Fate of Light Scalar Mesons
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ate of Light Scalar Mesons N. N. Achasov* Sobolev Institute for Mathematics, Novosibirsk, 630090 Russia *e-mail: [email protected] Received December 20, 2019; revised January 16, 2020; accepted January 29, 2020
Abstract—It is shown that all predictions for the light scalars, based on their four-quark nature, are supported by experiment. The future research program is outlined also. DOI: 10.1134/S1063779620040048
1. INTRODUCTION. OVERTURE Peter Higgs, C. R. Physique 8 (2007) 970–972: “Another example, which comes closer to the kind of symmetry breaking which is of interest in particle physics, is superfluidity. In 1947 Bogoliubov studied Bose condensation of an infinite system of neutral spinless bosons with short-range repulsive two-body interactions. Such a condensate is characterised by a ’macroscopic wave function’ (the order parameter) which is complex; its modulus squared is a measure of the observable condensate density, but its argument (which is unobservable) is arbitrary, thus breaking the symmetry of the dynamics under rotations of the boson wave functions in the Argand diagram. The shortrange interactions are represented in the second-quantised Hamiltonian by a term proportional to the square of the particle density, that is, to a quartic in the components of the scalar quantum field”. 2. OUTLINE The a0(980) and f0(980) mesons are well-established parts of the proposed light scalar meson nonet [1]. From the beginning, the a0(980) and f0(980) mesons became one of the central problems of nonperturbative QCD, as they are important for understanding the way chiral symmetry is realized in the low-energy region and, consequently, for understanding confinement. Many experimental and theoretical papers have been devoted to this subject. There is much evidence that supports the four-quark model of light scalar mesons [2, 3]. The suppression of the a00(980) and f0(980) resonances in the γγ → ηπ0 and γγ → ππ reactions,
respectively, was predicted in 1982 [4], Γa0γγ ≈ Γ f0γγ ≈ 0.27 keV, and confirmed by experi0 ment [1]. The high quality Belle data [5], Figs. 1, 2, allowed to elucidate the mechanisms of the σ(600), f0(980), and a00(980) resonance production in γγ collisions confirmed their four-quark structure. σ(600) = f0(500) ! Light scalar mesons are produced in γγ collisions via rescatterings, mainly via the γγ → π+ π− → f0(500) , γγ → K + K − → f0(980) a0(980) transitions, that is, via the four-quark transitions. As for a2(1320) and f2(1270) (the well-known qq states), they are produced mainly via the two-quark transitions (direct couplings with γγ ) [6–9]. As a result the practically model-independent prediction of the qq model g 2f2γγ : ga22γγ = 25 : 9 agrees with experiment rather well. As to the ideal qq model prediction g 2f0γγ : ga20γγ = 25 : 9 , it is excluded by experiment. Note also that the absence of J ψ → γf0(980), ρ a0(980), ωf0(980) decays in the presence of the intense J ψ → γf2(1270), γf2'(1525), ρ a2(1320), ωf2(1270) decays is at variance with the P -wave twoquark, qq , structure of a0(980)
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