  One of the major themes in the physics of condensed matter is unconventional
superconductivity in correlated electronic systems. The symmetry structure of
Cooper pairs is thought to be the key to the understanding of the pairing
mechanism of their superconductivity. In iron pnictide superconductors, the
precise symmetry of the order parameter as well its evolution with doping
remains highly controversial. There is no general consensus on the nature of
pairing in iron-based superconductors leaving the perspectives ranging from
$s^{++}$ wave, to $s^{\pm}$, and $d$ wave. Here we show that doping induced a
sudden change of the pairing symmetry from the $s^{++}$- to $d$-wave in the
BaFe$_{2-x}$Ni$_{x}$As$_{2}$ system with an $s^{\pm}$ state near the crossover
boundary at $x=0.15$. The presence of the $s^{\pm}$ state is of particular
scientific interest because an intermediate phase breaks the pairing symmetry
when the $s$- and $d$-wave phases come together to interfere. For $x <$ 0.15,
the temperature dependence of the London penetration depth, lower critical
field, and specific heat in the superconducting (SC) state provides strong
evidence for a two-band $s$-wave order parameter. Upon doping for $x >$ 0.15,
we find the temperature and magnetic-field contributions to the specific heat
in $T^{2}$ and $\sqrt{H}$, respectively, indicating line nodes in the SC gap.
These findings thus clarify the symmetry nature as well as the evolution of the
SC order parameter with doping in pnictide superconductors.
