Name: V. Shtabovenko Date: 05/27/19-07:05:08 PM Z
Hi,
well, you process (some Yukawa 2->2 ?) looks simple enough to
recalculate
it by hand and check whether the-so obtained result is correct. Then
it
should be
easy to see what goes wrong with your code. If it is a text-book
example, the correct
answer should be provided somewhere.
I don’t see immediate technical errors, but there is always a chance
that something
got messed up with the kinematics or the phase space integral
parametrization, i.e. things that one calculates separately and then
puts into FeynCalc code by hand.
If you can’t get the right result with a pen and paper calculation,
I’d
try Physics.StackExchange where you are more likely to get useful
feedback very soon.
At the moment it seems to me that your question is more about physics
that FeynCalc and Physics.SE is might be a much better place to ask
such
questions.
Cheers,
Vladyslav
Am 27.05.19 um 15:28 schrieb Daniel:
> Hello,
>
> I’m trying to calculate a total cross-section for a simples process
but I am getting negative values for some input masses:
>
> ScalarProduct[p1, p1] = mx^2;
> ScalarProduct[k1, k1] = mx^2;
> ScalarProduct[p2, p2] = mq^2;
> ScalarProduct[k2, k2] = mq^2;
> ScalarProduct[p1, p2] = (s - mx^2 - mq^2)/2;
> ScalarProduct[k1, p1] = -((t - 2 mx^2)/2);
> ScalarProduct[p2, k1] = -((u - mx^2 - mq^2)/2);
>
> Ma = (yx*yf)/(SP[k1 - p1] - m^2) Spinor[k1,
mx].Spinor[p1, mx] Spinor[
> k2, mq].Spinor[p2, mq];
>
> Ma = (yx*yf)/(SP[k1 - p1] - m^2) Spinor[k1,
mx].Spinor[p1, mx] Spinor[
> k2, mq].Spinor[p2, mq];
>
> MM = 1/4 Ma2 /. DiracTrace -> Tr /. k2 -> -k1 + p1 + p2 //
> ExpandScalarProduct // PropagatorDenominatorExplicit // Simplify
>
> Expand[MM /. u -> mx^2 + mq^2 - t - s ] //
ExpandScalarProduct //
> PropagatorDenominatorExplicit // Simplify ;
>
> ((-2 mq^2 + t) (-4 mx^2 + t) yf^2 yx^2)/(m^2 - t)^2
>
> Expand[MM /. u -> mx^2 + mq^2 - t - s /. t -> -px^2 (1 -
Cos[\[Theta]])] ;
>
>
> Integrate[(yf^2 yx^2 Sin(\[Theta])(-4 mx^2+px^2
Cos(\[Theta])-px^2) (-3 mq^2+mx^2+px^2
Cos(\[Theta])-px^2))/(m^2-px^2 Cos(\[Theta])+px^2)^2,
{\[Theta], 0, Pi}]
>
> The result of the above integral is:
>
> Sol12 = 1 - ((m^2 - 4 mx^2) (m^2 - 3 mq^2 + mx^2))/(
> px^2 (m^2 + 2 px^2)) - (-m^4 + 4 mx^2 (-3 mq^2 + mx^2) +
> m^2 (3 mq^2 + 3 mx^2 - px^2) +
> m^2 (-2 m^2 + 3 (mq^2 + mx^2)) Log[m^2])/(
> m^2 px^2) - ((2 m^2 - 3 (mq^2 + mx^2)) Log[m^2 + 2
px^2])/px^2;
>
> Then for some input masses it takes negative values:
>
> N[Sol12 /. mq -> 1 /. mx -> 100 /. m -> 1000 /. px ->
10]
> -0.000793574
>
> Thanks in advance for any help.
>
> Daniel
>