gamma fusion at e+e-

Asked by Roberto Franceschini on 2020-03-06

I am succesfully generating non-zero cross-sections for a a > f f~ at e+ e- colliders through

     3 = lpp1 ! beam 1 type
     3 = lpp2 ! beam 2 type

However if I try a resonant production a a > resonance the cross-section is always zero.

The resonance is very light compared to the CoM energy, so the only thing I can imagine is the particle being too narrow, but I would have said with the sintax 2>1 above the width should not even be called in the calculation.

1) A massive s-channel particle has a width set to zero. (it is light enough)
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s. (unclear if this applies for 3 = lpp1)
   3) The cuts are too strong. (no cuts on the resonance)
   Please check/correct your param_card and/or your run_card.

Am I asking something MG is not supposed to compute?

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
2020-03-08
Last reply:
2020-03-15

Could you send me the banner of the run?

Cheers,

Olivier

Sure! Thanks for looking into it!

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set group_subprocesses Auto
set ignore_six_quark_processes False
set loop_optimized_output True
set loop_color_flows False
set gauge unitary
set complex_mass_scheme False
set max_npoint_for_channel 0
import model sm
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
import model axion_model
generate a a > r0 /z
output Belle2/ee_aa2ALP
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Ah you are using the default dynamical scale which includes a cut-off when such scale is too small.
Additionally for photon from electron/proton the dynamical scale choice is use as the cutoff of the effective Photon approximation.
So I do not think that it makes sense to use the default dynamical scale in that case.

In 2.7.1, we have actually changed the behavior to force you to run in fixed factorization scale for your process.
(and we also force you to not use MZ for the cut-off scale)

Cheers,

Olivier

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