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## Main Question or Discussion Point

Hi. I'm reading some quantum field theory and I'm a bit rusty in my relativistic kinematics. I stumbled across the formula

[tex] E_1E_2 v_{rel} = ((p_1p_2)^2 - m_1^2m_2^2)^{1/2}[/tex]

where 1 and 2 are two collinearly colliding paritcles with their respective masses and [itex]v_{rel}[/itex] are their relative velocity. My question is; how is this relation derived?

[tex] E_1E_2 v_{rel} = ((p_1p_2)^2 - m_1^2m_2^2)^{1/2}[/tex]

where 1 and 2 are two collinearly colliding paritcles with their respective masses and [itex]v_{rel}[/itex] are their relative velocity. My question is; how is this relation derived?

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