# Elementary question on pion-proton scattering cross-section

Is $E_k$ the energy of the outgoing or incoming pion?

The first peak is supposed to be a delta baryon.

What does the graph tell us, experimentally? A pion of kinetic energy x comes in, then we look at the graph and find the cross-section, so then we have the probability of what happening? Or is $E_k$ the net loss in kinetic energy of the pion, and this graph tells us the distribution of its energy loss?

How is this graph constructed, what is measured?

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I am not sure but I think $E_k$ cannot be the incoming pion kinetic energy. From the graph, this energy is about $250\ MeV$. If the target protons are at rest, this is not enough energy to produce Δ resonances which have rest masses larger than $1\ GeV$ –  Dario Far May 1 '13 at 10:31
What reference did the graph come from? –  Qmechanic May 1 '13 at 10:42
Subatomic physics, Henley 3rd edition –  I am very happy May 1 '13 at 10:50
@DarioAlexander The proton is excited to a delta (as opposed to a whole new particle being created), so it is the difference of rest masses which matters. That's only around 290 MeV. –  dmckee May 1 '13 at 12:47
@dmckee, yes your right –  Dario Far May 1 '13 at 13:02