Comment 1 for bug 1818393

Dear Sam Witte.

Thank you for your mail.

I'll try to use your example to generate error message in next version
of  micrOMEGAs.

I guess the problem is caused by unitary gauge of Z' vector field.  We
have lost of precision because diagram cancellation.

Because your Z' interacts only with conserved current,  you can treat it
in Feynman gauge  without auxiliary ghosts. Just  write symbol G in
'aux' field in particle table. and call 'make'  before next launch of
micrOMEGAs main.

Other solution which  increases  energy limit is  the usage of other
types of  real numbers.

You can  open file CalcHEP_src/include/nTypes.h and   comment out one of
lines

//#define _LONG_      to use long double type in numerical calculations
//#define _QUAD16_    to use quadrouple precision for icc compiler.

In the second case you will need     icc compiler.  Use "make clean" and
"make"  for all micrOMEGAs package  after correction of  nTypes.h.

But this solutions are temporal. At larger  rate Mcd/mZ'  you again will
meet a problem with precision. The best way is to declare Z' as a gouge
particle.

Best

     Alexander Pukhov

  On 03.03.2019 13:12, Sam Witte wrote:
> Public bug reported:
>
> I'm encountering an error message while running n_calchep for a very
> simple model in which a Z' couples to first generation leptons (with
> purely vectorial couplings) and to a dirac dark matter candidate (with
> purely vectorial couplings). When the ratio of the dark matter coupling
> to standard model coupling is large (ie one has a dark sector-like
> model), and when the Z' mass is less than about 2 \times 10^3 M_{dark
> matter} (for example, take the dark matter mass of 3 TeV, the Z' mass of
> 0.1 GeV, dark matter couplings of 1, and SM couplings of 0.1), I find an
> error in the calculation of the cross section, with the error message
> reading 'Lost of precision in SQME'. I've tried adjusting things like
> the precision of the calculation, and I've tested this with 3 different
> model files (written by 3 different individuals).
>
> Have you encountered a problem like this before, and if so is there an
> easy fix?
>
> Thank you!
>
> ** Affects: calchep
> Importance: Undecided
> Status: New
>