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G. N. Shestakov

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

At least 37 records · Page 2Linked to original sources

Proposal to look for the anomalous isotopic symmetry breaking in central diffractive production of the $f_1(1285)$ and $a^0_0(980)$ resonances at the LHC

At very high energies, and in the central region ($x_F\simeq0$), the double-Pomeron exchange mechanism gives the dominant contribution to the production of hadrons with the positive $C$ parity and isospin $I=0$. Therefore, the observation of resonances in the states with $I=1$ will be indicative of their production or decay with the isotopic symmetry breaking. Here, we bear in mind the cases of the anomalous breaking of the isotopic symmetry, i.e., when the cross section of the process breaking the isospin is not of the order of $10^{-4}$ of the cross section of the allowed process but of the order of $1\%$. The paper draws attention to the reactions $pp\to p(f_1(1285)/f_1(1420))p\to p(π^+π^-π^0)p$ and $pp\to p(K\bar K)p\to p(a^0_0(980))p\to p(ηπ^0)p$ in which a similar situation can be realized, owing to the $K\bar K$ loop mechanisms of the breaking of isotopic symmetry. We note that there is no visible background in the $π^+π^-π^0$ and $ηπ^0$ channels. Observation of the process $pp\to p(f_1(1285))p\to p(π^+π^-π^0)p$ would be a confirmation of the first results from the VES and BESIII detectors, indicating the very large isospin breaking in the decay $f_1(1285))\toπ^+π^-π^0$.

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Observation of the isospin breaking decay $Υ(10860)\toΥ(1S)f_0(980)\toΥ(1S)ηπ^0$ with the Belle II detector

Using the Belle data on the decay $Υ(10860)\to Υ(1S)f_0(980)\toΥ(1S)π^+π^-$, we estimate the fraction of the process $Υ(10860)\toΥ(1S)f_0(980)\to Υ(1S)ηπ^0$, caused by the mixing of $a^0_0(980)$ and $f_0(980)$ resonances that breaks the isotopic invariance due to the $K^+$ and $K^0$ meson mass difference. With an instantaneous luminosity of $8\times10^{35}$ cm$^2\cdot$\,s$^{-1}$ at the SuperKEKB collider, one can collect with the Belle II detector about a hundred $Υ(10860)\toΥ(1S)ηπ^0$ events in the narrow region of the $ηπ^0$ invariant mass near the $K\bar K$ thresholds in a time roughly spent on the present Belle experiment.

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Interference phenomena in the decay $D^+_s\toηπ^0π^+$ induced by the $a^0_0(980)-f_0(980)$ mixing

Using the data on the decay $D^+_s\to f_0(980)π^+\to K^+K^-π^+$, we estimate the amplitude of the process $D^+_s\to\left[f_0(980)\to (K^+K^-+K^0\bar K^0)\to a^0_0(980)\right]π^+\toηπ^0π^+$, caused by the mixing of $a^0_0(980)$ and $f_0(980)$ resonances that breaks the isotopic invariance due to the $K^+$ and $K^0$ meson mass difference. Effects of the interference of this amplitude with the amplitudes of the main mechanisms responsible for the decay $D^+_s \toηπ^0π^+$ are analyzed. As such mechanisms, we examine the transition $D^+_s\toηρ^+\toηπ^0π^+$, which is observed in experiment, and the possible transition $D^+_s\to\left(a^0_0(980) π^++a^+_0(980)π^0\right)\toηπ^0π^+$. It is shown that the rapidly varying phase of the $a^0_0(980)-f_0(980)$ transition amplitude strongly influences on the interference curves.

