arXiv subjects

N. N. Achasov

Publications and source records attributed to N. N. Achasov.

At least 55 records · Page 3Linked to original sources

arXiv2013

Pion form factor and reactions e^+e^---> omega pi^0 and e^+e^---> pi^+ pi^- pi^+ pi^- at energies up to 2-3 GeV in the many-channel approach

Using the field-theory-inspired expression for the pion electromagnetic form factor F_π, a good description of the data in the range -10 pi^+ pi^- is obtained at energies up to 3 GeV. The inclusion of the VP and AP loops demands the treatment of the reactions e^+e^---> omega pi^0 and e^+e^---> pi^+ pi^- pi^+ pi^-. This task is also made, with the SND data on the omega pi^0 production and the BABAR data on the pi^+ pi^- pi^+ pi^- production, both in e^+e^- annihilation, upon taking into account ρ(770) and the heavier ρ(1450), ρ(1700), and ρ(2100) resonances. The problems with inclusion of the VP and AP loops are pointed out and discussed.

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arXiv2013

Comparative study of the production of scalar and tensor mesons in $e^+e^-$ collisions

The intensity of scalar $a_0(980)$, $f_0(980)$ and tensor $a_2(1320)$, $f_2(1270)$ mesons production at VEPP-2000 (BINP, Novosibirsk) and the upgraded DA$Φ$NE (Frascati, Italy) in the processes $e^+e^- \to a_0 (f_0$, $a_2$, $f_2) γ$ is calculated. The calculation was made with the help of VDM and the kaon loop model (for scalar mesons case). Note that the processes $e^+e^-\to a_2(f_2)γ$ have not been studied in the energy region of VEPP-2000. It turned out that in the VEPP-2000 energy region $1.7-2.0$ GeV $σ(e^+e^-\to a_2(f_2)γ)\sim 10$ pb, and $σ(e^+e^-\to a_0(f_0)γ)\sim 0.1$ pb. Photon angle distribution and the spin density matrices of $a_2$ and $f_2$ were calculated also.

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arXiv2013

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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arXiv2012

Light scalars in semi-leptonic decays of heavy quarkonia

We study the mechanism of production of the light scalar mesons in the $D_s^+\toπ^+π^- e^+ν$ decays: $D_s^+\to s\bar s e^+ν\to [σ(600)+f_0(980)] e^+ν\toπ^+π^- e^+ν$, and compare it with the mechanism of production of the light pseudoscalar mesons in the $D_s^+\to (η/η') e^+ν$ decays: $D_s^+\to s\bar s e^+ν\to (η/η')e^+ν$. We show that the $s\bar s\toσ(600)$ transition is negligibly small in comparison with the $s\bar s\to f_0(980)$ one. As for the the $f_0(980)$ meson, the intensity of the $s\bar s\to f_0(980)$ transition makes near thirty percent from the intensity of the $s\bar s\toη_s$ ($η_s=s\bar s$) transition. So, the $D_s^+\toπ^+π^- e^+ν$ decay supports the previous conclusions about a dominant role of the four-quark components in the $σ(600)$ and $f_0(980)$ mesons.

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arXiv2012

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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arXiv2012

Pion form factor in the range -10 GeV^2 < s < 1 GeV^2

Based on the field-theory-inspired approach, a new expression for the pion form factor F_pi is proposed. It takes into account the pseudoscalar meson loops $π^+π^-$ and $K\bar K$ and the mixing of $ρ(770)$ with heavier $ρ(1450)$ and $ρ(1700)$ resonances. The expression possesses correct analytical properties and describes the data in the wide range of the energy squared $-10{GeV}^2\leq s\leq1$ GeV$^2$ without introducing the phenomenological Blatt -- Weisskopf range parameter $R_π$.

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arXiv2012

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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arXiv2012

Analytical $ππ$ scattering amplitude and the light scalars-II

In the paper Phys. Rev. {\bf D83}, 054008 (2011) we constructed the $ππ$ scattering amplitude $T^0_0$ with regular analytical properties in the $s$ complex plane, describing both experimental data and the results based on chiral expansion and Roy equations. Now the results obtained during development of our work are presented. We dwell on questions dealing with the low $σ-f_0$ mixing, inelasticity description and the kaon loop model for $ϕ\to γ(σ+f_0)$ reaction, and show a number of new fits. In particular, we show that the minimization of the $σ-f_0$ mixing results in the four-quark scenario for light scalars: the $σ$(600) coupling with the $K\bar K$ channel is suppressed relatively to the coupling with the $ππ$ channel, and the $f_0$(980) coupling with the $ππ$ channel is suppressed relatively to the coupling with the $K\bar K$ channel. The correct analytical properties of the $ππ$ scattering amplitude are reached with the help of rather complicated background function. We also suggest much more simple background parameterization, practically preserving the resonance features, which is comfortable for experimental data analysis, but allows to describe the results based on chiral expansion and Roy equations only on the real $s$ axis.

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arXiv2011

Study of Light Scalars, the learned Lessons

Attention is paid to the production mechanisms of the light scalars that reveal their nature. In the linear sigma model it is revealed the chiral shielding of the σ(600) meson and shown that the σfield is described by its four-quark component. The ππscattering amplitude is constructed taking into account the σ(600) and f_0(980) mesons, the chiral shielding of σ(600), the σ(600)-f_0(980) mixing, and results, obtained on the base of the chiral expansion and the Roy equations. The data agree with the four-quark nature of σ(600) and f_0(980). It is shown, that the kaon loop mechanism of the $ϕ$ radiative decays into the light scalar mesons, which is ratified by experiment, is the four-quark transition and points to the four-quark nature of the light scalars. It is shown also, 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, which are produced via two-quark transitions γγ\to q\bar q, that points to the four-quark nature of the light scalar mesons, too. A programme of further investigations is laid down.

