如图,在直四棱柱\(ABCD\)\(-\)\(A\)\({\,\!}_{1}\)\(B\)\({\,\!}_{1}\)\(C\)\({\,\!}_{1}\)\(D\)\({\,\!}_{1}\)中,底面四边形\(ABCD\)为菱形,\(A\)\({\,\!}_{1}\)\(A\)\(=\)\(AB\)\(=2\),\(∠ABC=\dfrac{\pi }{3}\),\(E\),\(F\)分别是\(BC\),\(A\)\({\,\!}_{1}\)\(C\)的中点.
\((1)\)求异面直线\(EF\),\(AD\)所成角的余弦值;
\((2)\)点\(M\)在线段\(A\)\({\,\!}_{1}\)\(D\)上,\(\dfrac{{{A}_{1}}M}{{{A}_{1}}D}=\lambda .\)若\(CM\)\(/\!/\)平面\(AEF\),求实数\(λ\)的值.