Hi Florian,

I'm having an issue that is causing my Z' boson to have the same mass at the SM Z boson, and I am unsure why. The model has 3 Z-like bosons (Z, Zp, Zpp). The mixing of the bosons and pseudoscalars is given by:
I initially had the Goldstones for Z, Zp and Zpp to be SA[1], SA[2] and SA[3] respectively, but that would result in Z and Zp having the same mass. I have also tried various other assignments of the Goldstone bosons to the Z bosons, but I seem to always get two bosons with the Z mass.

Could SARAH/SPheno be accidently fixing the mass of two Z bosons? Or did I misconfigure something?

Hi,

what's the order of your masses? Might it be that the Z' is lighter than Z?
The reason is that SPheno takes the eigenvalues of the mass matrix for the neutral gauge bosons and assigns them to gamma,Z,Z',Z''. Thus, if there is a eigenvalue smaller than 91 GeV, your Z will get this mass and Z' will have 91 GeV. However, for the SM particles, the pole masses are applied in many calculations, i.e. for M_Z the MS or DR mass is overwritten by the pole mass. Thus, if the ordering was wrong, this will overwrite the small eigenvalue.

If that's the case, you need to define your basis as {VP,VZp,VZ,VZpp}.

Cheers,
Florian

Hi Florian,

Thanks! That was indeed the issue and changing the ordering does produce distinct masses.

So it seems that SPheno can't handle a transition from a low-mass Z' to a high mass Z'? That is, the code either handles one case or the other? Though I guess there are very few scenarios where this would become an issue...

Yes, the approach right now is for sure not optimal and breaks down indeed in the case M_Z ~ M_Z'. However, so far there was no demand to improve. So, I didn't put time into that.