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arXiv2011

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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arXiv2011

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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arXiv2011

Electromagnetic form factor of pion in the field theory inspired approach

A new formula for the pion form factor F_pi is proposed which takes into account the pseudoscalar meson loops and the mixing among rho(770), rho(1450), and rho(1700). The expression possesses correct analytical properties and can be used in both timelike and spacelike kinematical regions. The comparison is made with the existing experimental data on F_pi collected by the detectors SND, CMD-2, KLOE, and BaBaR restricted to energies below 1 GeV. A good description of all four data sets is obtained. In the spacelike region, upon substituting the resonance parameters found in the timelike one, the formula gives F_pi, in agreement with the measurements of the NA7 Collaboration.

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arXiv2011

The analytical $ππ$ scattering amplitude and the light scalars

In this work we construct the $ππ$ scattering amplitude $T^0_0$ with regular analytical properties in the $s$ complex plane, which describes simultaneously the data on the $ππ$ scattering, $ϕ\toπ^0π^0γ$ decay, and $ππ\to K\bar K$ reaction. The chiral shielding of the $σ(600)$ meson and its mixing with the $f_0(980)$ meson are also taken into account. The data agrees with the four-quark nature of the $σ(600)$ and $f_0(980)$ mesons. The amplitude in the range $-5 m_π^2 < s < 0.64$ GeV$^2$ also agrees with results, obtained on the base of the chiral expansion, dispersion relations, and the Roy equations.

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arXiv2010

Analytical properties of the $ππ$ scattering amplitude and the light scalar mesons

The $ππ$ scattering amplitude with regular analytical properties in the $s$ complex plane has been constructed. It describes simultaneously the data on the $ππ$ scattering, $ϕ\toπ^0π^0γ$ decay and $ππ\to K\bar K$ reaction. The chiral shielding of the $σ(600)$ meson and it's mixing with the $f_0(980)$ meson are taken into account also. The data agrees with the four-quark nature of the $σ(600)$ and $f_0(980)$ mesons. The amplitude in the range $-5 m_π^2 < s < 0.64$ GeV$^2$ also agrees with results, obtained on the base of the chiral expansion, dispersion relations and the Roy equations.

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arXiv2010

Troubles of describing multiple pion production in chiral dynamics

Generalized Hidden Local Symmetry (GHLS) model as the chiral model of pseudoscalar, vector, and axial vector mesons and their interactions containing also the couplings of strongly interacting particles with electroweak gauge bosons, is confronted with the ALEPH data on the decay $τ^-\toπ^-π^-π^+ν_τ$ and BABAR and CMD data on the reaction $e^+e^-\toπ^+π^-π^+π^-$. It is shown that both the invariant mass spectrum of final pions in $τ$ decay calculated in the GHLS framework with the single $a_1(1260)$ resonance and the cross section $e^+e^-\toπ^+π^-π^+π^-$ calculated in the above framework with the single $ρ(770)$ resonance, disagree with the experimental data. The modifications of GHLS model based on inclusion of two additional heavier axial vector mesons $a_1^\prime$, $a_1^{\prime\prime}$ in the $τ$ decay and the vector mesons $ρ^\prime$, $ρ^{\prime\prime}$ in $e^+e^-\toπ^+π^-π^+π^-$ are shown to be necessary for the good description of the above data.

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arXiv2010

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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arXiv2010

Confronting generalized hidden local symmetry chiral model with the ALEPH data on the decay tau^- to pi^+ pi^- pi^- nu_tau

Generalized Hidden Local Symmetry (GHLS) model is the chiral model of pseudoscalar, vector, and axial vector mesons and their interactions. It contains also the couplings of strongly interacting particles with electroweak gauge bosons. Here, GHLS model is confronted with the ALEPH data on the decay $τ^-\toπ^-π^-π^+ν_τ$. It is shown that the invariant mass spectrum of final pions in this decay calculated in GHLS framework with the single $a_1(1260)$ resonance disagrees with the experimental data at any reasonable number of free GHLS parameters. Two modifications of GHLS model based on inclusion of two additional heavier axial vector mesons are studied. One of them giving a good description of the ALEPH data, with all the parameters kept free is shown to result in very large $Γ_{a_1^\pm\toπ^\pmγ}$ partial width. The other scheme with the GHLS parameters fixed in a way that the universality is preserved and the observed central value of $Γ_{a_1^\pm\toπ^\pmγ}$ is reached, results in a good description of the three pion spectrum in $τ^-\toπ^+π^-π^-ν_τ$ decay.

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arXiv2010

The Z\to c\bar c\toγγ^{\ast}, Z\to b\bar b\toγγ^{\ast} triangle diagrams and the Z\toγψ, Z\to γΥdecays

It is expounded the approach to the Z\toγΨand Z\toγΥdecay study, based on the sum rules for the Z\to c\bar c\toγγ^{\ast} and Z\to b\bar b\toγγ^{\ast} amplitudes and their derivatives. The branching ratios of the Z\toγψand Z\toγΥdecays are calculated for different guesses as to saturation of the sum rules. The lower bounds of \sum_ψ BR(Z\toγψ) = 1.95\cdot 10^{-7} and \sum_Υ BR(Z\toγΥ) = 7.23\cdot 10^{-7} are found. Deviations from the lower bounds are discussed, among them the possibility of BR(Z\toγJ/ψ(1S))\sim BR(Z\toγΥ(1S))\sim 10^{-6}, that could be probably measured in LHC. The angle distributions in the Z\toγψand Z\toγΥdecays are calculated also.

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