing. For observing the jump, any very high quality $ηπ^0$ mass resolution is not required. The energy dependence of the polarization effect is expected to be rather weak. Therefore, it can be investigated at any high energy, for example, in the range from eight to 100 GeV.

hep-ph↗
arXiv2004

Proposed Search for $a^0_0(980)-f_0(980)$ Mixing in Polarization Phenomena

The $K^+$ and $K^0$ meson mass difference induces the mixing of the $a^0_0(980)$ and $f_0(980)$ resonances, the amplitude of which, between the $K^+K^-$ and $K^0\bar K^0$ thresholds, is large in magnitude, of the order of $ m_{K}\sqrt{m^2_{K^0}-m^2_{K^+}}\approx\sqrtαm^2_K$, and possesses the phase sharply varying by about 90$^\circ$. We suggest performing the polarized target experiments on the reaction $π^-p\toηπ^0n$ at high energy in which the fact of the existence of $a^0_0(980)-f_0(980)$ mixing can be unambiguously and very easily established through the presence of a strong jump in the azimuthal asymmetry of the $ηπ^0$ $S$ wave production cross section near the $K\bar K$ thresholds. The presented estimates of the polarization effect to be expected in experiment are to a great extent model independent.

hep-ph↗
arXiv2003

$ππ$ scattering S wave from the data on the reaction $π^-p\toπ^0π^0n$

The results of the recent experiments on the reaction $π^-p\toπ^0π^0n$ performed at KEK, BNL, IHEP, and CERN are analyzed in detail. For the I=0 $ππ$ S wave phase shift $δ^0_0$ and inelasticity $η^0_0$ a new set of data is obtained. Difficulties emerging when using the physical solutions for the $π^0π^0$ S and D wave amplitudes extracted with the partial wave analyses are discussed. Attention is drawn to the fact that, for the $π^0π^0$ invariant mass, m, above 1 GeV, the other solutions, in principle, are found to be more preferred. For clarifying the situation and further studying the $f_0(980)$ resonance thorough experimental investigations of the reaction $π^-p\toπ^0π^0n$ in the m region near the $K\bar K$ threshold are required.

hep-ph↗
arXiv2001

Effective Lagrangians Induced by the Anomalous Wess-Zumino Action and $I^G(J^{PC})=1^-(1^{-+})$ Exotic States

A simple dynamical model for the exotic waves with $I^G(J^{PC})=1^-(1^{-+})$ in the reactions $ρπ\toρπ$, $ρπ\toηπ$, $ρπ\toη'π$, $ ρπ\to(K^*\bar K+\bar K^*K)$, and in the related ones, is constructed beyond the scope of the quark-gluon approach. The model satisfies unitarity and analyticity and uses as a "priming" the anomalous non-diagonal $VPPP$ interaction which couples together the four channels $ρπ$, $ηπ$, $ η'π$, and $K^*\bar K+\bar K^*K$. The possibility of the resonance-like behavior of the $I^G(J^{PC})=1^-(1^{-+})$ amplitudes belonging to the $\{10\}- \{\bar{10}\}$ and $\{8\}$ representations of SU(3) as well as their mixing is demonstrated explicitly in the 1.3--1.6 GeV mass range which, according to the current experiments, is really rich in exotics.

hep-ph↗
arXiv2001

Mechanisms of the $f_0(980)$ production in the reaction $π^-p\toπ^0 π^0n$

The model of the pure one-pion exchange mechanism, which gives a good description of the GAMS results on the alteration of the $S$-wave $π^0π^0$ mass spectrum in the $f_0(980)$ region in the reaction $π^-p\toπ^0π^0n$ with increasing $-t$, is compared with the recent detailed data on the $m$ and $t$ distributions of the $π^-p\toπ^0π^0n$ events obtained by the BNL-E852 Collaboration. It is shown that the predictions of this model are not confirmed by the BNL data. Therefore the observed phenomenon should be explained by the different exchange mechanism. It is most likely to be the $a_1$ exchange mechanism.

hep-ph↗
arXiv2001

Effective Lagrangian induced by the anomalous Wess-Zumino action and the exotic resonance state with $I^G(J^{PC})=1^-(1^{-+})$ in the $ρπ$, $ηπ$, $η'π$, and $K^*\bar K+\bar K^*K$ channels

A simple model for the exotic waves with $I^G(J^{PC})=1^-(1^{-+})$ in the reactions $VP\to VP$, $VP\to PP$, and $PP\to PP$ is constructed beyond the scope of the quark-gluon approach. The model satisfies unitarity and analyticity and uses as a "priming" the "anomalous" nondiagonal $VPPP$ interaction which couples together the four channels $ρπ$, $ηπ$, $η'π$, and $K^*\bar K+\bar K^*K$. The possibility of the resonancelike behavior of the $I^G(J^{PC})=1^-(1^{-+})$ amplitudes belonging to the $\{10\}- \{\bar{10}\}$ and $\{8\}$ representations of SU(3) as well as their mixing is demonstrated explicitly in the $1.3-1.6$ GeV mass range which, according to the current experimental evidence, is really rich in exotics.

