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i have the following exercise:

At the HERA acceleratro of DESY in Hamburg electrons and protons were accelerated to E_e=27.5 GeV and E_p = 920GeV and brought to collision.

A)How large is the reaction energy in the centre-of-mass system?

B) What is the velocity of the HERA centre-of-mass System in the lab system?

A) I think the CMS is defined by[tex]\vec{p}[/tex]_e +[tex]\vec{p}[/tex]_p=0, am i right? ([tex]\vec{p}[/tex] is the three-momentum.

Therefore, i can calculated the sum of the two four-momentum.

[tex]s=(p_p+p_e)^2= (E_p+E_e)^2 [/tex]

Reaction energy ist then sqrt(E).

B) How can i solve this?

Thank you.