Why is impact parameter not directly measurable?

[says]

I'm trying to understand a few things about the kinematics of collision processes.

I guess it's because we calculate the scattered angle of the projectile and then back calculate to get a value for the impact parameter. Is this right?

 

[Vanadium 50]

Are you talking about elementary particles or billiard balls?

If the former, a direct measurement of impact parameter would require a trajectory that is good to 10^-16 meters or so over several meters. How do you propose to do that?

 

[says]

Can you simplify you answer...

 

[Vanadium 50]

Cab you answer my questions?

 

[mfb]

For high-energy physics: Try to measure the position of a particle "somewhere in the accelerator" with a precision of 0.000000000000001 meters.

 

[ChrisVer]

to be honest I don't get how you suggested that number (E-16 meters). Does it have anything to do with point-likeness?

 

[Vanadium 50]

A proton is 10^-15 meters across. If you want to measure the impact parameter to 10%, you need to control the collision geometry to 10% of a proton radius, or about 10^-16 meters.