WHIZARD

mu+ mu- -> gamma v vbar

Asked by Sheikh Farah Tabira

Hello,

for the process: mu+ mu- -> gamma v vbar, I am getting a cross section of ~75 pb and my collaborator using madgraph is getting ~2.5pb, and we are very confused as to why this big difference is arising. Below is the sindarin script I used.

Thank you!

code:

sqrts = 1000 GeV
alias nu = n1:n2:n3
alias nubar = N1:N2:N3
process mmvva = e2 , E2 => nu, nubar, A

compile

beams = e2 , E2

!beams_pol_density = @(-1), @(-1)
!beams_pol_fraction = 80%, 20%
?fatal_beam_decay = false
isr_q_max = 20 GeV
isr_alpha = 1./137.
epa_q_min = 5 GeV
epa_x_min = 0.01

integrate (mmvva)
simulate (mmvva)
show (results)

Question information

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Krzysztof Mekala Edit question
Solved by:
Sheikh Farah Tabira
Solved:
Last query:
Last reply:
Revision history for this message
Krzysztof Mekala (krzysztofmekala) said : 		#1

Dear Tabira,
the result you got from Whizard makes little physical sense because you did not include any cuts. Due to the physics of the process itself, energy cuts are mandatory to avoid divergences in the calculation (which you can probably spot easily in your Whizard log, check how large the integration errors are). By default, no cuts are applied in Whizard and they must be set by the User.

Also, if you want to use ISR/EPA, you should add such a statement to your beam definition (e.g. "beams = e2 , E2 => isr"). Otherwise, the structure functions are not applied at all. Please note that one should be careful while using ISR and generating exclusive photons in the Matrix Element calculations at the same time, as it may lead to double-counting (see e.g. [2004.14486]).

Best regards,
Krzysztof

Revision history for this message
Sheikh Farah Tabira (farahtabira) said : 		#2

thank you for your response. this was just for comparisons sake with my collaborators madgraph results. we were just trying to see if we would get the similar numbers as they did not implement any cuts (maybe madraph applies them by default?). I did use the following cuts:

cuts = all E > 8 GeV [A] and all E < 257 GeV [A]
 and all 170 degree > Theta > 10 degree [A]

and was getting about 3.3 pb.

Revision history for this message
Juergen Reuter (j.r.reuter) said : 		#3

Hi Tabira,
just to comment: MG5 uses a default cut on photons (as you should take from their manual), IIRC it's a pT_min of 10 GeV and |eta| < 2.5 or so. You should also compare the electroweak scheme, which, however, should be the same or very similar between MG5 and Whizard.
Cheers,
    JRR (Juergen)

Revision history for this message
Sheikh Farah Tabira (farahtabira) said : 		#4

Thank you.