Problem about changing the parameters

Hi,

In the standard model, the mass of the W is given by the following relation.
cmath.sqrt(MZ__exp__2/2. + cmath.sqrt(MZ__exp__4/4. - (aEW*cmath.pi*MZ__exp__2)/(Gf*sqrt__2)))
(This is the formula used in MG5, you can open any textbook to have the formula in a more readable format)

As you can see, you have a square root of a difference.
If that difference became negative, then the W mass become complex which is of course not possible.
Since the coupling depend of cos\theta_W, the various coupling became quite weird.
this is why the partial width of the Z can turn to be negative (I guess).

Saying the same stuff in a different way, your benchmark point is not physical.
If you want a Z mass that light, you need to modify other parameter like aEW and/or Gf.

Obviously, this might not fit what you are trying to do.

Cheers,

Olivier

> On Oct 25, 2016, at 14:07, zhicheng <email address hidden> wrote:
>
> New question #403468 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/403468
>
> Dear experts,
>
> I am checking basic command in MG5_aMC_v2_4_3. I tried to change the mass MZ. Here is my command:
>
> -------------
> generate p p > l- l+
> output mz
> launch
> 0
> set mz 61you
> set wz auto
> 0
> ------------
>
> But MG5 stopped with the following error message:
>
> ------------
> Command "generate_events run_01" interrupted in sub-command:
> "set max_npoint_for_channel 0" with error:
> Exception : Partial width for z > ta- ta+ negative: -0.00460390335949
> Please report this bug on https://bugs.launchpad.net/mg5amcnlo
> More information is found in 'MG5_debug'.
> Please attach this file to your report.
> ------------
>
> If I reset MZ to a higher value(for example 62,63,64 ...), everything is ok.
> Then, I change the version to MG5_aMC_v2_5_0_beta2. There is a similar problem. When MZ is greater than or equal to 76 GeV, everything is ok. But when MZ is less than 76 GeV. MG5 stopped again with a diferent message:
>
> ------------
> Command "generate_events run_01" interrupted with error:
> Exception : Mass should be real number: particle 24 (w+) has mass: (53.1784557653+3.93041442842j)
> Please report this bug on https://bugs.launchpad.net/mg5amcnlo
> More information is found in 'ME5_debug'.
> Please attach this file to your report.
> ------------
>
> Is this normal?
> Thanks
> zhicheng
>
>
>
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