# Drell Yan process

1. Jan 29, 2013

### JoePhysicsNut

https://particle.phys.uvic.ca/twiki/pub/Main/AlisonFaulkner/Drell-Yan.jpg [Broken]

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?

Last edited by a moderator: May 6, 2017
2. Jan 29, 2013

### Bill_K

Fig 4 in this paper shows a nice peak at the Z mass.

3. Jan 29, 2013

### Staff: Mentor

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

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.

4. Jan 29, 2013

### JoePhysicsNut

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?

5. Jan 29, 2013

### Staff: Mentor

$E \neq p$ (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.

6. Jan 29, 2013

### Hepth

It also means that for a photon with 4-momentum K, K^2 is NOT 0 as it would be if "on shell".

7. Jan 30, 2013

### andrien

it can not be,can you see why.

8. Jan 30, 2013

### Bill_K

This talk on the observation of Drell-Yan processes at the LHC includes in the discussion those in which W is the intermediate particle. Other references use the term "Drell-Yan-like."

EDIT: Especially see the third slide, "The Drell-Yan processes", in which the diagram given has an intermediate W.

Last edited: Jan 30, 2013
9. Jan 30, 2013

### andrien

So a u quark and d(bar) quark annihilate each other.

10. Jan 30, 2013