The invariant momentum-space volume element?

[pellman]

When we way that

$$\frac{d^3p}{p_0}=\frac{d^3p}{\sqrt{m^2+\vec{p}^2}}$$

is the invariant volume element, is that with respect to all Lorentz transformations or just proper orthochronous Lorentz transformations?

 

[WannabeNewton]

The latter

 

[pellman]

Thanks.