Problem with beam polarisation in equivalent photon approximation (EPA)
Dear authors,
I tried to generate a process:
alias quark = u:d:s:c:b
alias QUARK = U:D:S:C:B
process aeqqn = A, E1 => ( quark, QUARK, N1 )
using EPA in e+e- collisions at 1.5 TeV CLIC:
beams = e1, E1 => circe2 => epa, isr
epa_x_min = 0.013333333 # 10 GeV from 750 GeV beam
epa_mass = me
epa_q_max = 4 GeV
Without beam polarisation the calculated cross section is about 4790 fb.
When I set electron beam polarisation the cross section is exactly twice as large (9580 fb) for both -80% and +80% polarisation. Something has to be wrong in my opinion, as the unpolarised cross section should be the average of -80% and +80% cross sections (the positron beam is assumed not to be polarised). The cross section is the same for any polarisation setting. Even if I set it to 0%:
beams_pol_density = @(-1), @(+1)
beams_pol_fraction = 0%, 0%
it is 9580 fb. But when I remove these lines (comment them out) it is back to 4790 fb...
I use same circe2 file in all cases (standard CLIC file). I tested it with version 2.7.0 and 2.8.2 (same results).
I will be grateful for your reply. Which cross section number is the correct one?
Regards
Filip
Question information
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- WHIZARD Edit question
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- Wolfgang Kilian Edit question
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