The invariant momentum-space volume element?

pellman
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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?
 
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The latter
 
Thanks.
 

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