How to make MuPAD know a limit is zero?

When I type in the following code, MuPAD returns a complicated answer that includes the limit as x approaches negative infinity. How do I get it to realize that the quantity it returns labelled sigma3 is zero. I thought that by defining Ws and rho as real and positive it would realize this.

Thanks

    reset();    
    assume(U, Type::Real and Type::Positive);    
    assume(Wc, Type::Real and Type::Positive);
    assume(Ws, Type::Real and Type::Positive);
    assume(`ρ`, Type::Real and Type::Positive);
    A:=cos(2*U)+((Wc)/U)*sin(2*U);
    B:=exp((Ws/`ρ`)*(x+(2*`ρ`)));
    C:=cos(Q*((x/`ρ`)+2));
    S:=sin(Q*((x/`ρ`)+2));
    F:=U*sin(2*U)-Wc*cos(2*U);
    G:=A*B*(A*C+((1/Q)*F*S));
    R:=int(G, x=-infinity..(-2*`ρ`),IgnoreAnalyticConstraints);