# Drell Yan process

by JoePhysicsNut
Tags: drell yan
 P: 35 The Feynman diagram for Drell Yan has a gamma-star or a Z as the mediator. Does gamma-star simply mean a photon with a sizeable amount of energy? Is there a well-defined point in the energy of the mediator when it's a Z instead of a gamma-star? The peak of the Z resonance on an invariant mass plot is 91 GeV, but as a wide resonance it could also have less energy than that. If a W acts as the mediator, is it still classified as Drell Yan?
Thanks
P: 4,160
 Is there a well-defined point in the energy of the mediator when it's a Z instead of a gamma-star? The peak of the Z resonance on an invariant mass plot is 91 GeV, but as a wide resonance it could also have less energy than that.
Fig 4 in this paper shows a nice peak at the Z mass.
Mentor
P: 11,906
It highlights the fact that the photon is virtual (and therefore off-shell).

 Is there a well-defined point in the energy of the mediator when it's a Z instead of a gamma-star?
Both are possible, and their relative contribution depends on the energy of the process.

With a W, you need ##q \bar{q'}## (one up-type and one down-type quark). I don't know how this process is called.

P: 35
Drell Yan process

 Quote by mfb It highlights the fact that the photon is virtual (and therefore off-shell).

For massive particles, a real particle is one which lies on mass shell i.e. E^2-p^2*c^2=m^2*c^4. For a virtual particle the equals sign doesn't apply and one can get different values for m other than the true value of m.

However, a photon is massless, so what does it mean for a photon to be off-shell?
 Mentor P: 11,906 ##E \neq p## (or ##E\neq pc## if you don't like c=1) - something which is satisfied for real photons. Real Photons have the same equation like massive particles, they just have m=0 there.
PF Gold
P: 471
 Quote by JoePhysicsNut Thank you for your replies! For massive particles, a real particle is one which lies on mass shell i.e. E^2-p^2*c^2=m^2*c^4. For a virtual particle the equals sign doesn't apply and one can get different values for m other than the true value of m. However, a photon is massless, so what does it mean for a photon to be off-shell?
It also means that for a photon with 4-momentum K, K^2 is NOT 0 as it would be if "on shell".
P: 1,020
 If a W acts as the mediator, is it still classified as Drell Yan?
it can not be,can you see why.