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Strong isospin symmetry breaking in light scalar meson production

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S.L. Sobolev Institute for Mathematics, Siberian Branch of the Russian Academy of Sciences, prosp. akad. Koptyuga 4, Novosibirsk, 630090, Russian Federation

Isospin symmetry breaking is discussed as a tool for studying the nature and production mechanisms of light scalar mesons. We are concerned with isospin breaking effects with amplitude \sim\sqrt{m_{\rm d}-m_{\rm u}} (instead of the usual \sim (m_{\rm d}-m_{\rm u})), where m_{\rm u} and m{\rm _d} are the u and d quark masses, respectively, whose magnitude and phase vary with energy in a resonance-like way characteristic of the {\rm K}\bar {\rm K} threshold region. The review considers a variety of reactions that can reveal (or have revealed) experimentally the mixing of {\rm a}^0_0(980)- and {\rm f}_0(980) resonances that breaks the isotopic invariance due to the mass difference of the {\rm K}^+ and {\rm K}^0 mesons. The experimental results on the search for the {\rm a}^0_0(980)-{\rm f}_0(980) mixing in the {\rm f}_1(1285)\to {\rm f}_0(980)\pi^0\to\pi^+\pi^-\pi^0 и \eta(1405)\to {\rm f}_0(980)\pi^0\to\pi^+\pi^-\pi^0 decays have suggested a broader perspective on the isotopic symmetry breaking effects due to the {\rm K}^+{\rm K}^0 mass difference. It has become clear that not only the {\rm }a^0_0(980)-{\rm f}_0(980) mixing but also any mechanism producing {\rm K}\bar {\rm K} pairs with a definite isospin in the S wave gives rise to such effects, thus suggesting a new tool for studying the nature and production mechanisms of light scalars. Of particular interest is the case of a large isotopic symmetry violation in the \eta(1405)\, \to\,{\rm f}_0(980)\pi^0\,\to\,\pi^+\pi^-\pi^0 decay due to the occurrence of anomalous Landau thresholds (logarithmic triangle singularities), i.e., due to the \eta(1405)\to({\rm K}^*\bar {\rm K}+\bar {\rm K}^*{\rm K})\to({\rm K}^+{\rm K}^-+{\rm K}^0\bar {\rm K}^0)\pi^0\to {\rm f}_0(980)\pi^0\to\pi^+\pi^-\pi^0 transition (where the {\rm K}^* meson should be crucially considered to be of finite width).

Now text is available in Russian only.
Keywords: physics of light scalar mesons, isospin symmetry breaking, resonance mixing, reaction mechanisms, experimental investigations
PACS: 11.30.Ly, 12.38.Qk, 13.30.Eg, 13.75.Lb (all)
DOI: 10.3367/UFNe.2018.01.038281
URL: https://ufn.ru/en/articles/2019/1/b/
Citation: Achasov N N, Shestakov G N "Strong isospin symmetry breaking in light scalar meson production" Phys. Usp. 62 (1) (2019)

Received: 11th, December 2017, revised: 1st, January 2018, 16th, January 2018

Оригинал: Ачасов Н Н, Шестаков Г Н «Сильное нарушение изотопической симметрии при рождении лёгких скалярных мезонов» УФН 189 3–32 (2019); DOI: 10.3367/UFNr.2018.01.038281

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