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Manifestations of the $a^0_0(980)-f_0(980)$ mixing in $D^0\to K^0_Sπ^+π^-$ and $D^0\to K^0_Sηπ^0$ decays

Possible manifestations of the isospin-breaking mixing of the $a^0_0(980)$ and $f_0(980)$ resonances are analyzed in the $D^0\to K^0_S π^+π^-$ and $D^0\to K^0_Sηπ^0$ decays. It is shown that the $a^0_0(980)-f_0(980)$ mixing has the most influence on the $π^+π^-$ mass spectrum in the decay $D^0\to K^0_Sπ^+π^-$. Owing to the $a^0_0(980)-f_0(980)$ mixing, the $f_0(980)$ peak in $D^0 \to K^0_Sf_0(980)\to K^0_S π^+π^-$ can experience distortions comparable with its size. The effect essentially depends on the relative phase between the amplitudes of the $D^0\to K^0_Sf_0(980)$ and $D^0\to K^0_Sa^0_0(980)$ transitions.

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Comparative study of the scalar- and tensor-meson production in the reaction $γγ^*(Q^2)\to ηπ^0$

The prediction of the cross section $σ(γγ^*(Q^2)\to ηπ^0)$ based on the simultaneous description of the Belle data on the $γγ\to ηπ^0$ reaction and the KLOE data on the $ϕ\toηπ^0γ$ decay is presented. The production of the scalar $a_0(980)$ and tensor $a_2(1320)$ is studied in detail. It is shown that the QCD based asymptotics of the $γ^*(Q^2)γ\to a_2(1320)\toηπ^0$ cross section can be reached by the compensation of the contributions of $ρ(770)$ and $ω(782)$ with the contributions of their radial excitations in $Q^2$ channel. At large $Q^2$ the $a_2(1320)$ contribution is expected to be dominant, while at $Q^2=0$ it is similar to the scalars contribution.

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Mechanisms of the isospin-breaking decay $f_1(1285)\to f_0(980)π^0\toπ^+π^-π^0$

Estimated are the contributions of the following mechanisms responsible for the decay $f_1(1285)\to f_0(980)π^0\toπ^+π^-π^0$: 1) the contribution of the $a^0_0(980)-f_0(980)$ mixing, $f_1(1285)\to a_0(980)π^0\to (K^+K^-+K^0\bar K^0)π^0\to f_0(980)π^0\toπ^+π^-π^0$, 2) the contribution of the transition $f_1(1285)\to(K^+K^-+K^0\bar K^0)π^0\to f_0 (980)π^0\toπ^+π^-π^0$, arising due to the pointlike decay $f_1 (1285)\to K\bar Kπ^0$, 3) the contribution of the transition $f_1(1285)\to(K^*\bar K+\bar K^*K)\to(K^+K^-+K^0\bar K^0)π^0\to f_0(980)π^0\to π^+π^-π^0$, where $K^*=K^*(892)$, and 4) the contribution of the transition $f_1(1285)\to(K^*_0\bar K+\bar K^*_0K)\to(K^+K^-+K^0\bar K^0)π^0\to f_0(980)\toπ^+ π^-π^0$, where $K^*_0=K^*_0(800)$ (or $κ$) and $K^*_0(1430)$. These mechanisms break the conservation of the isospin due to the nonzero mass difference of the $K^+$ and $K^0$ mesons. They result in the appearance of the narrow resonance structure in the $π^+π^-$ mass spectrum in the region of the $K\bar K$ thresholds, with the width $\approx2m_{K^0}-2m_{K^+}\approx8$ MeV. The observation of such a structure in experiment is the direct indication on the $K\bar K$ loop mechanism of the breaking of the isotopic invariance. We point out that existing data should be more precise, and it is difficult to explain them using the single specific mechanism from those listed above. Taking the decay $f_1(1285)\to f_0(980)π^0 \toπ^+π^-π^0$ as the example, we discuss the general approach to the description of the $K\bar K$ loop mechanism of the breaking of isotopic invariance.