hep-ph↗
arXiv1999

Exotic $ρ^\pmρ^0$ state photoproduction

It is shown that the list of unusual mesons planned for a careful study in photoproduction can be extended by the exotic states $X^\pm(1600)$ with $I^G(J ^{PC})=2^+(2^{++})$ which should be looked for in the $ρ^\pmρ^0$ decay channels in the reactions $γN\toρ^\pmρ^0N$ and $γN\toρ^\pm ρ^0Δ$. The full classification of the $ρ^\pmρ^0$ states by their quantum numbers is presented. A simple model for the spin structure of the $ γp\to f_2(1270)p$, $γp\to a^0_2(1320)p$, and $γN\to X^\pm (N, Δ)$ reaction amplitudes is formulated and the tentative estimates of the corresponding cross sections at the incident photon energy $E_γ\approx 6$ GeV are obtained: $σ(γp\to f_2(1270)p)\approx0.12$ $μ$b, $ σ(γp\to a^0_2(1320)p)\approx0.25$ $μ$b, $σ(γN\to X^\pm N\toρ^\pmρ^0N)\approx0.018$ $μ$b, and $σ(γp\to X^-Δ^{++ }\toρ^-ρ^0Δ^{++})\approx0.031$ $μ$b. The problem of the $X^\pm$ signal extraction from the natural background due to the other $π^\pmπ^0 π^+π^-$ production channels is discussed. In particular the estimates are presented for the $γp\to h_1(1170)π^+n$, $γp\toρ'^{+}n\to π^+π^0π^+π^-n$, and $γp\toωρ^0p$ reaction cross sections. Our main conclusion is that the search for the exotic $X^\pm(2^+(2^{++}))$ states is quite feasible at JEFLAB facility. The expected yield of the $γN\to X^\pm N\toρ^\pmρ^0N$ events in a 30-day run at the 100% detection efficiency approximates $2.8\times10^6$ events.

hep-ph↗
arXiv1999

Two-Photon Production of Four-Quark States Up to the $J/ψ$ Energy

Evidence for an explicitly exotic state with isospin 2 and spin-parity 2^+ near the $ρρ$ threshold and nontrivial complementary indications of the unusual quark composition of the $f_0(980)$ and $a_0(980)$ states obtained from the reactions of two-photon formation of neutral meson resonances are discussed, together with puzzling phenomena in the channels $γγ\toρ^0ϕ$ and $γγ\toρ^0ρ^0$ at high energies.

hep-ph↗
arXiv1998

New explanation of the GAMS results on the $f_0(980)$ production in the reaction $π^-p\to π^0π^0n$

The observed alteration of the S-wave $π^0π^0$ mass spectrum in the reaction $π^-p\toπ^0π^0n$ with increasing $-t$, i.e., the disappearance of a dip and the appearance of a peak in the region of the $f_0(980)$ resonance as $-t$ increases, is explained by the contribution of the $π^-p\to f_0(980)n$ reaction amplitude with the quantum numbers of the $a_1$ Regge pole in the $t$ channel. It is very interesting that nontrivial evidence for the $a_1$ exchange mechanism in the reaction $π^-p\to π^0π^0n$ follows for the first time from the experiment on an unpolarized target. The explanation of the GAMS results suggested by us is compared with that reported previously. Two ways of experimentally testing these explanations are pointed out.

hep-ph↗
arXiv1997

Mechanisms of the Reaction $π^-p\to a^0_0(980)n\toπ^0ηn$ at High Energies

Over the last twenty years, it was assumed that the reaction $π^-p\to a^0_0(980)n\toπ^0ηn$ at high energies is dominated by $b_1$ Regge pole exchange. We show that the new Brookhaven and Serpukhov data on the $t$ distributions for this reaction can be explained within the framework of the Regge pole model only if the $ρ_2$ Regge pole exchange conspiring with its daughter plays a crucial role. The tentative estimates of the absolute $π^-p\to a^0_0(980)n\toπ^0ηn$ reaction cross section are obtained. The contributions of the one-pion exchange mechanism and the Regge cuts are also estimated.

hep-ph↗
arXiv1996

Mechanisms of the reaction pi^-p --> a^0_0(980)n --> p-^0 eta n at high energies

The main dynamical mechanisms of the reaction $π^-p \rightarrow a^0_0 (980)n \rightarrow π^0ηn$ at high energies, currently investigated at Serpukhov and Brookhaven, are considered in detail. It is shown that the observed forward peak in its differential cross section can be explained within the framework of the Regge pole model only by the conspiring $ρ_2$ Regge pole exchange. The tentative estimates of the absolute $π^-p \rightarrow a^0_0(980)n \rightarrow π^0ηn$ reaction cross section at $P_{lab}^{π^-} = 18$ GeV/c are obtained: $σ\approx200$ nb and, in the forward direction, $dσ/dt\approx940$ nb/GeV$^2$. The contribution of the one pion exchange, which is forbidden by $G$-parity and which can rise owing to the $f^0_0(980)-a^0_0(980)$ mixing, is also estimate. A role of the Regge cuts in the non-flip helicity amplitude is briefly examined and a conclusion is made that the contributions of the cuts have to be inessential in comparison with the conspiring $ρ_2$ Regge pole exchange.

hep-ph↗