Abnormal isospin violation and $a_0 -f_0$ mixing in the $D_s^+ \to \pi^+ \pi^0 a_0(980) (f_0(980))$ reactions

Abstract

We have chosen the reactions $D_s^+ \to \pi^+ \pi^0 a_0(980) (f_0(980))$ investigating the isospin violating channel $D_s^+ \to \pi^+ \pi^0 f_0(980)$. The reaction was chosen because by varying the $\pi^0 a_0(980) (f_0(980))$ invariant mass one goes through the peak of a triangle singularity emerging from $D_s^+ \to \pi^+\bar K^* K$, followed by $\bar K^* \to \bar K \pi^0$ and the further merging of $K \bar K$ to produce the $a_0(980)$ or $f_0(980)$. We found that the amount of isospin violation had its peak precisely at the value of the $\pi^0 a_0(980) (f_0(980))$ invariant mass where the singularity has its maximum, stressing the role of the triangle singularities as a factor to enhance the mixing of the $f_0(980)$ and $a_0(980)$ resonances. We calculate absolute rates for the reactions and show that they are within present measurable range. The measurement of these reactions would bring further information into the role of triangle singularities in isospin violation and the $a_0 -f_0$ mixing in particular and shed further light into the nature of the low energy scalar mesons.