WHIZARD

How to add an "OR" restriction in a given process?

Asked by David d'Enterria (CERN)

Hello,

I'm interested in the following process: e+e- -> Z Z* with Z->l+l- or Z->qqbar, below the ZZ threshold. Namely, e+e- -> 2l2j where the 2 charged leptons OR the 2 jets have an invariant mass at the Z boson mass (and the Z* decays also into leptons or jets, but off-shell so no mass reconstruction is possible). I naively tried the following syntax:

process eeZZ_2j2l = "e+","e-" => (lepton, lepton, jet,jet) { $restrictions = "1+2 ~ Z || 3+4 ~ Z" }

but it does not exist in Whizard.

Alternatively, adding these 2 processes:
process eeZZ_2j2l = "e+","e-" => (lepton, lepton, jet,jet) { $restrictions = "1+2 ~ Z" }
process eeZZ_2j2l = "e+","e-" => (lepton, lepton, jet,jet) { $restrictions = "3+4 ~ Z" }
would lead to a double counting of processes.

How can one implement an "OR" in the restrictions as above?

Thanks again.

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WHIZARD Edit question
Assignee:
Thorsten Ohl Edit question
Solved by:
David d'Enterria (CERN)
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David d'Enterria (CERN) (dde) said : 		#1

Apologies for asking the question before trying out other alternatives. I realize now that the above syntax does not seem to require either an onshell Z boson (just a Z or Z* boson exchanged). so I'll try to go through requiring explicitly a Z boson in the final state (followed then by its decay "unstable Z (....)")

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Thorsten Ohl (thomega) said : 		#2

Hi David,

there used to be a "||" operator implementing a logical OR in the syntax for process restrictions. However, since it acted on the recursive representation of the amplitudes and not on the corresponding set of Feynman diagrams, it did not do what one would expect using ones physics intuition. Therefore, I removed it again.

Note that the particles are counted starting with the initial state particles. In e+ e- -> mu+ mu- u ubar, the electrons and 1+2, the muons 3+4 and the quarks 5+6.

Note that the $restrictions only affect the amplitudes and are completely separate from the phase space sampling. As you noticed in your second post, in

  "e+","e-" => (lepton, lepton, jet,jet) { $restrictions = "3+4 ~ Z && 5+6 ~ Z" }

diagrams will be selected that have two Zs in the intermediate state, but the the kinematics will not be restricted so that the invariant masses are close to MZ (of course, the cross section will have a resonance there).

If I understand you correctly, you could just generate the complete process and apply the corresponding cuts. If you really need to speed things up, you could remove all photon propagators with $restrictions = "!A", when you cut out the low invariant mass events anyway.

Cheers,
-Thorsten

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David d'Enterria (CERN) (dde) said : 		#3

Hi Thorsten,

Thanks for your prompt feedback.

- Maybe it would be worth to add a mention in the manual that explains the reason behind the absence of a logical OR in the list of restrictions (other whizard users may have the same question?)

- Thanks for your clarification about the counting convention of the particles. From the manual examples in the process restrictions, it wasn't absolutely clear for me that the counting 1+2 started with the initial particles (adding the example: "In e+ e- -> mu+ mu- u ubar, the electrons and 1+2, the muons 3+4 and the quarks 5+6" would maybe avoid misunderstandings).

Thanks again,

David