Traces of the new a_0(1780) resonance in the J/psi rightarrow phi K^+K^-(K^0bar{K}^0) reaction

Abstract

We study the J/psi rightarrow phi K bar{K} decay, looking for differences in the production rates of K^+ K^- or K^0 bar{K}^0 in the region of 1700–1800 MeV, where two resonances appear dynamically generated from the vector-vector interaction. Two resonances are known experimentally in that region, the f_0(1710) and a new resonance reported by the BABAR and BESIII collaborations. The K bar{K} should be produced with I=0 in that reaction, but due to the different K^{*0} and K^{*+} masses some isospin violation appears. Yet, due to the large width of the K^*, the violation obtained is very small and the rates of K^+ K^- or K^0 bar{K}^0 production are equal within 5%. However, we also find that due to the step needed to convert two vectors into K bar{K}, a shape can appear in the K bar{K} mass distribution that can mimic the a_0 production around the K^* bar{K}^* threshold, and is simply a threshold effect.