bild1=Plot[0,{mu,-2,0},AxesLabel->{"mu","x"},AxesOrigin->{-2,-2},
DisplayFunction->Identity];
bild2=Plot[0,{mu,0,2},PlotStyle->{{Dashing[{0.01}]}},
DisplayFunction->Identity];
bild3=Plot[Sqrt[mu],{mu,0,2},AxesLabel->{"mu","x"},
DisplayFunction->Identity];
bild4=Plot[-Sqrt[mu],{mu,0,2},AxesLabel->{"mu","x"},
DisplayFunction->Identity];
abb=Show[bild1,bild2,bild3,bild4,ImageSize->{400,300},
DisplayFunction->$DisplayFunction];