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Study of the $f_2(1270)$ and $a_2(1320)$ resonances in $γ^*(Q^2)γ$ collisions

We discuss studies of the $Q^2$ dependence of the $f_2(1270)$ and $a_2 (1320)$ production cross sections in $γ^*(Q^2)γ$ collisions at current and coming into operation colliders with a high luminosity. Changing the dominant helicity amplitude occurs in the reactions $γ^*(Q ^2)γ\to f_2(1270)$ and $γ^*(Q^2) γ\to a_2(1320)$ with increasing $Q^2$. This is caused by the coming of the QCD asymptotics. It is shown that the transition to the asymptotic behavior of QCD in the amplitudes $γ^*(Q^2) γ\to f_2(1270),a_2(1320)$ is provided by the compensation of the contributions of ground vector states $ρ$ and $ω$ in $Q^2$-channel with the contributions of their radial excitations.

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Isospin breaking decay $η(1405) \to f_0(980)π^0 \to 3π$

There are attempts in the literature to theoretically explain the large breaking of isotopic invariance in the decay $η(1405)$\,$\to$\,$f_0(980)π^0$\,$\to$\,$ 3π$ by the mechanism containing the logarithmic (triangle) singularity, i.e., as being due to the transition $η(1405)\to(K^*\bar K+\bar K^*K)\to(K^+K^-+K^0\bar K^0)π^0\to f_0(980)π^0\to3π$. The corresponding calculations were fulfilled for a hypothetic case of the stable $K^*$ meson. Here, we show that the account of the finite width of the $K^*$ ($Γ_{K^*\to Kπ}\approx50$ MeV) smoothes the logarithmic singularities in the amplitude and results in the suppression of the calculated decay width $η(1405)\to f_0(980) π^0\to3π$ by the factor of $6-8$ as compared with the case of $Γ_{K^*\to Kπ}$\,=\,0. We also analyze the difficulties related with the assumption of the dominance of the $η(1405)\to(K^*\bar K+\bar K^*K)\to K\bar Kπ$ decay mechanism and discuss the possible dynamics of the decay $η(1405)\toηππ$. The decisive improvement of the experimental data on the $K\bar K$, $ Kπ$, $ηπ$, and $ππ$ mass spectra in the decay of the resonance structure $η(1405/1475)$ to $K\bar Kπ$ and $ηππ$, and on the shape of the resonance peaks themselves in the $K\bar Kπ$ and $ηππ$ decay channels is necessary for the further establishing the $η(1405)\to3 π$ decay mechanism.

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Description of the ψ(3770) resonance interfering with the background

The parameters of the interfering $ψ(3770)$ resonance should be determined from the data on the reactions $e^+e^-$\,$\to$\,$D\bar D$ with the use of the models satisfying the elastic unitarity requirement. The selection of such models can be realized by comparing their predictions with the relevant data on the shape of the $ψ(3770)$ peak in the non-$D\bar D$ decay channels. Here, we illustrate this unitarity approach by the example of the most simple variant of the model of the mixed $ψ(3770)$ and $ψ(2S)$ resonances. When new high-statistics data become available, it will be interesting to test this clarity variant.

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Line shape of ψ(3770) in e^+e^-\to D\bar D

Interference phenomena observed in the $ψ(3770)$ resonance region in the reactions $e^+e^-$ $\to$ $ D\bar D$ are analyzed. To avoid ambiguities in the determination of the $ψ(3770)$ resonance parameters, when analyzing data between the $D\bar D$ and $D\bar Dπ$ thresholds, the amplitudes satisfying the elastic unitarity requirement should be used. In the lack of information on the $P$ wave of $D\bar D$ elastic scattering, the $ψ(3770)$ parameters, determined by fitting the $e^+e^-$ $\to $ $D\bar D$ data, can essentially depend on the model used for the total contribution of the resonance and background. The selection of the models can be toughened by comparing their predictions with the relevant data on the shape of the $ψ(3770)$ peak in the non-$D\bar D$ channels $e^+e^-$ $\to $ $γχ_{c0}$, $J/ψη$, $ϕη$, etc.

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f0(980) and a0(980) resonances near γγ\to K^+K^- and γγ\to K^0\bar K^0 reaction thresholds

High-statistics data on the reactions $γγ\to K^+K^-$ and $γγ\to K^0\bar K^0$ are the last missing link in investigations of the light scalar mesons $f_0(980)$ and $a_0(980)$ in photon-photon collisions. It is believed that $f_0(980)$ and $a_0(980)$ resonances exhibit their four-quark structure in these reactions in a vary peculiar way. The work estimates the feasibility of measurements of scalar contributions near $γγ\to K^+K^-$ and $γγ\to K^0\bar K^0$ reaction thresholds at modern colliders.

