How come relative velocity of the beams can be expressed by(adsbygoogle = window.adsbygoogle || []).push({});

[itex]v_{12} = \left| \vec{v}_1 - \vec{v}_2 \right| = \left|\frac{\vec{p}_1}{E_1} - \frac{\vec{p}_2}{E_2}\right|[/itex]

where [itex]\vec{p}_{1,2}[/itex] and [itex]E_{1,2}[/itex] is the momenta and energies of incoming particles, respectively?

Similar equation is in Peskin & Schroeder, p 105. The dimensions don't add up ...

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# Relative velocity of beams

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