CalcHEP

Four-Fermion Operator

Asked by starterism

when i use feynrules to output a calchep model file, it seems like i can't output a Four-Fermion Operator.

I don't know what's wrong, should i write the operator directly into the model file?

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Neil Christensen
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Alexander Pukhov (pukhov) said : 		#1

Four fermion operator in CalcHEP/micrOMEGAs is realized via
intermediate boson field with point-like propagator.
Particle with point-like propagator is described in CalcHEP particle
table writing "*" in "aux" column.

Should you use LanHEP instead of FeynRules, all 4-fermion operator
will be spited automatically.

Best
     Alexander Pukhov

  On 04/06/2016 06:07 PM, starterism wrote:
> New question #290037 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/290037
>
> when i use feynrules to output a calchep model file, it seems like i can't output a Four-Fermion Operator.
>
> I don't know what's wrong, should i write the operator directly into the model file?
>

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Alexander Belyaev (alexander.belyaev) said : 		#2

Just to confirm what Sasha Pukhov says --
LanHEP allows to implement contact interactions
starting from the Lagrangian.

If you need exampes of the LanHEP implementation, please let us know -- we are happy to provide them to you.

Alternatively you can do this via both FeynRUles and LanHEP using auxilary non-propagating fileds.

Hope this helps

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starterism (phywxh) said : 		#3

you mean I can use auxiliary non-propagating fields via Feynrules to get Calchep model file with 4-fermion operator?

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Best Neil Christensen (neil-christensen-qft) said : 		#4

FeynRules doesn't do this automatically for you. You have to introduce a new field and rewrite the Lagrangian so the 4-fermion terms are split into interactions of 2 fermions and this new field. FeynRules will then calculate the Feynman rules for this new Lagrangian and write the CalcHEP files. Before using them, you will want to modify the particle table and set the extra field to be auxiliary. As my colleagues said, LanHEP does all this for you, so that may be a good choice if you don't want to do this yourself. These things are discussed further in the manuals.

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starterism (phywxh) said : 		#5

Thanks Neil Christensen, that solved my question.