Cross section for two identical particles in the final state

[krishna mohan]

I was calculating the cross section for a particular decay and saw that I was off by a factor of half from the known value..


Now, the decay has two identical particles in the final state and I seem to remember having read somewhere that this requires a factor of half...but I am not able to find the statement in standard books like Peskin(P93-symmetry factors)...maybe it is given somewhere else in Peskin...

Could anyone give me a reference for the above statement?

 

[hamster143]

The rule is that you don't sum over diagrams that differ only by vertex permutation. In your case, two vertices corresponding to final state particles are identical.

 

[krishna mohan]

No...in this case there is only one vertex...I was considering the decay of the Higgs into two Z bosons...

But I know now why there is a factor of two...

In calculating the Cross section, we integrate over all the momenta of the two Z particles, say Z_1 and Z_2, with momenta denoted by p_1 and p_2...

But in doing so we count the points

( p_1 = x , p_2 = y ) and (p_1 = y, p_2 = x )

separately...although they are infact one and the same point in phase space since Z_1 and Z_2 are identical particles...