" run_01_log.txt " As to internal parameter assigned to be "NaN"

Dear Jeong Han Kim,

I don't know the specifics of your model or the process you are attempting to generate. However, concerning your last questions about how to handle customized behavior of a square root functions, I would like to remind you that the UFO format allows you for defining custom functions in the model function_library.py which can be, once defined, used throughout your parameters and couplings definition.

There is a catch however, related to the fact that the definition of the function is in python and there is no way to automatically translate it into a fortran or C++ equivalent function to be used when outputting your process in various languages. There is therefore a bit more work to be done on the MG5 side to implement this new function you will be defining.
This reasonably easy to do and we could even implement that into the next release if this function is general and has potential to be helpful for many models.

If this would be a viable solution for your issue, let me know and I can further help you.

Dear Valentin,

Thanks for the reply. I still need your assistance. It's again about "cmath.sqrt" function.

I have the "parameter.py" file (in my UFO file) which reads one of my parameter as:

AAA = Parameter(name = 'AAA',
                   nature = 'internal',
                   type = 'complex',
                   value = 'cmath.sqrt(-4)',
                   texname = '\\text{AAA}')

MG5 can't read 'cmath.sqrt(-4)' for the square-root of negative value( It just returned AAA = (NaN,NaN) ).
I want MG5 to translate it correctly (in this case MG5 should return AAA = (0 , 2.00000) ).

According to your advice, I should go to "function_library.py" to define the function. So I did it as:

square = Function(name = 'square',
  arguments = ('z',),
  expression = 'cmath.sqrt(z)')

And back to my "parameter.py" file to write:

AAA = Parameter(name = 'AAA',
                   nature = 'internal',
                   type = 'complex',
                   value = 'square(-4)',
                   texname = '\\text{AAA}')

But it didn't work.... Is there any mistake here?
Thanks

Cheers !

Jeong-han Kim

Hi Jeong,

Ok, I missunderstood what you wanted to do. I thought that you wanted to be able to have a square root function that dynamically select a given Riemann sheet depending on some other parameters.

So if you simply want a function square root returning the solution with positive imaginary part when called with a negative pure real, then I suggest that you simply always make sure that the argument is a complex parameters. For example, in your case you could write:

AAA = Parameter(name = 'AAA',
                   nature = 'internal',
                   type = 'complex',
                   value = 'cmath.sqrt(complex(-4,0))',
                   texname = '\\text{AAA}')

I would expect this to work.

Hi Valentin,

Thanks that works well :D
Let me ask you one more question as an extension to this problem. Suppose now we have an external parameter M = 300 defined. Then, as you said, I wrote such that:

AAA = Parameter(name = 'AAA',
                   nature = 'internal',
                   type = 'complex',
                   value = 'cmath.sqrt(complex(-M,0))',
                   texname = '\\text{AAA}')

It didn't work in MG5. Error message is the following:

ValueError : could not convert string to fload: -M

It's strange that 'cmath.sqrt(complex(-100,0))' works, but 'cmath.sqrt(complex(-M,0))' doesn't work... What should I do?..

Many Many Thanks !

Sincerely

Jeong-han

Hi Valentin !

Wow it worked !! I'm so happy.
Thank you very much

Cheers !

Jeong-han