Strong isospin symmetry breaking in light scalar meson production

Abstract

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 an amplitude (instead of the usual ∼ md − mu), where mu and md are the u and d quark masses, whose magnitude and phase vary with energy in a resonance-like way characteristic of the threshold region. We consider a variety of reactions that can experimentally reveal (or have revealed) the mixing of and f0(980) resonances that breaks the isotopic invariance due to the mass difference between K+ and K0 mesons. Experimental results on the search for mixing in f1(1285) → f0(980) π0 → π+ π− π0 and η(1405) → f0(980) π0 → π+ π− π0 decays suggest a broader perspective on the isotopic symmetry breaking effects due to the K+−K0 mass difference. It has become clear that not only the mixing but also any mechanism producing pairs with a definite isospin in an 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 breaking in the η(1405) → f0(980) π0 → π+ π− π0 decay due to the occurrence of anomalous Landau thresholds (logarithmic triangle singularities), i.e., due to the transition (where it is of fundamental importance that the K* meson has a finite width).