如图，三棱柱\(ABC-A_{1}B_{1}C_{1}\)的底面是边长为\(2\)的正三角形，侧面\(ACC_{1}A_{1}⊥\)底面\(ABC\)，且侧面\(ACC_{1}A_{1}\)为菱形，\(∠A_{1}AC=60°\)，\(E\)是\(BB_{1}\)的中点，\(F\)是\(AC_{1}\)与\(A_{1}C\)的交点.
\((1)\)求证：\(EF/\!/\)底面\(ABC\)；
\((2)\)求\(BC\)与平面\(A_{1}AB\)所成角\(θ\)的正弦值.