15. Given the function $f(x)=\frac{4 \mathrm{e}^{x-1}}{x+1}+x^{2}-3 a x+a^{2}-1$, if there exists $x_{0}>0$, such that $f\left(x_{0}\right) \leqslant 0$, find the minimum possible value of the real number $a$.