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 \gamma\gamma->q\bar q. Thus we get new evidence of the four-quark nature of the lower scalar states.Comment: 8 pages, 14 figures. The talk given by G.N. Shestakov at the International Seminar "Quarks-2010", Kolomna, Russia, June 6-12, 201

Interference phenomena in the decay Ds+→ηπ0π+D^+_s\to\eta\pi^0\pi^+ induced by the a00(980)−f0(980)a^0_0(980)-f_0(980) mixing

Using the data on the decay $D^+_s\to f_0(980)\pi^+\to K^+K^-\pi^+$, we estimate the amplitude of the process $D^+_s\to\left[f_0(980)\to (K^+K^-+K^0\bar K^0)\to a^0_0(980)\right]\pi^+\to\eta\pi^0\pi^+$, 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 $D^+_s \to\eta \pi^0\pi^+$ are analyzed. As such mechanisms, we examine the transition $D^+_s\to\eta\rho^+\to\eta\pi^0\pi^+$, which is observed in experiment, and the possible transition $D^+_s\to\left(a^0_0(980) \pi^++a^+_0(980)\pi^0\right)\to\eta \pi^0\pi^+$. 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.Comment: 7 pages, 7 figures; v2, replaced with the published version; typos corrected, clarifications and reference adde

Two-Photon Production of Four-Quark States Up to the J/ψJ/\psi Energy

Evidence for an explicitly exotic state with isospin 2 and spin-parity 2^+ near the $\rho\rho$ 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 $\gamma\gamma\to\rho^0\phi$ and $\gamma\gamma\to\rho^0\rho^0$ at high energies.Comment: RevTex, 5 pages and one PS figure. Talk presented by G.N. Shestakov at the International Workshop on e^+e^- Collisions from $\phi$ to $J/\psi$, Budker Institute of Nuclear Physics, Novosibiksk, Russia, March 1-5, 199

Proposal to look for the anomalous isotopic symmetry breaking in central diffractive production of the f1(1285)f_1(1285) and a00(980)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 $pp\to p(f_1(1285)/f_1(1420))p\to p(\pi^+\pi^-\pi^0)p$ and $pp\to p(K\bar K)p\to p(a^0_0(980))p\to p(\eta\pi^0)p$ 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 $\pi^+\pi^-\pi^0$ and $\eta \pi^0$ channels. Observation of the process $pp\to p(f_1(1285))p\to p(\pi^+\pi^-\pi^0)p$ would be a confirmation of the first results from the VES and BESIII detectors, indicating the very large isospin breaking in the decay $f_1(1285))\to\pi^+\pi^-\pi^0$.Comment: 5 pages, 4 figures; v2: title changed, comments and references adde

Description of the \psi(3770) resonance interfering with the background

The parameters of the interfering $\psi(3770)$ 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 $\psi(3770)$ 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 $\psi(3770)$ and $\psi(2S)$ resonances. When new high-statistics data become available, it will be interesting to test this clarity variant.Comment: 5 pages, 4 figures; v2: matches published version in Phys. Rev.

Mechanisms of the Reaction π−p→a00(980)n→π0ηn\pi^-p\to a^0_0(980)n\to\pi^0\eta n at High Energies

Over the last twenty years, it was assumed that the reaction $\pi^-p\to a^0_0(980)n\to\pi^0\eta n$ 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 $\rho_2$ Regge pole exchange conspiring with its daughter plays a crucial role. The tentative estimates of the absolute $\pi^-p\to a^0_0(980)n\to\pi^0\eta n$ reaction cross section are obtained. The contributions of the one-pion exchange mechanism and the Regge cuts are also estimated.Comment: Talk given at HADRON'9

Manifestations of the a00(980)−f0(980)a^0_0(980)-f_0(980) mixing in D0→KS0π+π−D^0\to K^0_S\pi^+\pi^- and D0→KS0ηπ0D^0\to K^0_S\eta\pi^0 decays

Possible manifestations of the isospin-breaking mixing of the $a^0_0(980)$ and $f_0(980)$ resonances are analyzed in the $D^0\to K^0_S \pi^+\pi^-$ and $D^0\to K^0_S\eta\pi^0$ decays. It is shown that the $a^0_0(980)-f_0(980)$ mixing has the most influence on the $\pi^+\pi^-$ mass spectrum in the decay $D^0\to K^0_S\pi^+\pi^-$. Owing to the $a^0_0(980)-f_0(980)$ mixing, the $f_0(980)$ peak in $D^0 \to K^0_Sf_0(980)\to K^0_S \pi^+\pi^-$ 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.Comment: 7 pages, 2 figures; v2: replaced with the published version; typos corrected, clarifications and reference adde

Effective Lagrangians Induced by the Anomalous Wess-Zumino Action and IG(JPC)=1−(1−+)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 $\rho\pi\to\rho\pi$, $\rho\pi\to\eta\pi$, $\rho\pi\to\eta'\pi$, $\rho\pi\to(K^*\bar K+\bar K^*K)$, 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 $\rho\pi$, $\eta\pi$, $\eta'\pi$, 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.Comment: Parallel session talk given at HADRON 2001, Protvino, Russia, August 28, 2001. LaTex, 5 pages, 1 figur

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

High-statistics data on the reactions $\gamma\gamma\to K^+K^-$ and $\gamma\gamma\to K^0\bar K^0$ 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 $\gamma\gamma\to K^+K^-$ and $\gamma\gamma\to K^0\bar K^0$ reaction thresholds at modern colliders.Comment: 3 pages, 1 figure, accepted in JETP Letter

Observation of the isospin breaking decay Υ(10860)→Υ(1S)f0(980)→Υ(1S)ηπ0\Upsilon(10860)\to\Upsilon(1S)f_0(980)\to\Upsilon(1S)\eta\pi^0 with the Belle II detector

Using the Belle data on the decay $\Upsilon(10860)\to \Upsilon(1S)f_0(980)\to\Upsilon(1S)\pi^+\pi^-$, we estimate the fraction of the process $\Upsilon(10860)\to\Upsilon(1S)f_0(980)\to \Upsilon(1S)\eta\pi^0$, 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 $\Upsilon(10860)\to\Upsilon(1S)\eta\pi^0$ events in the narrow region of the $\eta\pi^0$ invariant mass near the $K\bar K$ thresholds in a time roughly spent on the present Belle experiment.Comment: 5 pages, 2 figures; the comment is added, Ref. [6], that the using of the Flatte propagators for the $f_0(980)$ and $a_0(980)$ states with the masses under the $K\bar K$ thresholds requires the renormalization of the masses