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Drell Yan processby JoePhysicsNut
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#1
Jan2913, 10:41 AM

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The Feynman diagram for Drell Yan has a gammastar or a Z as the mediator. Does gammastar simply mean a photon with a sizeable amount of energy? Is there a welldefined point in the energy of the mediator when it's a Z instead of a gammastar? 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? 


#2
Jan2913, 11:06 AM

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#3
Jan2913, 11:08 AM

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It highlights the fact that the photon is virtual (and therefore offshell).
With a W, you need ##q \bar{q'}## (one uptype and one downtype quark). I don't know how this process is called. 


#4
Jan2913, 04:57 PM

P: 35

Drell Yan process
For massive particles, a real particle is one which lies on mass shell i.e. E^2p^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 offshell? 


#5
Jan2913, 05:00 PM

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##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
Jan2913, 05:47 PM

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#7
Jan3013, 12:08 AM

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#8
Jan3013, 04:56 AM

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


#9
Jan3013, 07:56 AM

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



#10
Jan3013, 10:35 AM

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Technically, DrellYan is only virtual photon exchange. However, especially at high mass, it is indistinguishable from Z exchange, so that is often included. The LHC experiments, by analogy, have called qqbar annihilation through a virtual W "DrellYan", but it is strictly speaking not correct.



#11
Jan3013, 10:46 AM

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I see from Wikipedia that Drell and Yan described this process in 1970. Electroweak theory (including the Z and W) was just getting off the ground at that time, so it seems natural that (a) they considered only photons, and (b) people later tended to generalize to similar processes involving the other gauge bosons.



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