CalcHEP

How to define a process

Asked by Angel

Hi,

I was trying to generate events using some model I found in some web page that goes like

gluon gluon ->Graviton->Vector boson, Vector Boson-> quark quark quark quark

So I thought of doing

Process: G,G->G*
Decay: G*->V,V
Decay: V->q,q
Composite: V=W+,W-
Composite: q=u,U,d,D,c,C,s,S,b,B

But I also could do:

Process: G,G->V,V
Decay: V->q,q
Composite:V=W+,W-
Composite: q=u,U,d,D,c,C,s,S,b,B
Remove: A,Z,h....(all particles that give me sub processes that I dont need)

Or I also coud do:

Process: G,G->q,qb,q,qb
Composite: qb=U,D,C,S,B
Composite: q=u,d,c,s,b
Remove: A,Z,h....(all particles that give me sub processes that I dont need)

I am getting different distributions between the first and the second one and different cross sections between the first two and the third one. Are these three ways of inputting the process supposed to give me the same result? if not which one should I use? I made sure that I excluded all the particles that I had to exclude and I checked the diagrams in the GUI.

Thanks.

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Neil Christensen (neil-christensen-qft) said : 		#1

The first example forces G* and W+,W- to be on-shell and drops the spin correlation for all three.

The second allows G* to go off-shell and keeps the spin-correlation for G*. W is still on-shell and the spin correlation is lost.

The third keeps all the spin correlation and allows all intermediate particles to go on-shell.

The distributions probably depend on the spin-correlation and that is why they are different.

For the cs, I would need to know more about what you did. If I had to guess, it is because you have not really done the same diagrams in the 3rd example or you have violated gauge invariance.

About which one to use, it depends on what you are trying to accomplish and how sensitive it is to spin-correlation. Based on what you have said, it appears that your distributions are sensitive to the spin-correlation. But, it sounds like from what you said that the spin-correlation of the W may not be important.

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Angel (campoverdeangelf) said : 		#2

Ok, I think I understand, what I dont write in the file is taken as off shell and the third way gave me a slightly bigger cross section than the first two because in the third one I include all events in which the Ws and the graviton goes off shell. I guess I have to use the second one, because we are planning to compare everything to data and data must include also the events with W's and Gravitons that are off shell.

When you say spin correlations I guess you mean that you throw away information about the spin and replace it with something random, like default, so that the angular distributions of the decay products might be different, right?

Revision history for this message
Neil Christensen (neil-christensen-qft) said : 		#3

Hi Angel,

When a particle is produced and then later decayed, its spins are summed over in the production matrix element and _independently_ averaged over in the decay matrix element. The spins are not connected between production and decay. In terms of Feynman diagrams, if the full diagram is included you use the full propagator which connects the spins of the production and decay. But, when you cut it and decay separately, this is lost.

Cheers,
Neil