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Light Scalar Mesons in Photon-Photon Collisions

The light scalar mesons, discovered over forty years ago, became a challenge for the naive quark-antiquark model from the outset. At present the nontrivial nature of these states is no longer denied practically anybody. Two-photon physics has made a substantial contribution to understanding the nature of the light scalar mesons. Recently, it entered a new stage of high statistics measurements. We review the results concerning two-photon production mechanisms of the light scalars, based on the analysis of current experimental data.

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η(1475) and f_1(1420) resonances in γγ* collisions and J/ψ\toγ(ρρ, γρ^0, γϕ) decays

The available data on the $Q^2$ dependence of the $γγ*(Q^2)\to K\bar Kπ$ reaction cross section in the energy region 1.35-1.55 GeV is explained by the $η(1475)$ resonance production in contrast to their conventional interpretation with the use of the $f_1(1420)$ resonance. It resolves theoretically the contradiction between the suppression of the $η(1475)\toγγ$ decay width and the strong couplings of the $η(1475)$ to the $ρρ$, $ωω$, and $γρ^0$ channels. The experimental check of our explanation requires definition of the spin-parity of the resonance contributions, $R$, in $γγ*(Q^2)\to R\to K\bar Kπ$ and in $J/ψ\to\gammaR\toγγ(ρ^0, ϕ)$. This will help to solve difficulties accumulated in understanding properties of the $η(1475)$ state and its nearest partners.

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Nature of Light Scalar Mesons in Bright Light of Photon-Photon Collisions

The surprising thing is that the light scalar meson problem, arising 50 years ago from the linear sigma model (LSM) with spontaneously broken chiral symmetry, has become central in the nonperturbative quantum chromodynamics (QCD), because it has been made clear that LSM could be the low energy realization of QCD. First, we review briefly signs of four-quark nature of light scalars. Then we show that the light scalars are produced in the two-photon collisions via four-quark transitions in contrast to the classic P wave tensor q\bar q mesons that are produced via two-quark transitions γγ->q\bar q. Thus we get new evidence of the four-quark nature of the lower scalar states.

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Nature of the a_0(980) Meson in the Light of Photon-Photon Collisions

New high-statistics Belle data on the reaction γγ\toπ^0ηare analyzed to clarify the two-photon production mechanisms and the nature of the a_0(980) meson. The obtained solution for the amplitude γγ\toπ^0ηis consistent with the chiral theory expectation for the πηscattering length, with the strong coupling of the a_0(980) to the πη, K\bar K, and πη' channels, and with a key role of the rescattering mechanisms a_0(980)\to(K\bar K+ π^0η+π^0η')\toγγin the a_0(980)\toγγdecay. Such a picture argues in favor of the q^2\bar q^2 nature of the a_0(980) meson and is in agreement with the properties of its partners, the σ_0(600) and f_0(980) mesons, in particular, with those that manifest themselves in γγ\toππ. An important role of the vector meson exchanges in the formation of the nonresonant background in γγ\toπ^0ηis also revealed. The preliminary information on the reaction π^0η\toπ^0ηis obtained.

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Nature of light scalars through photon-photon collisions

The surprising thing is that arising almost 50 years ago from the linear sigma model (LSM) with spontaneously broken chiral symmetry, the light scalar meson problem has become central in the nonperturbative quantum chromodynamics (QCD) for it has been made clear that LSM could be the low energy realization of QCD. First we review briefly signs of four-quark nature of light scalars. Then we show that the light scalars are produced in the two photon collisions via four-quark transitions in contrast to the classic $P$ wave tensor $q\bar q$ mesons that are produced via two-quark transitions $γγ\to q\bar q$. Thus we get new evidence of the four-quark nature of these states.

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