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## Homework Statement

I have an electron of 20 GeV and negligible mass that collides with a stationary proton (mc^2 = 9.38 GeV) and deflects at an angle of 5°. I'm asked to find the square of the four-momentum transfer, q

^{2}

## Homework Equations

q = P - P', where P/P' is a 4-momentum vector <p

_{x}, p

_{y}, p

_{z}, iE>

a "primed" quantity represents a value after the collision with the proton.

## The Attempt at a Solution

I took the quantity P-P' and squared it:

q

^{2}= (P-P')*(P-P') = P

^{2}+ P'

^{2}-2P*P'

I'm told the first two terms are negligible due to the electron's mass being negligible, but I'm not sure I see the sense in that.

Anyways, continuing on, I then have:

q

^{2}= -2P*P' = -2(p

_{x}*p

_{x}' + p

_{y}*p

_{y}'+p

_{z}*p

_{z}' -E*E')

or q

^{2}= -2

**p***

**p**' + 2E*E'

This is where I'm stuck. Should I also assume the product of the 3-momenta is 0 and carry on with the +2EE' term I'm left with? And what do I even do about the E' since I don't know that quantity?

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