CalcHEP

negative decay widths for new particles

Asked by vandana sahdev

I have made a model file in FeynRules and on running the files in calchep, the decay width for some of the new particles is coming out to be negative. For example, there are new quark-like particles, U and D, like the SM uL and dR, respectively. The decay width for the process, U -> W+, D is calculated to be -4.388*10^(-1). I calculated it using theEnter Process -> Square diagrams -> Make & Launch n_calchep -> total width.

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Alexander Belyaev
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Best Alexander Belyaev (alexander.belyaev) said : 		#1

Hello,

in which gauge did you create the model?
Default gauge is t'Hoof-Feynman one in CalcHEP,
however, you can switch to Physical(Unitary) one.

Negative width in t'Hooft-Feynman gauge is the sign of absense of gauge
invariance.

Regards,
Alexander

On 17/10/18 08:08, vandana sahdev wrote:
> New question #675273 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/675273
>
> I have made a model file in FeynRules and on running the files in calchep, the decay width for some of the new particles is coming out to be negative. For example, there are new quark-like particles, U and D, like the SM uL and dR, respectively. The decay width for the process, U -> W+, D is calculated to be -4.388*10^(-1). I calculated it using theEnter Process -> Square diagrams -> Make & Launch n_calchep -> total width.
>

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Revision history for this message
vandana sahdev (vandana.sahdev) said : 		#2

Thanks Alexander Belyaev, that solved my question.

Revision history for this message
vandana sahdev (vandana.sahdev) said : 		#3

Thanks! The problem of negative decay widths was solved when I put 'Force Unitary Gauge=ON' in calchep.